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\DOI{10.5802/crbiol.172}
\datereceived{2024-08-08}
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\begin{noXML}

\CDRsetmeta{articletype}{opinion}

\title{The Anthropocene and the biodiversity crisis: an
eco-evolutionary perspective}

\alttitle{L'anthropoc\`{e}ne et la crise de la biodiversit\'{e} : une
perspective \'{e}co-\'{e}volutive}

\author{\firstname{Philippe} \lastname{Jarne}\CDRorcid{0000-0002-6291-1999}}
\address{CEFE, CNRS, Univ Montpellier, IRD, EPHE, 1919 route de Mende,
34 293 Montpellier Cedex 5, France}
\email{philippe.jarne@cefe.cnrs.fr}

\begin{abstract}
A major facet of the Anthropocene is global change, such as climate
change, caused by human activities, which drastically affect
biodiversity with all-scale declines and homogenization of biotas. This
crisis does not only affect the ecological dynamics of biodiversity,
but also its evolutionary dynamics, including genetic diversity, an
aspect that is generally neglected. My tenet is therefore to consider
biodiversity dynamics from an eco-evolutionary perspective, {i.e.}{} 
explicitly accounting for the possibility of rapid evolution and its
feedback on ecological processes and the environment. I represent the
impact of the various avatars of global change in a temporal
perspective, from pre-industrial time to the near future, allowing to
visualize their dynamics and to set desired values that should not be
trespassed for a given time (e.g., +2 \textdegree C for 50 years from
now). After presenting the impact of various stressors (e.g., climate
change) on biodiversity, this representation is used to heuristically
show the relevance of an eco-evolutionary perspective: (i) to analyze
how biodiversity will respond to the stressors, for example by seeking
out more suitable conditions or adapting to new conditions; (ii) to
serve in predictive exercises to envision future dynamics (decades to
centuries) under stressor impact; (iii) to propose nature-based
solutions to the crisis. Significant obstacles stand in the way of the
development of such an approach, in particular the general lack of
interest in intraspecific diversity, and perhaps more generally a lack
of understanding that, we, humans, are only a modest part of
biodiversity.
\end{abstract}

\begin{altabstract}
Un aspect majeur de l'Anthropoc\`{e}ne est le changement global, par
exemple le changement climatique, qui est caus\'{e} par les
activit\'{e}s humaines et conduit \`{a} un d\'{e}clin drastique de la
biodiversit\'{e} et une homog\'{e}n\'{e}isation des
\'{e}cosyst\`{e}mes. Cette crise n'affecte pas seulement la dynamique
\'{e}cologique de la biodiversit\'{e}, mais \'{e}galement sa dynamique
\'{e}volutive, y compris la diversit\'{e} g\'{e}n\'{e}tique, un aspect
g\'{e}n\'{e}ralement n\'{e}glig\'{e}. Il est donc n\'{e}cessaire de
consid\'{e}rer la dynamique de la biodiversit\'{e} d'un point de vue
\'{e}co-\'{e}volutif, c'est-\`{a}-dire en tenant compte explicitement
de la possibilit\'{e} d'une \'{e}volution rapide et de sa
r\'{e}troaction sur les processus \'{e}cologiques et l'environnement.
Je repr\'{e}sente l'impact des diff\'{e}rents avatars du changement
global dans une perspective temporelle, de l'\'{e}poque
pr\'{e}industrielle au futur proche, ce qui permet de visualiser leur
dynamique et de fixer des valeurs souhaitables \`{a} ne pas
d\'{e}passer pour une p\'{e}riode donn\'{e}e (par exemple, +2
\textdegree C dans 50 ans). Apr\`{e}s avoir pr\'{e}sent\'{e} l'impact
de divers facteurs de stress (par exemple, le changement climatique)
sur la biodiversit\'{e}, cette repr\'{e}sentation est utilis\'{e}e pour
montrer la pertinence d'une perspective \'{e}co-\'{e}volutive : (i)
pour analyser comment la biodiversit\'{e} r\'{e}pondra aux stress
environnementaux, par exemple en recherchant des conditions plus
appropri\'{e}es ou en s'adaptant \`{a} de nouvelles conditions ; (ii)
pour servir dans des exercices pr\'{e}dictifs afin d'envisager les
dynamiques futures (d\'{e}cennies \`{a} si\`{e}cles) sous l'impact de
ces stress ; (iii) pour proposer des solutions \`{a} la crise
fond\'{e}es sur la nature. Il reste des obstacles importants sur la
voie du d\'{e}veloppement d'une telle approche, en particulier le
manque d'int\'{e}r\^{e}t g\'{e}n\'{e}ral pour la diversit\'{e}
intrasp\'{e}cifique, et peut-\^{e}tre plus g\'{e}n\'{e}ralement un
manque de compr\'{e}hension du fait que nous, les humains, ne sommes
qu'une modeste partie de la biodiversit\'{e}.
\end{altabstract}

\keywords{\kwd{Biodiversity}
\kwd{Anthropocene}
\kwd{Ecology}
\kwd{Evolution}
\kwd{Eco-evolutionary feedbacks}
\kwd{Prediction}
\kwd{Green solutions}}

\altkeywords{\kwd{Biodiversit\'{e}}
\kwd{Anthropoc\`{e}ne}
\kwd{\'{E}cologie}
\kwd{\'{E}volution}
\kwd{R\'{e}tro-actions \'{e}co-\'{e}volutives}
\kwd{Pr\'{e}dictions}
\kwd{Solutions vertes}}

\thanks{CNRS-Ecologie \& Environnement}

\maketitle

\vspace*{1pt}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{secI}

Biodiversity is classically defined as the variety of life on Earth,
from the within-species to the ecosystem level, including both biotic
and abiotic components and their complex interactions~\cite{Wilson1999}.
Resulting from more than three billion years of biological and
geochemical co-evolution, it has also largely contributed to the
Earth's geochemical history, adding colossal masses of, for example,
carbonaceous structures (coal, oil, etc.), particularly via
photosynthetic activity or the construction of vast coral reefs, over
the last billion years 
(see~\cite{Judson2017,Littleetal2023,Lynch2024,Szathmary2015} for a quick overview). 
Biodiversity is also the product of a history of\unskip\break biological
complexification and diversification, from the rise of the first living
structures around 3.5 to 4~billion years ago, to very large
multicellular organisms, via the appearance of multiple bacterial or
eukaryotic groups and increasingly complex ecological chains, often
under extreme living conditions 
(see \cite{Babajanyanetal2023,Lynch2024,Szathmary2015}).
This unfolding in time and space with the expansion of continents,
their collisions and their separation, has also been marked by
large-scale events, due to causes both internal and external to the
Earth (e.g., a giant meteorite impact at the Cretaceous--Paleogene
boundary 65~million years ago), freeing up evolutionary spaces and
leading to the domination of new groups 
(e.g., \cite{Butterfield2018,Jablonski2008,Sepkoski2020,Vermeij2017}).
As a consequence of this long ecological and evolutionary history,
biodiversity can be considered as a complex adaptive system or
network \cite{Holling1973,Levinetal2013,SoleLevin2022}. 


A (retrospectively) singular event took place 3 to 4~million years
ago, somewhere in Africa---from the lineage of the
African--Asian great apes, evolved hominid species that straightened up,
freeing their hands for tool use, and developing large brains and
intense social skills 
(e.g., \cite{KaplanRobson2002,AyalaCelaConde2017,Antonetal2014,Grabowskietal2015}). 
While these developments, and a few others, certainly did not make
these species more competitive in any particular respect (for example,
many of them were already present in other mammals; these hominids,
taken individually, were no match for a lion), they did make them
surprisingly plastic and adaptable to various environmental conditions.
The bushy evolution of these species resulted in the emergence of
(self-named) \textit{Homo sapiens}. Through its ability to manipulate
the environment (e.g., fire or stone) and to handle artifacts,
\textit{H.~sapiens} began to transform the planet, to such an extent
that Crutzen~\cite{Crutzen2002} proposed to call our era the
Anthropocene\footnote{``Civilization is living now under a syndrome of
too many people, with those in `developed' parts of the world consuming
an unfair share of Earth's resources, and all together using an
unsustainable fraction of our planet's natural capital'' \cite{Dirzoetal2022}.}.
The very notion of Anthropocene remains debated among scientists---for
example, it has not been adopted as an era by the International Union
of Geological Sciences~\cite{Watersetal2024}, but one can consider that
the human impact began about 50,000 years ago and became serious at the
beginning of the Holocene, some 11,000 years ago~\cite{Ellis2015,Ellis2021}.
This impact is today as powerful as that of geological or climatic
forces, in many ways much more rapid, and quite clearly the origin of
the current biodiversity crisis\break
\cite{TheConcordian2007,Ellis2015,Ellis2023,Folkeetal2021,Littleetal2023,TurveyCrees2019}.

