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Disturbance Type Affects the Distribution of Mobile Invertebrates in a High Salt Marsh Community
Margarita Brandt, Keryn Bromberg Gedan, and Erica A. Garcia

Northeastern Naturalist, Volume 17, Issue 1 (2010): 103–114

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2010 NORTHEASTERN NATURALIST 17(1):103–114 Disturbance Type Affects the Distribution of Mobile Invertebrates in a High Salt Marsh Community Margarita Brandt1,*, Keryn Bromberg Gedan1, and Erica A. Garcia2 Abstract - Salt marshes are frequently exposed to storm overwashes resulting in large deposits of sand and wrack at the margin of the high marsh and sand dune communities. On Cape Cod, MA, these disturbance-generated areas are dominated by burrows of the crab Uca pugilator and by nest entrances of the ants Formica subsericea and Tetramorium caespitum. We mimicked the effects of storm deposits through additions of sand and wrack and examined their effects on the distributions of the biotic structures of these organisms. We found that while crabs responded negatively to sand deposition, ants did so positively. We suggest that soil temperature and moisture explain these patterns. Wrack deposits extend higher the zone of moist soil and decrease evaporative stress for marine organisms such as crabs, whereas sandy areas tend to be drier and preferred by terrestrial ants. We conclude that disturbance type influences the distribution of these marine and terrestrial organisms over the ecotone. Introduction Disturbances and their effects on microclimate at the patch scale play a critical role in determining the distributions of organisms (Pickett and White 1985, Sousa 1984a). Ecological disturbances are generally measured by their effect on the frequency, spatial extent, and intensity of biomass removal of vegetation and/or sessile animals (Grime 1977). In contrast to sessile organisms, mobile animals can often avoid disturbance-associated mortality and can re-colonize disturbed habitats (Frid and Townsend 1989). Therefore, effects of disturbances on the distribution and fitness of mobile animals are often indirect, occurring due to disturbance-associated changes in habitat structure, resource quality and availability, and microclimate conditions (Sousa 1984b). Each of these changes in turn can affect habitat selection and/or re–colonization by mobile animals (Huff and Jarett 2007, Syms and Jones 2000). Salt marshes are frequently disturbed ecosystems. Disturbances are concentrated at habitat edges and play a major role in structuring these dynamic communities (Bertness and Ellison 1987). At the seaward edge, disturbance by wave erosion, aquatic predators, and ice scour (at high latitudes) is known to shape plant and animal distributions (e.g., cordgrass [Bertness 1999], mussels [Bertness and Grosholz 1985, Hardwick-Witman 1985, Lin 1990], periwinkle snails [Lewis and Eby 2002]). On the landward margin, wrack brought in by storms and high tides commonly results in plant mortality. Without a vegetative canopy, evaporation may cause the resultant bare patches to become 1Department of Ecology and Evolutionary Biology, Brown University, 80 Waterman Street, Box G-W, Providence, RI 02912. 2School for Environmental Research, Charles Darwin University, Darwin, Northern Territory 0909, Australia. *Corresponding author - m_brandt@brown.edu. 104 Northeastern Naturalist Vol. 17, No. 1 hypersaline, and even stress-tolerant plants such as Salicornia maritima Wolff and Jefferies (Slender Glasswort) and Distichlis spicata (L.) Greene (Marsh Spikegrass) are slow to recover (Bertness and Ellison 1987). The landward margin is an important zone that represents a transition from marine to terrestrial habitats. Previous work examining high-marsh disturbances in salt marshes has focused on effects on vegetation and succession (Bertness and Ellison 1987, Bertness and Shumway 1993, Valiela and Reitsma 1995). The effects of disturbances on mobile organisms at the landward margin of salt marshes are less well-known, although such effects could have important implications for cross-ecosystem transfer (Polis and Hurd 1996). In New England salt marshes, large wrack deposits occur in the spring, when senescent vegetation broken by winter ice builds mats and floats to the high marsh with spring tides. Wrack mats tend to break up after 2 to 4 weeks and often result in the death of underlying marsh vegetation (Bertness and Ellison 1987). Sand overwashes are caused when large storm surges push barrier beach sand across the marsh surface (Donnelly et al. 2001). Here, we examine the response to sand overwash and wrack deposition of 2 groups of mobile salt marsh ecosystem engineers, Uca pugilator Bosc (Atlantic Sand Fiddler Crab), and the ants Formica subsericea Say and Tetramorium caespitum L. (Pavement Ant). While crabs are marine and ants are terrestrial, they are similar in their ecological role in the high-marsh community, as both crabs and ants increase soil aeration and turnover within the top 25 cm of substrate by creating burrows (Allen and Curran 1974, Bertness 1985) and nest tunnels (Wilson 1971). Surface substrate is further modified by crab deposit-feeding (Montague 1980) and by ant moundbuilding (T. caespitum; Wilson 1971). We experimentally simulated wrack and sand deposition at the high-marsh boundary and examined the effects of these disturbances on the number of biotic structures of resident crabs and ants. Specifically, we asked 1) do crabs and ants respond similarly to the different disturbances? 