l 225 (1998) 53–68nsmn1.uh.edu/steve/cv/publications/nomann and pennings...journal of experimental...

LJournal of Experimental Marine Biology and Ecology,225 (1998) 53–68

Fiddler crab–vegetation interactions in hypersaline habitats

*Benjamin E. Nomann, Steven C. PenningsUniversity of Georgia Marine Institute, Sapelo Island, GA 31327, USA

Received 9 April 1997; received in revised form 20 July 1997; accepted 8 August 1997

Abstract

Abiotic conditions often change ecological interactions. Studies in areas with low to moderatesoil salinities have demonstrated a facultative mutualism between fiddler crabs and salt marshvegetation. In these habitats, fiddler crab burrowing increases plant growth, and plant roots helpsupport the walls of crab burrows. We looked for these interactions in hypersaline soils borderingunvegetated salt pans in a Georgia salt marsh. Crab burrows and vegetation cover were positivelyassociated. Neither crab removals nor burrow additions demonstrated a positive effect of crabs onvegetation. However, both vegetation removals and the addition of an artificial canopy (suspendedshadecloth) demonstrated a strong positive effect of vegetation on crab burrows. In contrast toprevious studies, we found no evidence that plants supported burrow walls. Instead, crabs likelyassociate with vegetation to avoid predators. Our results caution against extrapolating experimentalresults between habitats with different abiotic conditions. 1998 Elsevier Science B.V.

Keywords: Abiotic stress; Facilitation; Fiddler crab; Mutualism; Salt marsh; Uca

1. Introduction

Ecological interactions may change as a function of abiotic conditions (Dunson andTravis, 1991; Travis, 1996). Tansley (1917) provided one of the first examples of thisdependence: competitive dominance hierarchies of plant species were reversed depend-ing upon whether the soil was acidic or basic. Abiotic conditions have since been foundto mediate the outcome of ecological interactions as varied as competition (Moloney,1990), plant parasitism (Pennings and Callaway, 1996), and a plant–fungal mutualism(Kelrick and Nomann, unpublished data).

Facilitative interactions may be common and important in structuring communities

*Corresponding author. Tel.: 1 1 912 4852293; fax: 1 1 912 4852133; e-mail:[email protected]

0022-0981/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII S0022-0981( 97 )00209-8

54 B.E. Nomann, S.C. Pennings / J. Exp. Mar. Biol. Ecol. 225 (1998) 53 –68

(Bertness and Callaway, 1994; Callaway, 1995), however, species may shift betweencompetitive and facilitative interactions depending upon abiotic conditions. For example,Callaway and King (1996) demonstrated that aerenchymous plants may either facilitateor compete with neighbors depending on temperature. In New England salt marshes,either competition or facilitation may dominate plant interactions depending on soilsalinities (Bertness and Shumway, 1993; Bertness and Hacker, 1994).

Fiddler crabs (Uca spp.) are ubiquitous in salt marshes along the western Atlanticcoast (Teal, 1958). Facilitative interactions between crabs and low to middle marshvegetation (primarily Spartina alterniflora) have been described by Bertness (1985) andMontague (1980; 1982). Crab burrowing increases plant production through moderatingsoil conditions (e.g. increasing soil aeration, oxidation-reduction potential, and in situdecomposition of belowground plant debris). In soft sediments, plants appear to facilitatecrab burrowing by stabilizing the substratum, but in areas where vegetation forms denseroot mats, crab burrowing is effectively prevented.

Upper zones of southern salt marshes can become hypersaline, sometimes to theextent that vegetation cannot exist, and unvegetated salt pans form (Wiegert andFreeman, 1990). Fiddler crab–vegetation interactions have not been previously ex-amined in this habitat, but may differ from those described in low- to moderate-salinityhabitats. In hypersaline soils, plant productivity might be affected by salinity more thanby aeration or decomposition, and sediments might be stable enough that support ofburrows from plant roots was unimportant. We conducted a series of experiments todetermine if crabs and vegetation facilitated each other in hypersaline marsh habitats,and if so, whether the mechanisms were identical to those previously described inlower-salinity marsh habitats.

