Cascading predator effects in a Fijian coral reef ecosystem

Abstract

Coral reefs are among Earth's best-studied ecosystems, yet the degree to which large predators influence the ecology of coral reefs remains an open and contentious question. Recent studies indicate the consumptive effects of large reef predators are too diffuse to elicit trophic cascades. Here, we provide evidence that such predators can produce non-consumptive (fear) effects that flow through herbivores to shape the distribution of seaweed on a coral reef. This trophic cascade emerged because reef topography, tidal oscillations, and shark hunting behaviour interact to create predictable "hot spots" of fear on the reef where herbivores withhold feeding and seaweeds gain a spatial refuge. Thus, in risky habitats, sharks can exert strong ecological impacts even though they are trophic generalists that rarely feed. These findings contextualize the debate over whether predators influence coral reef structure and function and move us to ask not if, but under what specific conditions, they generate trophic cascades.

Figure 1

Reef topography and predation risk as a function of tide. The backreef at (a) low tide and (b) high tide. Deep, hard-bottom lagoons are habitable to large herbivorous and piscivorous (predatory) fishes throughout the tidal cycle. The upper surface of the reef (“reef top”), which has grown to mean low water mark in most places, is not accessible to these fishes at low tide. At high tide, the entire backreef becomes deeper, allowing all fishes, including large predators, access to all reef features. (c) Predator encounter rate (sightings/40 m²/hour; mean + s.e.m.) in the backreef at high vs. low tide each day (n = 5; paired t-test). The vast majority of predator sightings were sharks. Videos indicate that on average each 40 m² section of backreef is traversed by 4–5 sharks during each diurnal high tide cycle.

Figure 2

Herbivory as a function of tide. Rates of (a) seaweed removal by browsing fishes (grams/hour) and (b) substrate cropping by grazing fishes (bites/hour) in lagoons (grey bars) and on the reef top (black bars) at high vs. low tide. The reef top was not assayed at low tide, because large herbivores generally cannot access this substrate at low tide. Bars represent a mean (+s.e.m.) of five daily averages calculated for each location/tide combination. Herbivory rates in lagoons at high vs. low tide, and rates among locations at high tide, were each compared with a paired t-test.

Figure 3

Seaweed abundance in areas of high and low predation risk. Biomass of fleshy seaweeds (grams dry mass/0.25 m²; mean + s.e.m.) on reef tops (black bar) vs. in adjacent hard-bottom lagoons (grey bar) in three haphazardly selected lagoonal networks. Paired censuses (<1 m apart, n = 10 per site) were performed on comparable substrates open to seaweed colonization. Seaweed biomass in areas of high risk for herbivores (reef tops) vs. low risk (lagoons) was compared for each site with a paired t-test or Wilcoxon test.