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A Reliable Bioindicator of Anthropogenic Impact on the Coast of South Carolina
Mustafa R. Gül and Blaine D. Griffen

Southeastern Naturalist, Volume 17, Issue 2 (2018): 357–364

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Southeastern Naturalist 357 M.R. Gül, and B.D. Griffe 22001188 SOUTHEASTERN NATURALIST 1V7o(2l.) :1375,7 N–3o6. 42 A Reliable Bioindicator of Anthropogenic Impact on the Coast of South Carolina Mustafa R. Gül1,* and Blaine D. Griffen1,2 Abstract - Sandy beaches are frequently visited for recreational purposes. Although such recreational activities are economically beneficial, they cause disturbances to these habitats. Every ecosystem has unique properties; thus, ecosystem-specific species are often used as bioindicators of human disturbances. Here, we pioneer the use of an indirect burrow-counting technique on 20 sandy South Carolina beaches that experience different levels of human disturbance. Our results show that Ocypode quadrata (Atlantic Ghost Crab) is a reliable bioindicator of human disturbance on sandy beaches in South Carolina. The burrow density and width declined significantly as human disturbance increased. We also report that females outnumber males in highly disturbed sites. We conclude that the indirect burrow-counting technique employed in this study could minimize the cost and the effort of determining the human disturbances on coastal regions. Introduction Certain species have been defined as bioindicators because they are sensitive to ecological changes (Siddig et al. 2016). Collecting all possible data related to a habitat is not realistic (Carignan and Villard 2002, Pérez-Garciá et al. 2016); thus, scientists and managers have increasingly used bioindicators to examine a variety of natural and human impacts across spatial and ecological scales (Carignan and Villard 2002, Cortes et al. 2013, Siddig et al. 2016). Indicator species provide useful but limited information about a viable species or a species assemblage at local and regional scales (Cortes et al. 2013), which is particularly helpful in the development of conservation and management plans (Solomon et al. 2003). Selecting an appropriate bioindicator species is especially critical in coastal systems where evaluating human alterations in marine ecosystems is difficult (Vitousek et al. 1997). Thus, selection of an appropriate bioindicator species to determine the human impacts on long-stressed sandy beaches (Davenport and Davenport 2006, McLachlan et al. 2013) is important to assess the level and effects of human disturbance and determine if ecosystem services are being maintained (Barbier et al. 2011) in coastal regions. Invertebrates are often chosen as bioindicators because they are more sensitive to human stresses than vertebrate taxa in terrestrial and aquatic systems (Carginan and Vilard 2002). For this reason, many invertebrate taxa have been used as bioindicators in a number of sandy-beach regions including Donax spp. (clams; Defeo and de Alava 1995, Schlacher et al. 2008, Sheppard et al. 2009), Phaleria spp. 1Marine Science Program, University of South Carolina, Columbia, SC 29208. 2Current address - Department of Biology, Brigham Young University, Provo, UT 84602. *Corresponding author- Manuscript Editor: Matthew Heard Southeastern Naturalist M.R. Gül, and B.D. Griffe 2018 Vol. 17, No. 2 358 (coleoptera; González et al. 2014), Emerita spp. (mole crabs; Cardoso et al. 2016), and Ocypode spp. (ghost crabs; Schlacher et al. 2016a and citations). Ghost crabs, in particular, are used globally as bioindicators for sandy beaches (Schlacher et al. 2016a) due to their sensitivity to both human (Neves and Bemvenuti 2006) and natural impacts (Hobbs et al. 2008). Here, we examine the usefulness of Ocypode quadrata (Fabricius) (Atlantic Ghost Crab) as a bioindicator of human stress on South Carolina sandy beaches. The Atlantic Ghost Crab is the only ghost crab species found on the sandy beaches of the western Atlantic (Milne and Milne 1946), with a range extending from Rhode Island to Brazil (Williams 1984). The mono-specific nature of ghost crabs in the western Atlantic increases their utility as a bioindicator by eliminating any possibility of species or burrow misidentification (Pombo and Turra 2013). Atlantic Ghost Crabs also play an important ecological role in food webs on sandy beaches by consuming small invertebrates (Wolcott 1978), thus acting as an energetic link to higher trophic levels (Fisher and Tevesz 1979). Ghost crabs are often studied by examining their burrows on sandy beaches, because burrow width and abundance are well-established proxies for crab-population size and structure (Schlacher et al. 2016b). Examining the Atlantic Ghost Crab burrows provides a low-cost and efficient sampling design (Pombo and Turra 2013). Study-site Description We selected our study sites (Fig. 1) based on differences in the intensity of human visitors, whether or not the beach experienced mechanical cleaning, and Figure 1. Location of the study sites. HV = heavily impacted by people and vehicles, HI = heavily impacted by only people, MI = moderately impacted only by people, and P = pristine). We