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Temporal Variation in Molluscan Community Structure in an Urban New Jersey Pond
Eric J. Chapman, Robert S. Prezant, and Rebecca Shell

Northeastern Naturalist, Volume 19, Issue 3 (2012): 373–390

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2012 NORTHEASTERN NATURALIST 19(3):373–390 Temporal Variation in Molluscan Community Structure in an Urban New Jersey Pond Eric J. Chapman1, 2,*, Robert S. Prezant1, and Rebecca Shell1 Abstract - Barbour’s Pond is a 4.45-ha pond located in Garrett Mountain Reservation in Passaic County in northern New Jersey, one of the most densely populated regions in the United States. Despite its small size and surrounding urban sprawl, the shallow waters of this pond hold 18 species of molluscs. Monthly samples from March 2004 through March 2006 found the highest diversity in December 2004, and in January, June, and July 2005. Additional samples were taken in April 2007 and May 2010 to spot-check relative diversity years after the original sampling period. Total molluscan abundance was greatest in July and November 2004, possibly reflecting new late spring and autumn cohorts. Univariate statistics demonstrate that this pond has a temporally stable and diverse malacofauna. Analysis, of basic environmental parameters including temperature and pH, however, showed little correlation with molluscan diversity over time, underscoring the stable yet complex nature of biodiversity of this small urban pond. Introduction The potential ecological signifi cance of anthropogenic impacts on ponds has been an area of concern for decades (House and Fordham 1997, Schueler and Simpson 2001, Sriyaraj and Shutes 2001, Stoianov et al. 2000, Urban 2004, Zealand and Jeffries 2009), yet there have been few studies that followed discrete non-vertebrate populations over time. During this time, many natural ponds, particularly those in urbanized areas, have been backfi lled to create more land for housing and commercial needs (Gibbs 2000). In the late 1990s, Boothby and Hull (1997) reported that 61% of the 41,564 ponds reported in an ordinance survey of about 1870 in Cheshire, UK had vanished, with the greatest loss in urban areas. In parallel, small ponds have been created as reservoirs, farm ponds, and recreational habitats. These small freshwater lentic habitats, natural and created, can harbor complex communities (McNicol et al. 1987, 1995). Aside from the communities that reside within the ponds, many non-aquatic organisms also use ponds for food resources or for temporary sanctuary during migrations. The relative health of these environments and the concomitant health of the aquatic fauna in these complex systems are thus of critical importance well beyond the margins of the pond. A well-established body of literature suggests that small and isolated habitats (e.g., islands, ponds) would have relatively low species richness because of localized extinction events (vulnerability due to habitat size), quantitative 1College of Science and Mathematics, Department of Biology and Molecular Biology, Montclair State University, Montclair, NJ 07043. 2Western Pennsylvania Conservancy, Watershed Conservation Program, Indiana, PA 15701. *Corresponding author - echapman@ 374 Northeastern Naturalist Vol. 19, No. 3 limitations reflecting overall size of habitat, and challenges in inter-habitat recruitment (MacArthur and Wilson 1967). Indeed, Gledhill et al. (2008) confirm the negative effect of isolation on invertebrate and plant species richness in ponds, finding that the density of ponds within a localized area has a greater effect on within-pond biodiversity than does any other environmental parameter. Recent work, however, suggests that small habitats can enhance biodiversity in spite of the presumed perils associated with isolation and diminutive size (Scheffer et al. 2006). In fact, the habitat heterogeneity of ponds, in spite of overall size, can offer a mosaic of options for diverse communities (Zealand and Jeffries 2009). Urban ponds offer particularly interesting habitats in terms of relatively small sizes, relatively young ages (for created habitats), and perceived difficulties in recruiting in the face of possible localized extinctions due to anthropogenic contaminants and environmental disruptions and isolation. Thus, it might be assumed that small urban ponds harbor low species diversity. As this study will show, this was not the case for