2. Methods

2.1. Study site

Research was conducted in three marshes on Sapelo Island, Georgia, USA (Fig.1A,B). At each site we worked in a high marsh zone characterized by salt pans andextremely salt-tolerant vegetation. Located above the Spartina alterniflora Loisel zoneand below the Juncus roemerianus Scheele / terrestrial zone (Fig. 1C), the salt pans wereunvegetated flats in a patchy band parallel to the shore | 1–20 m wide. The vegetationfringing and forming patches within salt pans was dominated by Batis maritima L.,Distichlis spicata (L.) Greene, Salicornia bigelovii L., and S. virginica L (taxonomyfollows Radford et al., 1968). Five to 10% of high tides inundated this zone. Tidal waterdid not pond in the salt flats; however, soil pore-water steadily evaporated during long(15–25-day) intervals between flooding, resulting in extremely high ( . 100 ppt)concentrations of salts within the soil (Antlfinger and Dunn, 1979; Wiegert and Freeman,1990). The mud and sand fiddler crabs, Uca pugnax (Smith 1870) and U. pugilator(Bosc 1801) respectively, comprised | 95% of the crab individuals present in the highsalt marsh, with the remaining | 5% consisting primarily of Armases and Sesarma spp.(Teal, 1958; pers. obs.).

B.E. Nomann, S.C. Pennings / J. Exp. Mar. Biol. Ecol. 225 (1998) 53 –68 55

Fig. 1. (A) location of Sapelo Island, GA. (B) Location of study sites. (C) Typical marsh zonation pattern.

2.2. Crab distribution patterns

To characterize crab distribution patterns in relation to vegetation cover and soil2salinity, a 0.25-m quadrat, divided into 100 5 3 5-cm cells, was used to sample the

Marsh Landing salt pan and fringing vegetation on 19 August 1995. In each of three‘zones’, eight replicates were haphazardly located. The vegetation bordering the upperedge of the salt pan was sampled 0.5–1.0 m into vegetation (border) and 2.0–3.0 m intovegetation (vegetation), and the salt pan was sampled 2.0–4.0 m away from the saltpan–vegetation interface. Percent cover of above-ground vegetation and feeding pelletswas estimated as the amount needed to fully cover 5 3 5-cm cells (one cell 5 1%), andthe number of burrows present was recorded. One core of sediment, 15 cm in depth and1.85 cm in diameter, was collected in each quadrat to determine interstitial salinity.Cores were taken . 5 cm from the nearest crab burrow. Soils were usually too dry toobtain pore water directly. Instead, cores were weighed while wet, dried for 3 days at608C, and weighed again to measure initial water content. A known amount of distilledwater was added, and containers sealed to prevent evaporation. Samples were vigorouslystirred after 1 day, and the salinity of the supernatant measured with a refractometer after2 days. Original pore-water salinities were calculated based on the initial water contentof the soil, the volume of distilled water added, and the final salinity reading.

In order to explore temperature patterns that might influence fiddler crab behavior, soil


and air temperatures were measured at Marsh Landing between 1 and 2 p.m. on 23September 1996. Temperatures were measured 2.5 m into the salt pan, and 0.5 and 2.5 minto the vegetation. Independent measurements were taken | 7 mm below (n 5 15/zone)and | 10 mm above the sediment surface (n 5 15/zone). These two heights were chosento represent the burrow entrance and the approximate location of the body of a foragingcrab. Temperatures were only measured at one time on one day of the year but would ofcourse vary diurnally and seasonally. Moreover, actual temperatures of crabs would beinfluenced by additional factors such as body orientation and color; therefore, thesereadings represent only a preliminary estimate of the crabs’ thermal regime.

2.3. Manipulating crab density and plant cover

To determine whether plants facilitated crabs and/or vice versa, we conducted anexperiment where crab density and vegetation cover were independently manipulated.

To assess the influence of burrowing crabs on above-ground productivity of plants,crabs were removed from the vegetation fringing salt pans. Exclosures (1.0 3 2.0 3 0.3m (W 3 L 3 H )) were constructed of 0.65-cm (mesh size) Vexar plastic fencing attachedto wooden posts driven into the marsh. The fencing extended | 20 cm below the soilsurface to prevent burrowing crabs from entering plots. A 10-cm wide strip of 4-milclear plastic sheeting was attached to the upper edge of the exclosure to inhibitimmigration of climbing crabs. Because the exclosures cut rhizomatic connections ofplants, 1 3 2-m control plots were also cut along their perimeters to a depth of | 20 cm.Observations indicated that crabs could not crawl over the fence but that very smallcrabs could pass through the mesh.

