How do tropical forests support so many plant species? Moving away from traditional hypotheses, Sofia Gripenberg’s groundbreaking project targets broader spatial scales and includes detailed work on insects
Although ecological theory suggests that a few dominant species should monopolise rainforests, botanists routinely record more than one hundred tree species in plots no bigger than a football pitch. 50 years ago ecologists Janzen and Connell proposed a potential solution to this dilemma, which is now known as the “Janzen-Connell hypothesis”. A key prediction of this hypothesis is that the further a seed or seedling is from other members of its species, the higher its chance of surviving as it is less likely to be discovered and killed by specialist enemies such as insects and pathogens. As a consequence, plant individuals isolated from or surrounded by few members of their own species will do best, enhancing overall diversity. Although indirect evidence in support of the Janzen-Connell hypothesis has accumulated rapidly, some crucial aspects remain almost entirely unexplored. Importantly, there is a critical mis-match between the spatial scale at which ecologists have investigated the diversity-enhancing effects of plant enemies, and the spatial scale at which key processes are likely to operate.
Our Leverhulme-funded project, which focuses on the interactions between rainforest trees and insects attacking their seeds and seedlings, will address one untested component of the Janzen-Connell hypothesis. We hypothesise that the patchy distribution of many tree species in tropical forests has consequences for patterns of insect attack: insects will be less likely to find, disperse to and survive on trees that are spatially isolated from other individuals of the same species. Our project will map selected tree species from drone and satellite imagery and survey spatial patterns of attack by host specific insects and the survival of seeds and seedlings at our field site in Panama. The resulting data sets will be used to test key predictions about tree-insect interactions and their consequences for tree regeneration. Our findings will enhance our understanding of the processes generating the high levels of diversity of trees – and indirectly other species – in tropical forests. The project will also provide novel insights into the ecology of tropical insects at a time when there are major concerns about global insect declines and their wider consequences.