molluscs in Barbour’s Pond, a small urban pond in northern New Jersey. Many of our small ponds and wetlands remain in peril. The disappearance of habitat is occurring at an alarming rate and often with well-known consequences, including increased flooding, loss of biodiversity, and fewer “natural” habitats for human enjoyment (Fahrig 1997), especially in urban environments. Road runoff— including sedimentation from erosion, lawn-care products, and fertilizers— is often overlooked by the public as a potential problem for our aquatic systems and yet can impact large stretches of freshwater systems (Sriyaraj and Shutes 2001). Such run-off has been found to increase nitrogen load by as much as 45% (Wollheim et al. 2005). Climate change and the potential for greater localized precipitation could mean a growing issue of salt run-off during road de-icing in urban areas with dense networks of roadways. Schueler and Simpson (2001) argue that the unique issues associated with urban water systems warrant separation of these lentic systems into a distinct group, especially as related to water management. The current project focuses on the molluscan biota along the more exogenously vulnerable periphery of an urban pond, a faunal assemblage that could comprise an important portion of the food web for a variety of organisms including fi sh and birds, both of which are in turn heavily utilized by people for recreation (i.e., bird watching, fi shing, and hunting). The goals of this study were (1) to identify and quantify all species of molluscs found in the shallow waters of Barbour’s Pond, (2) to document the stability of these molluscan communities through time, (3) to examine possible correlations with community structure and measured environmental parameters, and (4) to consider the factors that could contribute to the overall diversity and stability, short and long term, of this urban freshwater pond. field Site Description Barbour’s Pond (40.9612°N 74.2287°W, elevation at center = 92.0 m), constructed in 1888, is fed by the Slippery Rock Brook and is the only lentic body of water found on Garrett Mountain Reservation in Passaic County, NJ. The pond is 2012 E.J. Chapman, R.S. Prezant, and R. Shell 375 located in the northeastern portion of New Jersey, approximately 20.0 km west of New York City in the Lower Passaic-Peckman River watershed. This watershed contains 9.0 ha of land, the majority of which has been modifi ed by development into a variety of land-use patterns (fig. 1). Percent land use was determined using 30-m resolution Landsat TM images (NOAA 2011). Land cover was classifi ed as developed (high, medium, and low intensity), forest (coniferous and deciduous), figure 1. Survey map with detailed land-use patterns at 30- x 30-m scale. 376 Northeastern Naturalist Vol. 19, No. 3 wetland, and other less common feature classes such as agricultural, barren land, or open water. The Lower Passaic-Peckman River watershed is a combination of high-density residential, commercial, and light industrial land uses, with over 60.0% of the watershed being classifi ed as developed. The city of Paterson, NJ is 1.0 km to the east, and has a declining population of just over 145,000 people (down about 5000 inhabitants since 2000) and a population density of 6667 people per square km (IDcide 2010), making it one of the most densely populated cities in the country. The Reservation lies less than 2.0 km to the east of the large Willowbrook Mall complex and less than 2.0 km to the west of the New Jersey Golf and Country Club. It hosts a variety of human activities with a focus on recreational usage (i.e., jogging, trail hiking, bird watching, fi shing, horseback riding, and frequent large crowds attending high school track and fi eld competitions). There is a paved road for vehicular traffi c that runs along the southern margin of the reservation within close proximity to the south side of Barbour’s Pond. The northern and eastern side of the pond is bordered by another paved public access road. The 229.9 ha that comprise Garrett Mountain Reservation are the only non-developed land in the area (fig. 1), except for an adjacent ≈40.5-ha tract of restricted-access reservoir property. Portions of the substratum in Barbour’s Pond have been modifi ed to increase recreational opportunities by adding large amounts of sand over the typical detritus and muck cover. Historically, a shallow, cemented area along the south rim was an