Crabs were removed from exclosures by hand and pitfall trap. Hand-collectionoccurred only when a crab was visually observed so as not to disturb the soil orvegetation. A single pitfall trap consisting of a | 700-ml glass jar was placed inside eachexclosure, with the jar’s mouth flush with the soil surface. Jars were emptied of crabsapproximately every 2 weeks, and the total number of crabs captured by both methodswas recorded.

To examine the influence of plants on crab burrowing and feeding, above-groundvegetation was removed from additional 1 3 2-m plots. Vegetation was cut to within | 1cm of the soil surface and reclipped as necessary to prevent regrowth.

Crab removal, vegetation removal and control treatments were grouped in eightblocks, each with one replicate of each treatment. Six blocks were located at MarshLanding and two at Cabretta. Within each block, plots were selected to maximize initialvegetation similarities, were located near each other ( | 0.5–2-m gaps between plots),and were randomly assigned to treatments. Because plots were so large, they en-compassed parts of both the ‘border’ and ‘vegetation’ zones described in Section 2.2.Vegetation was removed on 17–22 June 1995. Crabs were intensively removed from theexclosures by hand and pitfall from 8 July 1995 to 7 August 1995; pitfall trappingcontinued until 28 August 1996.

Plant and pellet cover and burrow data were recorded on 23–29 August 1995 and on225 June 1996 using two 0.25-m quadrats per 1 3 2-m plot. On the latter date, burrows

were classified into size categories: small (0.1–0.5 cm), medium (0.6–0.9 cm), and large


( . 0.9 cm). The two quadrats were placed to provide for a | 20-cm buffer between thesampled areas and pitfall traps and/or plot edges. On 23 and 28 August 1996 thevegetation from a 0.25 3 1.0-m area within each 1 3 2-m plot was harvested to within 1cm of the marsh surface, sorted according to species, dried at 608C, and weighed to thenearest 0.01 g. Two soil salinity cores, 2–3 cm in depth, were taken from each plantremoval and control plot on 18 September 1995.

2.4. Manipulating light intensity

To simulate light and cover conditions of vegetated areas in the absence of plantstems and roots, the salt pan surface was shaded. A single shaded and a single controlplot were placed in the salt pan within 2–3 m of each block of crab removal, vegetationremoval and control plots described above. After selecting plot locations, treatmentswere randomly assigned to plots. Shade cloths that reduced light by | 50% weresupported 15 cm over the 90 3 90-cm plots by wooden stakes, starting 17 June 1995.

2Burrow numbers and feeding pellet cover within a 0.125-m quadrat centered withineach plot were recorded on 18 September 1995 and on 25 June 1996. Soil salinity cores(2–3 cm in depth) were taken on 18 September 1995, and 20 August 1996.

2.5. Additional vegetation manipulations

Two other experiments were conducted in which we manipulated vegetation and/orshading to explore the relationship between vegetation and crab burrowing and feeding

In one experiment, we removed vegetation and/or shaded the substrate as describedabove in 0.5 3 0.5-m plots (n 5 12/ treatment /zone) in three zones at Lighthouse marsh:the salt pan, an area immediately above the salt pan dominated by Batis maritima, andan area immediately below the salt pan dominated by Salicornia virginica. In the saltpan we located control and shaded plots. In the two vegetated zones we located control,vegetation removal, and vegetation removal 1 shaded plots. Treatments were initiated on

215 August 1994, and sampled with a 0.125-m quadrat on 20 September 1995.In another experiment, we reciprocally removed two major components of the

vegetation fringing salt pans: dominant plants (i.e., Spartina alterniflora and Borrichiafrutescens) which dominate large zones of the marsh where salinities are moderate, andsalt tolerators (Batis maritima, Distichlis spicata, Salicornia virginica). Plots (0.5 3 0.5m, n 5 15/ treatment) were located in groups of three in vegetation mixtures near saltpans in the Marsh Landing and Lighthouse marshes, and randomly assigned to tworemoval and one control treatments. In these mixtures the ‘dominant plants’ hadscattered robust stems that did not form a dense canopy, whereas the ‘salt tolerators’ hadsmaller stems but formed a dense canopy over the soil surface. Thus, removingdominants affected structure more than shade, and removing salt tolerators affectedshade more than structure. Reciprocal removals were initiated on 30 March 1994 andmaintained by periodic weeding. Burrows were counted on 4 September 1995. Resultsfrom both marshes were similar and were pooled to increase statistical power.