This crisis can be defined as biodiversity general collapse and
homogenization---recall that the term biodiversity was coined
in the mid-1980s in response to the perception that biodiversity was in
crisis~\cite{Wilson1999}. There is little doubt that it is caused by
environmental (e.g., climate change) and biotic (e.g., biological
invasions) changes induced by human activities
\cite{TurveyCrees2019,Cowieetal2022,DiazMalhi2022,Dirzoetal2022,Rigaletal2023}.
These activities have created new and impactful environmental regimes,
so much so that biodiversity studies can hardly be conducted without
firmly integrating human impacts 
\cite{Folkeetal2021,Kefietal2022,Ellis2015,Currieetal2023,Ellis2023}.
It should also be highlighted, and regretted, that this crisis has
attracted far less attention than climate change in the general public,
stakeholders and politicians, although the two are more than
intertwined---the biodiversity crisis, in fact, covers all aspects of
the Anthropocene, including the climate
crisis~\cite{Diazetal2018,Dirzoetal2022,Kefietal2022,Portneretal2021,Rigaletal2023}. 

The Anthropocene has been the subject of a large number of studies,
particularly with regard to the Earth's biogeochemical functioning and
its impact on biodiversity. The ecological sciences in particular have
been widely mobilized to understand the biodiversity crisis
\cite{Ellis2015,DiazMalhi2022,Dirzoetal2022,Meyeretal2022},  but the
focus has been on species ecology, i.e.\ the human impact on the spatial
and temporal dynamics of individuals or ecosystems. However, less
attention has been paid to the impact on intraspecific diversity or
adaptive potential (in other words, how life can adapt to change), and
thus to longer-term consequences, i.e.\ considering the evolutionary
dynamics of biodiversity 
\cite{Carrolletal2014,GaitanEspitiaHobday2021,Stangeetal2021,Urbanetal2024}.
We note, however, that Folke et~al.~([\citenum{Folkeetal2021}], Figure~6)
mention genetic diversity as one of the planetary limits for which we
are in a high-risk zone, and that considering this diversity in
management practices has been addressed in the evolutionary literature
for several decades \cite{Soule1985,KinnisonHairston2007}. This major
change in ecological regime and conditions not only impacted
biodiversity per se, but also the conditions (forces, drivers and
processes) of its evolution---such a fast disruption by a single
species has never happened, and will have short- and long-term
evolutionary consequences. Moreover, even if it is rising, the
appreciation of the interaction between ecology and evolution, through
eco-evolutionary feedbacks, remains undervalued when it comes to
tackling the biodiversity crisis in the short term.

The general question addressed here is why we should take intraspecific
diversity and adaptive potential, and the eco-evolutionary feedback
loops that arise from them, into account in our understanding,
prediction and management of the biodiversity crisis in the
Anthropocene. The paper is structured around five main sections: in
Section~\ref{secII}, I briefly present the major avatars of the
Anthropocene, i.e.\ stressors such as climate change, setting them in a
short-term temporal perspective (e.g., how they have and will evolve
over decades, and how we can set targets for reducing their magnitude
and impact), before providing in Section~\ref{secIII} an analysis of 
the\unskip\break
ecological crisis and its impact on biodiversity through a (forward)
temporal perspective. In Section~\ref{secIV}, I outline the reasons why
biodiversity should be considered from an eco-evolutionary perspective,
including how species respond to environmental drivers and how
eco-evolutionary feedbacks are affected. Section~\ref{secV} is devoted
to predictions about future biodiversity, focusing on the short term (a
few decades). For example, how are predictions modified when evolution
is accounted for, and is this relevant? Or is the crisis too violent to
worry about evolution? Section~\ref{secVI} is devoted to action,
integrating an eco-evolutionary perspective in the various Nature-based
solutions to the biodiversity crisis, i.e.\ solutions that take
biodiversity into\break account.


\section{The many avatars of the Anthropocene}\label{secII}

Over its long-term evolution, biodiversity has never, in essence, been
stable, and has varied more or less rapidly under the influence of both
ecological and evolutionary processes. However, the recent irruption of
humans imposes/ed biotic or abiotic environmental changes at an
unprecedented speed---on time scales shorter by two to four
orders of magnitude 
\cite{Ellis2015,DiazMalhi2022,Dirzoetal2022,SoleLevin2022}.
Human activities emanate in many forms, and here we will focus on, and
take examples from, climate change, land/sea appropriation
(fragmentation and use), and biological invasions, among the most
impactful ``stressors'' on biodiversity\footnote{Pollution or water
use could also have been considered as well. The framework developed
below could be adapted to them with no major change.}. The latter two
have been in place since the emergence of human
societies 
\cite{Ellis2021,Ellis2015,Tilman2022}
and the former over the last century and half~\cite{Dirzoetal2022},
with marked and well-documented impacts, and are unlikely to stop for
several decades, even under future optimistic scenarios on human
activities.


The most discussed facet of the Anthropocene is climate
change 
(e.g.,
\cite{Folkeetal2021,Portneretal2021,Tilman2022,Urbanetal2024})---the
strongest and best-documented expression of global change, and probably
also the most keenly felt, as it directly affects biodiversity,
including humans, who experience not only an average increase in
\mbox{temperature,} but also more frequent extreme events, such as with marine
heatwaves that have about doubled in frequency since the 1980s
([\citenum{IPCC2023}], chapter 11; \cite{Otto2023}). It is therefore a
planetary force, omnipresent and multifaceted. The proven increase is
already 1.3~\textdegree C above the pre-industrial mean value (see
\cite{Schmidt2024}) on a global scale, with significant regional
variation: temperatures have risen more markedly on lands than on seas,
with e.g., the increase in Europe being already 2.3~\textdegree C
\cite{Vautardetal2023}. Marked temporal variation, both on the scale of
decades and within years, is also obvious. For example, 2023 has been
the hottest year ever, with 1.5~\textdegree C above the pre-industrial
mean value~\cite{Schmidt2024}. Intra-annual variation is also obvious,
with, for example, a more marked increase in spring in particularly
hard hit Western Europe. The impact on temperatures is largely coupled
with an impact on rainfall~\cite{IPCC2023,Tammetal2023}. But here
again, a key feature is spatiotemporal variability, which is
particularly important, as it is one of the driving forces behind the
functioning of biodiversity 
\cite{FutuymaKirkpatrick2018,RelyeaRicklefs2018}. See more details in
Appendix~1 (see Supplementary\break Materials).