ice-skating platform for winter recreational opportunities. Barbour’s Pond is created by an earthen gravity dam, Barbour’s Pond Dam, positioned along the southern edge of the pond. The dam allows a maximum discharge of 1206.0 cubic feet per second and retains a capacity of 202.0 acre feet. Typical water storage is 108.0 acre feet, draining an area of 155.4 ha (0.6 square miles). On the west side of the pond, there are solid rock formations that quickly drop off into deeper waters, which provide habitat for grazing hydrobiid snails. The northwest edge of the pond quickly transitions into a muddy bottom with a dense population of Nymphaea alba L. (European White Waterlily) and a small outflow creek. The north and northeast sides of the pond hold a dense array of Typha latifolia L. (Broadleaf Cattail) curving into a small (less than 1.0 m wide) inflow ephemeral stream. Substratum on the eastern side of the pond is a mix of sand and mud, often occupied by numerous resident Branta canadensis L. (Canada Goose) and Anas platyrhynchos L. (Mallard Duck). Methods field sampling Samples were taken from areas of Barbour’s Pond accessible through wading along the margins each month for 25 continuous months from March 2004 until March 2006. Sample sites were located along the northeastern border (muddy bank with mixed sand), the northern border along the exiting creek (muddy with Typha marsh habitat), and the northwestern border along the area of Nymphaea. Additional samples from these locations were taken in April 2007 and May 2010. The western rock faces (only accessible through diving) and southern shallow and highly impacted areas (cemented) were not sampled as part of the 2012 E.J. Chapman, R.S. Prezant, and R. Shell 377 continuous sampling effort. Sampling was initiated and completed during midafternoon hours during the fi rst week of each month. We sampled using dip nets with a 500-micron mesh size. To insure all habitats were fully sampled, we also hand-picked mollusks from fallen branches, rocks, and leaf debris for qualitative assessment. Two individuals sampled the same locations for one hour giving us a semi-quantitative sampling protocol that was repeated for the entire monitoring period of our survey. An integral part of our biological sampling regime focused on water-quality monitoring to aid in identifying potential trends in local water quality that could influence molluscan distribution, abundance, and/or diversity. Using hand-held meters, we measured pH (Oakton Pro 300), dissolved oxygen (YSI Model 85), and water and air temperature (YSI 85). During the April 2004 and May 2004 sampling period, the pH unit malfunctioned and was subsequently replaced by a similar hand-held unit (IQ150 system). This adjustment resulted in a data gap for those two months. All molluscs collected were promptly preserved in 70% ethanol (except for freshwater mussels which were identifi ed and immediately returned to their infaunal habitat). Specimens were identifi ed to species level with voucher specimens placed into the New Jersey State Museum. Data analyses Data were analyzed each month using univariate statistics including Margalef’s index (d), species richness (S), and Pielow’s evenness (J') and Shannon-Weiner diversity (H') indices. We employed the statistical program PRIMER (Version 5) to analyze Bray Curtis similarity indices for community structure and monthly occurrences over the 27 months sampled. Results Biological survey We collected a total of 18 species of molluscs from Barbour’s Pond (3216 individuals) from 10 families (Ancylidae, Hydrobiidae, Lymnaeidae, Physidae, Planorbidae, Pleuroceridae, Valvatidae, Viviparidae, Sphaeridae, Unionidae). Gastropod species outnumbered bivalves by a wide margin, with 14 species of gastropods recovered and only 4 species of bivalve, 2 of which were temporally common (Table 1). The six most frequently collected molluscs during the course of this study represented in excess of 88% of the total malacofauna found in Barbour’s Pond (Table 1). For the 27 months sampled, the fewest species recovered in a single sampling event was four in May 2004, while the greatest number collected in a single sampling was 12, which occurred six times (March and December 2004; January, June, and July 2005; and January 2006). The snails Helisoma trivolvis (Rams Horn Snail) and Amnicola limosa (Mud Amnicola) were the most common gastropods collected, albeit from different habitats. Helisoma trivolvis