2.6. Burrow effects on individual Salicornia bigelovii plants

To determine the effects of crab burrows on individual plants, we placed artificialburrows near individual Salicornia bigelovii plants. The experiment was conducted inMarsh Landing in S. bigelovii stands fringing a salt pan. Paired experimental and controlplants were greater than 5.5 cm from the nearest neighboring plant, were within 0.5 m ofeach other, and were similar in size. Using a portable electric drill, a single | 1.2-cmdiameter hole was drilled to a depth of 10 cm at a distance of 2 cm from the base of eachexperimental plant. A large diameter hole, similar in size to large burrows, was chosento obtain a maximum effect. Holes were drilled on 14 June 1995. Plants were checked1–3 times /week, deaths noted, and burrows redrilled as necessary until 5 September1995, when the surviving individuals were harvested, dried at 608C, and weighed to thenearest 0.01 g. We examined the effect of adding a burrow on survival and on final mass.Only cases in which both plants of a pair survived (n 5 4) were used in the analysis offinal mass.

2.7. Statistical analyses

Proportions were arcsine-squareroot transformed. Data sets in which the varianceincreased with the mean were ln transformed. Data were analyzed with t-tests orANOVA with Tukey means tests using Statistix (Analytical software, Tallahassee FL).

3. Results

3.1. Crab distribution patterns

Plant cover differed strongly between the three zones: plants were absent in the saltpan and more than twice as abundant in the vegetation zone than in the border zone (Fig.2A). Soil salinity was approximately two times greater in the salt pan than in thevegetation or border zones (Fig. 2B). Burrows were absent in the salt pan quadrats,present in the border zone, and common in the vegetation zone (Fig. 2C). Feedingpellets were about twice as abundant in the vegetation zone as in the salt pan or borderzones (Fig. 2D). Temperature patterns on 23 September 1996 differed depending onwhether readings were taken above or below ground. Although the differences wererelatively minor, below-ground temperatures were highest in the salt pan, whereasabove-ground temperatures were highest in the vegetation zone (Fig. 2E, ANOVA,interaction term, P , 0.0001).

3.2. Manipulating crab density and plant cover

During the first 38 days of the crab removal experiment, we removed 8463 crabs,including most of the large individuals, from each exclosure. Thereafter, we no longercounted the crabs that we removed but noticed that we were observing and removingonly small and medium-sized crabs. Examination of burrow densities indicated that we


Fig. 2. Plant cover, physical factors, and crab activity measures in three marsh zones (n 5 8/zone except fortemperatures, n 5 15/ location (air or soil) /zone). Soil temperatures were measured 7 mm below the soilsurface, to represent the burrow enterance; air temperatures were measured 10 mm above the soil surface, torepresent the body location of a foraging crab.

reduced the number of large burrows by . 75% but had no effect on the number ofsmall or medium burrows (Fig. 3). Small crabs were able to pass through the mesh andprobably grew into the medium-size class quickly enough that we could not reduce theirnumbers.

Vegetation cover and dry mass (all species were pooled) did not differ significantlybetween crab removal and control treatments (Fig. 4A,B).

Vegetation removal reduced crab burrow numbers by approximately 50% (Fig. 5A).Cover of feeding pellets was similar between treatment and control replicates in 1995,


Fig. 3. Burrow numbers in 1 3 2-m control and crab removal plots at Marsh Landing and Cabretta(n 5 8/ treatment). Error bars are61 SE.

but removal plots had one-third fewer pellets in 1996 (Fig. 5B). We observed nodifference between treatments in soil pore-water salinity (Fig. 5C).

3.3. Manipulating light intensity

Shading salt pans increased crab burrowing by | 5 times (Fig. 6A). The total numberof burrows present differed strongly between sampling dates, perhaps as a function oftemporal variability in soil salinity (Fig. 6C). Shaded plots had | 3 times the feedingpellet cover of controls in 1995, but no differences were observed in 1996 (Fig. 6B).

Fig. 4. Final (A) percent plant cover and (B) total above-ground dry plant biomass in 1 3 2-m control and crabremoval plots at Marsh Landing and Cabretta (n 5 8/ treatment). Error bars are61 SE.


Fig. 5. Burrow numbers, feeding pellet cover, and soil salinity during fall 1995 and summer 1996 in 1 3 2-mcontrol and vegetation removal plots at Marsh Landing and Cabretta (n 5 8/ treatment). Error bars are61 SE.

Soil salinity did not significantly differ between treatments in 1995, but shaded plotswere | 35% less saline than controls in 1996 (Fig. 6C).