Furthermore, it is hard to see how the average temperature could not
exceed the pre-industrial average by 2.1 to 3.5~\textdegree C by 2100
\cite{IPCC2023}, unless very marked socio-economic changes and
mitigation measures take place. Even in the latter situation, the
return to pre-industrial values could take centuries, if not millennia,
due to the inertia of the Earth's system 
\cite{Meyeretal2022,Zickfeldetal2013}. It is, of course, difficult to
predict future trends, as they will depend on factors that cause
temperature increase (e.g., the size of the world's human population)
or decline (e.g., effective efforts to reduce the injection of
greenhouse gases into the atmosphere). We can therefore envisage growth
leading to a plateau, or even to a decline if actions in this direction
are attempted. This can be visualized in Figure~\ref{fig1}, using the
idea of an ``overshoot period'' developed by Meyer et~al.\ 
\cite{Meyeretal2022}---the period during which temperature will exceed a
``desirable'' threshold value (for example, 2~\textdegree C over the
pre-industrial mean value). This period can last for varying lengths of
time (e.g., decades) before returning to conditions that are more
favorable to biodiversity, depending on the implementation of
mitigation actions and their temporality (Figure~\ref{fig1}), and may
even be of ``infinite'' length (centuries). The notion of overshoot
makes it possible to arbitrarily fix the duration of the effect in
models and scenarios, and to assess how biodiversity will respond to
these new conditions in terms of eco-evolution over several decades to
centuries. Note that (i) spatial variability in temperature increase
can be accounted for in this framework by considering local overshoots;
(ii) these curves may not necessarily be smooth, and may include local
peaks (e.g., extreme events); (iii) they may account for ``tipping
points and alternative states'' when the overshoot period is very long.
We will use this representation in what follows to understand the
eco-evolutionary impact of stressors on biodiversity, to predict its
future and to consider Nature-based solutions to global change. 

\begin{figure}
{\vspace*{4pt}}
\includegraphics{fig01}
{\vspace*{4pt}}
\caption{\label{fig1}Time curves of consequences of human activities, past and
future (modified from \cite{Meyeretal2022}). The curve baseline is the
global pre-industrial temperature, and they increase up to a desired
target temperature that should not be trespassed (2~\textdegree C here
for the sake of the argument). The continuous line represents a
situation in which this threshold is trespassed for some time (the
overshoot period), and then the temperature goes down below the target,
due to e.g., mitigation actions. The dotted line represents a situation
in which the temperature does not go down to the target, for example
because no strong mitigation actions have been set up, and remains high
(here in a plateau) for a period much longer than the overshoot period.
The option of continuous increase is not represented. The present is
indicated by\break ``Present''.}
\end{figure}

The impact of other stressors (space appropriation and invasive alien
species) on biodiversity is no less worrying and marked. Figure~\ref{fig1},
including the overshoot period idea, can be used, as for climate
change, to understand the dynamics of the consequences of these
stressors (detail in Appendix~2, in Supplementary Materials). They all
have a dynamic of strong growth compared to the pre-industrial era
(which began at various times depending on the stressor), and this
dynamic will reach a point that we do not wish to exceed---the
beginning of the overshoot period. This point is probably less easy to
determine than for temperature, for which we have a simple
determination criterion (here, ${+}$2~\textdegree C)---we can, however,
imagine that we do not wish to have more than ``a given number'' of
invasive alien species or an increase in lands devoted to agriculture
(Appendix~2, in Supplementary Materials). The duration of the overshoot
will also vary from stressor to stressor. For example, in the case of
invasive alien species, it is hard to imagine that we will be able to
go back to the initial situation, if we define the overshoot based on
the number of such species in a given location, but it is possible to
reason about the effects on invaded ecosystems. See Appendix~2 in
Supplementary Materials for more details.


These three stressors have been considered independently, but they
may act in concert, often synergistically and through feedback loops
\cite{Halpernetal2008,vanMoorseletal2023,Orretal2020}. 
Let us take two examples. The first combines the effect of climate
change and the exploitation of natural resources (overfishing as an
emanation of sea appropriation). Climate change is responsible for
major direct changes in phytoplankton, while fishing alters directly
the fish community that also influences phytoplankton. Both thus alter
phytoplankton and, ultimately, trophic networks with complex cascading
effects on the fish community and the opening to invasive alien
species~\cite{Lynametal2017}. The second is the emergence of new areas for
biodiversity as a result of global warming, for example at the glacial
margins towards the poles, with retreats that can be of the order of
km/yr~\cite{Batcheloretal2023}. These areas can be rapidly artificialized, for
example for shipping or oil exploitation, and subject to biological
invasions~\cite{Chanetal2019,Goldsmitetal2020}. For the first effect, we can imagine a
very rapid increase in dynamics (Figure~\ref{fig1}), even under those
extreme \mbox{environmental} conditions and the same type of dynamics as
observed elsewhere under less extreme conditions. Biological invasions,
on the other hand, might be slower and limited to a few species---these
dynamics can be inferred from post-glacial colonizations, particularly
the most recent (see e.g.,\cite{Gieseckeetal2017,Normandetal2011,Pedersenetal2016}).
The time scale is likely to be of the order of a few centuries. What we
can learn from these examples is that we should analyze in more details
combination of effects, but they seem to be synergistic rather than
antagonistic (e.g., \cite{Halpernetal2008,vanMoorseletal2023}).
The overshoot period (Figure~\ref{fig1}) could therefore be more
pronounced and longer, with a much longer return (if any) to
pre-anthropocene conditions. Whatever the case, stressors, taken
individually or in concert, create new ecological and adaptive
opportunities in the context of new complex adaptive systems, with
major impacts on biodiversity and its dynamics.


\section{The impact of human activities on\newline biodiversity}\label{secIII}

Human activities and the above-mentioned avatars of global change
impact biodiversity widely in many respects, and we here provide a
simple review and an evaluation of the incidence on both ecological and
evolutionary (in the next section) processes. This means distinguishing
effects on biodiversity itself vs.\ on ecological and evolutionary
forces and processes---a classical distinction in the ecological
sciences between patterns and processes 
\cite{FutuymaKirkpatrick2018,RelyeaRicklefs2018}.
Importantly, this presentation takes a time perspective: the effect of
stressors on biodiversity is certainly past and present, but will also
manifest itself in the future, because the overshoot period can last
for some time (Figure~\ref{fig1}), even if this effect was to be
stopped now. A currently apparently healthy species can indeed decline
soon, perhaps after trespassing an ecological tipping
point (see \cite{Kefietal2022}), such as in the classic study of excessive
phosphorus impact on lake eutrophication status~\cite{Carpenteretal1999}---we are
talking here about the debt (or legacy) of the crisis~\cite{Tilmanetal1994}.
Getting cues of this debt (so-called early warning signals) is
difficult, because of the intrinsic unpredictability of such dynamics,
but progress is being made on this side\break (see \cite{Kefietal2022}).


Biodiversity has probably never been as flourishing as it is today
(before Human impact)---it is quite clear in terms of species
numbers~\cite{Dirzoetal2022}. However, it has been struck by five major
extinction events---defined as short periods during which more 75\% of
species disappeared~\cite{Jablonski1991,Sepkoski2020} followed by
recovery over several million years. All these events are due to causes
external to life itself, either endogenous (e.g., volcanism) or
exogenous (e.g., meteorite) to Earth. Although we are far from such a
massive extinction event, human activities and their consequences have
reached a level on all geographical scales that makes the occurrence of
a sixth extinction not unlikely 
\cite{LeakeyLewin1995,Cowieetal2022,Dirzoetal2022,Rull2022}. One
originality of this crisis is that its source lies in both
environmental (e.g., climate) and biotic (e.g., biological invasions)
changes caused by human activities. Quantifying this crisis assumes
that we ``know biodiversity''. This is far from true, with optimistic
estimates of ca.\ 20\% of described species, i.e.\ two million out of an
estimated value of eight to 20~million living species 
\cite{DavidLecointre2022,Moraetal2011}. Available data (arguably
limited) focus on terrestrial animals (70,000) and plants (400,000)
with far less constituted datasets in the marine realm and in small
organisms, especially ``microbes''. Our knowledge of population
dynamics and of trait variation is even more limited.


The first and quintessential expression of the biodiversity crisis, and
its first definition, is species
extinction~\cite{LeakeyLewin1995,Woodruff2001}. A very conservative
estimate of the current extinction rate is ten times larger than the
background rate  \cite{Ceballosetal2015,Dirzoetal2022,SoleLevin2022}.
Seven hundred vertebrate species (at least), out of 70,000 described
species, have disappeared over the last half millennium, while a few
species are expected to \mbox{appear/vanish} over such a period through
speciation/extinction, so that the vertebrate extinction rate, assuming
a quasi-steady state of species number, might be 100~times higher than
the background rate. Importantly, species extinction has accelerated
over the last century: for example, two thirds of mammal extinctions
recorded over the last half millennium have occurred since 1900
\cite{SoleLevin2022}. In this biodiversity collapse, islands have often
paid a huge tribute. For example, 13 of the 22~native bird species from
Guam have disappeared since the introduction of a predator snake
\cite{FrittsRodda1998}, and half of the more than 700 terrestrial snail
species from Hawaii have experienced the same fate, producing a 7.5 to
13\% extinction rate since 1500, some thousand times higher than the
background rate---ironically, new species are described that have
already disappeared~\cite{Cowieetal2017,VanDooren2022}. It should also
be added that many species will have to pay the above-mentioned
extinction debt \cite{Tilmanetal1994}. Even under scenarios with a
temperature increase of 2~\textdegree C and a brief overshoot period,
10\% of species are at risk of extinction~\cite{Meyeretal2022}. The
magnitude and duration of this period are therefore critical\break values.


