Recreational impacts on the distribution of ghost crabs Ocypode quadrata fab.

BiologicalConsereatton20 ( 1981] 111 122

R E C R E A T I O N A L IMPACTS ON THE D I S T R I B U T I O N OF G H O S T CRABS OCYPODE Q U A D R A T A FAB. ALAN J. STEINER

Department of Forestry and Wild/lie Management, University of Massachusetts Amherst, Massachusetts 01003, USA & STEPHEN P. LEATHERMAN

National Park Service Cooperative Research Unit, University of Massachusetts, Amherst, Massachusett.~ 01003, USA ABSTRACT

Studies were conducted at Assateague Island, Maryland-Virginia, to determine the relative number of ghost crabs Ocypode quadrata Fab. on beaches subject to d~[Ji,rent recreational uses. The mean densiO, of crabs pet" O"1 ha plots was found to be 10 on an undisturbed beach, 19 on a pedestrian-impacted beach, 1 on a light (~[]~road vehicle (OR V)- and pedestrian-impaeted beach, and 0.3 in a heavv OR V-use beach. OR Vs could be adversely affecting the crabs directly by crushing or bm3'ing them or indirectO' by interfering with their reproductive ~Tcle or altering their environment. Vehicular disturbance probab O' results in fewer crabs or no reproduction at all, with new inhabitants migrating JEom undisturbed areas. Pedestrians appear to have no hart~['ul ~J]i,cts on ghost crabs; instead the crabs may he capitalising on the/m~d sct'aps scattered across the beach by bathers.

INTRODUCTION

Within the past few decades, off-road vehicle (ORV) and pedestrian traffic on the coastal ecosystem has increased greatly. The amount of impact inherent in these disturbances depends upon the type of use and the susceptibility of the system being affected. Each natural system has its own level of tolerance, and this sensivity must be determined in order to devise management strategies for alleviating or minimising the impact. Exposed beaches have few inhabitants, for only the most specialised creatures can live in sand where heavy surf is present. However, these species are a main food source of the aggregations of shorebirds and other forms of wildlife that forage on the beach (Britton, 1979). The food-web species that are most abundant on the eastern seaboard beach areas include the mole crab Emerita talpoida and the ghost 111 Biol. Conserv. 0 0 0 6 - 3 2 0 7 / 8 1 / 0 0 2 0 - 0 1 1 1 / $ 0 2 - 5 0 ~ A p p l i e d Science P u b l i s h e r s Ltd, E n g l a n d , 1981 P r i n t e d in G r e a t Britain

112

ALAN J. STEINER, STEPHEN P. LEATHERMAN

crab Ocypodequadrata Fab. The ghost crab has a nocturnal activity period in which it feeds and immerses itself in the ocean, whereas during the day it occupies burrows dug into the sand. Ghost crabs utilise the beach area most susceptible to impacts from ORVs and pedestrians. Therefore, due to its habitat, size, and abundance, the ghost crab was selected as the best indicator species for detecting detrimental effects of recreational beach use (Steiner & Leatherman, 1979). STUDY AREA

Assateague Island is a continuous, 60-km long barrier island off the southeastern coastline of the Delmarva Peninsula (Fig. 1). The island, which varies in width from

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BEACH

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Fig. 1.

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Location of the major land use/management zones of Assateague Island, Maryland-Virginia.

RECREATIONAL IMPACTS ON GHOST CRAB DISTRIBUTION

1 13

120 m to about 5 km, is backed by Sinepuxent Bay to the north and Chincoteague Bay to the south. Near the northern end of the island, the State of Maryland owns and administers the 280-ha Assateague State Park. The National Park Service manages the remaining land on the Maryland section of the island as a National Seashore. The Virginia section is controlled by the Fish and Wildlife Service as the Chincoteague National Wildlife Refuge. Most visitor use at Assateague Island is confined to the developed centres at T o m s Cove H o o k (VA) and North Beach (MD) (Fig. 1). However, significant visitor use takes place on or adjacent to Sinepuxent Bay at North Beach. A substantial number of people obtain ORV permits for surf-fishing and driving on the Maryland beach from North Beach south to the Maryland-Virginia state line and at Toms Cove Hook, Virginia. No ORV and only very light pedestrian traffic is allowed on the Wildlife Refuge beach from the Maryland-Virginia border to the northern end of Toms Cove Hook.

