Germination is a critical life history trait because it sets the conditions a plant experiences in life, both via abiotic and biotic conditions. Germinating at the wrong time can even lead to reproductive failure. Thus, we expect selection on germination strategies to match environmental conditions. However, the environment fluctuates along many dimensions like the timing of rainfall, frost, and temperature. These dimensions of variability may change in importance over a spatial gradient with important consequences for selection, community composition, and diversity. Here, we considered how such changes in the importance of environmental factors affect selection on germination and the ability of species to coexist along a gradient.
We modeled stochastic population growth of two species of annual plants in a fluctuating environment. The environment consists of two independent components, where the importance of each varies along a gradient. The fluctuating environment affects both species’ germination rates and growth during the year. Larger plants produce more seeds, but also demand more resources and therefore have the potential to affect growth through competition. By assuming rare mutations of small effect in the germination distribution, we found which germination strategies are present at evolutionary equilibrium along the environmental gradient.
Results/Conclusions
Using the assumptions of adaptive dynamics, we find that, with just one species along the gradient, selection favors strategies that germinate most when conditions favor large plants and germinate little when growth is poor. These patterns of growth differ along the gradient, which leads to a diversity of germination strategies along the gradient. However, a competing species that is introduced at low density generally evolves to have germination patterns uncorrelated with its competitor. This means that the introduced species evolves to germinate in times poor for growth. Despite the seemingly maladaptive response, uncorrelated germination responses are favored because they lead to low interspecific competition. We find that the strength of selection for uncorrelated germination increases with germination variance and resource demand. Thus, more productive and variable locations more greatly facilitate competitive displacement in germination strategies.
This study suggests that selection to reduce interspecific competition can lead to patterns similar to those of the introduction of species adapted to other locations. Indeed, the set of germination strategies are similar along the environmental gradient with competition between species. Further, it may be difficult to infer whether individuals are adapted to an environment.