Understanding processes responsible for latitudinal gradients in species richness is a key challenge for community ecology and has been the subject of intense study. The evolutionary speed hypothesis asserts this gradient is due to increased kinetic energy within the tropics, such that higher temperatures lead to increased evolutionary rates and higher species richness. Extensions of metabolic theory have formalized aspects of this hypothesis, but stop short of predicting kinetic effects of temperature on species richness. Species richness is a function of realized speciation and extinction rates, and both are ultimately ecological processes. To determine the kinetic effects of temperature on species richness, it is necessary to explicitly consider the role of ecological interactions. Here we merge temperature dependent mutation rates with an eco-evolutionary food web model in which organismal rates are scale allometrically and body size is allowed to evolve. This approach allows continuous feedbacks between evolution, ecology, and the ecosystem to dynamically influence diversification and realized ecosystem structure. Our model thus includes the minimal components necessary to determine if the evolutionary speed hypothesis is a plausible mechanism responsible for species richness gradients within the context of food web interactions. The mutation rate was scaled following a Boltzmann function whereby mutation rate is proportional to e-E/kT, where E is the mean activation energy of metabolic reactions (0.65 eV), k is Boltzmann’s constant, and T is temperature (K). The effect of temperature on species richness was analyzed by fitting a linear model to the regression of log-transformed richness against 1/kT. If speciation rate is limited by mutation rate and extinction rate does not vary with temperature, a slope of -0.65 is expected.
Results/Conclusions
We show that the quantitative effect of temperature on species richness is dependent on the stage of diversification and organismal traits: body size/trophic level and the degree of specialization and interference competition. Observed slopes, ranging from -0.65 to 0, overlapped with the central tendency of the empirical distribution. The steepest slopes emerged during early diversification and within trophic levels. In addition, steeper slopes were observed for consumers with broader niche breadth (i.e. generalists) and stronger interference. However through time nearly all slopes approached zero suggesting that temperature has little influence, via mutation, on equilibrium species richness. Extending our approach to include the role of potential energy and additional aspects of kinetic energy will provide critical insight into the processes that generate and maintain gradients in species richness.
