Blattella germanica was observed only inside buildings and Periplaneta americana was peri-domestic. As these two species were never found outside built-up areas, they have not been included in the following analyses.

Of the ten cockroach species found on La Réunion and the nine on Mayotte, six were recorded on both islands (Table 1). Three of the species collected on Mayotte had never been identified (Mayottella dimorpha Roth, Ylangella truncata Roth and Chorisoblatta denticulara Roth and have now been described. These new species could be endemic on Mayotte. However, we did not find Brachynauphoeta mayottensis, described by Bruijning in 1945 [7] as endemic on Mayotte. Chopard in 1957 [8] gave a list of nine species, including two endemic species, from La Réunion (Table 1). We added eight species to this list. We found one of the endemic species (Chorisoblatta chopardi), but not the other (Temnopteryx abbreviata) described without any precise localization. Four of the other species in Chopard's list were not found. Chopard's list included Pycnoscelus surinamensis. We collected two morphologically different types of females, one resembled Pycnoscelus surinamensis (parthenogenetic form) and the other could be Pycnoscelus indicus (sexual form). However, as no males were captured, precise identification was impossible. Therefore these species are referred to as Pycnoscelus sp. hereafter.

The number of species per site declined significantly when altitude increased on La Réunion (KW, p=0.0391, df=2) and on Mayotte (KW, p=0.0167, df=2). Altitude appeared to be a factor limiting the presence of cockroaches. Temperature declines rapidly when altitude increases. On La Réunion temperature varies on average 0.7 °C every 100 m. Cockroaches can be found above 250 m, but usually one species was observed on a given site, whereas 2.6 species per site were recorded on average between 0 and 100 m (Fig. 1). The distribution in relation to altitude varies with species. Some species, like Pycnoscelus sp., Neostylopyga rhombifolia and Symploce pallens were never found above 50 m, whereas Loboptera dimidiatipes and Balta longicercata can be found up to 900 m asl on La Réunion. On the other hand, Chorisoblatta chopardi was only found between 1300 and 1500 m asl (Fig. 2).

Cultivated lands were significantly richer in cockroach numbers and species than degraded forests, and very few cockroaches were found in natural forests (Fig. 3), on Mayotte (KW, p<0.0001, df=2) and on La Réunion (KW, p<0.0001, df=2). Level of the impact of human activity on different environments and species abundance are related. The greater the influence of human activity on the environment, the higher the number of cockroach species present. Number of species did not vary significantly between water edges and cultivated lands (MW, p=0.72) (Fig. 2). However, species variety was greater in water edges than in forests (MW, p=0.0003). Cultivated lands and water edges appear to favour establishment of several cockroach species. Cultivated lands on La Réunion allow a greater number of species to occupy the same site (MW, p=0.036). Average number of cockroach species per site in cultivated areas was 2.8 on La Réunion but only 1.6 on Mayotte (MW, p=0.036).

Most of the species recorded were present on cultivated sites (Table 2). Four species were never observed on cultivated lands. Chorisoblatta chopardi is strictly a forest species. It was found only in the high altitude moist Bébour forest on La Réunion, in holes in tree trunks and stumps. Ylangella truncata lives in large dead decaying trees on Mayotte. Although dead trees are more frequent in degraded forest zones, this species seems also to be a forest species. Symploce pallens and Neostylopyga were found in degraded forest zones and near habitations. They also use decaying dead wood as shelters. The two other endemic species recorded on Mayotte (Mayottella dimorpha and Chorisoblatta denticulara) were found exclusively on cultivated sites, but on a very restricted number of sites (respectively 5% and 4%, N=112). Loboptera dimidiatipes probably had the largest distribution. On Mayotte, it was found on cultivated land and in degraded forests (on 48% of the 112 sites). On La Réunion, it was found on cultivated land (on 28% of the 61 sites). Balta longicercata and Blattella lituricollis were found on cultivated land and on water edges on La Réunion, on cultivated land and in degraded forest on Mayotte. However, they were less frequent than Loboptera dimidiatipes. Balta longicercata was recorded on 11% of the Mayotte sites and on 31% of the La Réunion sites and Blattella lituricollis on 23% of the sites Mayotte and on 20% of the La Réunion sites. Blattella biligata was widely distributed on La Réunion, particularly on cultivated land where it was found on 53% of the sites. It was found only once on Mayotte, on a cultivated site. Margattea nimbata and Scalida latiusvittata were found on cultivated land and on water edges on La Réunion. Pycnoscelus sp. was found on cultivated land, in degraded forests, in dead trees and on areas near water. This species appears to be able to use a large variety of environments, as long as it can find loose substrate to burrow into.

