Transient dynamics during kelp forest recovery from fishing across multiple trophic levels

Abstract

Outcomes of management efforts to recover or restore populations of harvested species can be highly dependent on environmental and community context. Predator-prey interactions can alter recovery trajectories, and the timing of management actions within multi-trophic level harvest scenarios may influence the dynamics of recovery and lead to management trade-offs. Recent work using a generalist predator-prey model suggests that management promoting synchronized recovery of predators and prey leads to faster and less variable recovery trajectories than sequential recovery (predator or prey first). However, more complex communities may require different management actions to minimize recovery time and variability. Here, we use a tri-trophic level rocky reef community dynamics model with size-structure and fisheries at multiple trophic levels to investigate the importance of three ecological processes to recovery of fished communities: (1) size-structured predation, (2) non-consumptive effects of predators on prey behavior, and (3) varying levels of recruitment. We also test the effects of initiating recovery from community states associated with varying degrees of fishery-induced degradation and develop a simulation in which the basal resource (kelp) is harvested. In this system, a predator-first closure generally leads to the least volatile and quickest recovery, whether from a kelp forest, urchin barren, or intermediate community state. The benefits gained by selecting this strategy are magnified when recovering from the degraded community, the urchin barren, because initial conditions in the degraded state lead to lengthy recovery times. However, the shape of the size-structured predation relationship can strongly affect recovery volatility, where the differences between alternate management strategies are negated with size-independent predation. External recruitment reduces return times by bolstering the predatory lobster population. These results show that in a tightly linked tri-trophic level food web with top-down control, a predator-first fishery closure can be the most effective strategy to reduce volatility and shorten recovery, particularly when the system is starting from the degraded community state. Given the ubiquity of top predator loss across many ecosystems, we highlight the value of incorporating insights from community ecology into ecosystem management.

Keywords: ecosystem-based management; fishery closure; non-consumptive effects; recruitment facilitation; size-structured predation.