Differential Effects of Urbanization and Non-natives on Imperiled Stream Species
Scott A. Stranko, Susan E. Gresens, Ronald J. Klauda, Jay V. Kilian, Patrick J. Ciccotto, Matthew J. Ashton, and Andrew J. Becker
Northeastern Naturalist, Volume 17, Issue 4 (2010): 593–614
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2010 NORTHEASTERN NATURALIST 17(4):593–614
Differential Effects of Urbanization and Non-natives on
Imperiled Stream Species
Scott A. Stranko1,*, Susan E. Gresens2, Ronald J. Klauda1, Jay V. Kilian1,
Patrick J. Ciccotto1, Matthew J. Ashton1, and Andrew J. Becker1
Abstract - The distribution of imperiled stream fish, crayfish, salamander, and freshwater
mussel species of Maryland streams in relation to urban land cover and nonnative
species was investigated. Over the last 30 years, extinction or extirpation of 13
stream animal species (including the endemic Etheostoma sellare [Maryland Darter])
was observed within the Piedmont region of Maryland, where urbanization has spread
extensively outward from Baltimore and Washington, DC, and many non-native
species have become established. The presence of imperiled species in this area was
correlated with urbanization and non-native species occurrence. However, correlations
with land-cover data were stronger than with non-native occurrence. The majority of
sites with imperiled species contained less than 10% urban land cover and less than 5%
impervious land cover in their catchments. In contrast, stream reaches with non-native
species spanned the entire gradient of urban, agriculture, and forested land cover, with
the majority of non-native species persisting in streams with over 60% urban and 40%
impervious land cover. The persistence of rare species coincident with introduced species
in rural portions of Maryland indicates that habitat degradation (like that typically
accompanying urbanization) may be the most important threat limiting the distributions
of the rarest species that remain in these streams. Limits on urbanization in areas
with rare species are needed to maintain regional and global biological diversity. This
is particularly important in areas like Maryland that are anticipating extensive human
population and urban growth over the next 30 years.
Introduction
Rates of extinction and imperilment tend to be considerably higher for
aquatic taxa than for terrestrial taxa (Allan and Flecker 1993, Palmer et al.
2000, Ricciardi et al. 1998). In Maryland, for example, 88% (14 of 16) of
freshwater mussels and 41% (29 of 71) of native fish species are imperiled
(included on the state list of rare, threatened, and endangered species;
Maryland Department of Natural Resources 2007). Nearly 10% (6 of 71) of
Maryland’s native freshwater fishes are presumed extirpated or extinct, including
Maryland’s only endemic vertebrate, Etheostoma sellare (Radcliffe
and Welsh) (Maryland Darter) (Helfman 2007, Jelks et al. 2008, Raesly
1991). Providing adequate protection for extant imperiled species is crucial
to ensure the conservation of current stream biological diversity.
Landscape-level diversity (beta diversity; i.e., the differences in community
composition between different sites), is an important component
1Maryland Department of Natural Resources, 580 Taylor Avenue, C-2, Annapolis,
MD 21401-2352. 2Department of Biological Sciences, Towson University, 8000 York
Road, Towson, MD 21252. *Corresponding author - sstranko@dnr.state.md.us.
594 Northeastern Naturalist Vol. 17, No. 4
of regional biodiversity. Loss of this landscape-level diversity is being observed
in many terrestrial and aquatic habitats, and is referred to as biotic
homogenization (McKinney 2005; Rahel 2000, 2002; Taylor 2004). Human
activities cause biotic homogenization by increasing both the dispersal of
foreign species and local extirpation of native species. Although the impacts
of biotic homogenization on richness (alpha diversity) of a local assemblage
may be positive, negative, or neutral, the impacts serve to increase the
similarity of faunas across the landscape. Environmental stress and humaninduced
habitat degradation allow populations of tolerant native species to
expand (Rahel 2002) and favor proliferation of non-native species (Baltz and
Moyle 1993, Blair 2001, Byers 2002, Dukes and Mooney 2000, Dunham et
al. 2003, Knutson et al. 1999, Limburg and Schmidt 1990, Morley and Karr
2002). Faunas can also become less similar if different species are introduced
into separate locales, or species common to two regions are lost from
one region and not the other.
There are many potential mechanisms for species losses from one region
or another. Habitat, hydrologic, and chemical degradation that accompany
urbanization and biotic interactions with non-native species have been major
causes of aquatic species extirpation from regions of the United States. The
decline of 91–94% of imperiled fish species in the United States has been
attributed to habitat degradation, while nonnative fish interactions may have
affected 53 to 70% (Lassuy 1995, Reed and Czech 2005, Wilcove et al.
