Climatic niche breadth and species richness in temperate treefrogs
Patterns of species richness are often closely linked with climate, but the specific mechanisms by which species' climatic niches underlie large-scale richness patterns remain poorly understood. It has been hypothesized that reduced temperature seasonality in the tropics promotes the evolution of species with narrow temperature niche breadths, and that this hypothesis helps explain high tropical richness. However, the relationship between species' climatic niche breadths and species richness has yet to be tested. We have addressed this issue using treefrogs (Hylidae) in eastern North America.
Eastern North America.
We characterized climatic niches and niche breadths for all 24 hylid species in eastern North America using temperature and precipitation variables. We then examined the relationships between species richness, climatic niche positions and climatic niche breadths using phylogenetic comparative methods.
Species richness was negatively associated with mean climatic niche breadth, such that high-richness climates had species with narrower climatic niches. Our results also supported the roles of niche conservatism and the time-for-speciation effect in generating the relationship between climate and species richness in the region (more species in warm, wet regions that have been inhabited longer). Importantly, we show that the invasion of low-richness climates has occurred primarily through recent intraspecific niche expansion into these climates rather than evolution of species that are narrowly specialized for these conditions (although the two hylid clades studied showed somewhat different patterns).
We found that climatic zones with high species richness contain more species with narrower climatic niche breadths. Our results suggest that this pattern arose because narrow climatic niche breadths restricted the dispersal of most hylid species out of the ancestral, warm, moist climatic zones, allowing more time for speciation to build up higher species richness in these zones.