What determines species’ range boundaries?

Every species does not occur everywhere – though at first glance simplistic, this observation forms the foundation of the fields of ecology and evolution. But even after centuries of investigation, there is still not a single species for which we have identified completely all the environmental components that restrict that organism to its particular geographic location. This is due to the complex suite of factors involved in structuring the distribution of species, including abiotic conditions, dispersal limitation, adaptive potential, and interspecific interactions.

Of particular interest are those processes limiting range boundaries – why do we see some species’ ranges expanding rapidly while others stand remarkably still? Explanations for static range limits fall into two broad categories: a failure to disperse outside the range boundary or a failure to adapt to novel conditions existing therein. These novel environments have been explored primarily in the context of climatic variables such as temperature and precipitation, but environments are complex and comprised of multiple abiotic and biotic factors that often covary across space. It is increasingly acknowledged that we need to look beyond climate to gain insight into factors driving the formation of range boundaries.

I’m currently exploring these questions with Clarkia xantiana (Onagraceae), an endemic Southern California wildflower. The species is comprised of two parapatric subspecies (ssp. xantiana and ssp. parviflora) whose range limits have remained stable for many generations. Through a combination of range-wide environmental sampling, field transplants, and greenhouse experiments I’m investigating how biotic and abiotic factors may interact to constrain range expansion in the species.