However, even if impressive, this statement relies on an incomplete
diagnostic: (i) species are disappearing, but the number of individuals
per species (species abundance) is also collapsing. A striking example
are European birds with a decline of 25\% between 1980 and 2016, with a
major contribution of agriculture to this trend~\cite{Rigaletal2023}.
In insects, a marked decline in number (and also biomass) has been
reported (e.g., \cite{Seiboldetal2019}), but the trend might differ
among environments~\cite{vanKlinketal2020}, as also observed in marine
species \cite{Poloczanskaetal2016}, in a background of strong
methodological discussions \cite{Dornelasetal2022,Primacketal2018}.
(ii) Rapid reorganization of species assemblages (biodiversity
redistribution and homogenization) is occurring on a global
scale~\cite{Blowesetal2019,Pecletal2017}. This poses an ecological
problem, as this reorganization is largely due to opportunistic and
invasive species that might affect species diversity by increasing
extinction rates in local species (see point (i)). For example, marked
effects have been detected in the oceans with dramatic changes in
species composition in many regions, although not always associated
with a loss in species richness~\cite{Chenetal2011}. (iii)~All
stressors also have deep consequences on ecosystem functioning,
profoundly modifying interactions among species and between species and
their environments, an expression of which being ecological extinction.
To the extinction debt mentioned above, we add here an ecological debt:
species, although still extant, are no longer sufficiently abundant to
perform their functional roles~\cite{McCauleyetal2015} (and even worse
when they are extinct). For example, 40\% of plant species are
considered endangered~\cite{Enquistetal2019} with a substantially
decreased contribution to food chains and ecosystem processes. Of
course, other species, including invasive ones, can sometimes serve as
substitutes, an expression of ecological functional
equivalence~\cite{Davis2009}. This will be difficult if whole
communities and food chains reach tipping points and transition towards
degraded states, as is the case for lake
eutrophication~\cite{Carpenteretal1999} or
desertification~\cite{Kefietal2007}. Less visible, but not less
important, is the case of kelp forests, which occupy almost 30\% of
coastal ecosystems, and are declining fast and might soon reach a point
of no return~\cite{ColemanGoold2019}. (iv) Interaction between
stressors might lead to even more negative outcomes than their simple
addition. To the ecosystem-level example mentioned above, we should add
population-level examples with perhaps more directly visible effects.
For example, population shrinking might be compensated by immigration,
as classically shown in metapopulation theory \cite{Hanski1999}.
However, this assumes that appropriate habitats indeed support
populations contributing to migration, but such habitats are themselves
under pressure (e.g., because of land appropriation), reducing
metapopulation size, which could be further decreased by competition
with invasive alien species down to collapse \cite{Hanski1999}, as
shown in freshwater snails in the French Antilles
\cite{PointierDavid2004}. (v)~A small part of species has indeed
benefitted from human impacts, namely commensals and domesticates, with
a strong impact on biodiversity, and we should also modestly recognize
that the bacterial world remains a black box with regard to
biodiversity massive change. These two aspects are considered in more
detail in Appendix~3 (see Supplementary Materials). \looseness=1


We can conclude this section by stating that human activities have
markedly shaken biodiversity at all its organization levels, from
population to ecosystems, and spatial scales, with a major
reorganization of species, communities and ecosystems, even if we are
not (yet) experiencing a classical major extinction (i.e., 75\% species
extinction; \cite{Barnoskyetal2011,Cowieetal2022,Dirzoetal2022}). As a
consequence, most species are facing new environmental conditions to
which they are not necessarily adapted (new adaptive landscapes) and
are therefore under strong demographic pressure, possibly up to
ecological tipping points \cite{Folkeetal2021,Kefietal2022}, and
constraining evolutionary processes and potential, the scope of the
next section.


\section{Biodiversity and eco-evolutionary dynamics}\label{secIV}

At this point, we have not considered intraspecific diversity, and the
associated adaptive potential, in the biodiversity crisis. In fact, it
has been less considered than the higher levels of biological
organization considered above. However, genetic diversity is considered
one of the planetary limits for which we have reached a high-risk zone
\cite{Folkeetal2021}, but intraspecific diversity loss remains much
more difficult to document than biodiversity loss at the species level
\cite{Paulsetal2013}. Avatars of global change often result in a
decrease (collapse) in population size in impacted species and
ecosystems, which might lead to the loss of infraspecific diversity (as
species extinctions do), enhancing the role of stochastic processes,
such as genetic drift (e.g.,~\cite{CharlesworthCharlesworth2010}),
possibly driving populations in an extinction vortex, with a decreased
number of individuals leading to decreased genetic variation, itself
leading to a decreased number of individuals. This has been suggested
for some large mammal species, such as the
vaquita~\cite{Robinsonetal2022}. Theoretically, such loss can be
rescued by incoming gene flow, provided that immigration is not
impeded~\cite{CharlesworthCharlesworth2010,FutuymaKirkpatrick2018}.
Global change can also induce a new selection regime or the
mobilization of intraspecific diversity. There is for example wide
evidence that climate change induced both selective responses and
mobilization of phenotypic plasticity
([\citenum{GaitanEspitiaHobday2021,Martinetal2023,Urbanetal2024}] 
for recent perspectives), as detailed in Box~\Custref{1}{box1}. 
Another effect of global change,
the genetic counterpart of biota homogenization at species and
community level, is genetic mixing between entities (species or
lineages) that were previously separated, especially through the
introduction of invasive alien species. Such secondary contacts may
promote hybridization and introgression, and the emergence of
evolutionary novelties~\cite{McFarlanePemberton2019,Viardetal2020}. So
intraspecific diversity is not just lost, it is also being
restructured, especially in the context of large-scale biodiversity
hauling, similar to what we have seen above at the species and
community levels.


That global change affects all aspects of biodiversity is certainly an
incentive to take both an ecological and evolutionary stance on this
issue. After all, ecology and evolution are the core natural sciences
in the study of biodiversity dynamics 
(see \cite{CharlesworthCharlesworth2010,FutuymaKirkpatrick2018,RelyeaRicklefs2018}).
However, ecology has classically been seen as dealing with short time
scales, and evolution as operating on much longer time scales
\cite{Bassaretal2021,Fronhoferetal2023,Hendry2017}. Another way to put
it is from a perspective of biodiversity organization levels
(Figure~\ref{fig2}): populations play a central role in evolutionary
biology and, to a lesser extent, in ecology (population ecology), and
the latter becomes more important as we move towards the more
integrated and functional levels of biodiversity, i.e.\ communities and
ecosystems.


\begin{figure}
{\vspace*{7pt}}
\includegraphics{fig02}
\vspace*{4pt}
\caption{\label{fig2}Simplified diagram of the decreasing impact of evolutionary
theory (red triangle) in studies considering increasing levels of
integration of biodiversity (blue triangle), moving from populations
(in fact, genes and individuals; largely including population genetics
and evolutionary biology, but also population ecology) to communities
(community ecology) and ecosystems (ecosystem ecology). The blue
triangle could also be seen as an indicator of space and time, with
increasingly larger spatial and temporal scales (in general) towards
the figure bottom. Modified from\break Jarne~\cite{Jarne2022}.}
\vspace*{4pt}
\end{figure}