was most commonly found on soft sediments or on submerged twigs and logs; A. limosa, the most abundant species collected, was commonly found on hard substrata including submerged rocks, woody debris, and living aquatic angiosperms, and accounted for 754 of the 3216 total individuals collected or roughly 23% of the total molluscan fauna recovered. Pleurocera 378 Northeastern Naturalist Vol. 19, No. 3 virginica (Virginia Horn Snail), the third most common gastropod, had the widest cohort distribution year round, with small and large specimens regularly found in abundance just beneath a veneer of sandy mud or on submerged branches. Several species of mollusc were collected only rarely including Ferrissia fragilis (Fragile Ancylid), Lymnaea rustica, Gyraulus parvus (Modest Gyraulus), Micromenetus dilatatus (Bugle Sprite), and Pyganodon cataracta (Eastern Floater), each of which was found in less than 25% of the monthly samples. Bivalve occurrences were dominated by two species, both small sphaeriids, Pisidium casertanum (Ubiquitous Pea Clam) and Pisidium equilaterale (Round Pea Clam), which accounted for 91% of the bivalves collected in our 27-month survey. Water chemistry Three environmental parameters—pH, dissolved oxygen, and water temperature— were monitored throughout the duration of the sampling period. None of the parameters measured correlate with overall molluscan diversity (fig. 2). pH was quite variable in this pond and fluctuated from a low of 4.52 in December 2005 to a high of 9.47 in September 2005. The mean pH was 7.01 over the course of our sampling (Table 2). Overall, pH spikes (excessively high or low) had no discernable instantaneous effect on molluscs found in Barbour’s Pond. However, September 2005, with the highest pH recorded (9.47) during our study also showed one of the lowest diversity indices. Rather than reflecting a pH issue, this is most likely related to the very warm waters of the pond seen in August 2005. The next several months (pH of 6.78, 6.87, and 4.52, respectively) had no noticeable or significant change in species distributions for this pond (Table 3). Table 1. Occurrence of species found in Barbour’s Pond, NJ. Mo. = months collected out of 27 total surveyed, % = percent occurence, and n = number of individuals collected. Family Species Common name n Mo. % Hydrobiidae Amnicola limosa (Say) Ordinary Spire Snail 754 26 96.3 Viviparidae Bellamya chinensis (Gray) Chinese Mystery Snail 23 12 44.4 Viviparidae Campeloma decisum (Say) Brown Mystery Snail 15 6 22.2 Ancylidae Ferrissia fragilis (Tryon) Fragile Ancylid 10 3 11.1 Planorbidae Gyraulus deflectus (Say) Irregular Gyralus 32 7 25.9 Planorbidae Gyraulus parvus (Say) Modest Gyraulus 29 7 25.9 Planorbidae Helisoma trivolvis (Say) Ramshorn Snail 515 26 96.3 Lymnaeidae Lymnaea obrussa (Say) Golden Fossaria 56 11 40.7 Lymnaeidae Lymnaea rustica (I. Lea) No common name 3 2 7.4 Planorbidae Micromenetus dilatatus (Gould) Bugle Sprite 14 3 11.1 Sphaeridae Musculium partumeium (Say) Swamp fingernail Clam 100 19 70.4 Physidae Physa acuta (Draparnaud) Tadpole Snail; European Physa 258 24 88.9 Sphaeridae Pisidium equilaterale (Prime) Round Pea Clam 476 25 92.6 Sphaeridae Pisidium casertanum (Poli) Ubiquitous Pea Clam 545 24 88.9 Pleuroceridae Pleurocera virginica (Say) Virginia Horn Snail 290 24 88.9 Lymnaeidae Pseudosuccinea columella (Say) American Ear Snail 31 11 40.7 Unionidae Pyganodon cataracta (Say) Eastern Floater 1 1 3.7 Valvatidae Valvata tricarinata (Say) Three-keeled Valve Snail 64 16 59.3 2012 E.J. Chapman, R.S. Prezant, and R. Shell 379 figure 2. Environmental parameters show no relationship with any of the three diversity measures. d = Margalef’s index, H' = Shannon-Weiner diversity index, J' = Pielou’s evenness index. 380 Northeastern Naturalist Vol. 19, No. 3 Dissolved oxygen concentrations also varied considerably over the 27 months of monitoring, with values ranging from 2.10 mg/L in October 2005 to a high of 10.34 mg/L in April 2004 (Table 2). The general pattern found in Barbour’s Pond appears to be high values in spring when cool run-off from the small tributaries that feed the pond are greatest, and lowest values during the fall turnover of the pond. Dissolved oxygen values were relatively constant over the summer months of 2005 with values in the mid-4.00-mg/L range. Shallow-water temperatures fluctuated greatly in response to ambient air temperatures. In winter sampling, surface ice typically had to