1998). The high rate of imperilment among freshwater mussels (Ricciardi
and Rasmussen 1999) has been linked to poor land-use practices, habitat
and flow alteration, and invasive species (Bogan 1993, Brim Box and Mossa
1999, Ricciardi et al. 1998, Strayer 1999, Watters 2000). Poor land-use
practices and habitat degradation have also contributed to declines in stream
salamanders (Rohr et al. 2004, Southerland and Stranko 2008, Willson and
Dorcas 2003) and crayfishes (Taylor et al. 1996, 2007), but invasive species
may represent the most important threat to native crayfish diversity (Capelli
1982; Capelli and Munjal 1982; Holdich 1988; Perry et al. 2001, 2002;
Taylor et al. 1996, 2007). For all stream-dwelling taxa, habitat degradation
likely exacerbates the negative influence non-native species have on native
species (Moyle and Williams 1990). However, the relative importance and
differential effects of these stressors towards extirpations and imperilment
of aquatic fauna, in a regional context, are currently not well understood.
Despite the importance of imperiled (rare, threatened, and endangered)
fauna to regional biodiversity, rare species are often excluded in examinations
of broad landscape-scale alterations on stream quality due to the
paucity of records, which hampers rigorous statistical analyses. The deletion
of “rare” species is considered a legitimate “ecological transformation”
(McCune and Grace 2002) to prepare data for multivariate analyses whose
goal is data reduction, i.e., representation of taxa-rich community data in a
smaller number of synthetic axes. In such cases, the questions of interest
2010 S.A. Stranko et al. 595
focus on how the composition of groups of species may respond to environmental
factors, and thus rare species provide little information and add
more “noise” to the community response. In contrast, hypotheses regarding
richness and diversity emphasize rare species as much as common ones,
and deletion of any species would be inappropriate. Although species richness
metrics for a local habitat give equal weight to both rare and common
species, regionally rare and imperiled species do not contribute any extra
weight to total richness indices, even though they are much more important
for conservation than common, widespread species. The dearth of data and
distribution records for rare and endangered stream-dwelling species further
contributes to the problem. Data from the Maryland Department of Natural
Resources’ Maryland Biological Stream Survey (MBSS) provide a unique
opportunity to address this issue, with a large number of records for many
imperiled and introduced fish, amphibian, crayfish, and mussel species that
can be used to examine the impacts of landscape alteration on these taxa. The
purpose of this paper is to use the MBSS data set to describe differential patterns
of imperiled and non-native species distributions as they relate to land
cover and non-native species in Maryland’s Atlantic slope. As a case study,
we also examined patterns of land use and non-native species introductions
within watersheds coincident with apparent Maryland Darter extinction. We
hypothesize that extirpation/imperiled status of native species has a stronger
correlation with urbanization than with the introduction of non-native species.
In addition to testing this hypothesis, we document the degree to which
landscape alteration is correlated with differences in rare and introduced
species’ distributions in Maryland.
Methods
We used fish, crayfish, salamander, and freshwater mussel records from
all 2740 sites selected via stratified random sampling of first- through fourthorder
stream reaches by the MBSS during 1994–2007 east of the Appalachian
Mountains. The Ohio River drainage portion of Maryland, west of the Appalachian
Mountains, was not included because of major zoogeographical
differences between the Ohio drainage to the west and Atlantic Slope drainages
to the east (Hocutt and Wiley 1986). Data were collected using standard
MBSS protocols. A detailed explanation of MBSS sampling protocols can
be found in Stranko et al. (2007). In brief, backpack electrofishing, with two
passes in each 75-m-long site, was used to collect fishes, crayfishes, and
stream salamanders. Visual encounter surveys within the 75-m site for at
least 15 minutes were used to collect freshwater mussels and to supplement
the salamander and crayfish electrofishing catch. These data were used to
compile current Maryland stream assemblages by site and physiographic
region. The percent of forest, agriculture, and urban land-cover data from
the 2001 National Land Cover Data-set (NLCD; Homer et al. 2007) were
extracted for catchments upstream of each site, which were drawn by hand
using digital USGS 7.5-minute topographic quadrangle maps.
596 Northeastern Naturalist Vol. 17, No. 4
Extensive literature reviews (Cooper 1983; Harris 1975; Jenkins and
Burkhead 1994; Lee et al. 1976, 1980, 1981; Merideth and Schwartz 1960;
Ortmann 1909, 1919; Schwartz et al. 1963) were used to reconstruct historical
(before European settlement of North America) fish, crayfish, salamander,
and mussel assemblages in Maryland’s physiographic provinces east of
the Appalachian Mountains. Imperiled species are those on Maryland’s list
of rare, threatened, and endangered animals (Maryland Department of Natural
Resources 2007).
The relative contribution of species extirpations versus introductions
to biotic homogenization was estimated separately for three physiographic
regions: Coastal Plain, Piedmont, and Highland (Fig. 1). Data from the
Appalachian Plateau, Ridge and Valley, and Blue Ridge physiographic provinces
were combined to make the “Highland” region based on Southerland et
al. (2006), who found these three component provinces to be ecologically similar.
