The Role of Threespot Damselfish (Stegastes Planifrons)
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THE ROLE OF THREESPOT DAMSELFISH (STEGASTES PLANIFRONS) AS A KEYSTONE SPECIES IN A BAHAMIAN PATCH REEF A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Masters of Science Brooke A. Axline-Minotti August 2003 This thesis entitled THE ROLE OF THREESPOT DAMSELFISH (STEGASTES PLANIFRONS) AS A KEYSTONE SPECIES IN A BAHAMIAN PATCH REEF BY BROOKE A. AXLINE-MINOTTI has been approved for the Program of Environmental Studies and the College of Arts and Sciences by Molly R. Morris Associate Professor of Biological Sciences Leslie A. Flemming Dean, College of Arts and Sciences Axline-Minotti, Brooke A. M.S. August 2003. Environmental Studies The Role of Threespot Damselfish (Stegastes planifrons) as a Keystone Species in a Bahamian Patch Reef. (76 pp.) Director of Thesis: Molly R. Morris Abstract The purpose of this research is to identify the role of the threespot damselfish (Stegastes planifrons) as a keystone species. Measurements from four functional groups (algae, coral, fish, and a combined group of slow and sessile organisms) were made in various territories ranging from zero to three damselfish. Within territories containing damselfish, attack rates from the damselfish were also counted. Measures of both aggressive behavior and density of threespot damselfish were correlated with components of biodiversity in three of the four functional groups, suggesting that damselfish play an important role as a keystone species in this community. While damselfish density and measures of aggression were correlated, in some cases only density was correlated with a functional group, suggesting that damselfish influence their community through mechanisms other than behavior. Knowledge of the range in which a relative balance of biodiversity exists has potential for utilization in marine conservation. Approved: Molly R. Morris Associate Professor of Biological Sciences Dedication This thesis is dedicated to my husband, my daughter, my mother, and my dogs. Acknowledgments I would like to thank Dr. Gene Mapes and Dr. Royal Mapes for introducing me to my future and Dr. Donald Miles for his help and patience with data analysis. I especially thank Dr. Molly Morris for the suggestions and encouragement that led to both this paper and fieldwork. TABLE OF CONTENTS Page ABSTRACT 3 DEDICATION 4 ACKNOWLEDGMENTS 5 LIST OF TABLES 8 LIST OF FIGURES 9 I. INTRODUCTION 10 A. Importance of coral reefs 10 B. Intermediate Disturbance Hypothesis 10 C. Keystone species and their utilization 11 D. Damselfish as Keystone species 13 II. INFLUENCE OF DAMSELFISH ON CORAL REEF COMMUNITIES 14 A. Territories 14 B. Algae 15 C. Coral 22 D. Cryptofauna 28 E. Competitive Interactions 30 Damselfish 30 Herbivorous fish 32 Herbivorous urchins and seastars 35 F. Overview 36 III. MATERIALS AND METHODS 38 A. Study area 38 B. Experimental sites 39 C. Biodiversity 40 D. Aggression 43 E. Data analysis 44 IV. RESULTS 45 V. DISCUSSION 53 A. Functional groups 53 B. Biodiversity 58 C. Research applications 60 D. Direct conservation efforts at Three Sister’s Patch Reef 62 LITERATURE CITED 67 APPENDIX 76 A. Differences in coral reefs in different geographic regions 76 8 LIST OF TABLES Table Page 2.1 A partial list of phyla and classes found inside damselfish algal mats 29 (derived from Lobel 1980). 3.1 Taxonomic variables in each functional group, method and 42 order of observation, as well as the study from which they were derived. 4.1 Mean measures of aggression level and aggression index, as well as mean 45 percent cover algae within each threespot density. 4.2 Mean aggression level and aggression index as well as mean percent cover 47 coral variables and total coverage within each threespot density. The symbol (l) denotes live coral and (d) denotes dead coral. 4.3 Results of the main effects on all functional groups and all taxa. Results in 52 bold indicate those that were found to be significant. 9 LIST OF FIGURES Figure Page 3.1 Census plots on Three Sister’s Patch Reef. 40 4.1 Mean percent cover red, green filamentous, green branching, brown, and 46 total algae within each threespot population density. 4.2 The effects of mean aggression level at each threespot population density 48 on percent cover dead coral. 4.3 The effects of aggression index on percent cover live coral. 48 4.4 The effects of each threespot population density on mean percent cover 49 total coral. 4.5 The effects of aggression level on total percent cover of live and dead coral. 49 4.6 The effects of aggression index on total percent cover live and dead coral. 50 4.7 The affects of aggression index on the number of fish in each territory. 51 5.1 A comparison of the algae and coral functional groups at different 60 damselfish densities. 