However, work carried out over the last few decades has shown that
ecological time and evolutionary time can overlap. This is of course
the case for short-lived organisms such as bacteria 
([\citenum{Loreauetal2023}]; \mbox{Appendix 3} in Supplementary Materials), which
adapt very rapidly to environmental changes~\cite{Lenski2017}, but also
for organisms with slower life cycles, particularly when they are
subject to strong selection pressures, such as mosquito populations
treated with pesticides~\cite{Liu2015}. Rapid evolution has also been
observed in natural populations of species exposed to environmental
stress (e.g., Darwin's finches in the Galapagos; \cite{Grant2017}),
predation (e.g., guppies in Trinidad; \cite{ReznickTravis2019}) or
invasive alien species (e.g., snails in the West Indies;
\cite{Chapuisetal2017}). The time scale argument therefore no longer
holds, and a more general framework should take into account the fact
that ecology and evolution can be fast or
slow~\cite{Fronhoferetal2023}. We focus here on the fast time scale,
since centering on global change, acknowledging that ``fast'' might
depend on generation time and stressor intensity and therefore the rate
of global change stressors relative to this time
([\citenum{Fronhoferetal2023}]; Appendix 3 in Supplementary Materials for
the case of bacteria).
\pagebreak

\onecolumngrid
\bigskip
\unskip\noindent\begin{frtextbox}{Box 1---From global change and overshoot periods to eco-evolutionary loops}
\enlabel{box1}
\mbox{}

\vspace*{-10pt}

{\advance\baselineskip by .15pt

The ecological impact of global change has been widely documented
(e.g., \cite{Portneretal2021}), but its evolutionary consequences have
attracted less, and more recent, attention, presumably under the
(erroneous) idea that evolution is not concerned with such fast changes
(see main text). It is now quite clear that the evolutionary dynamics
should be considered, for example when analyzing the consequences of
climate change
\cite{GaitanEspitiaHobday2021,Martinetal2023,Urbanetal2024}. We use
here climate change and the overshoot idea (Figure~\ref{fig1}) to
address the ecological and evolutionary consequences of global change.


We consider a situation in which climate change (temperature) has a
negative impact on species demography, and that this situation lasts at
least for some time (the overshoot period; Figure~\ref{fig1}).
Temperature can affect survival or reproduction 
(see \cite{Regniereetal2012,Sageetal2015} for review in insects and plants).
Demographic decrease is nothing exceptional in general, but can take
here extreme values known as population bottlenecks, with both
demographic consequences (e.g., very biased sex ratio rendering
reproduction difficult) and evolutionary
consequences
(e.g., increased consanguinity, reduced evolutionary potential; 
\cite{Faconetal2011,KirkpatrickJarne2000}).
At the end of the overshoot period,
demographic recovery might be possible, but evolutionary recovery is a
much longer process, depending on mutation and recombination in the
absence of incoming gene flow. Although difficult to document, we
indeed have examples of species flourishing following recent
bottlenecks, though exhibiting low variation 
(e.g., \cite{Faconetal2008,Maderspacher2011}). 

Not independently of the effects on demography, temperature imposes new
selective pressures. Species that were are (more or less) at an
adaptive peak prior to environmental change should now track the new
conditions to adapt (Figure~\ref{fig3}). In more ecological terms, they
should move within their niche. They can then either be able to catch
up with change, or not (A vs.\ B in Figure~\ref{fig3}). At the end of
the overshoot period, those species that have been able to adapt are
again constrained to engage in a new round of adaptation, while those
that have not might no longer be maladapted. Adaptation (peak tracking)
can occur through various mechanisms, including genetic adaptation or
phenotypic plasticity~\cite{Arnold2023,WalshLynch2018}. Empirical
studies have shown that plasticity is a more common option than
adaptation \cite{MerilaHendry2014,Urbanetal2024}. The point is whether this
reserve of plasticity is abundant enough to go through the whole
overshoot period, and in any case, phenotypic plasticity is not the
magic bullet.

Migration is another option for peak tracking, noting that it can
strongly be constrained by global change---migration indeed requires
connectivity, which is restrained by land and sea appropriation.
Migration brings both genes and individuals to new locations that are
colonized when they are empty of the considered species, or reinforced
when that species occurs in very low numbers, contributing to an
eco-evolutionary response to, for example, temperature
\cite{Faillaceetal2021,NadeauUrban2019}. Populations at the
colonization front may even be selected for an increased migration rate
over a few generations, as shown in the invasive toad \textit{Rhinella
marina} in Australia \cite{Shineetal2021}.

Changes in demography, adaptation and migration will also lead to new
interspecific relationships (e.g., new competitors or parasites), and
interactions with the environment, and new eco-evolutionary feedbacks including
peak tracking. The guppy example developed in the main text is a good
example at local scale. On a larger scale, poleward migration with
increasing global temperatures is indeed leading to new communities and
species interactions under new environmental conditions
\cite{Walther2010,Zarzycznyetal2024}. These new interactions deeply affect demography,
and also have evolutionary consequences, as has been shown for example
for interactions between local and invasive species, leading to
variation in morphological traits 
([\citenum{Stuartetal2014}] in lizards from the Caribbean) or in life-history
traits 
([\citenum{Chapuisetal2017}] in freshwater snails from the same biogeographic area).
More complex eco-evolutionary dynamics are certainly at play, as shown
in guppies. These are situations in which adaptive tracking is also
more complex (compared to the simple situation depicted in
Figure~\ref{fig3}), since multiple traits, species and environmental
conditions may be at stake, and therefore multiple adaptive peaks.

}

\begin{Figure}
\includegraphics{fig03}
\Caption{\label{fig3}Variation of trait values when temperature changes
in time under the overshoot scenario. The upper part depicts time
curves and the overshoot period under temperature change as in
Figure~\ref{fig1}. The bottom part represents the temporal variation of
traits  (blue curves; adaptive peak in evolutionary terms;
\cite{Arnold2023}) with regard to temperature variation (red curves).
Strongly overlapping curves (e.g., leftmost figure) indicate excellent
fit between traits and temperature, while non-overlapping curves
indicate poor fit. Following a fast temperature change, the trait curve
is lagging behind (second figure from left to right in the bottom
part). During the overshoot period, peak tracking through adaptive
evolution or phenotypic plasticity may lag behind (A) or may be
efficient (B). At the end of the overshoot period (rightmost graphs in
the bottom part), individuals in (A) may indeed be (again) adapted to
temperature, while those in (B) are now far from the optimal trait
value and have to evolve again. Note that peak tracking is here
represented in a very simplified way. For example, many forces (e.g.,
antagonistic selection between different traits) might prevent
individuals from reaching the adaptive
peak (see \cite{Arnold2023,WalshLynch2018}).}
\end{Figure}

\end{frtextbox}

\vspace*{17pt}
\twocolumngrid

Beyond the issue of time scale, there are several reasons to consider
ecology and evolution together. First, the basic material of both is
made up of individuals (phenotypes), their numbers and traits, which
can be integrated from the within-species to the ecosystem level
(Figure~\ref{fig2}). The whole dynamic of these various levels is
driven by demography and individual fitness (leaving aside exchanges of
matter and energy). Individuals constitute groups at the local level,
which are connected by individual\unskip\break dispersal. This can be embedded in
the ``meta'' perspective with sets of populations, communities or
ecosystems functioning as metapopulations, metacommunities or
metaecosystems connected by migration 
\cite{Hanski1999,Leiboldetal2004,Loreau2010}. This brings us logically
to our second point: there is a clear analogy between the
forces/processes acting at these various biodiversity levels, for
example between genetic drift of alleles and ecological drift of
species, competition within and among species~\cite{Chesson2000}, or
gene flow of alleles and migration of species 
\cite{Hendry2017,Vellend2016}---an analogy described as a parallel view
on ecology and evolution by Huneman~\cite{Huneman2019}. Of course this
analogy has its limits; for example, natural selection at the
population level (interactions between alleles) cannot take the same
forms as the variety of interspecific interactions (mutualism,
parasitism, competition,\break predation{\,\ldots}). 


The parallel view on ecology and evolution is important, but does not
account for a deeper coupling due to eco-evolutionary feedbacks and
loops 
\cite{Fronhoferetal2023,Govaertetal2016,Hendry2017,JarnePinay2023}. In
the stronger version of loops, this occurs when an ecological driver
affects the trait evolution of organisms, and that evolution in turn
affects the ecological driver---this is what Hendry~\cite{Hendry2017}
calls eco-evolutionary feedbacks in the narrow sense. The long-term
study of  Reznick and colleagues on guppies in
Trinidad~\cite{ReznickTravis2019} provides an excellent example.
Guppies occupy rivers in which predation by other (invasive) fish is
low or high, and an evolutionary response of several traits (e.g.,
reproduction, morphology) to predation was detected. Selection here
derives from marked difference in population density. At high density,
guppies exert a strong pressure on their food sources, which in turn
affects primary productivity and the dynamics of other fishes that
compete with guppies, with an effect on guppy demography. This
eco-evolutionary loop involves the different levels of biodiversity
mentioned in Figure~\ref{fig2}, and set up over a few tens of
generations. 


