Restoration decisions in highly variable systems: Whether, when, and how to enhance kelp

Abstract

In highly variable ecosystems, natural variability can obscure signals of decline that warrant intervention, as there is often a lack of long-term data needed to distinguish between declines that are transient and likely to recover naturally and those that would be permanent without intervention. Implementing restoration for a species involves deciding on a threshold condition at which to intervene, duration that this threshold needs to be met before intervening, life stages at introduction, and amounts to introduce. Current bull and giant kelp restoration efforts in California were spurred by persistent kelp loss that started in 2013 following a disease outbreak among predators of the primary herbivore and a marine heatwave. We asked how decisions about intervention cue, threshold, wait time, life stage at introduction, and amount affect population outcomes against a backdrop of environmental variability. We analyzed additional context-dependencies on grazing and climate intensity. To address these questions, we built and parameterized a stochastic stage-structured model for bull and giant kelp, in which we simulated kelp enhancement approaches under a range of grazing and climate scenarios. In a global sensitivity analysis of kelp enhancement decisions, we found that grazing intensity and life stages at introduction were the most influential determinants of kelp biomass and persistence outcomes. The kelp enhancement approach involving temporarily introduced juveniles restored the germ banks of bull and giant kelp, but the two species differed in their responses to that intervention, highlighting that optimal intervention approach is species-dependent. Waiting longer for natural recovery before intervening reduced total intervention effort but also reduced biomass recovery potential. These results support empirical findings from kelp enhancement pilot projects in California and highlight context-dependencies that inform restoration decision-making in kelp forests and other highly variable ecosystems.