Predator declines and morphological changes in prey: Evidence from coral reefs depleted of sharks

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CITATION

Hammerschlag N, Barley SC, Irschick DJ, Meeuwig JJ, Nelson ER, Meekan MG. 2018. Predator declines and morphological changes in prey: Evidence from coral reefs depleted of sharks. Marine Ecology Progress Series, 586: 127-139.

ABSTRACT

Evidence from the wild as to the ecological and evolutionary consequences of top predator depletions remains limited, especially in marine systems. Given the pace and extent of predator loss, an understanding of these processes is important. Two sets of adjacent coral reef systems off north-western Australia have similar biological, physical and environmental conditions, but one of the reef systems has been exposed to nearly exclusive commercial fishing of sharks. Across reefs where sharks have been depleted, prey fishes have significantly smaller caudal fins and eyes compared to the reefs with intact shark populations (up to 40 and 46% relative difference in standardized means). These patterns are consistent across 7 teleost prey species (N = 611 individuals) that vary in behavior, diet and trophic guild. We hypothesize that these morphological patterns are primarily driven by differences in shark predation. Morphological differences are not consistent with plausible alternative explanations (habitat complexity, temperature, light, current, food availability, prey targets, competition) as primary drivers. These results provide field evidence of morphological changes in prey potentially due to predator depletions consistent with ecological predictions; specifically, predator loss caused a reduction in the size of prey morphological traits associated with predator detection and evasion. While our analysis cannot differentiate between rapid evolutionary change versus morphological plasticity due to shark depletions, either possible outcome would indicate that predator removals may have profound effects on body shapes of prey communities. This is particularly significant in the case of sharks, given that the consequences of their widespread removal have been a topic of significant speculation, debate and concern.

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EYE FOR AN EYE

Percent relative difference (%ΔV) in size-adjusted mean caudal fin area (sCFA) and eye area (sEA) at the Rowley Shoals compared to the Scott Reefs. Figure: Hammerschlag et al. 2018.

FUNDING & ACKNOWLEDGEMENTS

We thank the Western Australian Departments of Fisheries, Parks and Wildlife (DPaW), the Commonwealth Department of Sustainability, Environment, Water, Population and Communities for arranging permits to conduct research at the Scott Reefs and the Rowley Shoals. We acknowledge the crew on board the RV Solander, Kim Brooks, Paul Tinkler and numerous volunteers for aid in the field. Thanks also to Paddy Ryan who provided images for the figures. Special thanks to Sarah Hirth, Gammon Koval, Rachel Skubel and Marcelo Costa Lopez for help with some figures. This research was funded in part by the Australian Institute of Marine Science, an International Postgraduate Research Scholarship to SCB, the Save our Seas Foundation and the University of Western Australia, Perth, under UWA Ethics Approvals: RA3/100/1279, RA3/100/1172. We thank Ashley Frisch and several reviewers who provided valuable comments that improved our manuscript.