Global patterns in the effects of predator declines on sea urchins

Abstract

Latitudinal gradients in the strength of biotic interactions have long been proposed, but empirical evidence for the expectation of more intense predation, herbivory and competition at low latitudes has been mixed. Here, we use a meta‐analysis to test the prediction that predation pressure on sea urchins, a group of consumers with a particularly strong influence on community structure in the world's oceans, is strongest in the tropics. We then examine which biotic and abiotic factors best correlate with biogeographic and within habitat patterns in sea urchin responses to predation. Consistent with expectations, predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reef habitats (> 25° absolute latitude). However, latitude and temperature were weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. traits including behaviour, physical, and chemical defences) rather than large scale abiotic factors in determining variation in interaction strengths. Ecosystem‐shaping sea urchins such as Tripneustes gratilla, Diadema savignyi and Centrostephanus rodgersii were strongly impacted by consumers, indicating a tight coupling between predators of these species and their boom and bust prey. Anthropogenic activities such as over‐fishing, climate change and habitat destruction are causing rapid environmental change, and understanding how predation pressure varies with temperature, across habitats and among prey species, will aid in predicting the likelihood of ecosystem wide effects (via trophic cascades).

Experiments reveal limited top‐down control of key herbivores in southern California kelp forests

“…This is similar to other temperate and tropical regions in which predatory fish generally exert a stronger influence on urchins than do lobsters (Sheppard‐Brennand et al. ). Others have also suggested that fishing for spiny lobsters does not inevitably induce a trophic cascade (Guenther et al.…”

“…Trophic cascades are a common occurrence on rocky reefs globally (Shears and Babcock 2002, Lafferty 2004, Guidetti 2006, and our results suggest that, under certain circumstances, predatory fish, more so than lobsters, contribute to persistence of kelp forests in SC by consuming urchins. This is similar to other temperate and tropical regions in which predatory fish generally exert a stronger influence on urchins than do lobsters (Sheppard-Brennand et al 2017). Others have also suggested that fishing for spiny lobsters does not inevitably induce a trophic cascade ), yet the paradigm of top-down control by lobsters in SC has remained prevalent despite a lack of experimental evidence for such an effect.…”

“…Moreover, a recent meta‐analysis showed that predatory impacts on urchins increase with longer experimental duration and do not vary with the size of the experimental plot (Sheppard‐Brennand et al. ), so our short‐term, small‐scale field experiment may actually be a conservative estimate of urchin mortality.…”

Predation shapes invertebrate diversity in tropical but not temperate seagrass communities

“…Given the structural complexity of seagrasses and their associated species, it is likely that ample microhabitats exist in nature that are protected from predation, thereby providing refuges for predator‐susceptible species and increasing regional diversity (Freestone et al, ; Freestone & Osman, ). Patchy distributions of predators and prey (Sheppard‐Brennand et al, ) and uneven impacts of specialist predators (Mittelbach et al, ; Schemske, ) are likely to further contribute to compositional variability among prey communities in the tropics. In sessile marine invertebrate communities, non‐random community assembly, which can result from strong predator–prey interactions, can drive higher community variability (i.e., beta diversity) in tropical environments and contribute to regional coexistence (Freestone & Inouye, ).…”

Density‐dependent feedbacks, hysteresis, and demography of overgrazing sea urchins

“…Causes of sea urchin overgrazing varies across systems, but appears triggered by either environmental and/or trophic change (Tegner and Dayton 2000, Steneck et al 2002, Ling et al 2009a). Environmental change can facilitate large sea urchin recruitment events (e.g., Hart and Scheibling 1988, 2009b, while trophic change, observed to result via ecological overharvesting of sea urchin predators, can lead to urchin population increases in exceedance of overgrazing thresholds (reviewed by Steneck et al 2002, Sheppard-Brennand et al 2017.…”