METHODS AND MATERIALS

To measure ORV and pedestrian impacts on ghost crabs, four sites of differing ORV and pedestrian usage were censused over the summer of 1978 at Assateague Island. All four sites were located within Chincoteague National Wildlife Refuge (Fig. 2). Site 1, representing a heavy ORV use area, was located 50 m south of the ORV crossover on Toms Cove Hook. Most ORVs entering the beach turn south and therefore drive through this site. Vehicles travel across the entire length of the beach profile so that there is no concentrated section of use. Site 2, located 100 m north of the ORV crossover on Toms Cove Hook, is used by pedestrians with low-level ORV usage. The major zone of pedestrian use along this profile is on the foreshore and within 10 m of the berm, while the major ORV travel lanes are along the backshore. Site 3 is at a heavy pedestrian impact area with no ORV traffic. This site was located at the northern end of T o m s Cove Hook at a protected swimming beach, li is used intensively by people through the summer season; most sun bathers are concentrated near the berm. Site 4 was located l l k m north of T o m s Cove H o o k in the middle of the undisturbed (wild) Refuge beach. There is no ORV traffic in this area except for an occasional official Refuge vehicle. Pedestrian traffic is limited to a few beachwalkers. Crabs at these sites were censused biweekly at night (2100 0300 h) using circular, tenth-hectare (17.8m radius) plots with a powerful, hand-held flashlight. The number of crabs on the surface and within the circle's perimeter was counted. Three plots, selected randomly, were censused at each site during each census night. To observe possible differences in crab densities due to sampling time, censuses were taken twice on two consecutive nights starting at approximately 2100 and 0130 h, respectively. Milne & Milne (1946) noted that most crabs are out of their burrows by

114

ALAN J. STEINER, STEPHEN P. LEATHERMAN

-SITE 4

--SITE 3 TOMS COVE ~

Fig. 2.

-SITE 2 SIT 1

Location of the four fixed night census sites on the Chincoteague National Wildlife Refuge

RECREATIONAL

IMPACTS

ON GHOST

CRAB DISTRIBUTION

1]5

2200 h, which was the usual starting time o f the censuses. Significant differences were not observed between these censuses. In addition to t h e c e n s u s e s at the four fixed sites, counts were taken along the entire ORV-impacted sections o f Assateague Island using the same night census methods. Plots for these censuses were spaced 1.6 km apart along the length o f the beach. The M a r y l a n d beach was censused three times over the summer of 1978. while unfavourable tides and beach conditions allowed the Virginia beach to bc sampled only once. The Virginia night census is c o m p a r a b l e with a US Fish and Wildlife Service survey o f ghost crab burrows undertaken in 1973 and repeated on 27 28 June 1978 and 19 21 June 1979 (Britton, 1979, and pers. comm.). In the F W S censuses, 0.04 ha plots were used with plot centres situated so that the perimeter extended 0.3 m below the high tide mark. Plots were spaced 0.8 km apart along the entire Virginia section o f Assateague. One probable difference in procedure between the three F W S burrow censuses was that the 1979 census at T o m s Cove H o o k area was surveyed in the m o r n i n g before O R V or pedestrian traffic could cover up the burrows. This technique was not employed in the 1973 or 1978 censuses so that their estimates of ghost crab abundance cannot be considered totally reliable.

