Changing range boundaries in a changing environment

There are two neat articles about species distributions in the October issue of Ecology Letters, and they complement one another in an interesting way.

The first, by Murphy et al. (2010), examines the abundance of eastern North American tree species at their northern and southern limits. The idea here is that abundance at the limits may give us some information about the potential of a species to respond to changes in the environment. High abundance at the northern limits could increase capacity to migrate at the "advancing edge" while high abundance at the southern limits could allow populations to survive changes at the "trailing edge" of environmental change.

Based on their analysis, most species had greater abundances per area at the northern portions of their range (relative numbers within cells) suggesting higher "climate suitability" in the north. But, > 50% of the species also occupied fewer areas (lower "occupancy" - the number of cells) at both limits, which the authors say could negatively affect the species' ability to expand. So, they concluded that

"the most common response is likely to involve range erosion in the south and limited range expansion in the north, possibly leading to an overall reduction in range size."

Why might that be? The explanation is partially explained in the study by Burton et al. (2010), who discuss the importance of evolution at the advancing edge. These authors modeled dispersal, reproduction, and competitive ability and found that dispersal and reproduction are selected for at the advancing front (making the populations more suited for survival in a novel environment/area), at the cost of decreased competitive ability. This means that, when a competing species is present, a species ability to expand could be restricted because the edge populations are trying to maximize their ability to "get by" in their new surroundings.

The articles mesh nicely because the empirical observations of abundance by Murphy et al. (2010) can be directly related to the theoretical predictions by Burton et al. (2010) to explain how a species distribution could respond to changes in the environment. For these authors, I recommend collaboration to do some mechanistic modeling of North American tree species!

Murphy, H., VanDerWal, J., & Lovett-Doust, J. (2010). Signatures of range expansion and erosion in eastern North American trees Ecology Letters, 13 (10), 1233-1244 DOI: 10.1111/j.1461-0248.2010.01526.x

Burton, O., Phillips, B., & Travis, J. (2010). Trade-offs and the evolution of life-histories during range expansion Ecology Letters, 13 (10), 1210-1220 DOI: 10.1111/j.1461-0248.2010.01505.x