Modelling the Role of Diversity in Ecosystem Responses to Warming

Abstract

Anthropogenic climate change is disrupting the Earth’s physical environment, which in turn impacts ecosystems. Warming is a particularly important physical stressor, as organisms tend to be temperature sensitive due to the role of thermodynamics in controlling metabolic rates. Resilient ecosystems can withstand environmental changes and maintain function. High ecosystem function and resilience are often attributed to high ecosystem diversity. However, the mechanisms linking ecosystem function, resilience and diversity are far from well understood.

In this thesis, mathematical models of varying complexity are used to isolate processes relevant to the relationship between ecosystem diversity and resilience. The mechanisms investigated include ‘trait diffusion’ and ‘resource partitioning’. Trait diffusion alters the distribution of phenotypic traits in a population, generating diversity as well as influencing ecosystem function and resilience. Resource partitioning occurs when different species are specialised to different environmental niches, in this case, resources. This complementarity between species can lead to higher function and diversity. When trait diffusion is present, high initial diversity is found to lead to lower ecosystem function, whereas the opposite is the case with resource partitioning. With both mechanisms, emergent diversity is not clearly correlated with ecosystem function. Trait diffusion allows short-lived species to adapt to warming, whereas long-lived species rely on initial diversity.

More complex models are necessary for a sufficiently realistic representation of an ecosystem to make quantitative predictions. In this thesis, a size-structured dynamic energy budget model of fishes and invertebrates has been coupled to a spatially explicit model of corals and algae, to understand how warming impacts a coral reef ecosystem. Warming directly affects fishes and invertebrates by increasing their metabolic rate, resulting in changes to demographic processes such as growth rates. Indirect effects include the loss of reef habitat quality as coral bleaching reduces the availability of refuges. Modelled changes in biomass for +3⁰C of warming are found to be controlled predominantly by the direct effects of warming, and crucially for fisheries, the biomass of predators is modelled to decrease by at least 50% with +3⁰C of warming.