Predator coevolution and prey trait variability determine species coexistence

Abstract

Predation is one of the key ecological mechanisms allowing species coexistence and influencing biological diversity. However, ecological processes are subject to contemporary evolutionary change, and the degree to which predation affects diversity ultimately depends on the interplay between evolution and ecology. Furthermore, ecological interactions that influence species coexistence can be altered by reciprocal coevolution especially in the case of antagonistic interactions such as predation or parasitism. Here we used an experimental evolution approach to test for the role of initial trait variation in the prey population and coevolutionary history of the predator in the ecological dynamics of a two-species bacterial community predated by a ciliate. We found that initial trait variation both at the bacterial and ciliate level enhanced species coexistence, and that subsequent trait evolutionary trajectories depended on the initial genetic diversity present in the population. Our findings provide further support to the notion that the ecology-centric view of diversity maintenance must be reinvestigated in light of recent findings in the field of eco-evolutionary dynamics.

Keywords: Pseudomonas fluorescens SBW25; Tetrahymena thermophila; community dynamics; eco-evolutionary dynamics; genetic diversity; predator–prey interaction.

Figure 1.

Proportion of the P. fluorescens population over time. The rows represent the different bacterial population structures (rows in figure) for P. fluorescens: (a–c) full-diversity, (d–f) high-diversity and (g–i) ancestral P. fluorescens strain. The columns represent the different predator treatments that were also applied: (a,d,g) ciliates coevolved with bacteria, (b,e,h) naive ciliate and (c,f,i) no ciliates. The black line represents the proportion of P. fluorescens (mean ± s.e.), and the red line shows the equal proportion line as reference.

Figure 2.

Total densities for bacteria and ciliates. Rows represent data from the three different population structures: (a–c) full-diversity, (d–f) high-diversity and (g–i) ancestral P. fluorescens strain without diversity. Columns represent the three different predation treatments: (a,d,g) ciliates coevolved with bacteria, (b,e,h) naive ciliate and (c,f,i) no ciliates. Orange lines and points (mean ± s.e.) show total bacterial density measured by absorbance, and blue lines and squares represent ciliate densities (mean ± s.e.). Bacterial density is shown as optical density at 600 nm, and ciliate density as cells ml^–1, both normalized to 0–1 range.

Figure 3.

Divergence in trait space caused by the genetic diversity of P. fluorescens or predator evolutionary history (ellipses depict 95% confidence levels). Growth is the biomass yield in the absence of predation and defence is the effect of predation (0 = predation has no effect) on the biomass yield (both in optical density area units). (a–c) P. fluorescens and (d–f) E. coli clones isolated from the endpoint of a microcosm experiment.