Sedimentary and Ecological Interactions of Shoreline Management and Feral Horses on Cumberland Island, Georgia Public

Abstract

People have modified barrier islands in the eastern U.S. for centuries, whether through deforestation, agriculture, shoreline development, or introduction of non-native species. Cumberland Island (Georgia) is no exception in this respect. Before its current status as a U.S. National Seashore, a jetty was constructed in the late 1800s with the intent of stabilizing an inlet between Cumberland and Amelia Island (Florida). Moreover, non-native livestock were introduced repeatedly to the island, including horses released in the 1940s, which have been feral since. Despite the long-time presence of these human alterations, relatively little research has been done to investigate geological and ecological impacts of the jetty and feral horses on Cumberland, or whether they interact. For this research, I documented long-term changes in sedimentary processes caused by the jetty on the southeastern end of Cumberland Island, while also investigating effects of feral-horses (Equus caballus) in the same area. To document effects on dunes, salt marshes, and other near-coastal environments, I used aerial photos and GIS to map shoreline change related to the jetty and probable nearby feral-horse trails, which was supplemented by field work for baseline observations. My results showed a statistically significant difference in beach width north and south of the jetty, with the northern side wider. The southern shoreline stabilized about 30 years ago, whereas the northern shoreline continues to widen. More than 100 extensive and intersecting horse trails are also evident in dunes and salt marshes near the jetty. My primary conclusion is that the jetty and feral-horses had a combined and noticeable effect on sedimentological and ecological processes on the southeastern end of the island. The jetty interrupts longshore drift, resulting in less sand movement and deposition on the beach to its south. Moreover, long-term activities of the horses, such as browsing of vegetation (e.g., sea oats (Uniola paniculata) and trampling of coastal dunes, accelerated erosion in these environments. Both factors led to a thin, sand-deprived beach, radically altered salt marshes, and likely contributed to westward movement of windblown sand on the southern end of Cumberland Island. 

Table of Contents

INTRODUCTION ..............................................................................................................................1

METHODS ......................................................................................................................................4

RESULTS .......................................................................................................................................11

DISCUSSION .................................................................................................................................26

CONCLUSIONS ..............................................................................................................................32

REFERENCES .................................................................................................................................34

FIGURES AND TABLES

FIGURE 1. LOCATION OF CUMBERLAND ISLAND............................................................................1

FIGURE 2. TRACES OF HORSE FECES ON TRAIL IN SALT MARSHES.................................................7

FIGURE 3 DISTANCE TO 2018 SHORELINE FROM 1944-2018 SHORELINES...................................12

TABLE 1. DISTANCE TO 2018 SHORELINE FROM 1944-2018 SHORELINES....................................12

FIGURE 4. SHORELINE MIGRATION OF CUMBERLAND ISLAND 1948-2018..................................13

TABLE 2. SHORELINE MIGRATION 1940’S AND 2018....................................................................14

FIGURE 5. SHORELINES FROM TOPOGRAPHIC MAPS 1919-2018.................................................14

FIGURE 6. OLD JETTY IN HIGH MARSH..........................................................................................15

FIGURE 7. MAPS OF JETTIES 1919 AND 2017 ...............................................................................15

FIGURE 8. BEACH WIDTH BETWEEN 1974 AND 2017...................................................................16

FIGURE 9. DIFFERENCE OF BEACH WIDTH NORTH/SOUTH OF JETTY 1974-2017.........................16

FIGURE 10. AVERAGED BEACH WIDTH DIFFERENCE NORTH/SOUTH OF JETTY............................17

FIGURE 11. PHOTO OF THE JETTY ON CUMBERLAND ISLAND......................................................18

FIGURE 12. BEACH AREA NORTH AND SOUTH OF JETTY..............................................................18

FIGURE 13. AVERAGE BEACH AREA BETWEEN 1974-2003 AND 2005-2017.................................19

FIGURE 14. TOTAL HORSE TRAIL LENGTH.....................................................................................19

FIGURE 15. HORSE TRAILS FROM 1999 TO 2018..........................................................................20

FIGURE 16. LONG-TERM TRAIL AREA 1999 TO 2018....................................................................21

FIGURE 17. IMAGES OF HORSE EVIDENCE IN DUNES AND SALT MARSHES..................................22

FIGURE 18. IMAGES OF HORSE TRACKS AND FECES IN SALT MARSHES.......................................22

FIGURE 19. VEGETATION FOUND IN THE MARSHES.....................................................................23

FIGURE 20. HORSE HABITAT WAS FOUND IN THE MARITIME FOREST.........................................24

FIGURE 21A AND B. PERCENT VEGETATED DUNE AREA ..............................................................25

FIGURE 22. DUNE: SAND, AND VEGETATION AREA SOUTH OF THE JETTY...................................25

TABLE 3. DUNES AREA ADDED IN WESTWARD TRANSPORT.........................................................26

FIGURE 23. DUNE FIELD EXPANSION WEST..................................................................................26

About this Honors Thesis

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School	Emory College
Department	Environmental Studies
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Environmental Sciences Sedimentary Geology Remote Sensing
Mot-clé	Geology Feral Horses Ecology GIS Remote Sensing Jetty Sediments
Committee Chair / Thesis Advisor	Anthony J. Martin, Emory University
Committee Members	Michael Page, Emory University Carolyn Keogh, Emory University Susan Tamasi, Emory University

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