examined a total of 20 sites. Each human impact type is represented by 5 sites as shown on the map. Southeastern Naturalist 359 M.R. Gül, and B.D. Griffe 2018 Vol. 17, No. 2 presence or absence of vehicles. We considered the sites that are accessible by only boat (North Island 1 and 2, South Island 1 and 2, and Debordieu Island) as pristine because we observed no pedestrians. In order to determine the intensity of human disturbance on the beaches, we used binoculars to count the number of pedestrians on each site over a 2-h period from 0800 to 1000 over ~1 linear kilometer of beach. We considered the sites to be highly impacted if the number of pedestrians was greater than 50 per hour. The average number of pedestrians was 39.3(SD = ± 17.5) for moderately impacted and 60.2 ± 32.08 for highly impacted sites. Although recreational off-road vehicles are not allowed on South Carolinian beaches, some highly visited sites, such as Myrtle Beach, are mechanically cleaned. At some sites, municipality agents use vehicles to empty garbage bins and collect the litter on dunes. Moreover, security agents and life guards often use four-wheel– drive vehicles on the beaches. We also observed whether the sites were mechanically cleaned and whether other vehicles were present during night hours. Some other sites, such as Surfside beach, are highly visited during peak tourism months and are mostly the destinations for extended vacations due to local accommodations and entertainment. In contrast, sites like Pawley’s Island are mostly visited by locals and thus tend to experience little anthropogenic impact. Finally, we added some pristine sites to our study, such as South Island. These sites can only be accessed by boat and experience no anthropogenic impact. The North Myrtle Beach 1 and 2, Myrtle Beach 1 and 2 and Garden City Beach sites experienced mechanical cleaning and high disturbance by pedestrians; we considered them highly disturbed by vehicles and people. We classified the Surfside Beach 1 and 2, Sullivan’s Island, Isle of Palm 2, and Burkes Beach sites as highly disturbed sites by only people because they were not mechanically cleaned, and we observed no vehicles at those sites. We considered Pawley’s Island 1 and 2, Folly Beach, Isle of Palm 2, and Edisto Beach to be moderately impacted sites due to the fact that they experienced only human impacts (less than 50 pedestrians per hour). Methods To determine whether Atlantic Ghost Crab is a useful bioindicator species for South Carolina sandy beaches, we deployed 3 replicate rectangular transects (10 m x 5 m) parallel to the shore, randomly located within 2.5 m above and below the back-shore (i.e., dune) vegetation at each of the 20 sites. Transects were spaced ~200 m apart. We counted and used Vernier calipes to measure to the nearest 0.1 mm the width of the opening of all burrows within the transect. We sampled transects at each site just after sunrise to both diminish the impact of humans on the burrows and to allow individual crabs to return to their burrows following their nocturnal activities. In addition to anthropogenic impacts, morphological characteristics of sandy beaches such as sand compaction (Pombo and Turra 2013) and grain size (Defeo and McLachlan 2011) may also influence the density of Atlantic Ghost Crab burrows and the burrow width. We determined grain size at each site by sieving, which Southeastern Naturalist M.R. Gül, and B.D. Griffe 2018 Vol. 17, No. 2 360 is a widely used method for sand-dominated material (Cheetham et al. 2008, Folk 1980). Specifically, we collected three 500-g sand samples from each site, which we then sieved through a variety of mesh sizes (2 mm, 1 mm, 0.5 mm, 0.42 mm, 0.25 mm, 0.177 mm, 0.125 mm and 0.074 mm). We measured the sand compaction with a pocket penetrometer (AMS, E-280, Zoro, Inc., Buffalo Grove, IL) 5 times at each site. Sand resistance was extremely low; thus, we used an adaptor foot on the penetrometer to increase the surface area of the measurement by 16-fold. At the end of each measurement, we divided our result by 16 to obtain the true compaction. To determine if the sex ratio of Atlantic Ghost Crabs differed between sites, we caught all opportunistically encountered individuals using a dip-net around the strand line over a distance of ~1 km for 2 h during a time of day when crabs are often out of their burrows (between 0000 and 0200) . To examine which factors impacted burrow density and diameter, we used separate generalized linear mixed models (GLMM) with poisson and gaussian distributions, respectively, with impact level, sand compaction, grain size, and latitude as predictor variables for both models. We also included sampling day to control for seasonal and daily (i.e., weather) changes and transects nested within sites to control for the site differences as random effects in each model. We employed Akaike’s information criterion (AICc) values to determine the best model. When there was more than 1 model with ΔAICc < 2, these models were averaged (Burnham and Anderson 2002). We employed a chi-square test to examine the difference in sex ratio of observed Atlantic Ghost Crabs between sites with various impact levels. We conducted