A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 478 2013 NORTHEASTERN NATURALIST 20(3):478–492 Morphometry and Claw Strength of the Non-nativeAsian Shore Crab, Hemigrapsus sanguineus Andrew Payne1 and George P. Kraemer1,* Abstract - The invasive crab Hemigrapsus sanguineus (Asian Shore Crab) arrived on the northeast coast of the United States about fifteen years ago, and has attained high population levels at the expense of other resident crabs. Data collected between 1998–2012 at a low-energy, rocky intertidal site in the western Long Island Sound reveal continued Asian Shore Crab dominance. A body of research has suggested several reasons for the success of the Asian Shore Crab, including predation on resident crabs. We coupled morphometric data with measurements of claw closure force to model strength as a function of crab size and sex, enabling interspecific comparisons. The model provides indirect support for conclusions of an earlier study that suggested Asian Shore Crab dominance was achieved through predation on juvenile recruits of resident crabs such as Carcinus maenas (Green Crab). Asian Shore Crab males may have had more impact than females on Green Crabs due to the sexual dimorphism of Asian Shore Crab chelae and consequent strength disparity. Introduction Invasive species are of increasing concern to ecologists and, more recently, to natural systems economists (e.g., Aukema et al. 2011, Pimentel 2007). Invaders often have major impacts on the ecological structure and function of the receiving ecosystem (e.g., Strayer et al. 2006), effects that may ultimately manifest as evolutionary change (Freeman and Byers 2008). In the marine environment, invaders affect population levels of native species (Kraemer et al. 2007, Riisgård et al. 2012), modify and/or reduce access to required habitats (Kostecki et al. 2011), cause local extinctions of native species (Lafferty and Kuris 2011, Sommer et al. 2007), and alter the flux of ener gy and materials (Norkko et al. 2011). Hemigrapsus sanguineus De Haan (Asian Shore Crab), a native of the western Pacific Ocean, was first recorded in 1988 in southern New Jersey (Williams and McDermott 1990). Ballast-water discharge from commercial ships is the likely vector for the introduction of this non-native (McGee et al. 2006). Since its discovery, the Asian Shore Crab has spread into rocky shores from North Carolina through Maine (Gilman and Grace 2009). The Asian Shore Crab is omnivorous, feeding on amphipods, gastropods, bivalves, barnacles, sea grass, and macroalgae (Bourdeau and O’Connor 2003, McDermott 1998b), though it prefers animal prey to algae (Brousseau and Baglivo 2005, Griffen et al. 2011). Populations of Asian Shore Crab have risen to levels that argue for significant local and regional impacts, particularly in the southern section of its range (e.g., Bordeau and O’Connor 2003, Kraemer et al. 2007, Lohrer and Whitlatch 2002). 1Department of Environmental Studies, Purchase College (SUNY), 735 Anderson Hill Road, Purchase, NY 10577. *Corresponding author - george.kraemer@purchase.edu. 479 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 The species diversity and evenness of intertidal crab populations at a Rye, NY site, for example, have fallen greatly since the arrival of Asian Shore Crab in 1994 (Kraemer et al. 2007). Carcinus maenas L. (Green Crab) is another non-native species, long established on the northeast US coast, having arrived in the 1800s (Vermeij 1978). This omnivore has a broad diet comprising algae, annelids, mollusks, and crustaceans, including crabs (Ropes 1968). In Long Island Sound, intertidal Green Crabs have largely been replaced by the Asian Shore Crab (Delaney et al. 2011, Kraemer et al. 2007, Whitlatch and Lohrer 2002). The replacement stems from a host of factors (Griffen 2011, Jensen et al. 2002), but likely involves predation (Lohrer and Whitlatch 2002). Both Green Crabs and Asian Shore Crabs feed on the conspecific juveniles, as well as those of the other species (Lohrer and Whitlatch 2002). However, the Asian Shore Crab appears to be a more effective interspecific predator than the Green Crab, a factor Lohrer and Whitlatch (2002) believe underlies the Green Crab population decline. Cheliped size and strength have been implicated in the success of other decapod invaders (Garvey and Stein 1993). Interspecific differences in predation success may stem from differences in claw strength, since the force applied by claw closure determines the type and maximum size of prey a crab can consume (Preston et al. 1996). The Asian Shore Crab uses its claws to crush and chip away