RESULTS

D a t a obtained from the night census on the four fixed sites showed large differences in ghost crab densities between the sites (Table 1). When these data were tested by analysis o f variance (AN OVA) (Steel & Torrie, 1960), it was found that both site and date were significant factors (p < 0-01) in determining ghost crab abundance. A siteby-date interaction was also significant (p < 0-01), which indicates that the differences in crab densities from one site to another are not the same from date to date (Table 2). TABLE 1 NUMBERS OF GHOST CRABS OBSERVED IN THREE CIRCULAR PLOTS (RADIUS = CHINCOTEAGUE WILDLIFE REFUGE IN 11978

Date and Time

23 June, 210(~2400 24 June, 0200-0430 6 July, 2240~0130 20 July, 2145 0015 22 July, 0130-0345 5 August, 2140-2330 16 August, 2140-2330

17'8 m) AT

Sites 1

2

3

4

0, 0, 1

0,0,0 0,0,0 0,0,0 0,0, I 0, 1, 1 0,0,2

0, 1, 1

6, 10, 13

1,4, 7

0,1,2 0,1,1 0, 1, 1 0, 1,2 0, l, 2 1,1,3

10,13,15 14,16,17 8,9, 15 12, 16, 19 15, 16,20 46,58,65

1,2,4 4,7,17 0, 1, 1 13, 17, 17 10,20,27 17,20,24

1 16

A L A N J. STEINER, S T E P H E N P. L E A T H E R M A N TABLE 2 A N O V A Or GHOST CRAB DISTRIBUTION AT FOUR SITES IN WILDLIFE REFUGE

Source Mean Date Site D a t e , site W i t h i n cell a

D.F.

Sum. Sq.

Mean Sq.

F-Value

Prob (F)

1 6 3 18 56

5076-3 2424.3 5247-6 3626.2 606-7

5076-3 404-0 1749.2 201-5 10.8

468.58 37.30 a 161.46" 18.60 ~ 1.00

0 0 0

p < 0.005.

To determine if differences between sites were significant, an AN OVA test was run on single degree of freedom comparisons of the sites (Steel & Torrie, 1960). The results from this testing showed that significant differences (p < 0.005) existed between all of the sites except sites 1 and 2 (Table 3). Site 3, the protected swimming beach, had the highest density of crabs; population size was significantly greater than that found on the natural or control area (Site 4), located on the wild beach.

ANOVA

TABLE 3 OF GHOST CRAB DISTRIBUTION BETWEEN FOUR SITES AT CHINCOTEAGUE WILDLIFE REFUGE

Source Site 1 vs. 2 1 vs. 3 1 vs. 4 2 vs. 3 2 vs. 4 3 vs. 4 W i t h i n cell

df

Sum Sq.

Mean Sq.

F-Value

l l 1 1 1 1 56

4.67 3924-67 1030.10 3677-35 896.10 942.90 606.70

4.67 3924.67 1030.10 3677.35 896.10 942.90 10.83

0.43 363.404 95.38" 340.50 a 82.97 a 87.314

a p < 0.005.

The two ORV sites (1 and 2) had very low numbers of ghost crabs present and were significantly different from the natural (Site 4) and pedestrian (Site 3) areas. Although there was no significant difference between the two ORV sites, the more heavily used Site 1 had a lower crab density than Site 2. This finding may be due to heavier ORV use of Site 1 or the presence of people at Site 2. This same pattern of ghost crab density was also observed in the night census conducted along the Virginia beach (Fig. 3). Population densities were high along the wild beach (Zone A), highest at the swimming beach (Zone B), low at the area subject to light ORV and pedestrian use, and lowest at the heavy ORV use area (Zone C). Within the four fixed sites the date remained a significant factor in determining ghost crab abundance for Sites 3 and 4, while it was not significant for Sites 1 and 2. This indicates that the density of crabs present at the two ORV sites did not increase over the summer, as was the case in the control and pedestrian impact areas. The three night censuses taken on the Maryland section of Assateague showed low densities of crabs associated with ORV use (Fig. 4). In a similar fashion to Sites 1

RECREATIONAL IMPACTS ON GHOST CRAB DISTRIBUTION

I 17

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Fig. 3.

Densities of ghost crab per 0.1 ha for the night census of 19 June 1978 (2100-0100 h), Chincoteague National Wildlife Refuge beach.

118

Fig. 4.

