Antagonistic coevolution across productivity gradients: an experimental test of the effects of dispersal.

Coevolution commonly occurs in spatially heterogeneous environments, resulting in variable selection pressures acting on coevolving species. Dispersal across such environments is predicted to have a major impact on local coevolutionary dynamics. Here, we address how co-dispersal of coevolving populations of host and parasite across an environmental productivity gradient affected coevolution in experimental populations of bacteria and their parasitic viruses (phages). The rate of coevolution between bacteria and phages was greater in high-productivity environments. High-productivity immigrants ( approximately 2% of the recipient population) caused coevolutionary dynamics (rates of coevolution and degree of generalist evolution) in low-productivity environments to be largely indistinguishable from high-productivity environments, whereas immigration from low-productivity environments ( approximately 0.5% of the population) had no discernable impact. These results could not be explained by demography alone, but rather high-productivity immigrants had a selective advantage in low-productivity environments, but not vice versa. Coevolutionary interactions in high-productivity environments are therefore likely to have a disproportionate impact on coevolution across the landscape as a whole.

Increasing productivity accelerates host-parasite coevolution.

Host-parasite coevolution is believed to influence a range of evolutionary and ecological processes, including population dynamics, evolution of diversity, sexual reproduction and parasite virulence. The impact of coevolution on these processes will depend on its rate, which is likely to be affected by the energy flowing through an ecosystem, or productivity. We addressed how productivity affected rates of coevolution during a coevolutionary arms race between experimental populations of bacteria and their parasitic viruses (phages). As hypothesized, the rate of coevolution between bacterial resistance and phage infectivity increased with increased productivity. This relationship can in part be explained by reduced competitiveness of resistant bacteria in low compared with high productivity environments, leading to weaker selection for resistance in the former. The data further suggest that variation in productivity can generate variation in selection for resistance across landscapes, a result that is crucial to the geographic mosaic theory of coevolution.