2) how fast are these responses? and 3) what are the potential mechanisms driving these responses? Field-site Description The study was conducted in Summer 2007 and 2008 at the Lieutenant’s Island Marsh located in the Wellfleet Bay Wildlife Sanctuary of the Massachusetts Audubon Society (41º53'36"N, 70º0'56 " W). Lieutenant's Island is a back-barrier salt marsh. The site’s vegetation is that of a typical New England salt marsh, with Spartina alterniflora Loisel (Smooth Cordgrass) dominating the seaward edge, and Spartina patens (Ait.) Muhl (Marsh Hay Cordgrass) dominating the mid-zone of the marsh. Both Spartina species are replaced by a number of plants at the upland fringe of the high marsh: Salicornia maritima, Limonium carolinianum (Walt.) Britt. (Carolina Sealavender), and Suaeda maritima (L.) Dumort (Herbaceous Seepweed). Elymus repens (L.) Gould (Quackgrass) is also common along this fringe. The upland edge of the marsh borders sand dunes, vegetated by dune grasses, coastal shrubs, and occasional stunted trees. The nest entrances of Formica subsericea and Tetramorium 2010 M. Brandt, K. Bromberg Gedan, and E.A. Garcia 105 caespitum are conspicuous holes and mounds, respectively, and co–occur with Uca pugilator burrows at the high marsh/sand dune margin. Methods Natural distribution of crab burrows and ant nest entrances To examine the distributions of the biotic structures of ants and crabs, 8 transects were surveyed on August 22, 2007 at each of 4 different sites at Lieutenant’s Island. Sites were chosen haphazardly, were >20 m apart, and spanned an estimated area of 400 m2. We ran the transects from the upland margin perpendicularly to the shoreline until we reached dense vegetation, usually at 4–6 m, where it was not possible to quantify the biotic structures of ants and crabs. The number of crab burrows and ant nest entrances were counted in a 50- x 50-cm quadrat placed at 1-m intervals along each transect (distance between transects = 2 m). Only active crab burrows, with fresh evidence of crab feeding or excavation, were counted. Crabs generally occupy a single burrow (M. Bertness, Brown University, Providence RI, pers. comm.); thus, burrow density is a good estimate of crab density. However, ant nests may have had multiple entrances, so entrance number is not an equivalent for colony density but is a good proxy for territory expansion and foraging activity (Traniello and Levings 1986). The numbers of nest entrances of both ant species were pooled in our data analysis, as we were interested in looking at the responses of the ants as a functional group. Finally, elevation data were taken at each 1-m interval with surveying equipment and grounded by a global positioning system (GPS, ± 2.9 cm) benchmark established by a Trimble 4800 portable receiver. Effects of disturbance type on the distribution of ants and crabs We experimentally recreated 2 types of common marsh disturbances, wrack and sand deposition. The treatments consisted of adding wrack, sand, both together, or neither (unmanipulated control) to fifty-six 1- x 1-m plots (n = 14), situated at the high marsh/sand dune margin, where most crab burrows and ant nest entrances co-occurred. The 14 replicate plots spanned the overlapping habitat range captured by the transect surveys (see previous section). Wrack mats were taken from adjacent locations, and sand was brought from the adjacent barrier beach. Sand and wrack were spread over plots in a shallow, uniform layer. Wrack mats and sand deposits were approximately 1 cm thick. In combination plots, the sand and wrack additions were halved to make up a disturbance layer equal in depth to the single-disturbance treatments. The number of crab burrows and ant nest entrances were counted prior to wrack/sand additions and 7 times after experimental manipulation. In order to avoid edge effects, burrows and ant nest entrances were only counted in the central 0.25 m2 of the plots in all treatments. Six bamboo stakes were placed at the borders of the plots to hold the wrack mat together and ensure that it remained in place. Disturbance treatments were maintained and renewed as necessary to keep them as a press disturbance throughout the entire summer season, as some wrack mats usually persist throughout the summer months (Bertness and Ellison 1987); although sand overwash