The consequences of fishing-induced changes in predator size for predator-prey interactions
Body size has long been recognized as a key driver of species interactions and of an individual’s role in the ecosystem. Body size determines the amount, species, and sizes of prey resources an individual can consume, as well as its own susceptibility to predators. Human harvest of predators can result in severe truncations in predator body size that can have cascading consequences on food webs. Where small and large individuals of the same species differ greatly in their diets, as is common in aquatic systems, the absence of large predators may functionally eliminate a key predator-prey linkage.
Recently management agencies have begun to include size-based metrics as targets. As various harvest strategies differentially affect predator size and biomass, the research presented in this dissertation aims to understand the conditions under which truncations in predator size structure will result in additional loss of predator function than would be predicted from predator biomass alone, and where it will therefore be important to maintain predator size distributions. I specifically examine how the type of ontogenetic shift in diet (e.g. prey species or size class), and the shape of the diet switching function (e.g. gradual or abrupt) will affect the consequences of the loss of the largest predators, and the relative utility of various management strategies in maintaining predator function.
In Chapter 1, I examined the tradeoffs between fishery yield and predator function in the ecosystem when preferentially fishing the largest predators. I found that fisheries that delay harvest until large predator sizes maximize fishery yield but that this virtually eliminates predation on focal prey eaten late in life history when diet shifts are abrupt and occur at or after the size at maturity. In this case, there is a clear tradeoff between fisheries and ecosystem objectives. Instead, where shifts in diet toward late prey are more gradual, targeting the largest predators can achieve a win-win by maximizing yield and achieving predation rates similar to that with other strategies that harvest predators earlier. As such, the optimal fishing strategy to achieve both single-species and ecosystem benefits depends strongly on the interaction between the fishery selectivity pattern and the changes in predator diet with size.
In Chapter 2, I quantified the size-dependence of the predator-prey interaction between herbivorous sea urchins and one of their important predators in southern California kelp forests, California Sheephead. I further examined the consequences of changes in sheephead size and abundance in marine reserves at Catalina Island on size-specific urchin mortality in field predation trials. In my observations of predation of sheephead on urchins, sheephead smaller than 20cm TL do not eat urchins of any size. Thereafter, small sheephead only consumed small urchins, with larger sheephead sizes needed to successfully consume larger urchins, and the largest sheephead preferentially targeted the largest urchins. Inside marine reserves at Catalina, the greater abundance of large sheephead in combination with the observed size-specific capacities for urchin predation led to higher urchin mortality with marine reserve protection, particularly for the largest urchins. Ultimately, by restoring predator size structure, reserves may serve to enhance the resilience of southern California kelp forests.
In Chapter 3, I examined how variation in predator body size distributions and biomass affects the likelihood of size escapes in situations where predators begin eating prey at some threshold size and thereafter consume increasingly larger prey. We focus on California sheephead because of the size dependence of its interaction with herbivorous sea urchins (Chapter 2), and the natural variation in demography where sheephead achieve smaller maximum sizes but higher biomass in the south of its range. We evaluate the consequences of smaller predator body size on top-down control of urchin populations in two scenarios: 1) when overall predator abundance is the same as the population with larger body size, and 2) when predator biomass is the same. With the same numbers of predators, top-down control was significantly weakened by the lack of large sheephead. However, when sheephead biomass was maintained, the absence of large sheephead did not lead to greater urchin abundance, despite lower predation rates overall and much lower predation on large urchins. Higher predation rates on the smallest urchin size classes served as a bottleneck that kept total urchin population at similar levels and prevented a size escape for the largest urchins. This suggests that where predators switch prey size classes in the same species, the loss of the largest individuals does not inherently result in weaker top-down control, if biomass is maintained, but effective control is sensitive to prey growth rates.
The results of this research suggest that the ignoring shifts in predator size structure can under-estimate the effects of fishing on predator function, especially when large predators eat different species than their smaller counterparts. High predator biomass can compensate when diet shifts are to different prey size classes of the same species. Concordance between diet shifts and fishery selectivity can help identify where it will be important to consider changes in predator size in addition to biomass.