all statistical analyses in R, version 3.3.2 (R Core Team 2016). Results We found that human impacts had a significant influence on both burrow density and diameter of Atlantic Ghost Crabs on South Carolinian sandy beaches. Both the density (min–max = 0.3–1.146/m2) and opening diameter (min–max = 11.0–76.0 Table 1. summary of generalized linear mixed-effects models showing the outcomes of the best models explaining the changes of burrow diameter (mm) and burrow density (burrow/m2) compared to pristine sites. Coeff. = coefficient. HV = heavily impacted by people and vehicles, HI = heavily impacted by only people, and MI = moderately impacted by only people. Sampling days and transects nested within sites were random factors. An asterisk (*) indicates the significant values. Burrow diameter Burrow density Fixed factor Coeff. SE t P Fixed factor Coeff. SE Z P MI -9.91 3.57 -2.77 0.016* MI -0.45 0.13 -3.24 0.001* HI -42.91 10.67 -4.01 0.001* HI -0.78 0.18 -4.13 less than 0.001* HV -28.62 7.88 -3.63 0.002* HV -1.41 0.24 -5.8 less than 0.001* Compaction 193.01 93.51 2.06 0.060 Compaction 1.32 2.98 0.44 0.65 Grain size -19.89 83.14 -0.23 0.810 Grain size 1.19 1.11 0.17 0.86 Latitude 14.25 3.72 3.82 0.004* Latitude 0.04 0.1 0.45 0.64 MI X grain size 5.82 86.36 0.067 0.940 HI X grain size 789.54 251.79 3.13 0.008* HV X grain size -131.79 124.17 -1.06 0.301 Southeastern Naturalist 361 M.R. Gül, and B.D. Griffe 2018 Vol. 17, No. 2 mm) of the Atlantic Ghost Crab burrows declined significantly with increased human impact level (Table 1, Fig. 2). Latitudinal variation also significantly impacted the burrow diameter but not the density (Table 1). The burrow diameter significantly increased with the latitude. Sand compaction in the study sites varied between 0.071 kg/cm2 (South Island 1) and 0.288 kg/cm2 (Myrtle Beach 2) and the average Figure 2. Burrow measurements under different impact types. HV = heavily impacted by people and vehicles, HI = heavily impacted only by people, MI = moderately impacted only by people, and P = pristine. (a) burrow density (burrow/m2 ± SD) among impact types. The burrow density declines steadily with increasing disturbance. (b) Burrow diameter (mm ± SD) showed a similar, but weaker trend with burrow density, declining with increasing disturbance. Figure 3. Change in the sex ratio (male:female) of Atlantic Ghost Crab among impact types. HV = heavily impacted by people and vehicles, HI = heavily impacted only by people, MI = moderately impacted only by people, and P = pristine. Females outnumbered males with increasing disturbance. The sex ratio was similar between pristine and moderately impacted sites, and between the sites highly impacted only by humans and highly impacted by humans and vehicles. Southeastern Naturalist M.R. Gül, and B.D. Griffe 2018 Vol. 17, No. 2 362 sand-grain size at the sites was between 0.22 mm (Burkes Beach) and 0.38 mm (Pawley’s Island 2). The sex ratio (males/females) varied from 0.88 at sites highly impacted only by people to 1.64 in pristine sites (chi-squared test: χ2 = 659.1,8, P less than 0.0001). However, sites appeared to group into 2 distinct categories in terms of sex ratio; pristine and moderately impacted sites showed similar ratios (chi-squared test: χ2 = 0.10, P = 0.75) and sites highly impacted by people only and highly impacted by both people and vehicles showed similar ratios (chi-squared test: χ2 = 0.079, P = 0.99; Fig. 3). Discussion Here, we provided the first record of Atlantic Ghost Crab as a useful bioindicator species for human impacts on South Carolina sandy beaches. Atlantic Ghost Crabs provide useful information for human impacts by displaying a dramatic decline in burrow density and diameter in disturbed beaches. Similar decreases have been reported for Atlantic Ghost Crab on the coast of North Carolina at sites affected by off-road vehicles (Hobbs et al. 2008, Wolcott and Wolcott 1984), as well as in Texas (Maccarone and Mathews, 2007) and Brazil (Neves and Bemvenuti 2006). Our results are also consistent with the results of the studies of different ghost crab species in other regions of the world (see citations in Schalacher et al. 2016a). For conservation and management purposes, it is vital to have empirical biological data to make informed planning decisions (Solomon et al. 2003). For sandy beaches, these data are often obtained through the study of ghost crabs (Schalacher et al. 2016a and citations). We also present the first evidence that human disturbance impacts the sex-ratio of Atlantic Ghost Crabs. Further research is required to understand the consequences of the shift in this ratio and to understand the mechanism of anthropogenic impacts causing a decline in the Atlantic Ghost Crab population density and size structure on sandy beaches. Overall, in both marine and terrestrial systems, human disturbances have been increasing, and coping with those disturbances may be a serious challenge for their conservation and management. Every ecosystem is unique; thus, studies such as this one, which determine an area-specific bioindicator species (Solomon et al. 2003), are necessary for reliable and accurate management. 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