Altitude and temperature variations are related. Altitude influences cockroach distribution. Although some species, like Loboptera dimidiatipes and Balta longicercata, can be found at high altitudes, the general tendency of cockroach species is to remain near sea level, where the temperature is higher. Cultivated land constitutes a favourable environment where important populations of cockroaches can be found. The soil of cultivated areas has been ploughed and is loose enough for cockroaches to shelter in it. Another important aspect of this environment is the presence of abundant leaf litter offering shelter and food. Finally, these zones, particularly sugar-cane fields on La Réunion, are irrigated. Crops can also retain rain water with their leaves. The industrialisation of crops, particularly sugar-cane, on La Réunion appears to contribute to the presence of large cockroach populations. Altitude and cultivation are two factors which explain the distribution of cockroaches. Thus Loboptera dimidiatipes and Balta longicercata are found above 900 m asl due to the extension of cultivated land on La Réunion. On Mayotte, they were not present above 250 m asl because higher lands are rarely cultivated.

On La Réunion, Chorisoblatta chopardi did not appear to interfere with the other species as it occupies a very special habitat in the high altitude Bébour forest. The other species are simply not present in this habitat. The distribution of this species appears to have diminished since 1957 as it was then identified in the Bélouve forest, but we were unable to find it there again. Although nowadays the Bébour forest is protected by its status as a biological reserve, from all human disturbance, this habitat remains threatened by the invasion of plant pests like the giant bramble creeper (Rubus alceaefolius) [9]. The survival of Chorisoblatta chopardi is related to the protection of Bébour forest. Like other threatened and locally distributed insects, it is characterized by extreme ecological specialization.

On Mayotte, the three new species collected (Mayottella dimorpha Roth, Ylangella truncata Roth and Chorisoblatta denticulara Roth) could be endemic. Mayottella dimorpha and Chorisoblatta denticulara were found only on cultivated sites. They could come into competition with the other species present on cultivated sites. They were found only on few sites in the north-eastern part of Grande Terre and on Petite Terre, compared to Loboptera dimidiatipes, Blattella lituricollis and Balta longicercata that were distributed all over Mayotte. Crops on Mayotte are traditionally cultivated without pesticide use. Use of pesticides, if it develops, could present a threat for these species only found on cultivated land, their original habitat will probably remain unknown. Ylangella truncata occupies a special ecological niche: decaying trees form a limited closed microhabitat with few exchanges with outside. This species is not specific to a particular tree species, but to the level of decay of a large fallen tree trunk. Human activity is currently limited by law in the forests on Mayotte as deforestation contributes to erosion that could endanger the lagoon [6]. Deforestation would, at first, provide an important increase in potential habitats for Ylangella truncata and thereby a population increase. However, later this would lead to the destruction of its habitat and disappearance of the species. If the forest cover becomes too restricted, the number of available dead trees could drop below the level necessary for this species' survival.

Introduced species can be found in the wild, at least occasionally [10]. Loboptera dimidiatipes, Blattella lituricollis and Balta longicercata are widely distributed and well established on La Réunion and Mayotte. They are more frequent than the endemic species that use the same habitats. They appear to have succeeded colonisation and establishment, as their populations are self-sustaining. Although adult Loboptera dimidiatipes are apterous, this species has dispersed over Mayotte and La Réunion. Although Blattella lituricollis and Balta longicercata adults are winged and therefore present excellent adaptations for dispersal, they appear to be less frequent than Loboptera dimidiatipes, particularly on Mayotte. Blattella biligata is very frequent on La Réunion but was found only once on Mayotte, where it could be in the process of becoming established. As Symploce pallens was found only once on both islands, the species could also be in the process of becoming established. Pycnoscelus sp., considered to be an invading species [11], can be found on a great variety of sites, as long as the substrate is loose enough to burrow into. However, its population densities were generally low. Its density was higher in dead trees than in the other types of environment. It can be found in the same dead tree as Ylangella truncata, but the two species do not appear to mix. This microclimate formed by the tree trunk that provides a loose substrate also enables it to survive at higher altitudes. Species found on both islands (Loboptera dimidiatipes, Blattella lituricollis, Balta longicercata and Blattella biligata Pycnoscelus sp.) were found in different microhabitats indicating that they probably have less restricted ecological requirements than native species. These species are widely distributed over the Pacific and Indian Ocean islands [12–15]. Species with naturally small population sizes are more vulnerable to habitat modification simply because loss of even a small amount of habitat for a geographically restricted species could reduce numbers below sustainable levels [16].