The cryptic regulation of diversity by functionally complementary large tropical forest herbivores

Abstract

1. Tropical forests hold some of the world's most diverse communities of plants.Many populations of large-bodied herbivores are threatened in these systems, yet their ecological functions and contribution towards the maintenance of high levels of plant diversity are poorly known. The impact of these herbivores on plant communities through antagonistic seed and seedling predation has received much attention, whilst their relevance as seed dispersal agents has been largely overlooked in experimental studies.2. Here, we tested how two key and functionally distinct large generalist mammalian herbivore species (the tapir Tapirus terrestris -a solitary browser and generalist seed disperser, and the white-lipped peccary Tayassu pecari -a group-living generalist seed/seedling predator) affect spatiotemporal patterns of diversity of seedling communities in tropical forests. We conducted a long-term multi-region landscape-scale exclusion experiment across four regions of the Atlantic forest of Brazil, representing a functional gradient of defaunation where these species were either present and absent in isolation and in combination. 3.Our results indicate that mammalian herbivores have a substantial role in regulating beta diversity in space and time. Seedling recruitment was strongly limited by the presence of the seed/seedling predator species (the peccary), but the presence of the browser and seed disperser (the tapir) had null net effects. Alpha diversity of seedlings at the community level did not respond to large herbivore exclusion at any region, whereas beta diversity decreased only where both herbivores were simultaneously excluded. Surprisingly, the synergic positive effect of both herbivore types on beta diversity was linked to increased evenness amongst dominant plant species, and a simultaneous decrease in alpha diversity of rare species.4. Synthesis. Together, these results challenge the common perception that large tropical forest herbivores maintain tropical forest diversity through antagonistic interactions, suggesting instead a synergistic effect of antagonistic predation and mutualistic seed dispersal on regional compositional diversity and local community 280 | Journal of Ecology VILLAR et AL.

Landscape structure shapes tree seedlings’ diversity at multiple spatial scales in a fragmented tropical rainforest

“…Furthermore, the strong association between α-diversity with landscape composition and configuration; and β-diversity with landscape configuration and connectivity is consistent with landscape structure effects on birds and arboreal and terrestrial mammals [27,60,62,87,95]. These faunal assemblages regulate key ecological interactions, particularly seed dispersal and seed/seedling predation [24,93,102]. Therefore, the decline of forest-dependent fauna could alter seedling diversity and its long-term ecological functions, potentially compromising the ecosystem integrity of HMTLs.…”

“…Furthermore, the inverse relationship between 0 α-diversity and 1 β-/ 2 β-diversity through AI within similar buffer sizes (600-m and 500-m, respectively), suggests the absence of terrestrial mammals. Exclusion experiments have shown that terrestrial seed and seedling predators reduce seedling 0 α-diversity and the abundance of dominant species at the local scale [24,93]. Herbivorous mammal foraging also promotes β-diversity of dominant species through seed dispersal at larger spatial scales [24,26,94].…”

“…Exclusion experiments have shown that terrestrial seed and seedling predators reduce seedling 0 α-diversity and the abundance of dominant species at the local scale [24,93]. Herbivorous mammal foraging also promotes β-diversity of dominant species through seed dispersal at larger spatial scales [24,26,94]. Terrestrial mammals’ richness is positively associated with OGF in similar buffer sizes than the observed AI effects (564 m) [27].…”

“…In remaining forest patches, the seed rain is influenced by the surrounding secondary forests (SF), whereas seedling recruitment is sensitive to edge effects and habitat heterogeneity [9,23]. Furthermore, β-diversity is strongly affected by the functional complementarity between mammalian seed/seedling predators and seed dispersers [24,25]. Large-sized herbivorous mammals reduce the number of rare species and the abundance of dominant species at the local the scale, while dispersing the latter at the landscape scale [24,26].…”

“…Furthermore, β-diversity is strongly affected by the functional complementarity between mammalian seed/seedling predators and seed dispersers [24,25]. Large-sized herbivorous mammals reduce the number of rare species and the abundance of dominant species at the local the scale, while dispersing the latter at the landscape scale [24,26]. The persistence of herbivorous mammals in HMTLs relies on the functional connectivity provided by a structural contrast of matrix with OGF patches [27,28].…”