ALAN J. STEINER, STEPHEN P. LEATHERMAN

Ghost crab densities (1978) per 0.1 ha for night censuses of Maryland National Seashore.

and 2 on T o m s Cove Hook, the Maryland beach had only an occasional crab within the plot perimeters. Crab densities increased only at the southern end of the beach where ORV traffic was considerably lighter. However, densities at the very southern end of the Maryland beach were still appreciably lower than that found only 50 m to the south on the undisturbed Virginia wild beach. These low densities were shown to be significant (p < 0.01) when examined through a chi-squared test. A comparison of the three FWS burrow censuses (Fig. 5) indicates that there is a

1 19

RECREATIONAL IMPACTS ON GHOST CRAB DISTRIBUTION

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1979

Fish & Wildlife Service Burrow Censuses of Wildlife Refuge.

120

ALAN J. STEINER, STEPHEN P. LEATHERMAN

large amount of variability in ghost crab distribution patterns over time. In 1973, the crabs on the wild beach (Zone A), showed a high degree of aggregation in their associations, whereas in 1978 and 1979 they were more randomly spaced along the beach shore. Crab densities within the swimming beach (Zone B) and the ORV-used beach (Zone C) also displayed differences spatially and temporally. As previously mentioned, however, only the 1979 census could be considered reliable for the disturbance zones (B and C). The 1979 census showed that the highest mean density occurred in Zone B (Table 4), but it was not significantly greater than the mean density of Zone A. Zone C had a significantly lower density (p < 0.01) than the other two zones. TABLE 4 GHOST CRAB BURROW DISTRIBUTIONIN WILDLIFEREFUGE Year

Zone

1973

A B C A B C A B C

1978

1979

N 17 3 8 17 3 8 17 3 8

Min.

Max.

2 42 0 25 0 0 3 25 0

280 114 76 208 19 41 99 49 5

Mean 82-65 75.33 25.37 114.06 6.67 9.25 31.65 36.00 1.88

SD 89.039 36.295 27.588 45.903 10.693 13.792 25.617 12.124 1.885

SE 21.595 20.955 9.574 11-133 6-173 4.876 6.213 7.000 0.666

DISCUSSION

Observations on ghost crab abundance showed that the type and degree of human disturbance has a pronounced effect on population levels. Crab densities in areas subject to ORV traffic were quite low, as observed during the three night censuses of the Maryland beach, the night census of the Virginia beach, and the three FWS burrow surveys. All three FWS surveys and the night census along the Virginia beach showed that especially low densities occurred at the 'elbow' of Toms Cove Hook where the ORV crossover is located and traffic is concentrated. Previous studies on the effects of ORVs on ghost crabs (Leggett, 1975; US Department of the Interior, 1976; Florschuts & Williamson, 1978; Smith, 1978), while not producing quantitative data and therefore defensible results, have yielded similar trends. Studies of other invertebrates by Wheeler (1978) have shown that small infauna (substrate dwellers) are adversely affected by direct ORV impact. ORVs could be interfering with crabs in three ways: (1) by crushing or burying them inside their burrows as vehicles pass overhead, (2) by interrupting the reproductive cycle (Leggett, 1975), or (3) by modifying the beach habitat so that it can no longer support a ghost crab population. Brodhead & Godfrey (1979) found that vehicles travelling over the beach surface change the moisture content of the sand

RECREATIONAL IMPACTS ON GHOST CRAB DISTRIBUTION

121

and induce shear forces down to a depth of 25 cm. Disturbance by the churning tyres causes the dry upper sand to be mixed with the wetter sand below. The breaking up of this natural moisture barrier can lead to increased desiccation of the sand. Even if ghost crabs are unaffected by the physical pressure and shear stress created by the tyres, the lack of moisture would cause their gills to dry out, resulting in mortality. The small change in crab density with time at the two ORV sites in comparison with the very large increases at the other areas indicates ORV disturbance of the reproductive cycle (Fig. 6). Vehicular disturbance results in either few new crabs or no reproduction at all, with new recruitments coming from migrants leaving the undisturbed sites. It was also noticed that most of the crabs found within these two ORV sites were quite small or young (distinguishable from the adults by being spotted and sand-coloured instead of grey and yellow). Although this could have been due to the small number of crabs actually present at these sites, Leggett (1975) found that the size of ghost crab burrows was significantly smaller on an ORV impacted beach. Therefore, the life expectancy of crabs within ORV areas is probably shorter than normal. 60.