be broken in an effort to complete monthly sampling. Water temperatures were lowest in March, with temperatures ranging from 5.0 °C in 2004, 3.9 °C in 2005, and 3.7 °C in 2006. Overall temperatures ranged from a low of 3.7 °C in March 2006 to 38.8 °C in August 2005 (Table 2). Statistical measures Species counts appear to have a cyclical nature, with highest totals occurring in the summer and then again in winter months and with spring and fall having Table 2. Environmental parameters from Barbour’s Pond during March 2004–May 2010. * = Erroneous measurement, unit malfunctioned. pH DO (mg/L) Water temp (ºC) Mar-04 7.18 8.51 5.0 Apr-04 * 10.34 11.6 May-04 * 6.10 17.5 Jun-04 8.90 8.34 22.5 Jul-04 8.20 4.60 24.0 Aug-04 7.25 6.65 23.4 Sep-04 6.72 2.89 24.3 Oct-04 6.34 4.72 21.5 Nov-04 7.16 5.71 12.0 Dec-04 6.97 4.18 7.8 Jan-05 6.69 4.49 5.9 Feb-05 6.35 4.40 5.7 Mar-05 6.44 5.84 3.9 Apr-05 6.53 3.01 17.1 May-05 6.96 4.86 14.1 Jun-05 7.06 4.09 17.8 Jul-05 8.58 4.31 24.6 Aug-05 9.19 4.36 38.8 Sep-05 9.47 6.67 31.9 Oct-05 6.78 2.10 19.5 Nov-05 6.87 4.08 10.5 Dec-05 4.52 4.16 6.1 Jan-06 5.94 3.74 7.8 Feb-06 5.90 4.61 6.3 Mar-06 5.28 4.36 3.7 Apr-07 7.05 5.03 14.0 May-10 7.97 6.17 19.7 Mean 7.01 5.08 15.3 SE 0.25 0.37 1.89 2012 E.J. Chapman, R.S. Prezant, and R. Shell 381 comparatively lower diversity (fig. 3, Table 3). Margalef’s index values (d) were directly related to the number of species found in a given sample, with the lowest values occurring in months that had low numbers of both species and individuals recovered (Table 3). Evenness values (J') varied from a low of 0.698 in May 2010 to a high of 0.925 in September 2005. December 2004 and January 2005 had the highest Shannon-Weiner (H') values, respectively, followed by July 2005, June 2005, March 2004, and January 2006. Lowest diversity values during the two full years of study were found in May and June 2004, following a peak DO value of 10.34 mg/L in April 2004. Our most recent and temporally disconnected sample, taken in May 2010, showed the lowest diversity index to date, an H' of only 1.533. The April 2007 sample also had a low diversity index. Thus, of the six months holding the highest molluscan diversity, three were winter months, two summer months, and a single spring month. To identify possible changes in molluscan distributions through time in Barbour’s Pond, we analyzed monthly data with several similarity indices to examine both temporal community structure and species overlap. Data were Table 3. Univariate Statistics for Barbour’s Pond. S = species richness, n = total # of individuals recovered, d = Margalef’s index, J' = Pielou’s evenness index, and H' = Shannon-Wiener index. S n d J' H'(log2) Mar-04 12 129 2.263 0.870 3.119 Apr-04 7 43 1.595 0.918 2.578 May-04 4 23 0.957 0.902 1.805 Jun-04 6 72 1.169 0.750 1.940 Jul-04 10 222 1.666 0.781 2.595 Aug-04 10 133 1.840 0.813 2.702 Sep-04 6 92 1.106 0.877 2.266 Oct-04 8 156 1.386 0.755 2.266 Nov-04 10 258 1.621 0.766 2.543 Dec-04 12 114 2.323 0.896 3.212 Jan-05 12 126 2.274 0.914 3.277 Feb-05 7 37 1.662 0.767 2.153 Mar-05 7 39 1.638 0.763 2.141 Apr-05 10 113 1.904 0.887 2.946 May-05 9 182 1.537 0.904 2.867 Jun-05 12 167 2.149 0.891 3.194 Jul-05 12 98 2.399 0.904 3.242 Aug-05 9 87 1.791 0.850 2.694 Sep-05 5 83 0.905 0.925 2.148 Oct-05 10 173 1.746 0.793 2.634 Nov-05 7 96 1.315 0.867 2.434 Dec-05 11 96 2.191 0.839 2.903 Jan-06 12 108 2.349 0.844 3.026 Feb-06 10 74 2.091 0.832 2.765 Mar-06 10 80 2.054 0.843 2.801 Apr-07 10 218 1.671 0.732 1.686 May-10 9 195 1.517 0.698 1.533 Mean 9.15 119.04 1.745 0.836 2.573 SE 0.45 11.63 0.083 0.013 0.095 382 Northeastern Naturalist Vol. 19, No. 3 presented in a presence/absence (non-weighted) fashion to remove the bias of dominance of a single species in the data set (Clarke and Warwick 2001). Temporal community structure analysis results showed little support for a broad-scale seasonal grouping (fig. 4). The only two strong seasonal clusters were Nov 2004 and Oct 2005, which compared at 100% similarity, and a grouping of winter/spring months (Feb 2005, April 2004, and March 2005) that grouped together at 85.71%. All other comparisons for a seasonal component yielded little in terms of a positive relationship (i.e., high % similarity). figure 3. Temporal changes in molluscan diversity over time at Barbour’s Pond for the full consecutive 25 months plus the disjunct April 2007 and May 2010 samples. S =number of specimens recovered, n = number of species recovered, H' = Shannon-Weiner diversity index, J' = Pielou’s evenness index, d = Margalef’s index. 