Defaunation and fragmentation erode small mammal diversity dimensions in tropical forests

Figure 1: Best-fitting structural equation model (full direct + indirect model). Positive and negative pathways are indicated by black and gray lines, respectively. Arrow thickness is scaled to illustrate the relative strength of effects and significant coefficients are indicate with asterisks (*p < 0.05; **p < 0.01; ***p < 0.001). The marginal coefficients of determination (R2) is shown in the black circles for all response variables.​

By Ricardo S. Bovendorp, Mauro Galetti and Rafael Loyola

When 1992 Kent H. Redford’s famous paper suggested that the population collapse of large and mid-mammals would change drastically tropical forests leading to impoverished ecosystems, the term “empty forest” has called the attention of popular media. Empty forests has become a “symptom” in the diagnose of all ecosystems. Our paper recently published in Ecography shows, however, that in fact there is no empty forest.

We evaluated if size of forest patches, amount of habitat and defaunation – understood as the loss of large mammals in forest remnants – were responsible for (and by how much) the loss of small mammal diversity in tropical and subtropical Atlantic forests along the coast of Brazil. Based on an extensive fieldwork dataset of 283 mammal assemblages (adding up to 105 species), we showed that the extirpation of large-bodied mammals did lead to decline in the number of functionally disparate mammal species within communities (i.e. functional diversity), although the total number of species increased with forest size. Forest cover did not affect functional and phylogenetic diversity (see Figure 1).

Our results also indicate that the presence of medium and large mammals (probably acting as predators or competitors of small mammals) and forest patch size helped to retain species and functional diversity of small mammal communities in the region. Further, the total number of species in a community was critical to sustain phylogenetic diversity, and may have a pronounced influence on the ecological roles played by small mammals. Our results emphasize that landscape ecologists should consider the indirect effects caused by the extinction of competing or predator species on the number of species as well as on the roles or evolutionary relationships species show in different communities.

Given the unprecedented rates of defaunation worldwide, the results from our work may serve as a warning for cascading effects of large mammal extirpation on the diversity and roles played by megadiverse groups such as small mammals in the tropics.

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