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@
paconserve.org.
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:
http://www.census.gov/), 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. The authors would like to thank all Montclair State students
that helped with the monthly collections of malacofauna, especially Gina Quinones. The
mapping portion of this project was completed by Eli Long, GIS Specialist for Western
Pennsylvania Conservancy. All biological sampling was carried out under a permit from
the County of Passaic, NJ, with special thanks to Raymond J. Wright, Jr., Director of the
Passaic County Parks Department. The manuscript was substantially improved by the
editorial suggestions of Rob Dillon and Tim Pearce. Mollusc samples have been accessed
into the collection of the New Jersey State Museum in Trenton, NJ, and the authors thank
Jason Schein and David Parris for their assistance.
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