5.2 Marine Replenishment Zones and Scientific Monitoring Zones in Central 65 Andros National Park (Bahamas National Trust 2001). 10 I. INTRODUCTION It is estimated that thirty percent of all coral reefs world-wide are in critical condition (Tuxill 1998) and seventy percent of reefs are under direct threat from human activities (Carlton et al. 1999). A. Importance of coral reefs Coral reefs are among the most biologically complex ecosystems known. Yet in recent decades, the coral reefs of the Bahamas, as elsewhere, have been damaged by habitat alteration, pollution, and other human activities. Coral reefs provide food, medicine, income, and an intangible richness to our lives; however, destruction of this habitat is increasing and is among the most deleterious of any known ecosystem. Maintenance of species diversity and genetic variability are imperative in securing survival of these ecosystems. Biodiversity increases the probability that community members will respond differently to variable environmental conditions and perturbations, therefore reducing the risk of species and habitat extinction (Sole and Montoya 2001). Deterioration in reef ecosystems create an urgent need for research and further understanding of natural interactions for implementation of effective management and rehabilitation plans. B. Intermediate Disturbance Hypothesis The intermediate disturbance hypothesis, originally formulated by Paine and Vadas in 1969 and in large part by Connell in 1978, is described as “agents of physical disturbance or consumers at intermediate intensity enhance diversity by reducing competitive exclusion and preventing competitively superior species from attaining population sizes that are large enough to monopolize all of the limiting resources” (Wootton, 1998 p. 803). Therefore, at intermediate disturbance, the community becomes 11 a mosaic of patches at various succession stages of regeneration, allowing for a full variety of species diversity (Ricklefs and Miller 2000). The intermediate disturbance hypothesis was later elaborated by Huston (1979) to include the concept that “rates of population growth and competitive displacement of the species in the community as well as the extent of disturbance in the community are the primary explanations for the existence of highly diverse communities” (Ricklefs and Miller 2000, p. 612). These studies have shown the imperative role of intermediate disturbance in developing community structure. C. Keystone species and their utilization A “keystone” is the pivotal block of stone that secures the structure of an archway: without the stone, the archway falls. A keystone species is a species whose presence is essential to the diversity of life for a given ecosystem (Sole and Montoya 2001). Within a habitat, each species depends on other species for survival and, in turn, contributes to the overall condition of the habitat. Plants provide essential nutrients and energy to browsing and grazing animals and, ultimately, to the carnivores that feed on these herbivores. While each species contributes to habitat operation, some species apparently do more than others. One particular species may provide essential services that are unique. Without the work of these key species, the habitat changes significantly. When this keystone species disappears from its habitat, the environment changes causing loss of other resident species and, eventually, the intricate connections among the remaining residents begin to change dramatically. Identification of keystone species and insight into their behavior is fundamental in understanding the complex interactions of the animals of coral reef ecosystems and the 12 ways in which these species can play a role in proper management. One family playing a fundamental role in maintenance of coral reef diversity is the aggressive damselfish (Family Pomacentridae). Only limited research on the specific role of damselfish in small-scale disturbances has been done to the present in Bahamian reefs. The barrier reef on the east side of Andros Island, Bahamas offers a prime location for additional research on damselfish because it is the third longest barrier reef in the world (Lecard 2001), portions of this reef are currently being established as a division of Central Andros National Park (Bahamas National Trust 2002). The study of damselfish in this region and identification of their role in structuring the surrounding reef ecosystem will ultimately contribute to marine resource conservation and management in Andros, as well as on other reef tracts in other parts of the Bahamas and the Caribbean. McCook (2002) presented research showing how community based Marine Protected Areas where fishing is excluded (called no-take-zones) may