American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Extinction risk and lack of evolutionary rescue under resource depletion or area reduction”

Posted on

Steinar Engen and Bernt-Erik Sæther

We show that under r- and K-selection populations facing losses of habitats will not necessarily be rescued by evolution

During the last century human impact on natural populations has increased dramatically, by fragmentation, harvesting, pollution, introduced species, and climate change, often with the consequence that species are faced with reduced amount of resources and increased risk of extinction. It is commonly, and correctly, argued that species may adapt to such changing environments and thereby produce a rescue effect counteracting the negative impact of human activities.

In this paper Steinar Engen and Bernt‐Erik Sæther, at the Centre for Biodiversity Dynamics at The Norwegian University of Science and Technology, present a simple idea that force us to think differently about the possibility of rescue by Darwinian adaption to deteriorated environments. They apply their theory of continuously exchanging r‐ and K‐selection in fluctuating density dependent populations, previously worked out in collaboration with Russell Lande. At small population sizes the ability to grow quickly through large fecundity and short generation time is favored (r‐selection), while under large densities genes making individuals competitive for limited resources are selected for (K‐selection). Further, long‐term evolution must generate a genetic trade‐off between these properties, since otherwise evolution would lead to unlimited growth‐rates as well as competition ability. Engen and Sæther first show that the stationary fluctuations of mean phenotypes is the same before and after a given reduction of resources per individual. However, the interesting observation is what happens during a period of reduced resource availability. The number of individuals per resource unit will then increase, which according to the theory promotes K‐selection. The trade‐off between the growth rate r at small densities and the carrying capacity K then leads to evolution toward smaller r.

Long‐lived species may often have a small growth rate and a long return time to equilibrium. Engen and Sæther show through examples that reduction of resources through time then may even produce negative values of r by natural selection. Although this is an adaption to the environmental conditions at the time because competitive ability is important, the long‐run consequence is that the extinction risk may be much larger than in the case of no adaption. Hence, there is no rescue effect by Darwinian adaption to the changing environments, but rather a reduction in sustainability, opposite of common belief. Read the Article