Fecundity of trees and the colonization-competition hypothesis
Colonization-competition trade-offs represent a stabilizing mechanism that is thought to maintain diversity of forest trees. If so, then early-successional species should benefit from high capacity to colonize new sites, and late-successional species should be good competitors. Tests of this hypothesis in forests have been precluded by an inability to estimate the many factors that contribute to seed production and dispersal, particularly the many types of stochasticity that contribute to fecundity data. We develop a hierarchical Bayes modeling structure, and we use it to estimate fecundity schedules from the two types of data that ecologists typically collect, including seed-trap counts and observations of tree status. The posterior density is obtained using Markov-chain Monte Carlo techniques. The flexible structure yields estimates of size and covariate effects on seed production, variability associated with population heterogeneity, and interannual stochasticity (variability and serial autocorrelation), sex ratio, and dispersal. It admits the errors in data associated with the ability to accurately recognize tree status and process misspecification. We estimate year-by-year seed-production rates for all individuals in each of nine sample stands from two regions and up to 11 years. A rich characterization of differences among species and relationships among individuals allows evaluation of a number of hypotheses related to masting, effective population sizes, and location and covariate effects. It demonstrates large bias in previous methods. We focus on implications for colonization-competition and a related hypothesis, the successional niche-trade-offs in the capacity to exploit high resource availability in early successional environments vs. the capacity to survive lowresource conditions late in succession.
Contrary to predictions of trade-off hypotheses, we find no relationship between successional status and fecundity, dispersal, or expected arrivals at distant sites. Results suggest a mechanism for maintenance of diversity that may be more general than colonization-competition and successional niches. High variability and strong individual effects (variability within populations) generate massive stochasticity in recruitment that, when combined with "storage," may provide a stabilizing mechanism. The storage effect stabilizes diversity when species differences ensure that responses to stochasticity are not highly correlated among species. Process variability and individual effects mean that many species have the advantage at different times and places even in the absence of "deterministic" trade-offs. Not only does colonization vary among species, but also individual behavior is highly stochastic and weakly correlated among members of the same population. Although these factors are the dominant sources of variability in data sets (substantially larger than the deterministic relationships typically examined), they have not been not included in the models that ecologists have used to evaluate mechanisms of species coexistence (e.g., even individual-based models lack random individual effects). Recognition of the mechanisms of coexistence requires not only heuristic models that capture the principal sources of stochasticity, but also data-modeling techniques that allow for their estimation.
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