CAREER: Pachyderms to pathogens: the cascading effects of megafauna loss on tropical tree recruitment

Duke University, Durham NC

Investigators

Abstract

Worldwide, human activities such as hunting and farming are reducing the populations of the largest animals, also called "megafauna". A declining species of special concern is the African forest elephant. These animals help create and maintain forests throughout much of central Africa by dispersing tree seeds and recycling and spreading nutrients in their dung, and by eating vegetation and trampling or uprooting small trees, thus allowing more light to penetrate to the forest floor. It is still unknown how the loss of these ecological engineers will change the region's forests or if, in their absence, smaller mammals, insects and fungi that consume seeds and seedlings might more strongly determine the types of trees that grow there and consequently alter the ecosystem services, such as timber, medicine and food, that the forests provide to humans. The overarching goal of this research is to broadly define the impact of megafauna in general, and the African forest elephant specifically, on closed canopy forests. The project seamlessly integrates research with education and training of graduate students. This research tests the hypothesis that the depletion of megafauna has cascading effects on the remaining plants and animals. The research objectives are to: assess the relationship among trees and the organisms with which they interact; quantify the degree to which loss of elephants alters these plant-animal interactions and tree recruitment; and simulate population dynamics of elephant-dispersed tree species to compare extinction probability in simulated populations with and without elephants. The researchers will undertake three major experiments that correspond to the objectives. First, to measure the relative effects of plant enemy groups (such as fungi) on tree recruitment in intact forest, they will combine caging experiments and observations to monitor seedlings, saplings, juveniles and adult trees over the course of the study. Second, to evaluate how defaunation affects the relative importance of plant enemy groups and forest structure, composition and diversity, they will expand the caging experiments across a gradient of defaunation to evaluate how reductions in animal populations affect tree populations and communities. Third, they will forecast the population- and community-level effects of defaunation by building individual based models for specific tree species and modeling the effects of elephants on forest composition and structure. The research will determine whether, in elephant-reduced forests, the loss of large herbivores shifts the relative strengths of herbivory to alternate plant predators, and if this changes forest dynamics and ecosystem services. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.