Colonization-related tradeoffs and the maintenance of diversity in seed size

Background/Question/Methods

Interspecific tradeoffs involving colonization ability can contribute strongly to the maintenance of plant species diversity, and are often cited as a potential mechanism underlying high tropical forest diversity. The well-known competition-colonization tradeoff, between the ability to win a regeneration site after arrival and the ability to arrive, can in theory maintain very high species diversity, but only if there is strong competitive asymmetry among species, such that the best competitor present is highly disproportionately likely to win. Other, less-studied tradeoffs involving colonization ability can contribute to diversity maintenance given appropriate habitat heterogeneity, by facilitating habitat niche partitioning. Specifically, a tradeoff between fecundity and stress tolerance combined with corresponding variation in stress among regeneration sites can lead to coexistence between more tolerant species able to win high stress sites and more fecund species that are numerically more likely to win low stress sites. A tradeoff between fecundity and dispersal can similarly contribute to coexistence given spatial variation in the density of suitable regeneration sites. We use 20 years of data on seed arrival and 12 years of data on seedling recruitment into paired seed traps and seedling plots on Barro Colorado Island, Panama, to evaluate evidence regarding each of these tradeoffs.

Results/Conclusions

We find that there is a strong tradeoff between fecundity and seedling recruitment success mediated by seed size, but that recruitment patterns are not consistent with the strong asymmetry required for the classical competition-colonization tradeoff. Instead, the relative success of different size seeds among sites suggests that large seeds provide much more of an advantage in some sites than in others, consistent with a fecundity-tolerance tradeoff. We also find that overall among species, larger seed mass is correlated with shorter dispersal distances, consistent with a fecundity-dispersal tradeoff, although there is considerable variation in dispersal distance at any given seed size. Altogether, our results suggest that while the competition-colonization tradeoff does not operate in its classical form among woody plants in tropical forests, other colonization-related tradeoffs are present and important to species coexistence in these diverse communities.