We then estimated the change in similarity of species assemblages that has
occurred among these regions due to species extirpations and introductions,
using the approach of Rahel (2000). This method consisted of first comparing
the current assemblages of the three regions using Jaccard’s similarity index.
We then performed the comparison again without non-native species included
and with species presumed to be extirpated added into the assemblage. We
also compared the relative contribution of introductions and extirpations to
the current similarity between regions by calculating Jaccard’s index with a)
only extirpated species added or b) only non-native species removed.
Spatial association of imperiled and introduced species with catchment
land cover was determined for the Piedmont physiographic province using
Figure 1. Map of the study area showing the locations of biological sampling sites
and the three physiographic regions that were compared to examine potential biotic
homogenization of streams, by region, in Maryland.
2010 S.A. Stranko et al. 597
data from 846 MBSS sites. We focused on the Piedmont because it was the
only one of the three regions with a substantial amount (≥15%) of each major
land-cover type (forest, agriculture, and urban). A total of 160 sites had a
record of at least one imperiled species, whereas 581 sites had at least one
non-native species.
Because most of these sites included more than one type of land use,
and land-use types may be spatially correlated on a local scale, we treated a
catchment land-cover datum as a multivariate observation. Thus, our analyses
sought to detect shifts in the suite of land-use types associated with the
presence or absence of imperiled species. Multivariate analyses were conducted
using PC-ORD 5.1 software (McCune and Mefford 2006).
We used two approaches to analyze the circumstantial evidence provided
by distributions of imperiled and introduced species. Initially, we defined two
groups of watersheds, based on either the presence or absence of imperiled
species. We tested the hypothesis of no difference in the suite of watershed
land uses between these two predefined groups using a multi-response
permutation procedure (MRPP), which is a non-parametric test of differences
between two groups of multivariate observations (in this case, groups
of stream sites). The significance of the test is determined by repeated random
permutations of the data to yield a distribution of the test statistic under the
conditions of the null hypothesis. The advantages of the MRPP test is that
it does not require normally distributed data, nor homogeneity of variance
within groups (McCune and Mefford 2006).
We used an ordination technique, non-metric multidimensional scaling
(NMS), to provide a graphical representation of catchment land cover
for each stream, and to determine what shifts in land-use categories were
responsible for the results of the MRPP test comparing streams with and
without imperiled species.
In our second approach, we compared associations between imperiled
species and introduced species directly, using a contingency test (Zar 1999).
The hypothesis of independent distributions of imperiled and introduced
species across 846 Piedmont sites was tested using a “category 1” double
dichotomy contingency test, which assumes a random sample of stream
reaches in regards to presence of both imperiled and introduced species. We
were concerned, however, that spatial autocorrelation in the distribution of
these species within stream drainage networks and geographically in relation
to urban areas could bias the interpretation of this test. Therefore, we also
conducted a more conservative set of analyses using the Mantel’s statistic
for correlation between two matrices (Mantel 1967). The Mantel’s tests were
used to examine correlations in the distribution of these species groups,
while accounting for possible non-independence of sites due to geographical
proximity, drainage networks, or to spatial autocorrelation of habitat
features. Thus, we tested three null hypotheses of no correlation: 1) between
imperiled species and land use, 2) between non-native species and land use,
and 3) between the occurrence of imperiled species and non-native species.
598 Northeastern Naturalist Vol. 17, No. 4
Randomization tests (based on 1000 random permutations) were used to
establish the significance of each test.
We used a case study to determine if landscape-alteration thresholds correlated
with a presumed extinction were similar to thresholds correlated with
the presences and absences of imperiled species in the MBSS dataset. This
study involved the Maryland Darter and the two Maryland Piedmont watersheds
where it was known to occur. These two watersheds are Deer Creek
(37,700 ha) and Swan Creek (6820 ha). The Maryland Darter was found
only in these two Maryland watersheds and is presumed extinct because the
last record for the species was from 1988 (Raesly 1991). Land-cover data
were available from the Maryland Department of Planning for three years
spanning three decades: 1973, 1994, and 2000. Land-cover types in this data
set were digitized from aerial photographs and satellite images and urban
land use was verified using tax data from the appropriate time period. The
minimum mapping unit was 4 ha, meaning that a unique land-cover type
within a larger type must be at least 4 ha to be digitized separately. Presence
or absence of the Maryland Darter during each of these years was estimated
based on extensive surveys of historical habitats (Neely et al. 2003; Raesly
1991, 1992; US Fish and Wildlife Service 1985, 2007).
Results
Compared to historical estimates, total species and species within three
of the four taxonomic categories increased in number or stayed the same for
each physiographic region of Maryland (Appendix A). The only exception
was salamanders, for which richness decreased or stayed the same (Table 1).
