Demographic variation within spatially structured reef fish populations: when are larger-bodied subpopulations more important?

Environmental heterogeneity frequently induces spatial variability in somatic growth, which can cause inter-population differences in reproductive output among organisms for which fecundity is dependent upon body size. Mean asymptotic body size, L∞, varies among populations of several reef fish species. Deterministic models suggest L∞ has little effect on population growth, so subpopulations with larger L∞ may not have disproportionate effects in sustaining an open system. We used a stochastic simulation model to examine the potential role of a larger L∞ subpopulation in aspects of population dynamics beyond population growth under a range of assumptions about the prevailing recruitment relationships. We compared dynamics of a demographically homogeneous system with a system that included one subpopulation with 20% larger L∞. Despite the magnitude of the increase in L∞, mean population size and average time at large population sizes differed little between the homogenous system and that with the larger L∞ subpopulation. However, including the larger L∞ subpopulation did result in less time spent at very small population sizes, which could reduce extinction risks. Effects of the larger L∞ subpopulation were most pronounced when a deterministic recruitment cycle was imposed in combination with high stochastic variability in recruitment. This was due to regular series of poor recruitment years shifting the population structure toward older cohorts where differences in body size (and reproductive output) between the larger L∞ subpopulation and the other subpopulations were greatest. Differences were also greater when recruitment variability was regionally correlated. When recruitment variability was locally independent, the probability of system-wide declines was reduced because declines of individual populations at one time were replenished by unaffected neighbors in subsequent years. Our study suggests that variation in L∞ within a network of interconnected subpopulations may not be an important determinant of population behavior under certain conditions, but might be important in coping with periods of persistent, system-wide recruitment failure.