Funded Grants

Researcher: Ryan  Chisholm, Ph.D.

Grantee: National University of Singapore, Singapore

Researcher: Ryan Chisholm, Ph.D.

Grant Title: Modeling complex ecological communities: From small islands to tropical forests

Program Area: Studying Complex Systems

Grant Type: Scholar Award

Amount: $450,000

Year Awarded: 2016

Duration: 5 years

Modeling complex ecological communities: From small islands to tropical forests

Ecological communities are complex systems comprising potentially thousands of species with millions of interactions between them. Despite this fine-scale complexity, coarse-scale ecological patterns can be highly regular. For example, the relationship between area and species diversity exhibits consistent shapes across different ecosystems. How does this order arise and what processes drive it? Which ecological processes should we manage to conserve biological diversity effectively?

My research applies modeling techniques from mathematics and physics to large-scale ecological datasets to answer such questions. I focus on tropical forests, whose hyperdiverse tree communities have long been a source of biological fascination. I also study island systems, because islands act as microcosms whose relatively simple structure can reveal glimpses of fundamental ecological principles.

In recent years, my lab has focused on a key emergent property of ecosystems: species diversity. We have developed theoretical models of species diversity that predict phase transitions as an exogenous variable changes continuously. We recently confirmed the existence of such a transition in a dataset of islands from 100 archipelagos around the world, thereby resolving a classic ecological conundrum known as the “small;island effect”. Specifically, we showed that on small islands, species diversity is at a dynamic equilibrium determined by environmental constraints (niches), but that as island area increases past a threshold size, equilibrium diversity arises instead from a balance between immigration and stochastic extinction. This is satisfying not only because it shows how the dominant processes determining emergent ecosystem properties can change across scales, but because it unifies hitherto disparate bodies of ecological theory that, as we now see, describe phases on different sides of the transition.

In the coming years, I propose to develop my theory further by 1. Building mathematical models that unify my current island and mainland models. 2. Using larger archipelago datasets to test the theory’s island predictions further, including a prediction that there are in fact three possible phases mediated by two transitions. 3. Exploring the robustness of the theory’s predictions to the addition of strong environmental forcing terms, such as those arising from climate fluctuations. 4. Applying the insights gained from islands to understand the processes structuring mainland tropical forest diversity.

By understanding which factors are the dominant influences on biodiversity in different contexts, we will gain a better understanding not only of how our planet became home to such myriad life forms, but also of what processes must be managed in order to conserve this diversity.