Extirpations of species were highest (13) in the Piedmont and lowest (none) in
Highland streams. Twenty non-native species were found in Highland, 19 in
the Piedmont, and 13 in the Coastal Plain. Seventeen of the nineteen species
introduced to the Piedmont were also introduced in the Highland. Introductions
were dominated by fish species (19 of the 24 total non-native species).
Introductions of non-native species had a consistent homogenization effect
by increasing similarity among regions compared to historical estimates
(Fig. 2). Introductions also contributed more than extirpations to changes
in faunal similarity. The extirpation of different native species from the
Piedmont and the Coastal Plain resulted in greater dissimilarity rather than
homogenization. This decrease in similarity offset the five percent increase
in similarity caused by the same species having been introduced into the two
regions. The largest increase in overall taxonomic similarity (10%) occurred
between the Piedmont and Highlands (from 67% to 77% similarity). The increase
in similarity between the historically least similar regions (Highlands
and Coastal Plain) was also relatively large (37% to 44%).
Sixteen percent of sites with virtually no urban land cover (less than 1%; n =
266), contained at least one imperiled species (Fig. 3). Imperiled species
were found at only three sites with more than 20% urban land cover (n = 133)
and at no sites with more than 25% urban land cover (n = 126). The majority
2010 S.A. Stranko et al. 599
Table 1. Number of fish, mussel, crayfish, and salamander species in Maryland's Atlantic drainage
by physiographic region. Historical refers to species present prior to European settlement.
Number of species
Region Taxa Historical Current Introduced Extirpated
Highland
Fishes 41 58 17 0
Mussels 7 8 1 0
Crayfishes 3 5 2 0
Salamanders 7 7 0 0
Total 58 78 20 0
Piedmont
Fishes 54 63 16 7
Mussels 13 11 1 3
Crayfishes 5 7 2 0
Salamanders 7 4 0 3
Total 78 84 19 13
Coastal Plain
Fishes 53 61 10 2
Mussels 14 14 0 0
Crayfishes 5 7 2 0
Salamanders 5 4 0 1
Total 77 86 13 3
All regions
Fishes 74 88 17 3
Mussels 16 17 1 0
Crayfishes 8 12 4 0
Salamanders 10 8 0 2
Total 108 125 22 5
Figure 2. Change in the total proportion of similarity (Jaccard’s) from historical to
current biological assemblages in streams between three Maryland regions: Coastal
Plain (cp), Piedmont (pied), and Highlands (high), as well as the proportion of similarity
change attributable to extirpations and introductions.
600 Northeastern Naturalist Vol. 17, No. 4
of imperiled species (8 of 12) were found only at sites with less than 10%
urban land cover. Consistent with the lack of correlation between land cover
and non-native species, non-natives were found at about half (56%) of the
sites with more than 20% urban land cover. Nearly three quarters (14 of 19)
non-native Piedmont species were still found in heavily urbanized (≥60%
urban cover; n = 43 sites) Piedmont streams. Nearly half (48%) of the sites
with imperiled species also contained at least one non-native species. The
MRPP test showed significantly different catchment land-cover types for
streams inhabited by imperiled species vs. streams where imperiled species
were absent (P < 0.0001). The affect size (A = 0.014) indicates that the magnitude
of land-use difference was small. If the two groups of streams were
internally homogeneous in relation to land use—and thus quite distinctive—
the value of A would approach 1. In case of a random pattern of land use
across groups, A would be near zero, and could take lower values in the case
of extreme heterogeneity across groups (McCune and Grace 2002).
Figure 3. Cumulative proportion of stream sites in five land-use categories with imperiled
(a) and non-native (b) species represented with bars, along with numbers of
species shown with black diamonds. Sample sizes within each land-cover category
are: less than 1% = 266, less than 5% = 586, less than 10% = 666, less than 20% = 714; >20% = 133.
2010 S.A. Stranko et al. 601
The NMS ordination clearly arranged the stream sites according to the
degree of catchment covered by forest, agricultural, and urban land use
(Figs. 4, 5). Imperiled species were only collected from stream sites with
low (less than 25% land cover) urbanization (Fig. 4), whereas sites with non-native
species spanned the entire gradients of urban, agriculture, and forested land
cover (Fig. 5). The two ordination axes, representing major patterns of
variation in land cover, were interpreted using correlations of the original
land-cover categories with the stream score on a given axis. Axis 1 depicts
streams along a gradient from mostly agricultural (correlation coefficient r =
-0.91; low scores) to mostly urban land use (r = 0.90; high scores). Axis 2
portrays streams along a second gradient from agriculture (r = -0.50) to forested
land cover (r = 0.97). Overlay of symbols for the presence of imperiled
species (Fig. 4) on the ordination scores emphasizes that imperiled species
were collected only from streams with low catchment urbanization.
When spatial autocorrelation of streams was not taken into account, a
contingency test of independence of the co-occurrence of imperiled and
introduced species indicated that the distribution, i.e., presence or absence, of
Figure 4. Nonmetric multidimensional scaling (NMS) of urban, agriculture, and
forested land cover from the National Land Cover Dataset (NLCD). Filled circles
represent stream sites with imperiled species.
