Trends in the effects of kelp removal on kelp populations, herbivores, and understory algae

Abstract

Kelp forests provide habitat and resources to diverse organisms and provide valuable ecosystem services. However, marine deforestation due to wild kelp harvesting, among other drivers, is being observed worldwide. Studies assessing kelp removal effects often focus on the effects on kelp populations, although deforestation also impacts the organisms that interact directly or indirectly with kelp, including herbivores and algal assemblages. Using a meta-analytical approach, we estimated the magnitude and direction of kelp removal effects on kelp, invertebrate herbivores, and understory algae. We also tested if responses varied among functional groups of understory algae and whether results were influenced by the subtidal or intertidal distribution of the removed kelp species and the time elapsed since kelp removal. We observed a substantial decrease in kelp abundance, remaining for up to 4 years following kelp removal, with a larger decrease in subtidal kelp but no recovery observed in intertidal kelp over time. Invertebrate herbivore abundance showed no significant change over time. Understory algae abundance responded positively, although this effect tended to slightly decline over time following subtidal kelp removal. Canopy-, turf-forming and foliose algae were the most benefited, which raises concern about their potential to outcompete kelp. The early succession patterns and cascading effects within kelp forests illustrated here highlight the need for long-term studies to elucidate the long-lasting effects of kelp fisheries, which are scarce at present. There is also a need to consider kelp forests’ role in providing habitat and resources to improve predictive frameworks allowing kelp forest conservation and sustainable fisheries.