occurs less frequently, 106 Northeastern Naturalist Vol. 17, No. 1 its effects may be long-lasting and may cause vegetation burial and death (Hayden et al. 1995). Soil microclimate To examine the effect of surface disturbances on soil microclimate, we surveyed soil temperature and moisture in natural wrack and sand deposits at midday on July 14, 2008 (air temperature = 29 ºC). These data were meant to represent a relative measure of the effects of temperature and moisture caused by the sand and wrack treatments. The sampling date was chosen at the peak of summer and during a neap tide cycle so that differences in soil microclimates were maximized by hot and dry conditions. Soil temperature was measured with a Digi-Sense® Type K thermocouple and surface probe (Oakton Instruments, Vernon Hills, IL) at the soil surface and 1 cm underground (“sub-surface”) in wrack (n = 20) and sand (n = 20) natural deposits at the high margin of the marsh. Soil moisture was measured in a small soil core, 2.5 cm diameter by 8 cm depth. Soil cores were collected from sand (n = 8) and wrack (n = 8) deposits at the same time as temperature measurements. Cores were stored in plastic sample bags until transported to the lab, where they were immediately weighed, dried at 110 ºC for 2 days, and re-weighed. Gravimetric soil water content was calculated as a ratio of water mass lost per soil dry weight (Jarell et al. 1999). Data analysis Differences between treatments before wrack/sand additions were analyzed using 1-way ANOVAs. After experimental manipulation, differences between treatments were analyzed using repeated-measures analysis of variance (rm-ANOVA), and post-hoc comparison tests were used with a Bonferroni-adjusted critical P-value (to differentiate treatments and to control for multiple comparisons across the 7 sampling dates). Additionally, treatment differences within each time period were analyzed using 1-way ANOVAs, and post-hoc Tukey tests were used for multiple comparisons. In the figures, the responses of the number of crab burrows and ant nest entrances to disturbance type are displayed as percent (%) increase or decrease relative to initial conditions (prior to wrack and sand additions). We used a 2-way ANOVA nested design to compare soil temperatures between wrack and sand deposits with depth (surface, subsurface) nested within deposit type. Post–hoc Tukey tests were used for multiple comparisons. Finally, we used 1-way ANOVA to compare soil water content between wrack and sand deposits. Results Natural distribution of crab burrows and ant nest entrances In the transect survey of the high marsh, crab burrows and ant nest entrances were found as close as 1 m from the wrack line and extended into dense vegetation at lower marsh elevations (Fig.1a). While the number of crab burrows was relatively constant throughout the marsh elevation, ant nest entrances were more abundant at 2–3 m from the wrack line and 2010 M. Brandt, K. Bromberg Gedan, and E.A. Garcia 107 Figure 1. Natural distribution of crab burrows and ant nest entrances (a) The number of crab burrows and ant nest entrances per 0.25 m2 quantified in a distributional survey (mean ± SE; n = 4 sites, note y-axis is log scale) (b) The range and mean for height above mean sea level (m) in quadrats where crab burrows and/or ant nest entrances were present. 108 Northeastern Naturalist Vol. 17, No. 1 declined toward the sea (Fig. 1a). Similar densities of crab burrows and ant nest entrances co-occurred in a 2-m strip of open substrate near to the wrack line (Fig. 1a) and overlapped in elevation (Fig. 1b). Effects of disturbance type on the distribution of ants There were no significant differences in the numbers of ant nest entrances across the plots prior to wrack and sand additions (1-way ANOVA: F3,52 = 1.22, P = 0.311). After treatments were established, we found a significant treatment effect in the number of ant nest entrances in the wrack, wrack and sand, sand, and control plots (rm–ANOVA: F3,52 = 5.38; P = 0.03, between subjects, disturbance type effect). Post-hoc tests revealed that the sand addition treatment had a higher number of ant nest entrances (P < 0.018; Fig. 2a) than the other treatments. For 5 of the 7 sampling dates, the number of nest entrances increased more than 100% relative to the initial conditions (Fig. 2a). In addition, all treatments varied through time similarly, as ant nest entrances decreased during the highest tides (rm-ANOVA, within subjects, time effect: F6,312 = 10.2; P < 0.001; Fig. 2a). Effects of disturbance type on the distribution of crabs There were no significant differences in numbers of crab burrows across the treatments prior to wrack and sand additions (1-way ANOVA: F3,52 = 0.59, P = 0.624). However, the number of crab burrows was significantly different between treatments after experimental manipulation (rm-ANOVA: F3,52 = 2.89; P = 0.04, between subjects, disturbance-type effect). Sand addition plots