602 Northeastern Naturalist Vol. 17, No. 4
imperiled and introduced species were independent of each other (with Yates
correction χ2= 0.634; 0.25 < α < 0.50). In contrast, the Mantel’s test, which
does account for autocorrelation among sites, gave a significant, but low,
negative correlation between the presence of imperiled and introduced species
(r = -0.07, P < .001). Consistent with the results of the MRPP test on land-use
differences, the Mantel’s test gave a significant positive correlation of the
presence of imperiled species and the suite of catchment land-cover types (r =
0.14, P < 0.001). Land cover was not significantly correlated with the presence
of non-native species according to Mantel’s test (r = 0.02, P = 0.13).
The last record for the Maryland Darter in the Deer Creek watershed was
from 1988 and the last record from the Swan Creek watershed was from 1965.
Land-use data from the Maryland Department of Planning for 1973, 1994,
and 2000 indicate that urbanization in the Deer Creek and Swan Creek watersheds
increased over time, and replaced forested and agricultural land uses
(Table 2). Deer Creek land use from 1973 provided the only land-use data for
the period when the Maryland Darter was still known to occur in one of the
two watersheds (Deer Creek). This was also the only year for either watershed
with land-use data showing the urbanization to have been less than 10%. As
Figure 5. Nonmetric multidimensional scaling (NMS) of urban, agriculture, and
forested land cover from the National Land Cover Dataset (NLCD). Filled circles
represent stream sites with non-native species.
2010 S.A. Stranko et al. 603
described above, most imperiled species are currently limited to catchments
with less than 10% urbanization. While urban land use increased over the period
when the Maryland Darter disappeared, all of the non-native species that
we found in the Deer Creek (n = 58 sites) and Swan Creek (n = 11 sites) watersheds
during 1994–2007 had been initially introduced over 50 years (Lee et al.
1976) prior to the species’ presumed extinction.
Discussion
Non-native species, urbanization, and many other factors contribute
to the decline and loss of native species. Our findings illustrate, however,
that many rare, threatened, and endangered species persist in portions of
Maryland together with non-native species, but none occur in urban areas.
By comparing statistical analyses with and without accounting for spatial
autocorrelation, we acknowledge the presence of multiple confounding environmental
factors. Thus, we emphasize that we are fully aware that one cannot
prove anything using distributional data; however, one can build a case with
circumstantial evidence—part of the “strength of evidence” approach used
in stressor identification by the Environmental Protection Agency (Cormier
et al. 2002, 2003). We do not intend to definitively conclude that the cause
for the spatial and temporal patterns of species distribution and extirpation
that we report can be solely attributed to urbanization, or that non-native
species did not contribute to these patterns. However, our findings do lend
support to those of other studies that document the drastic effects of even
low levels of urbanization on stream species diversity (Angermeier et al.
2004, Klein 1979, Lucchetti and Feurstenburg 1993, Marchetti et al. 2006,
Wang et al. 2001, Weaver and Garman 1994). Although many studies have
documented negative correlations between non-native species and rare species
(e.g., Lassuy 1995, Miller et al. 1989, Moyle 1976, Tyus and Saunders
2000, Wilcove et al. 1998), we found the correlation to be weak (r = 0.07).
Furthermore, non-natives occurred commonly (48% of sites) with imperiled
species at sites with little urban land cover. However, since non-native
species are present throughout urban and non-urban watersheds, we were not
Table 2. Percent urban, agriculture, and forest land cover for three different years in two watersheds
where the Maryland Darter was historically known to occur.
Land cover (%)
Watershed Year Urban Agriculture Forest
Deer Creek 1973A 4 62 34
(Last seen in 1994 11 43 46
1988) 2000 12 56 32
Swan Creek 1973 18 41 41
(Last seen in 1994 41 1 58
1965) 2000 41 1 58
AYear when the Maryland Darter was presumed to still live in the Deer Creek watershed, where
the last collection was made in 1988.
604 Northeastern Naturalist Vol. 17, No. 4
able to investigate the affect of urbanization, in the absence of non-natives.
We acknowledge that the combined effects of habitat degradation and competition
with non-native species may have cumulative effects on many native
species (Moyle and Williams 1990).
Despite the fact that agriculture also alters watersheds, there was no
obvious pattern with either imperiled or non-native species occurrences
and the percentages of agriculture or forested land cover in this study. This
result could be, in part, because possible legacy effects from past agriculture
land use (Brush 2009) potentially eliminated stream species that could
not tolerate extensive sedimentation from agricultural run-off that occurred
before distributional data were available (Harding et al. 1998). Conversely,
urban land cover has only recently begun spreading away from the major
metropolitan centers of Baltimore and Washington, DC. Thus, we may be
witnessing the extirpations of sensitive species as they face, for the first time,
the increased flood frequency, erosion, and inputs of potentially toxic chemicals
and sediment associated with urban run-off (Walsh et al. 2005). Having
mostly the same non-native species in the highly urbanized Piedmont and the
primarily rural Highland region, adds to the weight of evidence supporting
the concept that urbanization may have contributed to many recent extirpations,
although introduced species and other factors likely also influenced
this distributional pattern.
