Herbivore community function shapes resilience and bistability of coral reefs

TL;DR

This paper shows how the diversity of herbivorous fish functions affects coral reef resilience and stability in the face of overfishing.

Contribution

The study introduces a model incorporating herbivore functional groups to reveal how their diversity influences coral reef resilience and alternative stable states.

Findings

Evenness in herbivore functional groups is crucial for reef recovery under increasing fishing pressure.

Grazer-dominated reefs may remain in degraded states even after fishing stops.

Models omitting functional groups miss key dynamics like loss of alternative states in browser-dominated communities.

Abstract

Ecological communities globally are shifting to degraded states, motivating research into attributes supporting resilience or leading to alternative stable states. Coral reef communities are particularly vulnerable as they are facing myriad anthropogenic impacts that contribute to shifts away from coral dominance, motivating much research on whether these shifts are gradual and reversible transitions versus alternative stable states. Empirical studies demonstrate functionally-diverse herbivorous fish communities support coral reef resilience to anthropogenic impacts. However, few coral reef models incorporate herbivore and algal functional groups and quantify their effects on reef resilience and alternative stable states. We built a coral reef model that includes herbivorous fish functional groups and their algal targets and explored how this expansion affects predictions of resilience and alternative stable states under different scenarios of human impacts. We found evenness across the functional groups of herbivores is increasingly important for reef recovery as fishing pressure increases, and recovery is promoted when algal communities begin with more turf than macroalgae. Our findings also highlight that models omitting functional groups that comprise real communities will miss crucial phenomena, such as loss of alternative stable states for browser-dominated communities, or continued presence of alternative stable states in grazer-dominated communities even when fishing pressure is removed entirely. This work advances our ability to understand and predict coral reef resilience and alternative stable states in the Anthropocene, and provides guidance for conservation policy on fishing practices.

We examine the question “how does functional group composition of herbivorous fishes and their food resources (algae) shape the resilience of coral reefs in response to overfishing?” We did this using a mathematical model that explicitly incorporates functional groups of herbivorous fish (grazers, browsers) and their algal targets (turf, macroalgae). One key finding is that the community composition of herbivore functional groups modulates – and can qualitatively alter – the resilience of reefs when subject to human disturbance. Specifically, we found evenness of functions (grazing vs browsing) within herbivore communities is increasingly important for reef recovery as fishing pressure rises. Another novel finding is that reefs with grazer-dominated communities may persist in degraded algal-dominated states even if fishing pressure is entirely removed. Our findings highlight the importance of monitoring for skewed function in herbivore communities and implementing fishing policy to protect functional diversity and preserve coral reef resilience. Broadly, we show ecosystem response to anthropogenic stressors can be drastically impacted by community composition, and that functional ecology provides a pathway to understanding these impacts. Here, we find ecosystem resilience is determined by the relative abundance of functional groups of consumers and the resources they target, highlighting the importance of considering these groups in conservation and fisheries policy.

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