the shells of prey. As Asian Shore Crabs increase in size, the size of prey consumed also increases (Brousseau et al. 2001). Net prey energy yield can influence prey consumption (Elner and Hughes 1978). Claw strength may also determine diet range; prey with greater shell strength such as Mercenaria mercenaria L. (Northern Quahog) and the Littorina littorea L. (Common Periwinkle) are only consumed by the largest Asian Shore Crabs (Bourdeau and O’Connor 2003). The assumption here is that allometries of claw size and strength with Asian Shore Crab body size are neutral to positive, as is generally the case for other decapods (McLain et al. 2003). Greater claw strength may also explain why Asian Shore Crabs are capable of consuming larger-than-self Green Crabs (Lohrer and Whitlatch 2002). Adult Asian Shore Crabs have an advantage in agonistic encounters with Green Crabs; even when the Green Crab was larger by 5 mm of carapace width (CW), its chances of successfully consuming Asian Shore Crab were only 50% (Lohrer and Whitlatch 2002). Like many crabs that consume hard-shelled prey (e.g., mollusks), the Green Crab is heterochelous, with a slow, crusher claw and a fast, cutter claw (Juanes et al. 2008). These two claws differ not only in operational speed, but also strength, with the crusher claw being the more powerful (Taylor et al. 2009). The Asian Shore Crab is homochelous, with left and right claws similar in morphology and presumably adapted to serve similar functions. However, the Asian Shore Crab is sexually dimorphic, with males’ claws larger and possessing greater mechanical advantage than those of females (McDermott 1999). Sexual dimorphism likely extends to claw strength; males can open larger mollusks than females of the same size (Bordeau and O’Connor 2003). A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 480 The goal of this study was to better understand the ecology of the Asian Shore Crabin its western Atlantic habitat and further explore the reasons underlying the success of this invader. We had two main objectives: 1) to describe the morphological changes in claw size and strength during development, comparing males and females; and 2) to create a model relating crab size to claw strength to enable comparison of the Asian Shore Crab and the now-scarce Green Crab. In addition, we report crab population density and diversity for this highly impacted site, updating records from 1998–2005 (Kraemer et al. 2007). Materials and Methods Population levels of Asian Shore Crab have been monitored since 1998 in the intertidal zone on the western end of Long Island Sound at Read Wildlife Sanctuary (Rye, NY; 40°57'57.81"N, 73°40'6.67"W). We sampled three cross-intertidal transects during the first spring low tide in June of each year. At 2-m intervals from low water to the sandy dune face (≈43 m horizontally), all crabs within a 0.49 m2 quadrat were captured (Kraemer et al. 2007). In addition, eight quadrats were sampled each year along a horizontal transect in the mid-intertidal zone, the region of highest Asian Shore Crab abundance. For the study of claw strength and related morphology, Asian Shore Crabs were collected by hand during low tides between May and July of 2009 at Read Wildlife Sanctuary. A total of 351 Asian Shore Crabs were collected haphazardly at a range of vertical elevations within the intertidal zone. For each crab, the CW was measured at its maximum, just behind the anterior edge, and the wet biomass was recorded. Asian Shore Crabs of all sizes were collected, ranging from 5.9 to 40.5 mm CW. Gravid females were excluded from analysis. Recently molted crabs (those with soft carapaces) were also rejected. At this site, the Green Crab is now present at very low density, making this species very difficult to find. Therefore, only 11 Green Crabs were included in the morphometric study, of which five were found in 2009, and another six in 2007–2008. Green Crab carapace widths ranged from 34.9 mm to 46.6 mm. After capture, crabs were placed in plastic bags over a layer of ice in a cooler and transported back to the lab. The CW, mass, and sex of each crab were recorded while the crabs warmed to room temperature (≈22–24 °C). To restrain the crab, plastic-coated wire was looped through holes in a basal Plexiglas plate. The pollex (immovable finger) of the claw was then inserted into the loop and the wire tightened to hold the claw in place. An Imada DPS-11 tensometer was attached to the dactyl (movable finger) of the claw directly behind the dark-colored dactyl tip using twenty-pound-test (9-kg-test) fishing line (attempts using a bare wire loop