The Journal of Ecology has recently published new research by Villar et al. “The cryptic regulation of diversity by functionally complementary large tropical forest herbivores.”
Read more about their work in this post & Part 1, written by authors Nacho Villar & Mauro Galetti.
To Janzen or not to Janzen?
The standard Janzen-Connell hypothesis (JCH) and its subsequent ramifications (e.g. Terborgh 2012) suggest that natural specialized enemies, such as invertebrates, parasitic fungi and bacteria, maintain plant diversity in tropical forests by suppressing resource abundance in a density dependent manner – thus promoting coexistence as a “side effect”. However, what is the expected impact of a generalist consumer like tapir (mutualist) or a WLP (antagonist) on diversity structure? This is what we set out to discover in the first ever analysis of our dataset. We examined the effect of experimental exclusion on seedling recruitment, and alpha and beta diversity, across the first seven years of the experiment. We found that tapirs and WLPs lead to increased dissimilarity between communities (larger beta diversity) only in the region were both co-occurred, but we found no increase in local plant species diversity (alpha diversity) with any herbivore type, either in isolation or in combination. WLPs strongly suppressed seedling recruitment and, as expected from a solitary animal at low densities, tapirs had a negligible impact on recruitment.
Clearly, these results suggest that the JCH does not apply to large generalist consumers. Furthermore, they suggest a functionally complementary relationship between tapirs and WLP. In our article, we speculate about the possibility that antagonistic disturbance from WLPs facilitates recruitment of seeds dispersed by tapirs, so that feedbacks between mutualisms and antagonisms synergistically increase dissimilarities amongst communities (beta diversity) at the landscape level. This is a strongly speculative mechanistic model, but our best hypothesis given our detailed knowledge of the natural history of the system.
More to come
We urgently need more experimental studies in forest ecosystems, tackling synergies and redundancies amongst large vertebrates in tropical forests. Such studies can inform conservation practice, before these species and their ecological functions vanish from tropical forest habitats. The characterization of these functions and their synergies can allow classical ecological models to be tested, and the development of fascinating new ecological theories. More analyses of our dataset are on their way, and they will certainly provide more surprises about the functional roles of these charismatic and fundamental species.
Nacho Villar, Universidade Estadual Paulista (UNESP), Brazil
Mauro Galetti, Universidade Estadual Paulista (UNESP), Brazil & University of Miami, USA
Read the full paper online: The cryptic regulation of diversity by functionally complementary large tropical forest herbivores (free to view for a limited time)
Read the press release from São Paulo State University (UNESP): https://www.britishecologicalsociety.org/large-herbivores-increase-biodiversity-tropical-forests/