Le peuplement de Mammifères Insectivores des forêts du Nord-Est du Gabon
Authors: Brosset André 
Published: November 2022

The structure and dynamics of the community of rain forest Insectivora of the Belinga-Makokou region (N-E Gabon) are described. This community can be considered as typical of other sites in central and western Africa. One species of otter shrew (Potamogalinae) was found along the forest-streams, and eleven species of shrews (Soricinae) were observed living on the forest floor, or among the forest litter.

Shrews were collected by non selective trapping. The results of this trapping, combined with field observations, illustrate the species richness and diversity of this community.

This shrew community may be characterized as follows. No spatial segregation of species in the different patches of the forest mosaic was noted, nor was there any evidence of segregation on the basis of activity cycles. Ecological separation of these species is a function of body size, there being no (or very little) overlap in the average body size between the 11 species. The size of each shrew species is correlated with the strength of its jaws, and its ability to move through forest litters of different texture. This, in turn, affects the type of prey accessible to each species. Some species display a more specialized morphology, with long limbs and a long tail providing them with the ability to leap.

One species made up about 50 % of all the individuals present in the community, with other species being far less common, or even rare. One species, rather scarce in primary forest, was very common in human settlements. Groups of twin-species were noted amongst the large and middle sized species. Compared to their palearctic relatives, African shrews occurred at low densities in African forests, the overall catch by non selective-trapping being about 10 times smaller than it is in western France, for instance.

The population dynamics of these shews display some striking features. In primary forest, fecundity was very low, and only 1 to 2 embryos (av.1.8) were found in pregnant females, with the exception of the numerically dominant species which bore an average of 3.1 embryos. One species which lives both in forest and human settlements produced more young when living in houses than in field conditions. One community once locally decimated by trapping, re-established itself only very slowly — the first species to return being the formerly more numerous one.

Some peculiarities of the structure and dynamics of this community are discussed. It is difficult to believe that the regular size distribution of these shrews is just a chance event. The «nul hypothesis», disregarding any ordination of species in the community, is not confirmed by statistical analysis, and is actually incompatible with the fact that all the known forest communities of African shrews display the same pattern of organisation, although the other sympatric communities belonging to different taxa have different structural patterns. Originally, the community of shrews at Makokou was probably shaped by competition, but now its organisation and resilience seem to be maintained by some homeostatic mechanism regulating the ratio between rare and common species — perhaps without competition playing any role.

The long legs and the leaping ability of some shrew species, seem to be related not to some advantage in the pursuit of a prey, but to their anti-predatory strategy. This becomes particularly obvious when a Shrew escapes the attack of army ants (genus Anomma). Army ants indeed seem to be the major predators of the invertebrate and small vertebrate faunas of the forest litter in this area.

Understanding the evolutionary mechanisms which maintain the structure and resilience of this community requires consideration of the question of abundant versus rare species. One does not yet quite understand why some particular species are rare, and other not. Competition for territories, or a seldom realized ecological niche, do not explain the scarcity of some African forest shrews. The more numerous species at Makokou have the larger geographical distribution, are ecologically more opportunistic, are more fecund, and can invade new or empty habitats more rapidly, than the rarer species. In some other similar sites, a same species can remain numerically dominant, whereas it becomes scarcer elsewhere, regardless of the distance separating the populations. Conversely, shrew species which are rare at Makokou, are also rare elsewhere. established itself only very slowly — the first species to return being the formerly more numerous one.

Some peculiarities of the structure and dynamics of this community are discussed. It is difficult to believe that the regular size distribution of these shrews is just a chance event. The «nul hypothesis», disregarding any ordination of species in the community, is not confirmed by statistical analysis, and is actually incompatible with the fact that all the known forest communities of African shrews display the same pattern of organisation, although the other sympatric communities belonging to different taxa have different structural patterns. Originally, the community of shrews at Makokou was probably shaped by competition, but now its organisation and resilience seem to be maintained by some homeostatic mechanism regulating the ratio between rare and common species — perhaps without competition playing any role.

The long legs and the leaping ability of some shrew species, seem to be related not to some advantage in the pursuit of a prey, but to their anti-predatory strategy. This becomes particularly obvious when a Shrew escapes the attack of army ants (genus Anomma). Army ants indeed seem to be the major predators of the invertebrate and small vertebrate faunas of the forest litter in this area.

Understanding the evolutionary mechanisms which maintain the structure and resilience of this community requires consideration of the question of abundant versus rare species. One does not yet quite understand why some particular species are rare, and other not. Competition for territories, or a seldom realized ecological niche, do not explain the scarcity of some African forest shrews. The more numerous species at Makokou have the larger geographical distribution, are ecologically more opportunistic, are more fecund, and can invade new or empty habitats more rapidly, than the rarer species. In some other similar sites, a same species can remain numerically dominant, whereas it becomes scarcer elsewhere, regardless of the distance separating the populations. Conversely, shrew species which are rare at Makokou, are also rare elsewhere.