Diversité spécifique et coexistence en forêt tropicale : l’occupation des milieux par une guilde d’oiseaux insectivores en Guyane
Authors: Thiollay Jean-Marc 
Published: November 2022

The structure and distribution of a guild of 43 species of small foliage gleaners in the lower strata have been compared between 7 localities of primary rain forest in French Guiana, using a sample plot census method. This is the second step (see Thiollay, 1986) in a longterm study of the the determinants of the maintenance of bird species diversity in a neotropical rain forest.
This subset (10 %) of a larger bird community has a structure similar (although more even) to that of the whole avian population, with a majority of relatively rare and patchily distributed species. The human hunting pressure, in otherwise undisturbed primary forest, tends to increase the abundance of dominant species, while some rare species decrease and eventually disappear. A similar pattern of indirect effects of hunting pressure (reduction of species richness, diversity, and evenness, increase of dominance index) had already been found amongst other non game species. In submontane forest (500-850 m), the species richness decreases, but not the other parameters of the guild structure.
The frequency of most species varies among localities, independently of the proportion of the different habitats available. The rarer the species the more uneven its distribution ; within particular habitats, a species abundance is more variable in marginal than in optimal habitats, as if it was more sensitive to interspecific competition in low density or suboptimal situations.
Only habitat parameters have been measured. Along this axis, most species’ niches appear relatively narrow, densely packed, overdispersed over the full range of available habitats, and often widely overlapping, while retaining pronounced specific differences. Likewise, foraging heights are finely subdivided between species, but less so within multispecies flocks, where antipredator advantages far exceed the risks of competition. However, microhabitat selection may change according to the time of the day, seasons and localities, probably under the influence of prey availability, interspecific competition and pressure of predators.
Among several ecological correlates of rarity, the specialization in a very localized habitat accounts for the low overall density and the very patchy distribution of half the rare species. There is no significant correlation between niche width and abundance, but the commonest species tend to have the widest geographical distribution. Species originating from the Guianan Pleistocene refuge show no tendency to be more abundant in Guiana than those that originaled in more distant refuges. The low rate of cooccurrence of congeneric, or ecologically similar species (outside multispecies flocks), and a negative correlation (although non significant) between the abundance of species members of such pairs of co-occurring species, as well as their sharp niche segregation, all indicate a limited niche similarity, and competitive exclusion. This points out the probable importance of interspecific competition in this «oversatured» community. The low density of most species, probably originating from limiting environmental factors (including food availability), favors a higher species packing, which in turn reduces the specific densities through increased competition.