had significantly fewer crab burrows than the wrack addition plots (P = 0.0037). In addition, the number of crab burrows changed through time (rm-ANOVA: F6,312 = 2.38; P = 0.049, within subjects, time effect), but treatments varied with a different pattern (rm-ANOVA: F18,312 = 2.80; P = 0.001, within subjects, time*disturbance type interaction), increasing in control plots and decreasing in wrack addition plots over the last 2 sampling dates (Fig. 2b). Soil microclimate We found that soil temperature varied significantly with deposit type and depth (2-way ANOVA, nested model: F3,39 = 91.92, P < 0.0001). Soil temperature was almost 5 °C higher in sand deposits (mean = 34.77 °C), compared to wrack deposits (mean = 29.79 °C) (2–way ANOVA, deposit-type effect: F1,39 = 118.87; P < 0.001). Soil temperature also varied with depth (2-way ANOVA, depth effect within deposit type: F2,39 = 6.65; P = 0.0035). Soil temperature was higher in the surface than in the subsurface in sand deposits, while it was similar in wrack deposits (post–hoc Tukey tests: P < 0.005; Fig. 3a). Finally, soil water content was almost 4 times higher in wrack deposits (1-way ANOVA: F1,15 = 162.30; P < 0.001; Fig. 3b) than in sand deposits. Discussion The greater number of ant nest entrances found in sand addition treatments suggest that this type of disturbance favors the distribution of ants. The higher soil temperature found in sand deposits (Fig. 3a) is an ideal 2010 M. Brandt, K. Bromberg Gedan, and E.A. Garcia 109 Figure 2. The % change (increase/decrease) of the number of ant nest entrances (a) and crab burrows (b) (mean ± SE, n = 14) relative to the initial conditions in response to disturbance type over the course of the experiment. Asterisks indicate where the treatments significantly differed (P < 0.05) according to 1-way ANOVAs within each time period. High-tide heights are shown for reference. Tide heights were downloaded from the NOAA/NOS/CO-OPS (2007) and converted from feet to meters. 110 Northeastern Naturalist Vol. 17, No. 1 environment for ants such as Tetramorium caespitum, which are known to prefer temperatures of up to 40 °C (Hölldobler and Wilson 1990). In addition to warmer temperatures, sand deposits also showed dryer conditions (Fig. 3b). We suspect that tidal inundation may increase the appeal of sand Figure 3. Soil microclimate conditions. (a) Soil temperature (mean ± SE) in sand (n = 20) and wrack (n = 20) natural deposits, with soil depth (n = 10, surface; n = 10, subsurface) nested within deposit type. Different letters correspond to significantly different groups after post-hoc Tukey tests (P < 0.05). (b) Soil water content (% mean ± SE) in sand (n = 8) and wrack (n = 8) natural deposits. 2010 M. Brandt, K. Bromberg Gedan, and E.A. Garcia 111 deposits to ants. Although a targeted experiment or greater temporal resolution is needed to fully explore this hypothesis, our experimental data do suggest a greater reliance of ants on sand deposit areas during spring tides (asterisks, Fig. 2a). The last sampling date is an exception, as treatments significantly differed during a neap tide; however, sand deposits continued to affect nest-entrance densities positively. The treatment effect in the densities of crab burrows was due to the difference between sand and wrack addition plots (P = 0.0037). For the first 5 sampling dates, crabs considerably favored the wrack addition treatment (Fig. 2b). Like the ants, the crabs likely responded to disturbance effects on microclimate, but in the opposite direction, preferring the cool, moist wrack areas to warm, dry sand areas (Fig. 2b; Fig. 3a, b). For Sand Fiddler Crabs, temperatures above 40 °C are lethal (50% mortality from 1 hour exposure; Orr 1955, Teal 1959, Wilkens and Fingerman 1965). In open areas, Uca pugilator lives at or above its thermal tolerance limits and must periodically retreat into its burrow to cool itself (Montague 1980, Wilkens and Fingerman 1965). Consequently, we suggest that the higher soil water content and lower temperatures found in wrack deposits (Fig. 3b) allows a small expansion of crabs’ occupancy range. Although our data support that crab and ant preferences of wrack- and sand-disturbed areas are due to microclimate effects, we must consider other hypotheses. For example, soil salinities were likely affected by disturbances, with salinity higher in the warm, dry conditions of sand deposits than in the cool, moist conditions of wrack deposits. However, if ants were sensitive to higher salinities, we would have seen at least some positive response to wrack. In addition, Uca pugilator is tolerant of extremely high salinities (Green et al. 1959), and hypersalinity would not have precluded crabs from inhabiting sand deposits. Another possibility is that crabs may have preferred wrack-disturbed areas for protection from predators. In southern marshes, Nomann and Pennings (1998) suggested that crabs favor vegetated areas to open