OOS 12-1 - Modeling the effects of variable disturbance regimes on kelp forest community dynamics

Background/Question/Methods
Along the coast of southern California, kelp forest ecosystems are dominated by the foundation species giant kelp (Macrocystis pyrifera). Giant kelp populations are highly dynamic, and a major driver of their dynamics is annual loss from winter storms. Due to giant kelp’s foundational role in these systems, disturbance-driven fluctuations in its abundance may have important consequences for the stability of associated kelp forest communities. In this study, we sought to quantify the direct and indirect (kelp-mediated) effects of disturbance frequency and severity on understory communities. We used mathematical modeling to investigate the effects of changing disturbance regimes on kelp forest community dynamics, with a focus on kelp-mediated competition between major benthic community guilds. We parameterized our model for a southern California kelp forest using long-term data collected by the Santa Barbara Coastal LTER and compared its predictions to observed time series.

Results/Conclusions
We found that giant kelp’s dynamic response to interannual variation in disturbance caused the benthic community to fluctuate between benthic macroalgal- and sessile invertebrate-dominated states. Frequent, severe disturbance events favored benthic macroalgal dominance, as limited kelp recovery resulted in high benthic irradiance and promoted macroalgal growth. These results were consistent with the empirical time series. The model also predicted that direct damage to the benthos was important for post-disturbance community dynamics, although it was ultimately the removal and subsequent recovery of kelp that determined the outcome of benthic competition. This study provides an example of how pairing theoretical models with long-term empirical data can improve our understanding of key drivers of kelp forest ecosystem dynamics, thus providing insight into the stability of community states under changing disturbance regimes.