What determines species’ range boundaries?

The study of species’ geographic distributions, especially limits to those distributions, lies at a fruitful nexus of ecology and evolutionary biology. At these distributional limits, the ecological interactions that determine population mean fitness across and beyond the range limit collide with the evolutionary limits to adaptation. Species’ geographic distributions comprise the spatial extent of their populations, and vary greatly in size, shape, and the arrangement and abundance of populations contained therein. However variable these distributions are, they all are bounded by an invisible perimetric line on the landscape beyond which populations of that species cannot be found, i.e., the species’ geographic range limit. Why are individuals able to persist on one side of this border but are excluded from regions directly adjacent?  This deceptively simple question is a perennial one that underlies many foundational questions about ecological interactions and adaptation.

Most species’ are likely to be distributed over complex environmental gradients comprising changes in multiple abiotic and biotic variables. Transplant experiments, ideally combined with manipulations of putatively important environmental variables, are the most powerful method to test the relative influence of these variables on the location of the species’ range limit. In my dissertation, I used a variety of greenhouse and field experiments with the California native plant Clarkia xantiana ssp. xantiana (Onagraceae) to untangle complex environmental gradients and quantify the relative influence of both biotic and abiotic factors on plant lifetime fitness inside and outside the subspecies’ geographic range limit.

A wordcloud of my dissertation (defended September 2019)

My first two chapters focused on how two biotic interactions (herbivory and pollination) interact with an abiotic gradient in precipitation to set the range limit of C. x. xantiana. You can read more about this work here:

Benning, JW, Eckhart, VM, Geber, MA, and Moeller, DA. 2019. Biotic interactions contribute to the geographic range of an annual plant: herbivory and phenology mediate fitness beyond a range margin. American Naturalist, 193:786-797. pdf

Benning, JW and Moeller, DA. 2019. Maladaptation beyond a geographic range limit driven by antagonistic and mutualistic biotic interactions across an abiotic gradient. Evolution, 73:2044-2059. pdf

My second two chapters focused on spatial variation in plant-soil interactions and asked whether this variation might contribute to maladaptation outside the range limit, or local adaptation of plant populations within the range. These manuscripts are in press:

Benning, JW and Moeller, DA. 2020. Microbes, mutualism, and range margins: testing the fitness consequences of soil microbial communities across and beyond a native plant’s range. New Phytologist, in press.

Benning, JW and Moeller, DA. 2020. Plant-soil interactions limit lifetime fitness outside a native plant’s geographic range margin. Ecology, in press.