QUANTIFYING HERBIVORY ON CORAL REEFS: 

 JUST SCRATCHING THE SURFACE AND STILL BITING OFF MORE THAN WE CAN CHEW 



Robert S. Steneck 

 Department of Zoology, University of Maine at Orono 

 Darling Center Marine Laboratory, Walpole, Me 04573 



ABSTRACT 



Herbivory was quantified using six different techniques simultaneously at nine 

 discrete sites along a coral reef system on the Caribbean island of St. Croix. In 

 order to study diverse assemblages of herbivores and algae, functional groups were 

 used for both. The groups are based on shapes and structural properties of the 

 algae and feeding capabilities of the herbivores. 



Herbivory was most frequent and intense in the shallow forereef sites where an 

 average of over 5,000 herbivorous fish bites per meter square per hour was recorded. 

 Although most of these were from herbivorous fishes that do not denude primary 

 subtratum (i.e., small damself ishes), this site also had the highest frequency of 

 grazing from herbivores capable of denuding (i.e., yellowtail damselfish and tangs) 

 and excavating the calcareous substratum (parrotf ishes, urchins and limpets). 

 Herbivory from all sources decreased in backreef, shallow algal ridge, and deep 

 wall-reef habitats. The latter sites had the lowest levels of grazing. 



Herbivory on macrophytes was assessed using a Thalassia bioassay technique but 

 the results at the forereef sites contradicted those of all other techniques. 

 Caution is suggested in applying this technique as a single measure of herbivory. 



INTRODUCTION 



The process of herbivory is generally thought to be of primary importance to the 

 distribution and abundance of benthic algae on coral reefs (reviewed by Lubchenco 

 and Gaines 1981). While the units of measure and methods for determining the 

 abundance of algal prey are well established, no such convention exists for 

 determining the impact of their herbivorous predators. Measurements of percent 

 cover, biomass, or number of individuals when applied to assemblages of reef 

 dwelling herbivores are of dubious meaning for quantifying herbivory. For instance, 

 how many foraging urchins equal the impact of a 20 kg parrotfish? The "apples and 

 oranges" involved here result from trying to force units and techniques which are 

 designed to determine patterns in herbivore abundance on the process of herbivory. 



Ecological processes are factors that result in observed patterns. It is 

 generally assumed that the abundance of herbivores corresponds with their impact on 

 algae. This assumption has never been tested. In this paper, I will report on 

 several techniques used simultaneously on a single reef system in order to measure 

 the impact of a diverse assemblage of herbivores on an assemblage of reef-dwelling 

 algae. I will also provide an argument for considering this topic at a "functional 

 group" level so that herbivores with similar effects and algae with similar 

 ecological properties are treated together. 



ORGANISMS, STUDY SITES, AND METHODS 



Functional Groups 



Since herbivory involves the interaction of two diverse groups .of organisms, 

 "functional group" subdivisions will be used. Algae have been subdivided into such 

 groups based on shared anatomical and morphological characteristics (see Littler and 

 Littler 1980, Steneck and Watling 1982). For the purposes of this paper, I will 

 simplify these subdivisions to three groups: 1) ALGAL TURFS (diverse, microscopic 



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