| A major goal in ecology is to understand how processes observed on single habitat patches "scale up" to predict population- or metapopulation-scale dynamics. This issue is especially critical for marine species with widely-dispersing, planktonic larvae. I addressed this question by investigating the behavior, growth, and mortality of juvenile bluehead wrasse, Thalassoma bifasciatum, a small planktivore, on reefs around the Caribbean island of St. Croix. At a small spatial scale (centimeters), bluehead wrasse enjoyed safety-in-numbers: per-capita mortality was substantially lower in larger groups. This effect did not scale up, however, and mortality was density-dependent at the spatial scale of entire reefs (Chapter 1). The relative safety of groups may explain why settlers occurred in groups more often than expected by chance, but group sizes may be limited by competition for prey. Grouped fish spent more time foraging than solitary fish, but dietary and otolith analyses revealed that grouped fish obtained the same number of prey and grow slower than solitary fish despite foraging more. Behavior and growth were also affected by the local abundance of copepod prey, which varied consistently across reefs (Chapter 2). Of even greater interest is spatial variation in the recruitment and abundance of a major wrasse predator, which closely followed the spatial pattern of bluehead wrasse recruitment. Presumably this occurred because larvae of both species are affected by the same oceanographic forces. This spatial correlation in recruitment is noteworthy because predator density strongly affected the form and intensity of density-dependent mortality among recently settled wrasse (Chapter 3). Since the majority of marine metapopulation models assume that density dependence is spatially homogenous, models incorporating spatially correlated settlement of predators and their prey produce strikingly different results. Populations with consistently low larval settlement experience much weaker density-dependence and are consequently far more important to the persistence of the metapopulation than high-settlement, high-predation populations (Chapter 4). Far from supporting the idea that small-scale processes scale up, I have identified several new potential sources of large-scale, oceanographically-driven variability that may affect the fate of individual fish and entire metapopulations. |