Species introductions have resulted in greater species richness in Maryland’s
Atlantic drainage compared to assemblages present before European
settlement. This finding is consistent with other studies (Gido and Brown
1999, Hobbs and Mooney 1998, Sax and Gaines 2003) showing increasing
diversity with species introductions. However, in Maryland’s Piedmont, the
number of apparent extirpations was substantial and greater than in the Coastal
Plain or Highland regions. Some studies have reported greater differentiation
of regional faunas, rather than homogenization, resulting from extirpations
and/or introduction of different species in different regions (McKinney
2005, Taylor 2004). In this study, the contrary effects of extirpations and introductions
combined together resulted in no net difference in similarity of
Piedmont and Coastal Plain faunas compared to their historical assemblages.
Nevertheless, there were local extirpations of particular species from each
region, indicating that a lack of overall change in similarity, by itself, does not
represent sustained biotic integrity at the local scale. Regardless of whether
homogenization or differentiation occurs, local extirpations are the most obvious
evidence of biodiversity loss, and extinctions are irreplaceable.
The apparent extinction of the Maryland Darter is an example of permanent
global biodiversity loss. This species exhibited traits common to many
other severely imperiled stream fishes: endemism, small geographic distribution,
ecological specialization, preference for benthic habitats, and small
body size (Angermeier 1995, Burkhead et al. 1997, Etnier 1997). While
these ecological traits may have made the long-term persistence of the Maryland
Darter questionable even in the absence of human disturbances, these
2010 S.A. Stranko et al. 605
same traits likely also made the Maryland Darter more sensitive to even
minor human disturbances (Helfman 2007) resulting from incremental increases
in urban development. The weight of evidence from this study, which
documents the apparent loss of the Maryland Darter and disappearance of
13 stream species from the highly urbanized Piedmont region, indicate that
maintaining Maryland’s current stream species diversity may require strict
limitations on urban development to certain areas.
Many studies concur that sensitive taxa can be eradicated from streams
at even low levels of urbanization (e.g., <5% impervious land cover; Angermeier
et al. 2004, Southerland et al. 2005, Yoder et al. 2000). Urbanization
has been shown to be correlated with increases in stream temperature (Galli
1991, Klein 1979, Schueler 1994, Stranko et al. 2008, Wolman and Schick
1967) and sediment (Fox 1974, Swarts et al. 1978) as well as less stable
habitat (Booth and Jackson 1997, May et al. 1997), compared to streams in
undeveloped areas. There are many factors which could be responsible for
the local eradication of a rare species. However, while biological indices
provide measures of ecosystem response to a gradient of disturbance, these
aggregated indices are not very effective at identifying which factors directly
caused an impairment or the loss of a species (Allan 2004). This limitation
makes it even more difficult to design remediation and restoration strategies.
Indeed, we are not aware of any study that documents the improvement of an
urban stream to pre-urban condition following an attempt at rehabilitation.
The process of rehabilitating instream habitat may itself be perceived by the
biota as a long-lasting disturbance (Tullos et al. 2009). Rehabilitation of urban
streams by planting riparian buffers and re-engineering instream habitat
often gives only short-term results, with modest improvements in biotic conditions,
unless catchment-wide actions are taken to divert stormflow runoff
from entering streams and to intercept chemical pollutants in runoff (Booth
2005). Stormwater detention ponds provide only imperfect solutions to this
problem: the cost of a detention pond large enough to reduce both peak flow
as well as duration of stormflow inputs is prohibitive in established urban
areas (Booth and Jackson 1997). Given the cost and difficulty of catchmentwide
projects to disconnect impervious surfaces and stream channels in
developed areas, preservation of catchments harboring biologically sensitive
and imperiled stream fauna should receive the highest priority.
Although the disproportionate sensitivity of certain “intolerant” species
to urbanization has been recognized, most published limits of stream
biology to urbanization have been based on correlations with biological index
scores (Klein 1979, Morgan and Cushman 2005, Schueler 1994, Wang et
al. 2001). Loss of a single species may not result in a substantial change to an
index that combines information about many species. For example, fish index
of biotic integrity (IBI) scores (Southerland et al. 2006) from Deer Creek in
1996 and a tributary to Swan Creek in 1997 were rated as “Good” (≥4 on a
scale of 1–5). In fact, the IBI score of 5.0 in the Swan Creek tributary, where
the Maryland Darter had not been collected for over 30 years, was the highest
606 Northeastern Naturalist Vol. 17, No. 4
possible score for this multi-metric assemblage index. Regardless of how
highly generalized biological index scores rate streams, the apparent extreme
sensitivity of the regionally rarest species makes knowledge of their distributions
and ecological requirements vitally important to wise land-use planning
in order to maintain both regional and even global biological diversity.
