Large wild herbivores slow down the rapid decline of plant diversity in a tropical forest biodiversity hotspot.
The UN declaration of the Decade of Ecosystem Restoration 2021-2030 emphasizes the need for effective measures to restore ecosystems and safeguard biodiversity. Large herbivores regulate many ecosystem processes and functions; yet, their potential as a nature-based solution to buffer against long-term temporal declines in biodiversity associated with global change and restore diversity in secondary forests remains unknown. By means of an exclusion experiment, we tested experimentally the buffering effects of large wild herbivores to avert against long-term biodiversity collapse in old-growth and secondary tropical forests in the Atlantic Forest of Brazil where sapling abundance and species richness declined c. 20% over the course of 10 years. The experiment comprised 50 large herbivore exclosure-open control plot pairs (25 at the old-growth forest and 25 at the secondary forest), where 2 m2 were monitored in every plot during a 10-year period. Large herbivores were able to decelerate diversity declines and compositional change in the species-rich old-growth forest, but only decelerated compositional change in the secondary forest. In contrast, declines in species richness and abundance were unaffected by large herbivores on either forest. The buffering effects of large herbivores were strongly nonlinear and contingent on the initial level of diversity at the patch scale: highly diverse communities suffered the strongest collapse in the absence of large herbivores. Thus, larger buffering effects of large herbivores on the old-growth forest are the logical consequence of large herbivores buffering the many high diversity plant communities found in this forest. Conversely, as the secondary forest held fewer high diversity patches, buffering effects on the secondary forest were weak. Synthesis and applications. Our study indicates that large herbivores have moderate yet critical effects on slowing down community change and diversity loss of highly diverse plant communities, thus suggesting that the conservation of (and potentially trophic rewilding with) large herbivores is a fundamental nature-based solution for averting the global collapse of the strongholds of biodiversity. Its buffering effects on biodiversity loss operate at very small spatial scales, are likely contingent on successional stage and are most effective in old-growth or high diversity secondary forests.