Resetting predator baselines in coral reef ecosystems

Darcy Bradley,Kydd Pollock,Eric Conklin,Bruce E Kendall,Amanda Pollock,Jennifer E Caselle,Douglas J Mccauley,Yannis P Papastamatiou,Steven D Gaines

doi:10.1038/srep43131

Abstract

What did coral reef ecosystems look like before human impacts became pervasive? Early efforts to reconstruct baselines resulted in the controversial suggestion that pristine coral reefs have inverted trophic pyramids, with disproportionally large top predator biomass. The validity of the coral reef inverted trophic pyramid has been questioned, but until now, was not resolved empirically. We use data from an eight-year tag-recapture program with spatially explicit, capture-recapture models to re-examine the population size and density of a key top predator at Palmyra atoll, the same location that inspired the idea of inverted trophic biomass pyramids in coral reef ecosystems. Given that animal movement is suspected to have significantly biased early biomass estimates of highly mobile top predators, we focused our reassessment on the most mobile and most abundant predator at Palmyra, the grey reef shark (Carcharhinus amblyrhynchos). We estimated a density of 21.3 (95% CI 17.8, 24.7) grey reef sharks/km2, which is an order of magnitude lower than the estimates that suggested an inverted trophic pyramid. Our results indicate that the trophic structure of an unexploited reef fish community is not inverted, and that even healthy top predator populations may be considerably smaller, and more precarious, than previously thought.

Highlights

Given that none of the estimated Bayesian credible intervals (CIs) in the model that included all covariates overlapped zero, we report results from the full model only
We estimate that grey reef shark density at a near-pristine coral reef is 21.3 sharks/km[2], which translates to a total population size of 8344 sharks (Supplementary Table 1)
The latter estimate led to the controversial suggestion that pristine coral reefs have inverted trophic pyramids, with disproportionally large top predator biomass

Summary

Introduction

The latter estimate provided support for the surprising and controversial idea that historical marine food webs may have had an inverted trophic pyramid with larger biomass at the top of the food web than at trophic levels below[21,22,23] If this vision of past ocean ecosystems was correct, and if it applied to other ocean geographies and habitats, the implications for ocean management and the need for ocean restoration would be profound. One path to resolution is to solve the empirical sampling challenges and see if better estimates from pristine habitats in the ocean support a different view of unexploited ocean food webs To this end, we set out to re-estimate shark abundance and density at the same unfished coral reef – Palmyra atoll – that inspired the idea of inverted trophic biomass pyramids in coral reef ecosystems. We discuss the management implications of inflated baseline abundance and density estimates for shark populations

Methods

Results

Conclusion