from tensometer to dactyl were unsuccessful; crabs did not resist tension when applied with bare wire). Once the crab had closed its claw, tension was applied gradually, and the force necessary to re-open the claw was recorded. The maximum force applied by both left and right claws was recorded, alternating with each crab the side first measured. After measuring the claw strengths, each crab was placed in a separate container at -20 °C. Later, claws were detached 481 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 from the body at the chela-cheliped junction, and the mass of each claw measured. Strength measurements and morphological characters were measured for 147 female and 204 male Asian Shore Crabs in this study (the difference between sexes reflects the preponderance of males at larger sizes; Kraemer et al. 2007). Of the six Green Crabs captured for claw strength measurements, only five provided data (the sixth did not respond by closing its claws). From these, only measurements from the larger, crusher claw are reported. Microsoft Excel software was used for statistical summarization, plotting, generation of regression lines, and testing of sample means, while homogeneity of regression slopes was evaluated using R software. Regressions were performed on linearly related variables, and on log-log transformed power relationships. Results Although the population of Asian Shore Crabs at the Rye site has continued to fluctuate from year to year in the mid-intertidal zone, Asian Shore Crab population densities measured in June have averaged more than 100 crabs m-2 over the past 10 years (Fig. 1). Before the Asian Shore Crab population increase in 2000, intertidal crab species richness, determined from three yearly cross-intertidal transects (total of 54.3 m2 sampled each year), averaged 3.5 species (SD = 0.8; Fig. 2). The average richness of intertidal transects dropped by more than 50% to 1.7 species (SD = 1.0; most quadrats contained only Asian Shore Crabs) during 2000–2012. Green Crabs, in particular, became scarce during the same period (Fig. 2), dropping from an average of 3.8 individuals per 54.3-m2 transect (SD = 3.2) to 0.5 per transect (SD = 1.0). Figure 1. Population density of Hemigrapsus sanguineus (Asian Shore Crab) estimated in the mid-intertidal zone (characterized by highest intertidal densities) at Read Wildlife Sanctuary (Rye, NY). Error bars represent one standard deviation around the means. A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 482 The biomass of male and female Asian Shore Crabs was predictably related to size, measured as CW (Fig. 3). The scaling appeared to differ slightly by sex (power exponent = 3.04 vs. 2.91 for males and females, respectively). However, the difference in slope was not significant (F1,253 = 2.67, P = 0.103). Green Crab biomass may scale differently with size than Asian Shore Crab biomass (exponent = 2.64), though small sample size precludes certainty. Male and female Asian Shore Crab claws are clearly sexually dimorphic, with claw growth from the post-settlement stage differing in males and females. Claws of males increase as a power function (exponent = 1.27; F1,190 = 15,299, P < 0.0001; Fig. 4A), while those of females increase linearly with overall body mass (F1,143 = 9004, P < 0.0001). Claws of females remain in constant proportion to body mass (≈2.2%, F1,143 = 0.78, P = 0.380; Fig 4B), while males’ claws grow to constitute an increasingly large fraction of body mass. A power curve with an exponent of 0.27 provided the best fit of the data representing male mass and percent of total biomass constituted by chelae (F1,190 = 671, P < 0.0001). Youngof- the-year males (10–12 mm) already possess claws that constitute 3–4% of body mass, and this fraction grows to 10% for the largest males (35–40 mm CW). A paired sample t-test determined there was no significant difference between the strengths of the right and left claws (ratio of strength [R:L] average = 1.1, median = 1.0; t = 0.05, df = 155, P > 0.05). Therefore, the strongest measured claw strength for individual crabs was used in the remaining analyses. Claw strengths of female Asian Shore Crabs ranged from 1–7 N, while strengths of males ranged from 1–11 N. As male and female Asian Shore Crabs grew larger, their claws also developed greater strength. The exponents in the power relationship between size (CW) and claw strength for males and females (1.59 and 1.56, respectively) did not differ significantly (F1,166 = 0.01, P = 0.91; Fig. 5). Regressions were highly Figure 2. Brachyuran species richness and number of Carcinus maenas (Green Crab) captured per cross-intertidal transect at Read Wildlife Sanctuary (Rye, NY). Estimates were obtained from three transects per year (area sampled per year = 54.3 m2). 