Removal of intertidal grazers by human harvesting leads to alteration of species interactions, community structure and resilience to climate change

Abstract

Extreme fluctuations in abiotic conditions can induce a biological stress response (e.g. bleaching) detrimental to an organism's health. In some instances, organisms can recover if conditions are alleviated, such as through cooccurrence with other species that confer protection. Biodiverse, multitrophic communities are increasingly recognised as important promoters of species persistence and resilience under environmental change. On intertidal shores, the role of grazers as top-down determinants of algal community structure is well recognised. Similarly, the harvesting of grazers for human consumption is increasingly prevalent with potential to greatly alter the community dynamics. Here, we assess how differences in harvesting pressure of grazers under three management regimes (no-take; managed access; open-access) alters the trophic interactions between grazers, and algal communities. Grazer density and body size frequencies were different among regimes leading to changes in the photosynthetic performance and recovery of crustose coralline algae (CCA) post-bleaching, as well as their presence altering the strength of interactions between species. The exclusion of grazers from patches using cages led to different emergent communities and reduced negative correlations between taxa. The absence of larger grazers (>9cm) at the managed access site led to macroalgal overgrowth of bleached CCA negatively affecting its recovery, whereas no-take or open-access led to a moderated algal growth and a shift from competitive to facilitative interactions between algal species. Given that CCA play an important role in the population growth and development of other species, the choice of management measure should be carefully considered before implementation, depending on objectives.

Population responses to harvesting in fluctuating environments

“…This led to overgrazing of kelp, and consequently impacted the numerous species that depend on the habitat created by kelp forests (Estes & Duggins 1995, Silliman & Angelini 2012. In this way, harvesting of a single species can have serious impacts on species that are not themselves harvested, destabilize whole communities and seriously reduce their resilience to climate change (Chaverra et al 2019). Moreover, dispersal or migration can extend the interspecific effects of harvest to areas that are not harvested.…”

Connectivity modelling informs metapopulation structure and conservation priorities for a reef‐building species

“…The biodiversity associated with biogenic reef habitats is often higher than the surrounding environment and can vary with the evolution and ecological status of these habitats (Dubois et al, 2002; Jones et al, 2018). Despite their important role in ecosystem structure and functioning, biogenic reef habitats are considered among the most threatened globally (Beck et al, 2011; Chaverra et al, 2019) and their effective protection requires an in‐depth understanding of their population dynamics, connectivity and ecology (Knights et al, 2014; Piet et al, 2015).…”

Seascape genomics reveals population isolation in the reef-building honeycomb worm, Sabellaria alveolata (L.)

“…Intertidal communities have been proposed as particularly suitable for studying the impacts of climate change, as many species already exist at the limit of their thermal tolerance ranges [ 21 , 22 ]. Ecosystem engineers provide and modify habitat for other species and are thus disproportionately important to diversity, ecological functioning, and conservation in a changing climate [ 1 , 14 , 23 – 25 ]. Therefore, understanding how littoral ecosystem engineers have adapted to the local environment they experience is key to understanding not only their future survival in a changing climate, but also the potential survival of intertidal ecosystems as a whole.…”