Interspecific interactions and range margins: contrasts among interaction types

Background/Question/Methods

Ecologists seek to anticipate how species interactions shift a species’ range margins (the limit of its geographic distribution). There is a great diversity of types of species interactions and at present, we lack a clear understanding of which species’ interactions most influence species’ range margins. To resolve this, we synthesize results from a broad array of models of pairwise species interactions to ask 1) Which species interactions most influence species’ range margins and 2) How many parameters must be measured to anticipate a species’ range margin.

Here we focus on one species and analyse where its range margin will be in the face of a second species. Interactions may benefit (+), harm (-), or have no effect (0) on either the focal species or the second species. We use this framework to contrast the effects of all interaction types on range margins, notably competition, commensalism, amensalism, mutualism and predation. Mathematically, we derive an expression for the range margin using a variant of an invasion analysis.

Results/Conclusions

We show striking parallels in the effects of all species interactions on species’ distributions. In answer to our questions: 1) Any species interaction that influences the focal species can dramatically shift its range margin. 2) The range margin typically depends on how each species responds to the abiotic environment, and how much species 1 benefits from species 2. Other details are frequently irrelevant.

A counterintuitive consequence of these results is that the effect of species 1 on species 2 is typically irrelevant. Hence, the range margin will be identical when the second species is a competitor, predator or amensalist. This suggests that many different ecological interactions must be considered when we make predictions about changes in species' distributions.