483 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 significant (males: F1,130 = 384, P < 0.0001; females: F1,37 = 35.5, P < 0.0001). Males of all sizes were stronger than equivalently sized females. The disparity in strength ranged from male Asian Shore Crabs being about 50% stronger than females for small young-of-the-year (10 mm CW) to 120% stronger at the size of the largest females (38 mm). The sex-based difference in Asian Shore Crab strength was due to differences in claw size. When claw strength was examined as a function of claw mass (Fig. 6), the strength of male and female claws varied in a similar fashion (i.e., exponents were not significantly different; F1,221 = 1.08, Figure 3. Relationship between size (as carapace width [CW; mm]) and biomass (as fresh weight [FW; g]). A. Hemigrapsus sanguineus (Asian Shore Crab) females (non-gravid females only). Biomass (FW) = 0.000542*(CW)2.91; R2 = 0.987). B. Hemigrapsus sanguineus males. Biomass (FW) = 0.000444*(CW)3.02; R2 = 0.993). C. Carcinus maenas (Green Crab)males and females pooled for regression (Biomass (FW) = 0.000892*(CW)2.65; R2 = 0.852). A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 484 P = 0.30). In both cases, the regressions were highly significant (males: F1,128 = 433, P < 0.0001; females: F1,92 = 99.9, P < 0.0001). A comparison of the claw strengths of similarly sized Asian Shore Crabs and Green Crabs was not feasible because of the scarcity of Green Crabs. Mitchell et al. (2003), however, recorded claw strength of Green Crabs employing methods similar to ours. The allometric equations relating CW to claw strength were used to compare the (stronger) crusher claws of Green Crabs (Mitchell et al. 2003) Figure 4. Claw biomass in male and female Hemigrapsus sanguineus (Asian Shore Crab) as a function of crab body mass (FW; g). A. Claw biomass as a function of crab body mass. Claw biomassmale = 0.0491*(body mass)1.265, R2 = 0.988; claw biomassfemale = 0.0221*(body mass) – 0.0014, R2 = 0.984. B. Claw biomass as a percent of crab body mass. Percent claw biomassmale = 0.0491*(body mass)0.265, R2 = 0.779; Percent claw biomassfemale = 0.022*(body mass); slope for females not significantly different than zero. 485 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 with claws of Asian Shore Crabs (this study). The claws of male Asian Shore Crabs are predicted to be stronger than the crusher claws of similarly sized female Green Crabs (Fig. 7). Additionally, the model predicts that claws of male Asian Shore Crabs are stronger than claws of male Green Crabs at CW less than 15 mm (Fig. 7). Claws of female Asian Shore Crabs are predicted to be stronger than the crusher claws of similarly sized female Green Crabs when both species are smaller than 23 mm CW. Female Asian Shore Crabs are also predicted to be stronger than similarly sized male Green Crabs, though only when both individuals are smaller than 9 mm CW. Discussion The population density of Asian Shore Crabs in the mid-intertidal zone of the Rye, NY site has ranged from 70–160 crabs m-2 between 2002–2012. The mid-intertidal average (110 crabs m-2) is roughly 12-times the combined density of all crabs during the first population census (1998; 9 crabs m-2). Across the entire intertidal zone, total crab density is now ≈50-times greater than in 1998. This increase derives from an increase in the Asian Shore Crab because this species now constitutes >99.5% of all crabs captured in the intertidal zone. The increase of the Asian Shore Crab population has clearly occurred at the expense of other crabs, evidenced by the drop in brachyuran species richness, and has undoubtedly driven other, undocumented ecological changes. In addition, the fact that Asian Shore Crabs are now more numerous than all crabs combined at the start of the population expansion (i.e., 1998–1999) suggests that energy Figure 5. Claw strength as a function of crab size (measured as carapace width [CW]). Note the log10 scaling. Slopes of two regression lines do not differ significantly. Hemigrapsus sanguineus (Asian Shore Crab) females: Strength (N) = 0.0376*(CW)1.44, R2 = 0.490. Asian Shore Crab males: Strength (N) = 0.0497*(CW)1.59, R2 = 0.747. A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 486 and materials have been rerouted from other components of the local ecosystem through the Asian Shore Crab. Greater certainty of this conclusion would be possible with measures of total biomass, but crab sizes were not uniformly recorded during the first two years of this study. Asian Shore Crabs possess claw strengths that are within the range reported for other decapods. Yamada and Boulding (1998) reported claw strengths of Figure 6. Hemigrapsus sanguineus (Asian Shore Crab) claw strength as a function of claw mass. Female: Strength (N) = 10.49*(CW)0.488, R2 = 0.521. Male: Strength (N) = 10.89*(CW)0.449, R2 = 0.774. Slopes are not significantly different. 