EVALUATION OF LONG-TERM HABITAT AND COLONIAL WATERBIRD DYNAMICS OF SHAMROCK ISLAND, NUECES COUNTY, TEXAS by Carolyn Gorman and Elizabeth H. Smith Center for Coastal Studies Texas A&M University-Corpus Christi 6300 Ocean Drive, NRC 3200 Corpus Christi, Texas 78411 prepared for Thomas Calnan Coastal Biologist Coastal Management Division Texas General Land Office 1700 North Congress Avenue Austin, TX 78701-1495 October 16, 2001 EVALUATION OF LONG-TERM HABITAT AND COLONIAL WATERBIRD DYNAMICS OF SHAMROCK ISLAND, NUECES COUNTY, TEXAS EXECUTIVE SUMMARY Shamrock Island is a naturally formed island located west of Mustang Island, along the eastern portion of Corpus Christi Bay, Texas. This 30-ha island is comprised of a diversity of coastal habitat types and is an extremely important nesting site for colonial waterbirds. Shamrock Island became a Preserve through acquisition by The Nature Conservancy of Texas (TNCT) in 1995. This action as well as a conservation easement owned by the Texas General Land Office ensures perpetual protection of the island from development. During the latter part of 1990s, a restoration project was designed to minimize erosion along the northern and western portions of the island. A comprehensive review of historic studies on Shamrock Island was undertaken to assess existing knowledge of the natural and anthropogenic changes of Shamrock Island. GIS interpretation of the areal extent of unvegetated habitat, and the analysis of the colonial waterbird database on ground-nesting species abundance, were conducted to assist in the development of a management strategy for the island. A decadal analysis of digitized aerial photographs from 1980, 1990, and 1999 reaffirmed that sediment was eroding on the north end of the island and accreting along the southern shore. Additionally, digitized aerials from 1998 and 1999 were selected to assess pre- and post- restoration habitat extent. Since the early 1950s, when the land bridge to Mustang Island was severed, accelerated erosion has occurred along the northern beach face of Shamrock Island. Such erosion not only threatens vital bare-ground nesting habitat, but also endangers the pristine habitat of the interior lagoons and seagrass beds, as breaching may occur. Unvegetated habitat extent increased by four hectares between 1998 and 1999, primarily from the creation of the 2.35-ha feeder beach, but abundance of bare-ground nesting species decreased. Based on assessment of colonial waterbird dynamics, ground- nesting species abundance is not directly related to availability of habitat, but may be related to a variety of limiting factors (i.e., food source availability, amount of precipitation, competition for space, etc.). Continued documentation of island/habitat changes should be implemented within a long-term monitoring program. Aerial photography of the entire island, as well as selected portions of the island (e.g., northern peninsula, southern peninsula, western beach) at a larger scale, should be obtained at yearly intervals for multi-scale habitat change assessments. Colonial waterbird censuses should also be continued with additional surveys to document overall nesting success of key species. Protection strategies should be implemented to eliminate disturbance to nesting birds. In addition, the connection between nesting and adjacent foraging habitat use should be investigated to determine other factors responsible for breeding success. i ACKNOWLEDGMENTS This research was supported by the Texas General Land Office through the Center for Coastal Studies and Texas A&M University-Corpus Christi. Historic data synthesis would not have been possible without the collaborative assistance of The Nature Conservancy of Texas, Texas General Land Office, U.S. Fish and Wildlife Service, Texas Parks & Wildlife, and Shiner, Moseley, and Associates. Many thanks go to the people who assisted with GIS software interpretation, particularly, Rick Hay, Emily Lundblad, John Wood, Beau Hardegree, Felicia Serchia, and Jane Crowson. We are also grateful to Lee Elliott for his contributions in the colonial waterbird census data analysis and to friends and colleagues who assisted with field work and data synthesis. We are indebted to comprehensive manuscript reviews by Drs. Wes Tunnell and David McKee and final report compilation by Suzanne Dilworth. ii TABLE OF CONTENTS Page EXECUTIVE SUMMARY ……………………………………..…………………… i ACKNOWLEDGMENTS ……………………………………………………..… ii LIST OF FIGURES ……………………..………………………………………. v LIST OF TABLES ………..……………………………………………………… vii CHAPTER I INTRODUCTION AND OBJECTIVES ………………………… 1 II STUDY AREA ………………………………...……………………. 3 III HISTORIC DATA SYNTHESIS ………………………………... 7 INTRODUCTION …………………………………………. 7 METHODS ………………………………………………… 7 RESULTS ………………………………………………….. 