Smaller and bolder fish enhance ecosystem-scale primary production around artificial reefs in seagrass beds.

Abstract

Effective management of wild animals requires understanding how predation and harvest alter the composition of populations. These top-down processes can alter consumer body size and behavior and thus should also have consequences for bottom-up processes because (1) body size is a critical determinant of the amount of nutrients excreted and (2) variation in foraging behavior, which is strongly influenced by predation, can determine the amount and spatial distribution of nutrients. Changes to either are known to affect ecosystem-scale nutrient dynamics, but the consequences of these dynamics on ecosystem processes are poorly understood. We used an individual-based model of an artificial reef (AR) and reef fish in a subtropical seagrass bed to test how fish body size can interact with variation in foraging behavior at the population and individual levels to affect seagrass production in a nutrient-limited system. Seagrass production dynamics can be driven by both belowground (BGPP) and aboveground primary production (AGPP); thus, we quantified ecosystem-scale production via these different mechanistic pathways. We found that (1) populations of small fish generated greater total primary production (TLPP = BGPP + AGPP) than large fish, (2) fish that foraged more increased TLPP more than those that spent time sheltering on ARs, and (3) small fish that foraged more led to greatest increases in TLPP. The mechanism by which this occurred was primarily through increased BGPP, highlighting the importance of cryptic belowground dynamics in seagrass ecosystems. Populations of extremely bold individuals (i.e., foraged significantly more) slightly increased TLPP but strongly affected the distribution of production, whereby bold individuals increased BGPP, while populations of shy individuals increased AGPP. Taken together, these results provide a link between consumer body size, variation in consumer behavior, and primary production-which, in turn, will support secondary production for fisheries. Our study suggests that human-induced changes-such as fishing-that alter consumer body size and behavior will fundamentally change ecosystem-scale production dynamics. Understanding the ecosystem effects of harvest on consumer populations is critical for ecosystem-based management, including the development of ARs for fisheries.