Understanding how interactions between abiotic environmental factors determine population growth is a critical challenge in ecology, made pressing by rapid and wide-spread anthropogenic change. The individual effects of temperature and resources (nutrients, light, water availability) on population growth are well documented. However, the interactive effects of these factors on growth is poorly understood, despite their influence on everything from species distributions to the flow of energy and materials through ecosystems. Without a sufficiently mechanistic ecophysiological framework, models of community and ecosystem processes (including those used to forecast primary production) must rely on parsimonious, but potentially misleading assumptions. We present an integrative mathematical framework for understanding the interactive effects of temperature and resource availability on population growth. Using this framework, and an extensive compilation of data on the growth of auto- and heterotrophic microbes, we answer a series of fundamental questions. At the species level, these include: (1) how light and nutrient limitation affect optimum temperatures for growth, and (2) how auto- and heterotrophs differ in their thermal sensitivity, and (3) what tradeoffs constrain thermal tolerance. At the community level, we also explore how resource limitation affects the scaling of maximum interspecific growth rates with temperature.
Results/Conclusions
At the species level, we show that: (1) optimum temperature varies unimodally with light availability and declines with nutrient limitation; (2) birth rates of heterotrophs increase more strongly with temperature than those of autotrophs, consistent with theory; and (3) the shape of species’ thermal performance curves are constrained by a novel tradeoff between optimum temperature and the ratio of species’ temperature-independent death rate to their birth rate at a reference temperature. At the community level, our model reproduces a well-documented empirical pattern: light limitation prevents maximum interspecific growth rates from increasing with temperature. Collectively, our results and new modeling framework offer an enhanced understanding of interactive environmental effects on both individual microbial species and entire communities. While we focus on microorganisms, these results likely apply to a wide range of ectothermic organisms. Continued integration of physiology and ecology is essential to anticipating the ecological effects of changing abiotic environments, including modified temperature regimes and the widespread redistribution of inorganic nutrients, driven by human actions.