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Seasonal distribution of ghost crab densities at the four monitoring sites on the Chincoteague Wildlife Refuge.

Pedestrians, in contrast to ORVs, appear to have no harmful effects on ghost crabs. Instead the density of crabs at the swimming beach was significantly greater than that of the natural beach. A possible reason for this finding is that people using the beach leave scraps of food upon which the crabs feed. In the pedestrian area, crabs were observed even in the foredunes; whereas at the other sites, nearly all the crabs were concentrated near the beach berm. This dispersion of crabs over the entire beach profile might be due to their scavenger feeding habits. Food scraps left by the bathers would be scattered all over the entire beach profile instead of being distributed only in the tidal area, where the crab's natural food supply is located.

122

ALAN J. STEINER, STEPHEN P. LEATHERMAN

In addition to distribution differences, the percentage of large crabs was higher in the protected swimming area than at any of the other sites. This difference may have been due to the difficulty of spotting small crabs along the tracked beach at night, whereas the terrain at the other sites was much more regular, permitting easier night viewing. It is also possible that the large food quantity permits faster growth or that pedestrian impact is more detrimental to smaller crabs. CONCLUSIONS

Human usage of the exposed beach habitat was found to have a pronounced effect on ghost crab densities. A monitoring experiment at four fixed sites showed a significant relationship to occur between the type of recreation impact and number of ghost crabs observed at the sites studied. This same relationship was observed on censuses conducted along entire sections of the beach front. Areas subjected to ORV traffic had significantly lower densities and a smaller physical size of crabs. The life expectancy of the populations in these impacted areas is rather short. ORVs could be interfering with the ghost crabs by (1) crushing or burying them inside their burrows as vehicles pass overhead, (2) by interrupting the reproductive cycle, or (3) by modifying the soil moisture of the beach habitat. Pedestrian impacts, in contrast to ORVs, had a beneficial effect on ghost crab densities. Food scraps left behind provided a large food source for the crabs and resulted in higher densities than found on wild, undisturbed areas. REFERENCES BRITTON, E. E. (1979). Evaluation o f public use impacts upon nesting shorebirds and the beach habitat on Chincoteague National WiMlife Refuge. Chincoteague National Wildlife Refuge, Chincoteague, VA, US Fish and Wildlife Service. BRODHEAD,J. M. B. & GODFREY, P. J. (1979). The effects of offroad vehicles on coastal vegetation in the Province Lands, Cape Cod National Seashore, Massachusetts, final report. University of Massachusetts, Amherst, National Park Service Cooperative Research Unit Report, No. 32, 240 pp. FLORSCHUTS, O. & WlLLIAMSON,N. F. Jr. (1978). Public and wildliJe use on beaches of Pea Island National Wildlife Refuge. Washington, DC, US Fish and Wildlife Service. LEGGET'r,A. P. Jr. (1975). A population and behavioral study o f the ghost crab, Ocyp0de quadrata (Fab.), at the Back Bay National Wildlife Refuge: A continuation. Old Dominion University, VA, Department of Biological Sciences (unpublished report). MILNE, L. J. & MILNE, M. J. (1946). Notes on the behaviour of the ghost crab. Am. Nat., 80, 362-80. SMITH,T. J. (1978). Public and wildlife use on beaches of Pea Island National Wildlife Refuge: Addendum to the final report Asheville, NC, US Fish and Wildlife Service. STEEL, R. G. D. & TORRIE,J. H. (1960). Principles andprocedures of statistics. New York, McGraw Hill. STEINER, A. J. & LEATHERMAN, S. P. (1979). A preliminary study of the environmental effects of recreational usage on dune and beach ecosystems of Assateague Island (Final Report). NPS-CR U Rep. No. 44, 101 pp. US DEPARTMENTOF THE INTERIOR (1976). Alternative beach access regulations, Back Bay National Wildlife Refuge, Virginia. Washington, DC, US Fish and Wilcllife Service. WHEELER, N. (1978). Effects of off-road vehicles on the infauna of Hatches Harbor, Cape Cod National Seashore. University of Massachusetts, Amherst, National Park Service Cooperative Research Unit Report No. 28, 57 pp.