Acknowledgments
We thank D. Boward, R. Hilderbrand, R. Morgan, and three anonymous reviewers
for reviewing this document; P. Angermeier for reviewing an early draft and
providing recommendations; and R. Hilderbrand for guidance and assistance with
generating land-use data. We also extend our gratitude to the long list of diligent,
hard-working MBSS field-sampling crew members, seasonal employees, interns,
and volunteers who collected the majority of the data presented in this study. This
study was funded by State Wildlife Grant funds provided to the state wildlife agencies
by US Congress, and administered through the Maryland Department of Natural
Resources’ Natural Heritage Program.
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Appendix A. Stream-dependent fish, freshwater mussel, salamander, and crayfish species in
Maryland's Altlantic drainage by physiographic region. N = native/common; R = imperiled
(listed on Maryland's list of rare, threatened, and endangered species); X = presumed extirpated;
I = introduced/non-native. H = Highland, P = Piedmont, and CP = Coastal Plain.
Scientific Name Authority Common Name H P CP
Fishes
Acantharchus pomotis Baird Mud Sunfish R
Alosa aestivalis Mitchell Blueback Herring N N
Alosa mediocris Mitchell Hickory Shad N N
Alosa pseudoharengus Wilson Alewife N N
Alosa sapidissima Wilson American Shad N N
Ambloplites rupestris Rafinesque Rock Bass I I
Ameiurus catus Linnaeus White Catfish N
Ameiurus natalis Lesueur Yellow Bullhead N N N
Ameiurus nebulosus Lesueur Brown Bullhead N N N
Anguilla rostrata Lesueur American Eel N N N
Aphredoderus sayanus Gilliams Pirate Perch N
Campostoma anomalum Rafinesque Central Stoneroller N N
Carassius auratus Linnaeus Goldfish I I I
Catostomus commersoni Lacepède White Sucker N N N
Centrarchus macropterus Lacepède Flier R
Channa sp. Snakehead I I
Clinostomus funduloides Girard Rosyside Dace N N N
Cottus caeruleomentum Kinziger, Raesly, Blue Ridge Sculpin N N N
and Neely
Cottus girardi Robins Potomac Sculpin N N
Cottus sp. Checkered Sculpin N
Cyprinella analostana Girard Satinfin Shiner N N
Cyprinella spiloptera Cope Spotfin Shiner N N
Cyprinus carpio Linnaeus Common Carp I I I
Enneacanthus chaetodon Baird Blackbanded Sunfish R
Enneacanthus gloriosus Holbrook Bluespotted Sunfish N
Enneacanthus obesus Girard Banded Sunfish N
Erimyzon oblongus Mitchell Creek Chubsucker N N N
Esox americanus Gmelin Redfin Pickerel N N
Esox luscius Linnaeus Northern Pike N N
Esox masquinongy Mitchell Muskellunge N N
Esox niger Lesueur Chain Pickerel I N
Etheostoma blennioides Rafinesque Greenside Darter N
Etheostoma caeruleum Storer Rainbow Darter I
Etheostoma flabellare Rafinesque Fantail Darter N
Etheostoma olmstedi Storer Tessellated Darter N N N
Etheostoma sellare Radcliffe & Maryland Darter X X
Welsh
Etheostoma vitreum Cope Glassy Darter X R
Etheostoma zonale Cope Banded Darter I
Ethestoma fusiforme Girard Swamp Darter R
Exoglossum maxillingua Lesueur Cutlip Minnow N N N
Gambusia holbrooki Girard Eastern Mosquitofish N N N
Hybognathus regius Girard Eastern Silvery Minnow N N N
Hypentilium nigricans Lesueur Northern Hogsucker N N
Ictalurus punctatus Rafinesque Channel Catfish I I I
Lampetra aepyptera Abbott Least Brook Lamprey N
Lampetra appendix DeKay American Brook Lamprey X R
Lepisosteus osseus Linnaeus Longnose Gar R
2010 S.A. Stranko et al. 613
Scientific Name Authority Common Name H P CP
Lepomis auritus Linnaeus Redbreast Sunfish N N N
Lepomis cyanellus Rafinesque Green Sunfish I I I
Lepomis gibbosus Linnaeus Pumpkinseed N N N
Lepomis gulosus Cuvier Warmouth R
Lepomis macrochirus Rafinesque Bluegill I I I
Lepomis megalotis Rafinesque Longear Sunfish I I
Lepomis microlophus Günther Redear Sunfish N N
Luxilus cornutus Mitchell Common Shiner N N N
