Evolutionary ecology of species´ ranges

What determines the limits to a species niche and therefore its geographic range? Over the last hundred years, the theory for single well mixed populations has been well developed. However, in nature, populations are distributed over space and finite in numbers. The theory for spatially structured populations under selection and random genetic drift is still fairly rudimentary, to a large extent because such problems are hard to solve formally. A mathematical theory of adaptation in natural populations and, especially, of range expansion, provides an understanding of immediate ecological processes, such as changes in a species range as environment changes over time, or the spread of invasive species. The interface between ecology and evolution is critical in understanding the evolution of the species range, yet the processes are rarely modeled together. I propose to develop a comprehensive theory of a species range, modeling jointly ecological and evolutionary dynamics. First, I will broaden the current theory of a species` range, by concentrating on intra-specific dynamics in spatially varying environments. Then, I will allow the environment to vary both in space and time, to characterise the core parameters that predict when a species` range contracts, expands or shifts due to changing conditions. As conditions change over time, species adapt and change their range or go extinct. The proposed project would be the first study of adaptation to spatially and temporally changing environments that combines evolutionary dynamics including the evolution of genetic variance with ecological dynamics and stochastic sampling. It is important to consider temporal and spatial change jointly, as their effects interact: a species adapted to a wide range of conditions across its habitat maintains higher genetic variance locally, and hence can sustain higher rates of temporal change as it lags less behind the optimum. Furthermore, a consistent model of species` range margins must include stochastic effects: when adaptation starts to fail, population size decreases, and the effect of stochastic sampling (genetic drift) dominates. By including temporal change, and allowing genetic variance to evolve in response to both the environment and genetic drift, I will develop a general theory that can be applied for example to address a species` response to climate change. Finally, by extending the theory to co-evolution of multiple species, I will connect the theory of evolution of species` ``fundamental`` and ``realized`` niches (where ``fundamental`` niche describes the set of conditions that allows species` to persist on its own in contrast to ``realized`` niche, which determines the habitat occupied in the presence of competitors).

All species have restricted distributions. Yet a joint coherent theory explaining the span and borders of species ranges has been missing both due to the complexity of the problem and the disparate traditions in ecology and evolution. This FWF project (Elise Richter, V 458-B25) explains that just two compound parameters, important for both ecology and evolution of species, are fundamental to the stability of their range: the environmental heterogeneity and the size of the local population. The theory includes both population and evolutionary dynamics, and elucidates the role of dispersal for adaptation across a species range. Dispersal has conflicting effects: on the beneficial side, it increases the genetic variation that is necessary for adaptation and counters the loss of genetic diversity due to genetic drift. However, dispersal also carries a cost: it may swamp adaptation to local conditions. This interplay is crucial for the evolution of a species` range. The novel theory demonstrates that adaptation across a species range depends on two dimensionless parameters: i) the fitness cost of dispersal - a measure of environmental heterogeneity, and ii) the strength of genetic drift - a measure of the reduction of genetic diversity. The more heterogeneous is an environment, the more challenging it is to expand into, and the lower the genetic diversity, the more limited is the scope for potential adaptation. Together, these two parameters define an expansion threshold: adaptation fails when the number of individuals accessible by dispersal within one generation is so small that genetic drift reduces genetic diversity below that required for adaptation to a heterogeneous environment. This threshold provides a testable prediction for the formation of a range margin and for the collapse of a species` range. Interestingly, the parameters are independent of the genetic architecture, offering a wide scope for empirical tests of the theory across many species. Furthermore, the theory explains that the benefits of dispersal into marginal populations are likely to outweigh the costs, offering a theoretical base for management strategies of threatened populations. References: Polechov J. (2018) Is the sky the limit? On the expansion threshold of a species` range. PLoS Biology. doi: https://doi.org/10.1101/234377