2012 E.J. Chapman, R.S. Prezant, and R. Shell 383 Individual species distributions, on the other hand, revealed probable assemblages for this pond (fig. 5). A group of six common species, two congener bivalves and four gastropods, was found to be associated together 91.23% of the time: Amnicola limosa, Helisoma trivolvis, Pleurocera virginica, Pisidium casertanum, Pisidium equilaterale, and Physa acuta (Tadpole Snail or European Physa). figure 4. Bray-Curtis similarity indices on presence/absence transformed data showing temporal community structure and clusters over time. figure 5. Bray-Curtis similarity dendrogram on presence/absence transformed data showing dominant molluscan species assemblage and taxonomic cluster of six species (in rectangle). 384 Northeastern Naturalist Vol. 19, No. 3 Discussion Schueler and Simpson (2001) define urban lakes as satisfying a series of “operational criteria” that include small size, shallow depth, strong influence of watershed, impervious cover (i.e., development), management strategy geared for recreation, and unique hydrology. Pond habitats have a variety of roles in an urban landscape. Ponds are frequently the focal point of parks (Gledhill et al. 2008) and are often heavily used for recreational activities. Parks, associated ponds, and open spaces also have significance in human health and well being (Lees and Evans 2003). Urban parks specifically are considered as strong links in bringing nature back to children (Johnson and Hurley 2002). With their growing importance in our increasingly urbanized environments and as more Americans are moving into cities (2010 US Census:, the likelihood of growing negative impacts on these ponds is also enhanced. Monthly surveys over two contiguous years and disjointed post-surveys have allowed for a better understanding of molluscan communities, distributions, and population fluctuations with regard to temporal structure and physical parameters in Barbour’s Pond, an urban pond that fi ts Schueler and Simpson’s defi nition well. No obvious defi nitive temporal trends surface from this study except that both winter and summer months show relatively high diversity (Table 3). There was a temperature spike in August 2005 followed by a total count of fi ve species recovered the following month (Tables 2, 3). Those recovered represent the most abundant and, perhaps correlatively, hardiest species found in this pond, including Amnicola limosa, Helisoma trivolvis, Pleurocera virginica, Physa acuta, and Pisidium casertanum. It should be noted that the taxonomy of several groups of freshwater molluscs remains in flux and is evolving much more rapidly than the taxa themselves. Small sphaeriid bivalves are often diffi cult to defi nitively identify to species, not just because of their small size but also due to shell variability, allometric variability with growth, variation in characteristics used in nomenclature, and ongoing arguments in the literature that continue to seek resolution over specifi c taxa (Bailey et al. 1983, Dyduch-Falniowska 1983, Holopainen and Kuiper 1982, Korniushin 1998, Lee and Ó Foighil 2003). This taxonomic challenge is also true for the gastropods. We have found two seemingly distinct morphs of Lymnaea (Lymnaea [Fossaria] obrussa and L. rustica). Dillon et al. (2006), however, notes that, because of this family’s flexibility in shell morphology, most of the nominal species within this genus have been assigned to Lymnaea humilis by Hubendick (1951). Overall effects of temperature are often diffi cult to ascertain from a shortterm survey, and we do not have a quantitative evaluation of the communities that could reside in deeper water refuges with more stable temperatures. The results observed here suggest that temperatures over 35 °C could be fatal for some molluscs in the shallow waters of this pond. How often temperatures of this magnitude occur and the relative vulnerability of various species to thermal stress and associated anoxic conditions need to be examined more widely before defi nitive predictions can be made relating water temperature to biodiversity of freshwater 2012 E.J. Chapman, R.S. Prezant, and R. Shell 385 molluscs. As early as 1930, Boycott (1930) defi ned habitats in England that would have diverse molluscan communities as having clean, slow-flowing water, a modicum of submerged angiosperms, and