patches in order to avoid predators. While we did not observe predators feeding on Sand Fiddler Crabs, this may be a factor in the crabs’ selection of wrack-disturbed areas. The crabs’ preference for wrack-disturbed areas diminished in the last 2 sampling dates, and the densities of crab burrows increased greatly in control plots (Fig. 2b). Coincidentally, vegetation cover increased greatly in these final weeks of the summer season. Plots that were on average 85% bare earlier in the season became dominated by the forbs Suaeda maritima and Salicornia maritima in a matter of days (M. Brandt, unpubl. data). This new vegetation may be one of the reasons that crabs left wrack areas and colonized control plots. As the summer ended, air temperatures decreased and the thermal stressreduction effect of wrack may have become less beneficial. The control areas with new vegetation would have provided a mixture of shady and sunny spots, thereby providing thermal options more suitable to the crabs in the changing season. Alternatively or in addition, crabs may have exhausted the local food supply under wrack mats and sought higher quality forage. Salt marsh microalgal concentrations tend to be higher in the open where light intensities are higher (K.B. Gedan, unpubl. Data; Whitney and Darley 1983). 112 Northeastern Naturalist Vol. 17, No. 1 The 2 disturbance types mimicked in this experiment, wrack and sand deposition, may have similar effects on the salt marsh plant community—plant death and patch formation (Bertness and Ellison 1987, Brewer et al. 1998, Hayden et al. 1995)—yet elicited different responses from the mobile organisms in the community. Ants responded positively to sand addition, but not to wrack deposition. Conversely, crabs responded positively to wrack deposition and not to sand; however, the crabs’ responses were more subtle than the ants’ (Fig. 2). These experimental findings correspond to our transect data, where a greater number of ant nest entrances were found at higher, less frequently inundated, elevations in the marsh. Both ant and crab responses were extremely rapid. While plant death due to wrack deposition can take a month, the responses of ants and crabs occurred within 15 days of the initial manipulation. The small scale of our experiment (1- x 1-m plots) may have increased the speed of response by facilitating rapid recruitment from unaffected areas on plot perimeters, but this scale was on par with the size of natural disturbances, which often span mere centimeters in elevation and several meters of spatial area (Bertness and Ellison 1987, Brewer et al. 1998). Additionally, responses to disturbances may have been particularly strong in this comparison of crabs and ants due to the positioning of our study site at a marine–terrestrial boundary and our choice of focal organisms, one marine and one terrestrial. Here, the potential was high for organisms to cross into suitable habitat during undesirable conditions in the adjacent habitats. Effects of disturbances on mobile organisms may be more detrimental in habitats that are more isolated or part of a patchy landscape. Due to the major effects of these disturbances on soil microclimate, we suggest that these mobile species did not respond directly to the disturbances, but rather to the effects of the disturbance on microclimate, specifically soil temperature and water content. A key component in the response of mobile organisms to disturbances is that they frequently escape the direct effects of disturbance, but, as we and others have found, the indirect effects can be consequential (Siddon and Witman 2003). Both ants and crabs are habitat engineers, with different effects on the ecological community (Jones et al. 1994). We expect that, in addition to the direct effects of wrack and sand disturbances on plants, indirect disturbance effects on mobile ecosystem engineers may have feedbacks for the plant community. Here, at the beginning of the season, wrack deposition favored the Sand Fiddler Crab, a deposit feeder that is known to promote plant and soil fauna production (Bertness 1985), whereas sand deposition favored patch colonization by ants, which have more subtle physical effects on the soil community, such as increasing soil turnover (Wilson 1971). Further research is needed to close the link between mobile organisms’ responses to disturbances and resultant feedbacks on community resilience and postdisturbance community composition. Acknowledgments We would like to thank A. Chiriboga, C. Holdredge, D. MacCombie, P. Flombaum, and T. Savage for help in the field. Also, thanks to A. Ellison for identifying ant 2010 M. Brandt, K. Bromberg Gedan, and E.A. Garcia 113 vouchers. We appreciate the permission of R. Prescott and Massachusetts Audubon Society to allow us to conduct research at the Lieutenant’s Island Marsh. M. Bertness, J. Witman, D. Morse, J.A. Iriarte, and J. 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