3.4. Additional vegetation manipulations

Shading the Lighthouse salt pan increased burrow numbers 3–4 times (Fig. 7A).Shade plots had 30–55% greater feeding pellet cover than control plots (Fig. 7B), andsoil salinities were 20–50% lower in shaded than control plots (Fig. 7C). In vegetatedzones, vegetation removal 1 shade plots consistently had more burrows than vegetationremoval plots, with control plots intermediate (Fig. 8A). Feeding pellets were mostabundant in vegetation removal plots, and least abundant in control plots (Fig. 8B).


Fig. 6. Burrow numbers, feeding pellet cover and soil salinity in 90 3 90-cm shade and control plots at MarshLanding and Cabretta (n 5 8/ treatment). Error bars are61 SE.

Salinity was 15–35% lower in vegetation removal 1 shade plots than in the othertreatments (Fig. 8C).

Removing the more robust ‘dominant’ salt marsh species from vegetation mixtureshad little effect on burrow numbers, but removing the more abundant ‘salt tolerators’significantly reduced burrow numbers (Fig. 9).

3.5. Burrow effects on individual Salicornia bigelovii plants

Adding artificial burrows near individual S. bigelovii plants had no effect on survivalrates (Fig. 10A) but significantly reduced final mass of survivors (Fig. 10B).


Fig. 7. Burrow numbers, feeding pellet cover, and salinities in 0.5 3 0.5-m shade and control plots atLighthouse (n 5 12/ treatment). Error bars are61 SE.

4. Discussion

In contrast to Montague (1982) and Bertness (1985), our study found no evidence thatcrab burrowing facilitated plant growth. In fact, our burrow additions appeared to reduceplant mass, although the sample size for this test was small. In previous studies, thepositive impact of crabs on marsh plants was likely mediated through burrow effects onsoil oxygenation, decomposition rates, and nutrient concentrations (Montague, 1982;Bertness, 1985). We suggest that these impacts are largely irrelevant to vegetationsurrounding salt pans, where extremely high soil water salinities (Antlfinger and Dunn,1979; Wiegert and Freeman, 1990; this paper) likely exert the primary control onvegetative production. There remains the possibility that we failed to remove enoughcrabs to see an effect; however, burrows of large crabs (which probably have the greatestimpact on sediment chemistry because of their disproportionate size) were reduced by


Fig. 8. Burrow numbers, feeding pellet cover, and soil salinities taken in 0.5 3 0.5-m control, vegetationremoval 1 shade, and vegetation removal plots at Lighthouse (n 5 12/ treatment). Error bars are61 SE.

over 75% in exclosures relative to controls late in the experiment, and probably more soduring earlier periods of intensive crab removal. This removal rate compares favorablywith the 30–50% reduction achieved by Bertness (1985), who found large effects ofcrabs on vegetation.

Superficially, our result that plant cover facilitated crab burrowing appears similar tothat of other studies (Bertness and Miller, 1984; Bertness, 1985), but the mechanism wasprobably different. Previous studies focused on the importance of vegetation inproviding structure for burrows, but vegetation might also affect crabs through severaladditional mechanisms including altering salinity and temperature through shading,modifying intraspecific territorial interactions by limiting display distance, or supplying


Fig. 9. Burrow numbers in 0.5 3 0.5-m dominant species removal, salt tolerator removal and control plots atLighthouse (n 5 30/ treatment). Error bars are61 SE.

cover to hide from predators. Although we did not experimentally address all of thesepotential mechanisms, our results and observations suggest that vegetation surroundingsalt pans acts primarily as refugia for Uca from predators.

We found no evidence that Uca uses vegetation in the salt pan region to provideburrow support. Burrows in the salt pan region were not located immediately along plantstems (pers. obs.). We found greater burrow numbers in shaded versus control salt panplots despite a lack of structure that could support burrows in either plot type. Moreover,clipping above-ground vegetation resulted in a rapid drop in burrow number, despite the

Fig. 10. Proportion of Salicornia bigelovii surviving (initial n 5 40/ treatment) and above-ground dry weight ofsurviving S. bigelovii control–experimental pairs (n 5 4 pairs survived) in the burrow-addition experiment atMarsh Landing. Error bars are61 SE.


fact that underground stems and large roots which could have provided burrow supportremained intact for months (pers. obs.).