Margariscus margarita Cope Pearl Dace R X
Micropterus dolomieu Lacepède Smallmouth Bass I I
Micropterus salmoides Lacepède Largemouth Bass I I I
Moxostoma erythrurum Rafinesque Golden Redhorse N N
Moxostoma macrolepidotum Lesueur Shorthead Redhorse N N
Nocomis micropogon Cope River Chub N N
Notemigonus crysoleucas Mitchell Golden Shiner I I N
Notropis amoenus Abbott Comely Shiner R R R
Notropis bifrenatus Cope Bridle Shiner X X
Notropis buccatus Cope Silverjaw Minnow N
Notropis chalybaeus Cope Ironcolor Shiner R
Notropis hudsonius Clinton Spottail Shiner N N N
Notropis procne Cope Swallowtail Shiner N N
Notropis rubellus Agassiz Rosyface Shiner N
Noturus gyrinus Mitchell Tadpole Madtom N
Noturus insignis Richardson Margined Madtom N N N
Oncorhynchus clarkii Richardson Cutthroat Trout I
Oncorhynchus mykiss Walbaum Rainbow Trout I I
Perca flavescens Mitchell Yellow Perch N N N
Percina bimaculata Haldeman Chesapeake Logperch R
Percina notogramma Raney & Hubbs Stripeback Darter X R
Percina peltata Stauffer Shield Darter N N
Percopsis omiscomaycus Walbaum Trout-perch X
Petromyzon marinus Linnaeus Sea Lamprey N N
Pimephales notatus Rafinesque Bluntnose Minnow N N
Pimephales promelas Rafinesque Fathead Minnow I I I
Pomoxis annularis Rafinesque White Crappie N N N
Pomoxis nigromaculatus Lesueur Black Crappie I I I
Rhinichthys atratulus Hermann Eastern Blacknose Dace N N N
Rhinichthys cataractae Valenciennes Longose Dace N N
Salmo trutta Linnaeus Brown Trout I I I
Salvenilus fontinalis Mitchell Brook Trout R R R
Sander vitreus Mitchell Walleye N N
Semotilus atromaculatus Mitchell Creek Chub N N N
Semotilus corporalis Mitchell Fallfish N N N
Umbra pygmaea DeKay Eastern Mudminnow N
Mussels
Alasmidonta heterodon I. Lea Dwarf Wedgemussel X N
Alasmidonta undulata Say Triangle Floater N N N
Alasmidonta varicosa Lamarck Brook Floater N N
Anodonta implicata Say Alewife Floater N N
Elliptio complanata Lightfoot Eastern Elliptio N N N
Elliptio fisheriana I. Lea Northern Lance N
Elliptio lanceolata I. Lea Yellow Lance X N
Elliptio producta Conrad Atlantic Spike N N N
Lampsilis cardium Rafinesque Plain Pocketbook I I
614 Northeastern Naturalist Vol. 17, No. 4
Scientific Name Authority Common Name H P CP
Lampsilis cariosa Say Yellow Lampmussel N N N
Lampsilis r. radiata Gmelin Eastern Lampmussel N N
Lasmigona subviridis Conrad Green Floater N X
Leptodea ochracea Say Tidewater Mucket N
Ligumia nasuta Say Eastern Pondmussel N
Pyganodon cataracta Say Eastern Floater N N N
Strophitus undulatus Say Creeper N N N
Utterbackia imbecillis Say Paper Pondshell N N
Crayfishes
Cambarus acuminatus Faxon Acuminate Crayfish R
Cambarus b. bartonii Fabricius Common Crayfish N N N
Cambarus diogenes Girard Devil Crawfish N N
Cambarus dubius Faxon Upland Burrowing Crayfish N
Fallicambarus fodiens Cottle Digger Crayfish N
Orconectes limosus Rafinesque Spinycheek Crayfish N N
Orconectes obscurus Hagen Allegheny Crayfish R R
Orconectes rusticus Girard Rusty Crayfish I I
Orconectes virilis Hagen Virile Crayfish I I
Procambarus acutus Girard White River Crawfish N
Procambarus clarkii Girard Red Swamp Crawfish I
Procambarus zonangulus Hobbs & Hobbs Southern White River I
Crawfish
Salamanders
Cryptobranchus a. Daudin Eastern Hellbender X
alleganiensis
Desmognathus fuscus Green Northern Dusky Salamander N N N
Desmognathus monticola Dunn Seal Salamander N
Desmognathus ochrophaeus Cope Allegheny Mountain Dusky N
Salamander
Eurycea bislineata Green Northern Two-lined N N N
Salamander
Eurycea l. longicauda Green Longtail Salamander N N
Gyrinophilus p. Green Northern Spring Salamander N
porphyriticus
Pseudotriton m. montanus Baird Eastern Mud Salamander X N
Pseudotriton r. ruber Latreille Northern Red Salamander N N N
Siren lacertina Linnaeus Greater Siren X X I