Considering eco-evolutionary dynamics seems therefore relevant when
analyzing the effect of global change on the time scale of a few
decades. The duration of the overshoot period is certainly critical,
since it will determine whether an evolutionary response can be mounted
and then affect the aspect of global change (e.g., increased predation
in guppies) that is at the origin of this response. This is further
elaborated in Box~\Custref{1}{box1}, connecting environmental change and traits through
the idea of adaptive/ecological peak tracking. It might also be that
eco-evolution proceeds along a weaker version of loops, i.e.\ that the
ecological driver of evolution is not in itself affected by the
evolutionary response, but that other ecological factors are
\cite{Hendry2017}. In the guppy example, it might well be that guppy
populations have also been affected by climate change, but it is
unlikely that guppy evolution impacts, at least directly, the climate.


The guppy example shows how eco-evolutionary coupling could be studied
in practice, and we certainly lack similar examples in which the full
eco-evolutionary loop has been considered 
\cite{Farkasetal2013,Hendry2017,JarnePinay2023}. However, this remains
a ``simple complex adaptive system''. Studies on much wider spatial
(and impact) scales are required with presumably weaker loops and
causalities, and a multitude of ecological and evolutionary factors at
play---see Box~\Custref{1}{box1} for climate change and Appendix~2 in
Supplementary Materials for other stressors. Even if we know that
evolution can serve in actions dedicated to the limitation of global
change---we have good examples of evolutionary rescue of
populations~\cite{Bell2017,Carlsonetal2014}, its overall impact at the
planetary scale in the face of strong drivers such as climate change
remains poorly understood from an eco-evolutionary perspective.
Bridging this gap is critical for both predicting future dynamics and
finding solutions to global change, issues that we will consider in the
next sections.

\section{Predictions integrating eco-evolutionary dynamics}\label{secV}

The framework used above implicitly appeals to the notion of
prediction, since the general idea is to understand how biodiversity
will react to the multiple avatars of global change by projecting into
the near future---the notion of overshoot (Figures~\ref{fig1} and
\ref{fig3}) does indeed lead us to address the question in terms of
predictions. Prediction is, in fact, a general question in
science---sometimes associated with the very existence of a science,
and the biodiversity sciences are no exception, whether ecology 
\cite{Marisetal2018,OliverRoy2015,Petcheyetal2015}, evolution
\cite{Reiskindetal2021,Shaw2019,Urbanetal2024},  or
eco-evolution~\cite{Hendry2023}.


In a somewhat caricatural way, we will distinguish in the context of
the present analysis two forms of prediction, namely explanatory vs.\ 
anticipatory  ([\citenum{Marisetal2018,Mouquetetal2015}]; see also
\cite{Hendry2023} for a more complex classification).
The former corresponds to the construction of knowledge about a given
object, and can be built on the basis of different approaches, based on
hypothesis testing or induction. For example, we may want to test the
effect of temperature on the reproductive rate of a species, making the
hypothesis that, up to a certain temperature, an increase in
temperature leads to a higher reproduction rate. An experimental
approach will enable us to evaluate this hypothesis, and quantify the
impact of temperature, thus building up knowledge about the biology of
this species. Of course, the knowledge built up by this approach is not
absolute, and is subject to variation, uncertainty and therefore
re-evaluation. It can also be considered as a proxy, when, for example,
we assume that the temperature--reproduction relationship in the
population studied can be considered valid for other populations from
the same species or for those from closely related species. 


Anticipatory prediction concerns the future, and is built on the
consolidated knowledge derived from the explanatory approach. From this
knowledge, it is possible to construct predictions. Taking the previous
example, we can use the temperature--reproduction relationship to
predict the reproductive rate of the species under consideration if the
temperature increases by 2~\textdegree C (Figure~\ref{fig1}). This
prediction can be built from a variety of approaches, including
narratives, scenarios, models or
projections 
(see~\cite{Coreauetal2009,Hendry2023,Mouquetetal2015,OttoRosales2020}),
depending on the objectives and amount of available information and
based on one (or more) stressors supporting the projection---for
example, a temperature rise or the arrival of an invasive exotic
species. This is not a simple exercise, depending on the quality and
quantity of data, or the uncertainties about the projected stressor
(what temperature increase?). The strengths, weaknesses and
difficulties of this approach are detailed in Mouquet
et~al.~\cite{Mouquetetal2015}, Maris et~al.~\cite{Marisetal2018}, Elliott
Graves \cite{ElliottGraves2019} and Hendry \cite{Hendry2023}, and
beyond the scope of the current paper. Below we discuss only some
aspects that are relevant to short-term eco-evolutionary predictions
(Figure~\ref{fig1}). 


Even if they are associated with uncertainties, the Intergovernmental
Panel on Climate Change routinely produces several scenarios for
predicted temperatures, e.g., of the 21st century, including the
functioning of geo-hydro-climato cycles (\url{https://www.ipcc.ch/}).
Carrying out the same type of exercise for biodiversity is somewhat
more difficult, since the aim is not simply to predict an environmental
indicator such as temperature (with of course its uncertainties), but
rather biodiversity per se (e.g., the distribution or demography of
species, the arrival of invasive species) which itself depends on
environmental parameters. Going back to Figures~\ref{fig1}
and~\ref{fig3}, we can see the parameters to be quantified as a basis
for predicting biodiversity's fate, and some of the difficulties that
will arise: (i) the magnitude and dynamics of environmental variation.
The IPCC scenario was used to set a target value (2~\textdegree C
temperature rise); (ii) the duration of the overshoot, if it is not
infinite (or very long); (iii) the dynamics of the return to
``normal''.


Predicting the future of biodiversity is essentially based on
ecological approaches that do not take into account the possibility of
evolution. This is already an extraordinarily complex exercise
\cite{Marisetal2018,Mouquetetal2015,OliverRoy2015,Petcheyetal2015},
particularly when multiple stressors affect
biodiversity~\cite{Coteetal2016,Petcheyetal2015}. A source of
complexity, noted above, is that these stressors will affect a whole
range of species, and give rise to new communities and ecosystems.
Predictions are based, as mentioned above, on various approaches, but a
massive appeal is made to models, for example to predict species
distributions~\cite{Bucklinetal2015,ElithLeathwick2009}. These models
can be correlational, for example based on the relationship between the
distribution of a species and the temperature envelope corresponding to
that distribution, or mechanistic, when we are able to integrate
biological parameters, such as migration capacity, which will define
the ability to reach favorable environmental conditions. It is even
possible to use heuristic models, and an imprecise prediction approach
\cite{ElliottGraves2020}, or even a combination of models. None of
these modeling approaches is perfect with possible trade-offs between
alternative models, as highlighted by Levins~\cite{Levins1966} in the
early days of ecological modeling, nor do they necessarily answer the
same questions.


Prediction must also incorporate the possibility of adaptation through
evolution, particularly as species will often find themselves facing
moving environmental conditions (Figure~\ref{fig3}). This means
including forces or characteristics such as standing genetic variation,
mutation or phenotypic plasticity to track these peaks, and improve the
level of adaptation---we saw earlier that this process can be rapid.
Including the possibility of evolution will certainly add variance to
species responses, and unpredictability. However, this contribution is
rarely taken into account in anticipating and managing (see
Section~\ref{secVI}) the impacts of different global change
avatars~\cite{Reiskindetal2021}. Evolutionary biology is indeed
generally rather shy when it comes to predicting future
trajectories~\cite{OttoRosales2020,Shaw2019}. Day \cite{Day2012} even
suggests, on the basis of an approach close to G\"{o}del's
incompleteness theorem, that prediction in evolution is intrinsically
limited.