487 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 similarly sized (8.7 g) West Coast crabs Hemigrapsus nudus Dana (Purple Shore Crab), Cancer productus Randall (Red Rock Crab), and Lophopanopeus bellus Figure 7. Allometric models relating carapace width (CW; mm) to claw strength (N) for Carcinus maenas (Green Crab) (Mitchell et al. 2003) and Hemigrapsus sanguineus (Asian Shore Crab) (this study). A. Male H. sanguineus vs. male and female C. maenas (Green Crab) crusher claws. B. Female H. sanguineus vs. male and female C. maenas crusher claws. Small insets present model output from small crabs (young of the year to second year). A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 488 Stimson (Blaclclaw Crestleg Crab) at 4.5, 12.2, and 25.5 N, respectively. Our model of claw strength indicates that Asian Shore Crabs of similar biomass (8.7 g) possess a claw strength of 4.8–9.8 N (range encompasses females and males of that size). According to our data, male Asian Shore Crabs are stronger than similarly sized female Green Crabs, and stronger than similarly sized male C. maenas during the Asian Shore Crab’s first year (i.e., CW is less than ≈ 15 mm). This finding strengthens the link between the Asian Shore Crab invasion and the recorded declines of intertidal Green Crab populations in CT and NY (Lohrer and Whitlatch 2002). Although adult Green Crabs are, in general, much larger and stronger than those of Asian Shore Crabs (Lohrer and Whitlatch 2002), the unexpected consequence (the Green Crab decline) is inevitable if the Asian Shore Crab preys heavily on juvenile Green Crab recruits. Our findings suggest a mechanism explaining the results of Lohrer and Whitlatch (2002), who demonstrated that Asian Shore Crabs and Green Crabs have an equal chance of successfully preying on one another only when the Green Crab is 5 mm larger. We believe the size differential in predation outcome derives, in part, from the strength differences reported here. However, our data also suggest claw strength alone does not provide the full explanation of the dominance of Asian Shore Crabs over Green Crabs. A 40-mm Green Crab has a strength of ≈7 N, while 35-mm male Asian Shore Crabs generate a closure force of 15 N. We estimate that Asian Shore Crabs with claws as strong as a 40-mm Green Crabs are only 21 mm (male). Since this size difference exceeds the 5 mm predation differential reported by Lohrer and Whitlatch (2002), the larger body mass of Green Crabs may also play a role in determining the outcome of agonistic encounters between these two species. In addition, Asian Shore Crab carapaces are more resistant to being crushed than those of Green Crabs (MacDonald et al. 2007). Aggression in crabs tends to be correlated with chela size (Vermeij 1977). Informal observations by G. Kraemer over 15 years have identified the Asian Shore Crab as the most pugnacious of the crabs present at the Rye, NY site at the 1997 outset of study (Asian Shore Crab, Green Crab, Cancer irroratus Say [Atlantic Rock Crab], Eurypanopeus depressus Smith [Flatback Mud Crab], and Libinia emarginata Leach [Portly Spider Crab]). Differences in strength and aggressiveness may also have caused the displacement of Green Crab juveniles by Asian Shore Crabs from under sheltering rocks, exposing the former to greater risk of predation by terrestrial and marine predators during emersion and immersion, respectively (Jensen et al. 2002). For Asian Shore Crabs, the strength to claw-mass relationship did not differ for males and females, arguing that the claws differ not in structure (mechanical leverage) or musculature, but in size alone. Male and female non-claw biomass scaled similarly with CW (i.e., overall biomass differences between males and females of the same CW are driven by the sexual dimorphism in claw size). Increased claw size and strength are tied to selection for increased fitness via several possible mechanisms: increased chance of success in agonistic encounters over habitat or other scarce non-reproductive resources, more fruitful foraging, 489 A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 and/or greater reproductive output if the males compete