Coexistence of aggregation strategies in outbreak species

Background/Question/Methods Many species of non-social animals aggregate to acquire resources for survival and reproduction. By aggregating, organisms may create more favourable environmental conditions or procure resources which are not attainable when acting alone. However, if the resources are to be shared, aggregation can increase competition among individuals and diminish gains from synergistic effects from aggregation. Hence, individuals working in groups make trade-offs between the costs and benefits of aggregation, giving rise to optimal group size. The advantage of aggregation behaviour depends on the environment surrounding the individuals and population density. When resources are readily available and population density is high, aggregation likely results in overcrowding and severe competition for the resource. As a result, population size influences whether an aggregation strategy is selected for. The interplay between population dynamics and aggregation makes it difficult to establish which aggregation strategy will evolve through natural selection. In this paper, our objective is to investigate the best aggregation strategy when population density wildly fluctuates as in many outbreak species. We developed a population dynamics model where aggregation mechanisms are incorporated, and performed evolutionary simulations.

Results/Conclusions

The results from simulations show that, when any single strategy is unstable, a mixed strategy can be the evolutionary stable strategy although each individual has one fixed strategy during its life time. The results demonstrate that mixed strategies can be evolutionarily selected for due to realized population stability and also suggest that interactions between apparent competitors can actually be mutualistic as well as competitive, depending on the environment, namely population density in this case. In our model, high short-term payoff of one strategy is selected for individual-level benefit, while another strategy is selected for a long-term advantage of population persistence. We conclude that population dynamics can feedback to individual-level selection and realize the coexistence of different aggregation strategies.