The origin of species richness patterns along environmental gradients: uniting explanations based on time, diversification rate and carrying capacity

Aims: Patterns of species richness, such as the remarkable biodiversity of tropical regions, have been documented and studied for centuries. However, their underlying evolutionary and ecological causes are still incompletely understood. A commonly stated paradigm in the literature is that high richness in some habitats is directly caused by one of three competing explanations: (1) greater time-for-speciation (earlier colonization), (2) more rapid diversification rates (faster speciation relative to extinction) or (3) higher carrying capacity. However, these three explanations have been relatively little studied using theoretical approaches (especially in terms of comparing all three). Furthermore, empirical studies give conflicting results about their relative importance. Here, we use simulations to study the processes that drive richness patterns along environmental gradients. Location: Globally applicable. Methods: We use individual-based and trait-based modelling of eco-evolutionary dynamics to simulate the evolutionary radiation of a clade across five habitats with differing ecological conditions, and track patterns of species richness within and between habitats over time. We specifically address the roles of time and diversification rates in explaining richness patterns and the potential impact of carrying capacity. Main results and conclusions: Contrary to the widespread paradigm, we find that variation in carrying capacity can underlie differences in diversification rates and time-for-speciation among habitats. Therefore, carrying capacity is not a competing, alternative explanation for richness patterns. We also find that the time-for-speciation effect dominates richness patterns over short time-scales, whereas diversification rates dominate over longer time-scales. These latter observations can help reconcile the seemingly conflicting results of many empirical studies, which find that some patterns are explained by time and others by differences in diversification rates.