Competitive Response Hierarchies for Germination, Growth, and Survival and Their Influence on Abundance

Abstract

Among plants, differences in competition intensity and in the ranking of competitive ability for traits such as germination, growth, and survival may suggest that the importance of competition for population success varies with the trait measured. If a species is a good competitor when measured by one component of fitness (e.g., seedling growth), but a poor competitor when measured by a different component of fitness (e.g., seedling survival), understanding the relative importance of each component of fitness for persistence in the community is critical to understanding how communities are structured. Using a field and a garden experiment in southeastern Michigan, we generated competitive response hierarchies among 4–8 old-field perennials for five components of fitness (germination, seedling growth, seedling survival, adult growth, and adult survival). We examined how the overall response to neighbors changes among components of fitness, the concordance of hierarchies within and among components of fitness, and the correlations between competitive hierarchies and natural abundance. We found little to no overall effect of neighbors on germination and seedling survival, indicating a high tolerance (= strong response competitive ability) of species for neighbors as measured by these components of fitness. We found a strong effect of neighbors on seedling growth, and on adult survival and growth, indicating poorer response competitive ability overall for these components. Although rankings of competitive ability were concordant among species across all components of fitness, certain demographic parameters were more consistent in their rankings than others. Specifically, competitive response hierarchies based on size were strongly concordant, while those based on survival were not. This suggests that future studies may effectively estimate response competitive ability for growth with a single estimate, but that estimates of response competitive ability for survival at both the seedling and adult stages may be required. Finally, competitive response rankings based on germination and seedling growth were most strongly correlated with abundance, suggesting that these components of fitness more strongly influence success in the community than do seedling survival and adult growth and survival.