relatively warm waters. Zealand and Jeffries (2009) note that water hardness and conductivity appear most signifi cant in distribution of snail communities; however, in their study of 52 ponds in northern England, they found six ponds absent of snails with “no consistent character to the ponds lacking snails”. The ponds in the latter study had a mean species richness of 2.44, a lower diversity than those found in other European ponds (see Brönmark 1985, Costil 1994, Costil et al. 2001, Pip 1986). The six Barbour’s Pond molluscan species found almost every one of the 27 months sampled include A. limosa (26 months), H. trivolvis (26 months), P. virginica (24 months), P. equilaterale (25 months), P. casertanum (24 months) and P. acuta (24 months) and form a temporal cluster. The molluscan species in this pond seem mostly to be not limited by seasonal fluctuations but are dominated by several resilient taxa with rarer species found intermittently. This heterogeneity of relative abundance could reflect migrations deeper into non-sampled portions of the pond (Amyot and Downing 1997), deeper burial into the substratum (beneath the upper 4–5 cm sampled), stochastic immigration from other nearby ponds, or a reflection of a breeding cycle that reduces the representation of a population to fertilized, over-wintering eggs (which were not sampled in this survey) for part of the year. For over 35 years, MacArthur and Wilson’s (1967) concept of island biogeography has been used to variously explain diversity on islands and in isolated lakes and ponds. This concept suggests that the ultimate number of taxa found on an island (isolated habitat) would be the result of an equilibrium between immigration of new species and extinction. Lassen (1975) found evidence of this equilibrium in a meta-analysis of 86 lakes and ponds in Denmark, both in eutrophic and oligotrophic systems. He noted that oligotrophic lakes had steeper species-area curves likely correlated with higher extinction rates. Similarly, small ponds run higher risks of faunal extinctions, including short-term species losses that are balanced through eventual immigration. Immigration into Barbour’s Pond is poorly understood. Molluscan immigration into a relatively isolated pond could be achieved via several routes: flotation during flood periods (an unlikely event for Barbour’s Pond as it is located atop Garrett Mountain); aerial dispersal from attachment (byssal or mucus) to bird feathers or scales or by passing unharmed through vertebrate digestive systems (see review of both topics in Green and figuerola 2005); import via human activity; or migration from an adjacent lake or pond via stream. A single stream flows into Barbour’s Pond from the northeast corner. This stream is very narrow and sluggish, only flowing with any speed or strength after a major rain event. The outflow, at the northwest corner of the lake has a more substantial and constant flow, delivering water to the New Street Reservoir and on to the Passaic River. Nevertheless, Lewis and Magnuson (2001) have clearly determined that gastropods use streams as “dispersal corridors” and that diversity will increase in isolated highland lakes that are stream-connected to other lakes; these feeder and outflow streams may serve as a dispersal corridor. 386 Northeastern Naturalist Vol. 19, No. 3 With signifi cant rain storms, there could be a temporary, enhanced molluscan biodiversity, including less tolerant species (for this particular pond), that dissipates over short periods of time. Of the 18 total species collected, only one is an invasive, Bellamya chinensis (Gray) (Chinese Mystery Snail). There are numerous other non-indigenous molluscan species affecting overall conservation goals and species distributions within New Jersey, most notably Corbicula fluminea (Müller) (Asiatic clam) (NJDEP 2008), which is commonly found in surrounding lotic and lentic systems (Prezant and Chapman 2006, Vazquez and Perera 2010). Considering the dispersal properties of juvenile C. fluminea (Prezant and Chalermwat 1984), and the presence of inflow and outflow streams plus regular visits by fi shermen who often use fi eld-collected clams as bait, it is likely only a matter of time before this invasive bivalve fi nds its way into Barbour’s Pond. Barbour’s Pond is regularly stocked with a number of different fi sh species for recreational purposes, several of which are known to be glochidial hosts for New Jersey