Our initial sampling suggested that burrow numbers were correlated with soil watersalinity; however, our experiments altered burrow numbers without consistently affect-ing soil water salinity. In general, shading by vegetation or shadecloths would beexpected to reduce evaporation and moderate soil salinities (Bertness, 1991; Bertness etal., 1992; Bertness and Hacker, 1994). These effects do occur at our sites (Pennings,unpubl. data) but were not always obvious in these experiments, perhaps because manyof our shade and clearing plots were quite small (0.5 3 0.5 m) and/or because thevegetation was often quite scrubby and did not heavily shade the substratum.

It is possible that vegetation and shading might alter burrow numbers by mediatingthe temperature regime. Air temperatures were lower in the salt pan than in vegetation,probably because vegetation reduced airflow and absorbed radiation; however, airhumidity was probably lower and desiccation stress higher in the salt pan. Soil surfacetemperatures were slightly lower in vegetation than in the salt pan, probably due tovegetation shading the soil. Temperatures were measured on only one date and were notmeasured at depth; however, it is possible that differences in burrow temperature mightconstrain fiddler crabs to vegetated areas. Examining this hypothesis would requireextensive monitoring of temperature and humidity regimes experienced by actual crabs.

Some workers have suggested that Uca densities are limited by territorial behavior(Zucker, 1981; Mueller, 1983). If so, removing vegetation might have facilitatedintraspecific interactions, resulting in enlarged territories and lower burrow densities.However, this hypothesis would predict no effect of shade cloth on burrow numbers.Since we observed a large effect of shade cloth on burrow numbers, we believe thatterritoriality was not the primary mechanism controlling burrow density in ourexperiments.

Vegetative structure has repeatedly been shown to protect prey from predators in avariety of systems (Stoner, 1979; Coen et al., 1981; Savino and Stein, 1982, 1989;Werner et al., 1983; Anderson, 1984; Ryer, 1988; Pennings, 1990). Fiddler crabs oftenfeed in salt pans—as evidenced by the presence of feeding pellets in unshaded salt panplots in our study—but flee when approached, suggesting avoidance of predators.Predation pressure on fiddler crabs is probably intense. Raccoons frequent these marshesand their faeces regularly contain the remains of Uca spp. (pers. obs.). Channel bass,Sciaenops ocellata, blue crabs, Callinectes sapidus, and mud crabs, Eurytium limosum,also prey on fiddler crabs (Shanholtzer (1973) cited in Montague, 1980; Kneib andWeeks, 1990). Further, certain birds feed heavily on fiddler crabs (Petit and Bildstein,1987; Watts, 1988; Frix et al., 1991; Ens et al., 1993), and focus their activities in saltpans, possibly due to the ease of walking and attacking in unvegetated habitats. During 8days of observations over 2 years totaling 336 foraging white ibis, Eudocimus albus (L.),we noted that . 85% of the birds were feeding in salt pans or in low-lying, sparsevegetation. Birds almost never fed in tall, dense vegetation when unvegetated areas wereavailable, even though tall, dense vegetation comprised the majority of the marsh area.Similarly, we have consistently observed a variety of gull species feeding in salt pans,but not in dense vegetation. These observations suggest that a primary reason crabsburrow in vegetated areas of Sapelo Island marshes is to reduce their risk of predation.


The nature of ecological interactions can be strongly affected by abiotic factors(Dunson and Travis, 1991; Stephens and Bertness, 1991; Callaway and King, 1996;Travis, 1996). Our results suggest that crab–vegetation interactions in salt marshes arenot the same in all habitat types. Working in higher, more hypersaline habitats thanprevious workers, we found crab facilitation of vegetation to be absent, and vegetativefacilitation of crabs to be strong but driven by different mechanisms than suggested byprevious workers. Our findings caution against the over-extrapolation of results fromsingle experiments to other situations where abiotic factors may differ.

Acknowledgements

We thank M. Grant, T. Page, C. Pullen, and C. Richards for field assistance, C.Richards for use of her Lighthouse plots, and D. Casey for Fig. 1. Ten percent ($1000)of research was funded by the US Department of energy’s (DOE) National Institute forGlobal Environmental Change (NIGEC) through the NIGEC Western Regional Center atthe University of California, Davis (DOE Cooperative Agreement No. DE-FCO3-90ER61010). Financial support does not constitute an endorsement by DOE of the viewsexpressed in this article. Support for Benjamin Nomann in the summer of 1995 wasprovided by the Sapelo Foundation through the University of Georgia Marine InstituteStudent Intern Program. This is contribution number 800 from the University of GeorgiaMarine Institute.

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