Synthesis: “A practical guide to the study of distribution limits”
Yvonne Willi and Josh Van Buskirk (June 2019)
A practical guide to studying distribution limits, emphasizing recently proposed genetic constraints
Constraints on adaptation are an important explanation for the tremendous diversity of species on Earth. This may seem surprising – after all, adaptation also leads to the evolution of new species. But adaptive limits prevent species from evolving ever-larger ecological niches, and hence ensure that species are confined to restricted geographic distributions. This, in part, explains why different species occur in different geographic regions. This argument suggests that the causes of range limits are a key to understanding the maintenance of biodiversity. According to theory, there are several plausible explanations for range limits, but empirical tests are too limited to reach any general conclusions. This article is a guide to the study of range limits that should be applicable to any organism. It emphasizes the degradation of habitat quality near the edge of the range, the consequences of small population size and fragmentation at the edge, genetic factors that may obstruct adaptation at the range edge, and the importance of synthesizing different explanations within an eco-evolutionary framework.
Factors that limit the geographic distribution of species are broadly important in ecology and evolutionary biology, and understanding distribution limits is imperative for predicting how species will respond to environmental change. Good data indicate that factors such as dispersal limitation, small effective population size, and isolation are sometimes important. But empirical research highlights no single factor that explains the ubiquity of distribution limits. In this article, we outline a guide to tackling distribution limits that integrates established causes, such as dispersal limitation and spatial environmental heterogeneity, with understudied causes such as mutational load and genetic or developmental integration of traits limiting niche expansion. We highlight how modeling and quantitative genetic and genomic analyses can provide insight into sources of distribution limits. Our practical guide provides a framework for considering the many factors likely to determine species distributions and how the different approaches can be integrated to predict distribution limits using eco-evolutionary modeling. The framework should also help predict distribution limits of invasive species and under climate change.