unionids. Oncorhynchus mykiss (Walbaum) (Rainbow Trout), a known host of Margaritifera margaritifera (L.) (Eastern-River Pearl Mussel; Cordeiro and Bowers-Altman 2003), are stocked in November for public angling opportunities; other fi sh recorded from the pond include Ictalurus punctatus (Rafi nesque) (Channel Catfi sh), a host of Utterbackia imbecilis (Say) (Paper Pondshell; Rogers- Lowery and Dimock 2006), and Lepomis gibbous L. (Pumpkinseed), a known host of Anodonta implicata (Say) (Alewife Floater) and Pyganodon cataracta among others (Cordeiro and Bowers-Altman 2003). Pyganodon cataracta was found in one sample taken for this study and has also been noted on other occasions by the authors. Given the frequent stocking of host fi shes into Barbour’s Pond and the large area of lake bed not surveyed to date, it is possible that small populations of other of these unionids will establish themselves, or indeed have already been established. Though the location of Barbour’s Pond within a county park, which is essentially a green oasis surrounded by highly developed properties, should provide protection from common forms of anthropogenic perturbations seen in urbanized areas (i.e., draining, back fi lling, and cut-off from recharging sources of water), we suspect the pervasiveness of other urban effects in part influences pond pH and dissolved oxygen levels. Given this anthropogenic influence, the malacofaunal richness identifi ed was not expected. Similar surveys of the malacofauna of West Point Military Academy in nearby New York State (Prezant and Chapman 2004) revealed considerably lower molluscan diversity in comparable lentic bodies, as did work on larger lakes in Denmark which contained an equal or lower number of gastropods (Lassen 1975). A total of only 18 species of gastropods were found in the 115 southern Swedish ponds sampled by Brönmark (1985), with a maximum richness of 14 species and an average of just under 8 species. In the UK, Zealand and Jeffries (2009) recovered only 1–3 species of snails from 46 of 52 ponds they sampled, with a maximum number of 7. The latter authors found no correlation of species distributions with conductivity and only a very weak negative correlation with pH. Importantly, they found that similarity of gastropod communities decreased with distance between ponds. 2012 E.J. Chapman, R.S. Prezant, and R. Shell 387 And so, what is driving the high molluscan diversity in this small urban pond in New Jersey? Reports by Brown and Lomolino (2000), Whittaker (2000), and Scheffer et al. (2006) are beginning to show that the concept of island biogeography from MacArthur and Wilson’s pivotal work (1967) may be over-extended. In fact, small isolated habitats, even those degraded by human environmental impacts, can harbor high species richness. These phenomena could reflect localized microhabitat diversity, biological control over species richness, or small regional interspecifi c (biological) regulation of communities (Urban 2004). Specifi c to ponds, clusters of several small ponds house greater numbers of species than solitary ponds of larger size (Oertli et al. 2002). Because ponds are vulnerable to rapidly shifting environmental features both seasonally and across years, there can be heterogeneity of local microhabitats based not just on physical structure (both manmade and natural in ponds within parks), but on longer-term temporal structure (including, in urban ponds, changes induced as parks are modifi ed to suit human needs). It would be assumed that many of the taxa in these communities are “generalists”, conserving traits that allow them to tolerate a range of rapidly shifting environmental parameters. Variability across differing environmental landscapes within the pond can be reflected in broad (across the pond) diversity and shifting temporal populations. This variability reflects the mosaic of habitats within the pond at any one moment in time as well as across time and, in turn, the possibility of a diverse biota with a relatively stable sub-community of a few taxa across time. The influence of biological accommodation within this broad community, including predator “influence”, is yet to be examined but will certainly play an important role in defi ning fi nal habitat diversity. Acknowledgments This research would not have been possible without the generous fi nancial support of Montclair State University. 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