7 DISCUSSION ……………………………………………… 18 IV GIS INTERPRETATION & BIRD DATABASE ANALYSIS … 21 INTRODUCTION …………………………………………. 21 METHODS ………………………………………………… 22 RESULTS ………………………………………………….. 25 DISCUSSION ……………………………………………… 46 VII CONCLUSIONS AND RECOMMENDATIONS ………………. 59 LITERATURE CITED ………………………………………………………….. 63 APPENDICES …………………………………………………………….……… 69 APPENDIX A – RESIDUALS ……………………………. 71 APPENDIX B – TAXONOMIC CLASSIFICATION ……. 73 APPENDIX C – BIRD CENSUS GRAPHS ……………… 77 iii LIST OF FIGURES Figure Page 1. Texas coast with study site, Shamrock Island, Corpus Christi Bay, Nueces County ...……………………………………………………… 4 2. A timeline of events occurring at Shamrock Island or within its vicinity ………………………..……………………………………….. 8 3. Shamrock Island, Texas, depicting approximate locations of historic study sites ….…………………………………………………………... 11 4. Topographic profiles from historic transect study sites on Shamrock Island, Texas ..…………………………………………………………. 12 5. Digitally-enhanced images of Shamrock Island, Texas ………………. 15 6. Wetlands classified on or in the vicinity of Shamrock Island, Texas .…………………………………………………………………. 17 7. Control points and measurements established during Trial Method 1 of rectifying aerial photographs of Shamrock Island, Texas .………... 26 8. Control points and polygon established from using Trimble GPS device for Trial Method 2 of rectifying aerial photographs of Shamrock Island, Texas .……………………………………………… 27 9. Georeferenced aerial images of Shamrock Island ..…………………... 29 10. Digitized aerial image of Shamrock Island in 1980, showing delineation of habitat types .…………………………………………... 32 11. Digitized aerial image of Shamrock Island in 1990, showing delineation of habitat types .…………………………………………... 33 12. Digitized aerial image of Shamrock Island in 1998, showing delineation of habitat types …………………………………………... 34 13. Digitized aerial image of Shamrock Island in 1999, showing delineation of habitat types …………………………………………... 35 14. 1999 aerial image of northern portion of Shamrock Island, Texas, illustrating the repositioning of the northern arm between 1980 and 1999 ……………………………….……………………………... 36 iv 15. 1999 aerial image of southern extent of Shamrock Island, Texas, illustrating accretion along southeastern peninsula of island over a 19-year period ….……………………...………………………. 36 16. Perimeter/area ratio according to habitat type for each digitized aerial image, Shamrock Island, Texas ………………………………… 38 17. Georeferenced aerial image of Shamrock Island in 1999, comparing unvegetated habitat delineation through a) digitization, b) ground - truthing, and c) ground-truthing polygon overlain onto digitized polygon .…………………………………………………….. 39 18. Primary water level comparison between date of aerial photograph (16 March 1999) and date unvegetated habitat was ground-truthed (7 December 1999), according to Ingleside and Packery Channel tidal gauges ……………………………………………………………. 40 19. 1989 aerial image of Shamrock Island, Texas, illustrating areas formed as a result of dredged material deposition ……….…………… 41 20. Total number of nesting pair values according to nesting guild on Shamrock Island, Texas …….………………………………………... 42 21. Total number of nesting colonial waterbird species utilizing Shamrock Island, Texas between 1973 and 1999, with indication of observer and comparison of the percentages of laughing gulls versus unvegetated and vegetated nesters .…………………………… 43 22. Habitat delineation of Shamrock Island, Texas in 1999, with depiction of colony sites for: unvegetated nesters in 1998, 1999, and 2000; Laughing Gulls in 1999 (only those colonies in close association with unvegetated nesters); and Black Skimmers in 1980 ………………………………………………………………….. 44 23. Number of nesting pairs of unvegetated nesters compared with areal extent of unvegetated habitat ……………………………………. 45 24. Comparison of the number of unvegetated nesters counted in colonial waterbird census to precipitation in May …………………………...… 46 v LIST OF TABLES Table Page 1. Results adapted from SMA (1986) depicting rates of retreat (ft/yr) of the northeastern peninsula of the island and of the northwestern beach face of Shamrock Island, Texas ………..……………………… 13 2. List of unvegetated nesters on Shamrock Island showing 1) the average distance found between nests within the colony and 2) other species who may also nest within the same colony …….……………. 25 3. Coordinates of Trimble GPS ground control points used in second trial method of rectifying aerial photographs of Shamrock Island, Texas ………………………………………………………………..… 28 4. Quantified area (m²) of landscape features associated with Shamrock Island, Texas ………………………………………………………….. 31 5. Areal extent of habitat by year-image at Shamrock Island, Texas ……. 37 6. Perimeter, area, and perimeter to area ratios of habitat for Shamrock
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