The study of eco-evolutionary dynamics relies on both theoretical and
empirical approaches. We developed here aspects that are of major
interest with regard to prediction. From a theoretical point of view,
evolutionary biology does have some tools at its disposal for making
predictions of the kind required in Figures~\ref{fig1} and~\ref{fig3}.
The simplest approach might be to use narratives sensu Otto and Rosales
\cite{OttoRosales2020}, as a basis for mathematical models, or perhaps
more simply, scenarios. More detailed approaches can be based on
models. A widely used approach to build projections on the evolution of
traits is quantitative genetics, generally deemed valid in the short
term (a few tens of generations), but not beyond, due to the rapid
temporal variation in variance and covariance structures between traits
\cite{Shaw2019,Steppanetal2002,WalshLynch2018}. Furthermore, the
approach is species-centric, and it seems difficult to generalize it to
the community level (but see \cite{Whithametal2020}). Another widely
used approach is adaptive dynamics~\cite{Champagnatetal2006} which has
for example been used to model the evolution of
dispersal~\cite{Massoletal2011}, but considers evolution on a time
scale too long to be relevant in the context of global change. Lion
et~al.~\cite{Lionetal2023} recently proposed a hybrid approach that
takes into account elements of both quantitative genetics and adaptive
dynamics, which could enable medium-term predictions.


Prediction in evolution can also rely on empirical approaches. For
example, Mallard et~al.~\cite{Mallardetal2023} suggests that phenotypic
evolution can be predicted at 50~generations in a model species, the
nematode \textit{Caenorhabditis elegans}, even if predicting the
evolution of given traits is difficult, for example because of genetic
drift. Long-term monitoring is especially worthwhile in this endeavor,
whether in the laboratory~\cite{Lenski2017} or under natural conditions
\cite{Chapuisetal2017,ReznickTravis2019}. Retrospective approaches are
also possible, for example in the context of resurrection ecology, i.e.\ 
``the revival of long-dormant organisms via hatching of dormant life
stages such as seeds, eggs, and \mbox{spores/cysts}''~\cite{Weideretal2018}.
Comparing their traits (and fitness) with those of current entities
allows to quantify evolutionary responses to stressors. For example,
Acoca-Pidolle et~al.\  \cite{AcocaPidolleetal2024} suggest that a plant,
the field pansy, evolved higher selfing rates in response to pollinator
decline, based on a resurrection approach (see, for a review,
\cite{Cheptou2024}). The next step is to move towards approaches that
integrate both ecology and evolution to derive
predictions~\cite{Hendry2023}, including all biodiversity levels, as we
have seen above that eco-evolutionary approaches may give different
results from purely ecological or evolutionary approaches. Such an
eco-evolutionary approach can be integrated into schemes of future
environmental variation of the type shown in Figures~\ref{fig1}
and~\ref{fig3}. The ecological and evolutionary issues raised in
Box~\Custref{1}{box1} should indeed be considered (e.g., peak tracking,
adaptation to lower temperatures at the end of overshoot), but
producing eco-evolutionary predictions comes with additional
challenges. To cite just a few. The first is that we do not have enough
work coupling ecological and evolutionary approaches, as mentioned
above (Section~\ref{secIV}), perhaps because this requires long-term
approaches (several decades in plants and animals), which makes it
difficult to build predictions taking into account all the associated
pitfalls \cite{Hendry2023}. The second is that we should be clearer
about whether or not evolution, which facilitates peak tracking during
the overshoot period (Figure~\ref{fig3}), will also drive biodiversity
into trajectories that make adaptation to new conditions at the end of
this period more difficult (i.e., evolutionary traps;
\cite{Schlaepferetal2002}). Taking again the example of increased
selfing rates with pollinator decline~\cite{AcocaPidolleetal2024}, one
might ask how pollinators are affected if such an evolutionary
trajectory is taken by many plant species, and how to integrate such a
genuine eco-evolutionary loop into predictions. A third aspect are
multiple stressors~\cite{Orretal2020}---we have essentially used the
example of climate change (Figure~\ref{fig1}), but predictions in
practical cases will often require considering it in connection with,
say, invasive alien species. This means more complexity, and probably
less precise predictions, opening up the issue of the kinds of
predictions we need for
action~\cite{Hendry2023,Marisetal2018,Mouquetetal2015}.


\section{Implementing eco-evolutionary solutions to the biodiversity crisis}\label{secVI}

A wide range of engineering solutions have been proposed to cope with
various aspects of the Anthropocene crisis (e.g., mitigation of
stressor impacts, adaptation to their effects), but a good part,
especially the grey ones (solutions involving artificial structures,
such as dams against floods), can cause more problems than they solve,
have only a short-term local impact, and do not address the future of
biodiversity at all. More generally, it is unlikely that the human mind
will ``demiurgically'' produce solutions to all problems through this
type of approach~\cite{SoleLevin2022}, and that some form of
``degrowth'' (e.g., reduction of stressors) should not have to be
achieved, coupled with nature-based solutions\footnote{According to
IUCN, nature-based solutions are actions to address societal challenges
through the protection, sustainable management and restoration of
ecosystems, benefiting both biodiversity and human well-being
(\url{https://www.iucn.org/resources/issues-brief/ensuring-effective-nature-based-solutions}).}, 
to get actual positive results (see \cite{Folkeetal2021}). These
approaches are nothing new, and have been developing for almost half a
century in conservation biology~\cite{Soule1985} or ecological
restoration \cite{ClewellAronson2013}, relying on concepts such as
ecosystem services or, more recently, nature's contributions to
people~\cite{Diazetal2018}, and considering focal species (as typically
in conservation biology) or ecosystems (in restoration ecology). A huge
literature has developed on these subjects, and we will not elaborate
further. By definition (see footnote~3), nature-based solutions
should return benefits to society, the economy and biodiversity
\cite{Eggermontetal2015,Seddon2022}---the novelty relies on the fact
that solutions should be good for the targeted diversity (e.g., a
species to be saved), but also for the untargeted one (e.g., saving the
targeted species should not impact negatively the recipient ecosystem).


These approaches have been based on two pillars: the ecological
sciences and the humanities and social sciences. As for prediction, the
evolutionary principles had limited penetrance, with minor integration
of the role of intraspecific diversity and adaptive potential, although
their relevance has been highlighted in conservation biology
\cite{CookSgro2016,GaitanEspitiaHobday2021,Heuertzetal2023,SarrazinLecomte2016}, 
in restoration ecology \cite{Mijangosetal2015}, but also in
socio-ecological~\cite{Currieetal2023} or ecosystem
services~\cite{Rudmanetal2017} thinking. In fact, all human
interventions (direct or indirect) are evolutionary in essence, because
they modify the adaptive landscape in which species are living, but
this needs to be explicitly recognized and implemented. As 
Mr.~Jourdain, Moli\`{e}re's famous character from \textit{Le Bourgeois
gentilhomme}, eventually realized that he was speaking in prose without
knowing it.


A first step would be for intraspecific diversity and adaptive
potential to be taken into account less timidly in international
discussions and regulations, for example for those sustainable
development goals (\url{https://sdgs.un.org/goals}) involving
biodiversity, as highlighted by D\`{i}az and Mahli~\cite{DiazMalhi2022}.
It is encouraging to see that genetic diversity is mentioned in the
Kunming-Montreal Agreement (COP15; see
\url{https://www.cbd.int/article/cop15-final-text-kunming-montreal-gbf-221222}).
Intraspecific diversity and adaptive potential also play a critical
role in nature's contributions to people~\cite{DesRochesetal2021}, and
probably in nature-based solutions, although their role remains to be
formally evaluated. For example, moving individuals in assisted
migration or species translocation programs means moving their genes,
so a lack of concern for the genes being moved might have (negative)
consequences, such as moving individuals that are not adapted to local
conditions \cite{Brodieetal2021,RicciardiSimberloff2021}. Another
example is genetic rescue of populations, which might, as already
noted, be as important as demographic rescue, through the input of
genes from outside \cite{Bell2017}. The role of intraspecific diversity
should thus be explicitly recognized, assessed and, if relevant,
implemented. There is therefore wide room (and necessity) to include
them in nature-based solutions. An eco-evolutionary approach would then
involve taking into account both ecological and evolutionary
processes/forces, and ideally evaluating possible eco-evo feedbacks.


