Global patterns in the effects of predator declines on sea urchins

Submitted by editor on 27 September 2017. Get the paper!
The venomous long-spined sea urchin, Diadema setosum, gathers together on a coral reef to spawn.

By Hannah Sheppard Brennand

Biologists have long been fascinated by the differences between tropical and temperate regions on Earth. There are striking global patterns in species richness for many plants and animals, and latitudinal gradients in the strength of biological interactions are often put forward as an explanation for this. The prevailing view has been that interactions such as predation, herbivory, parasitism and competition are more intense at low latitudes due to comparatively stable weather conditions and population sizes.


Empirical evidence for latitudinal gradients in interaction strength, however, has been mixed. In recent years, there has been considerable focus on quantifying large scale patterns in herbivory and predation, particularly in the context of predicting how these interactions are likely to change in a warming world.

The venomous long-spined sea urchin, Diadema setosum, gathers together on a coral reef to spawn.


Predation plays a fundamental role in determining community structure. Vital to our understanding of how variation in the strength of predation shapes ecosystems on large scales is understanding predator effects on those species that have large impacts on ecosystem structure and function.


In marine ecosystems, sea urchins are one such group – relatively small organisms with a large ecological footprint, capable of denuding kelp forests and keeping coral reefs clear of algal overgrowth. Predator control of sea urchins is one of the most striking examples of trophic cascades on Earth (the well-known sea otter-urchin-kelp interaction) and any changes to how sea urchin populations are regulated by predators are predicted to have considerable effects.


To test for a latitudinal gradient in predation pressure on sea urchins, we used a meta-analysis of experimental studies where predators were either excluded via active removal or physical barriers, or studies where sea urchin populations were contrasted between marine reserves (with high predator populations) and comparable unprotected areas.


Our literature search yielded 420 measures of sea urchin abundance and/or survival from 262 experiments from both hemispheres, conducted primarily on coral and rocky reefs. Direct measures of predation and the change in sea urchin abundance between high and low predation areas were then contrasted to latitude, which is a proxy for global variation in temperature, primary productivity and nutrient availability.


Consistent with theoretical predictions of more intense predation in the tropics, we found evidence for a latitudinal gradient in predation pressure on sea urchins. Coral reef systems experienced consistently high predator effects in contrast with rocky reef systems, where predation pressure decreased with increasing latitude. However, the susceptibility of sea urchins to predation was better predicted by prey traits. Sea urchins are well defended against predators with defences including long spines, toxins, and biting pedicellariae, but the production of defences varies strongly among different sea urchin groups. This suggests that global patterns in the strength of predation on sea urchins could arise from the uneven distribution of species of varying susceptibility.


Biogeographic patterns are likely to be driven by a combination of factors and variation in both predator and prey traits could further help to explain patterns in predation pressure.  Differences in gape, hunting range and behaviour, and physical size and strength will impact predator success and could explain the strength of predator impacts in the tropics.


Our results indicate significant top down control of sea urchins across all marine habitats considered. Consequently, any anthropogenic activities, such as overfishing, that lead to predator declines are likely to be associated with dramatic increases in sea urchin populations, with subsequent declines in primary producers.