for access to females. Competition for resources is not likely to be the primary driver of the evolution of larger male claws in Asian Shore Crabs because males and females occupy similar intertidal habitat, and are presumably under similar re source constraints. Claw strength is correlated with diet; those species with the largest fraction of the diet comprising hard (i.e., shelled) prey have the strongest claws (Schenk and Wainwright 2001,Yamada and Boulding 1998). While claw strength is an important prerequisite for feeding on hard-shelled prey, the applied stress (i.e., force per area) determines prey shell-failure (Schenk and Wainwright 2001). Though apparently similar, the dentition and occlusal patterns of male and female Asian Shore Crabs and Green Crabs were not compared quantitatively in this study. Male and female Asian Shore Crabs appear to have similar dentition and diets, but females could not open clams, and males could (Brousseau et al. 2001). Larger Asian Shore Crabs also consumed mussels at greater rates than small crabs. Because smaller Asian Shore Crabs constitute the majority of the population (Kraemer et al. 2007) and are not strong enough to consume significant numbers of hard-shelled prey, most male and female Asian Shore Crabs rely on similar diet. The Asian Shore Crab male-female similarity in diet, and the similar habitats occupied by the two sexes suggest, therefore, that the sexual dimorphism in claw size derives from a male reproductive advantage accruing to those males possessing larger, stronger claws. This benefit should outweigh the material and energetic costs associated with the construction and maintenance of the more massive male structure (Lee 1995). Hartnoll (1974) suggested that intersexual size dimorphism begins to develop at sexual maturity. However, for Asian Shore Crabs, such dimorphism appears to be developed earlier since the positive allometric growth of the male chelae begins shortly after recruitment from the plankton (CWs as small as 5 mm), and sexual maturity, at least in the female, occurs at a minimum size of ≈12 mm CW (G.P. Kraemer, unpubl. data). Male and female Asian Shore Crabs differ in both average size and claw strength. Sexual dimorphism may also have anti-predator benefits. Bildstein et al. (1989) demonstrated an aversion by a bird predator to male crabs compared with females, and to clawed males compared with declawed males. The observed numerical dominance of male Asian Shore Crabs in the largest size classes (>26 mm) could be generated by a sex-based difference in susceptibility to predation, a possibility yet to be tested. The Asian Shore Crab was preadapted for successful establishment at the Rye, NY site through a combination of life-history characteristics (broad environmental tolerances, high reproductive output and dispersal). Strength and aggression have undoubtedly played roles in the disappearance of the Green Crab at the Rye site, but other factors were likely involved. Although the Asian Shore Crab is dominant, its population size relative to that of the Green Crab varies from 16:1 to 525:1 within Long Island Sound (Delaney et al. 2011). The A. Payne and G.P. Kraemer 2013 Northeastern Naturalist Vol. 20, No. 3 490 decline of the Green Crab may also have been influenced by the interspecific difference in time required to attain sexual maturity. The Asian Shore Crab produces its first brood of embryos at ≈12 mm CW when the crabs are less than seven months old (Epifanio et al 1998); Green Crabs must survive 2–3 years, depending on temperature, before maturing. Shelter influences survival, as it reduces both the stresses associated with intertidal emersion and predator efficacy and impact. Availability and use of substrate (Beck 2000) may differ for the invader and resident crabs in a way that may have contributed to the success of the Asian Shore Crab, though the paucity of residents make a test of this possibility a difficult and ethically tenuous proposition. Acknowledgments We are grateful for financial support from Lucille Werlinich and the School of Natural and Social Sciences (Purchase College). Field assistance was provided by many people, but in particular by Anthony Alves, Nicole Mazur, and Jessica Rassmusen. Susan Letcher conducted the ANCOVA evaluating regression slopes. The bulk of the work presented here represents the Senior Project research of A. Payne, in partial fulfillment of the requirements for the BA degree in Environmental Studies. Literature Cited Aukema, J.E., B. Leung, K. Kovacs, C. Chivers, K.O. Britton, J. 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