Monitoring Functional Groups of Herbivorous Reef Fishes as Indicators of Coral Reef Resilience

20 August 2009 | Downloads - publication

A practical guide for coral reef managers in the Asia Pacific Region
 

INTRODUCTION
Coral reefs are the most structurally complex and taxonomically diverse marine ecosystems on earth, providing ecosystem goods and services for millions of people worldwide. These reefs are seriously threatened by a variety of anthropogenic threats, particularly overexploitation of marine resources, destructive fishing practices and runoff from poor land use practices. Over half of the world’s reefs have already been lost or are under threat from these activities, with serious and widespread declines in reef health reported from around the world.

Climate change also represents a new and increasing threat to coral reefs and associated ecosystems. Major threats include rising sea temperatures, rising sea levels and changes in ocean chemistry. Urgent action is now required to halt or reverse these threats and declines in coral reef health. One approach to the protection of reefs during this period of change is to manage for resilience.

CORAL REEF RESILIENCE
Resilience is the ability of an ecosystem to absorb shocks, resist phase shifts and regenerate after natural and human-induced disturbances. For coral reefs, it is the ability of reefs to absorb recurrent disturbances, and rebuild coral dominated systems rather than shifting to algal dominated systems. Coral reef resilience will be increasingly important in future as disturbances become more frequent and severe with climate change.

Several key factors are critical for maintaining coral reef resilience. They are predominantly factors that facilitate coral recruitment and survivorship, including the availability of coral larvae, good water quality, conditioning by biological agents and a stable, consolidated substratum. In contrast, factors that negatively affect coral recruitment and survivorship include a lack of larval supply, loose rubble or unconsolidated substratum, thick algal mats or large stands of macroalgae, some sessile invertebrates (e.g. soft corals), and poor water quality (particularly runoff of sediments and nutrients from poor land use practices).

ROLE OF FUNCTIONAL GROUPS OF HERBIVOROUS REEF FISHES
Herbivores play a critical role in coral reef resilience by limiting the establishment and growth of algal communities that impede coral recruitment. On coral reefs in the Indo-Pacific Region, fishes are the dominant group of herbivores, while both echinoids and fishes are both important in the Caribbean. Major families include surgeonfishes, parrotfishes, rabbitfishes and rudderfishes.

Herbivorous reef fishes are diverse and do not constitute an ecologically uniform group. They comprise several functional groups that differ in terms of how they feed, what they consume, and their impact on the underlying substratum. This study focuses on four functional groups of herbivorous reef fishes that each play different and complimentary roles in coral reef resilience: scrapers/small excavators, large excavators/bioeroders, grazers/detritivores, and browsers.

MONITORING FUNCTIONAL GROUPS OF HERBIVOROUS REEF FISHES
Coral reef monitoring has traditionally focused on monitoring the status of coral communities and populations of conspicuous species, particularly fisheries species (fish and invertebrates). While these measures provide useful information on the current status of coral reef communities and associated fisheries, they do not provide information on the status of key ecological processes that are essential for maintaining coral reef resilience.

Developing new metrics for monitoring coral reef resilience that are process oriented is an urgent priority for the improved management of coral reefs. Monitoring coral reef resilience will require a combined approach to monitoring key ecological processes, and functional groups that contribute to these processes including:
• Coral population dynamics (size structure and patterns of recruitment).
• Factors that influence coral recruitment and survivorship, particularly water quality, substratum consolidation, and benthic communities (particularly macroalgae).
• Factors that influence the establishment and growth of macroalgal communities, particularly functional groups of herbivorous reef fishes.

Methods for assessing and monitoring coral reef resilience have been developed by the International Union for Conservation and Nature (IUCN) Working Group on Climate Change and Coral Reefs. This document represents the first attempt to develop a monitoring program that is specifically designed to monitor key functional groups of herbivorous reef fishes as indicators of coral reef resilience. Even though it is based on the best available information, it is important to remember that the science underpinning these methods is still new and developing. Further research is now required to address knowledge gaps and refine monitoring methods.