Using again Figure~\ref{fig1} as a guide, such eco-evolutionary
practices could be used to deal with the magnitude of the stressors
(here, up to 2~\textdegree C), the overshoot period duration, and the
dynamic of environmental targets. In practice, this means avoiding
genetic and ecological drift (i.e., diversity loss), understanding the
impact (strength and direction) of natural selection, which assumes
quantifying the link between phenotype and fitness
\cite{Arnold2023,WalshLynch2018}, and eventually importing genes and
individuals to cope with temperature increase, at least for overshoot
duration. This also means considering the associated issues of local
adaptation and genetic exchanges with local
individuals~\cite{Bell2017,Carlsonetal2014}. At the population level,
population size and fragmentation are useful guiding
indicators~\cite{GaitanEspitiaHobday2021,Heuertzetal2023,Hobanetal2022}. At the overshoot end, importing
again new genes might be useful to cope with a come back to
pre-overshoot temperature. Reasoning (and acting) at the community
level is a further challenge, as intraspecific diversity should be
evaluated and monitored in the target species (e.g., translocated), but
also in the recipient communities---simply counting species in the
latter will not be informative about their genetic diversity, given
that diversities within and among species are not necessarily
correlated~\cite{Lamyetal2017,Vellend2016}. Both at the population and
community level, it is quite clear that the fast advances in genomics
over the last decade, including full genome sequencing and
environmental DNA approaches, offer powerful tools for characterizing
(epi)genetic
diversity~\cite{Heuertzetal2023,Stangeetal2021,Taberletetal2018}.
However, phenotyping is also critical and complementary to connect this
diversity to the phenotype and fitness, and therefore to fast
environmental change \mbox{(Figure~\ref{fig3}).} The major challenge will then
be to \mbox{evaluate} whether conservation or restoration actions drive the
target species and ecosystems into virtuous eco-evolutionary loops, as
again nature-based solutions should benefit biodiversity.


Implementing eco-evolutionary (nature-based) solutions to the
biodiversity crisis is associated with major locks, especially when it
comes down to managers and policy-makers (and the general public): (i)
considering that appropriate biodiversity should be used in
nature-based solutions, i.e.\ considering basic principles of ecology,
such as adaptation to a given environment, is already a first challenge
in a short-termist world~\cite{Eggermontetal2015,Seddon2022}.
Considering intraspecific diversity goes a step beyond, because the
role of this diversity is underappreciated, or even unknown, except
perhaps for species of direct interests to humans (e.g., crops, pets)
or for concepts such as consanguinity; (ii) Nature-based solutions 
require consensus and democracy, since they also aim at
well-being and improved social
contexts~\cite{Nesshoveretal2017,Seddon2022}. On a large geographic
scale, this means international cooperation{\,\ldots}\ in a world that is
not always eager to cooperate, even to deal with massive challenge such
as climate change. For more local issues, discussion, acceptance of
non-scientific sources of knowledge, and co-construction seem
essential~\cite{DesRochesetal2021,Diazetal2018,Seddon2022}. Such an
approach would be possible if we had time ahead of us; however, the
problems are already right in front of us, and we may have to accept
the implementation of solutions based on uncertain predictions and
limited scientific knowledge, as already mentioned.


\section{Conclusion and perspectives}


Although the Anthropocene has not been adopted as an era by the
International Union of Geological Sciences~\cite{Watersetal2024}, the
human impact on the planet, its climate and its biodiversity is massive
and multifaceted, and its magnitude is likely to increase further, as
there are few signs that humanity is ``putting the brakes on its
activities''. The focus, quite rightly, on the climate crisis somewhat
overshadows that of biodiversity, the main aspects of which we have
outlined in this paper. Yet the biodiversity crisis is having a massive
impact on human life, and in turn on various facets of the
Anthropocene, starting with climate change. The various aspects of the
\mbox{biodiversity} crisis, from its analysis to \mbox{predictions} 
and \mbox{solutions,}
have been addressed by essentially ecological approaches---this is
absolutely vital. However, biodiversity (including humans) continues to
evolve, in the Darwinian sense, and it is therefore necessary to take
evolutionary processes into account throughout this chain of work. My
tenet in this article is that biodiversity should be considered from an
eco-evolutionary perspective, while acknowledging the major role played
by human activities. Biodiversity evolution can indeed feed back on
ecology (including stressors), so an eco-evolutionary perspective is
required, with its feedbacks and loops 
\cite{Fronhoferetal2023,Hendry2023,JarnePinay2023}.
The notion of environmental overshoot (Figure~\ref{fig1}) then provides a
heuristic temporal perspective on the impact of the different avatars
of global change, such as climate change (see also Appendix~2 in
Supplementary Materials). The overshoot duration, whatever the
environmental stressor considered, will indeed be long enough to
require taking into account genetic diversity and evolutionary
potential, even for long-lived species. This can be achieved by
considering temporal trait variation and the factors responsible for
their evolutionary dynamics, as represented in a very simple way in
Figure~\ref{fig3}. This academic exercise can set a basis to analyze the
dynamics of biodiversity, but also to predict its fate and to implement
solutions to the biodiversity crisis, for example in nature-based
solutions. 


In conclusion, I would like to highlight a few issues and problems: (i)
Biodiversity is faced with a multitude of stressors, often acting
synergistically, that need to be considered together
\cite{Coteetal2016,Halpernetal2019,Orretal2020}. An interesting
proposal is to develop a ``networked'' and temporal vision of these
effects (which are not necessarily the same if they occur in sequence
or together, with possible transgenerational effects;
\cite{Orretal2020}). This is consistent with the notion of overshoot,
which includes the possibility of adaptation over dozens of
generations. (ii) Integrating an evolutionary perspective is not easy,
and even less so for an eco-evolutionary perspective---socio-economic
anthropocentrism is a major obstacle, as is the disconnect between
humans and ``nature'' \cite{Loreau2023}. Powerful socio-economic
interests are at stake, which are pushing us not to act or to propose
technological solutions that will/may pose more problems than they will
solve in the near future
\cite{Blasiaketal2023,Folkeetal2021,Littleetal2023}. It is essential to
recognize the scale of the problems posed by global change for
biodiversity, to discuss them democratically, and to try \textit{ad
minima} to behave decently towards
biodiversity~\cite{Chapinetal2022,Diazetal2018,Folkeetal2021}---perhaps
do what is possible, even if hopeless, as an ethical
imperative~\cite{VanDooren2022}. Building a new pact with ``nature'' is
certainly more ambitious, yet necessary (e.g.,~\cite{Loreau2023}),
setting it on human resilience~\cite{Ellis2023} and on transformative
change, i.e.\ fundamental, system-wide reorganization, therefore calling
for widely\unskip\break different socio-economic perspectives
(\url{https://www.ipbes.net/transformative-change}). (iii) It is
questionable whether the ecology and evolution research community is
structured to tackle the problems posed by global change on
biodiversity (see \cite{Carrolletal2014,Urbanetal2024}). It has been
involved in the management of environmental problems for several
decades, for example with the emergence of conservation biology in the
1980's \cite{Soule1985}, and proposes applicable solutions integrating
ecological and evolutionary
processes~\cite{CookSgro2016,GaitanEspitiaHobday2021,SarrazinLecomte2016}. However,
research in these fields remains largely underfunded (see
\cite{Urbanetal2024}), and is perhaps destined to remain so. The reason
for this may lie in the fact that ecology and evolution are
revolutionary sciences, in the sense that they have taught us two bad
news: we are just one part of biodiversity, one ape among
many~\cite{Darwin1859}, but a hyper-powerful and destructive ape that
does not seem very concerned about the necessary stewardship of Earth.
Yet an eco-evolutionary vision seems more necessary than ever to find
solutions, in conjunction with other approaches, particularly
socio-economic ones \cite{Currieetal2023,Ellis2023,Levinetal2013}, and
to map out a future less gloomy than the one we can envisage today.


\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}
PJ's work is supported by CNRS-Ecologie \& Environnement.

\section*{Acknowledgements} 
The author thanks T. Giraud and J.-D. Lebreton for inviting to write
this paper with a wild card on the topics to be addressed, and E.
Fronhofer, T. Giraud, P.~Huneman, F. Viard, and an anonymous reviewer
for important comments on the manuscript.

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\section*{Supplementary data}
Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

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