2009 NORTHEASTERN NATURALIST 16(1):101–112
Stream Macroinvertebrate Communities in Paired
Hemlock and Deciduous Watersheds
James J. Willacker, Jr.1,2,*, William V. Sobczak3, and Elizabeth A. Colburn4
Abstract - Tsuga canadensis (Eastern Hemlock) is a common forest species that is
declining throughout its range in the eastern United States because of the invasion
of an exotic forest pest, Adelges tsugae (Hemlock Woolly Adelgid). This pest kills
infected trees, and over time, infected stands are replaced by deciduous forests. The
conversion of forests from hemlock to deciduous species is predicted to impact the
hydrology, chemistry, and biology of associated headwater streams. In this study,
we examined the macroinvertebrate communities of two adjacent headwater streams
with differing hemlock influence in central Massachusetts. Abundance, taxa richness,
diversity, and unique taxa were generally greater in the deciduous stream.
Differences in the distribution of functional feeding groups were observed: the
hemlock stream had a greater percentage of collector-gatherers while the deciduous
stream had a greater percentage of shredders and predators. These findings suggest
that macroinvertebrate communities in streams draining hemlock and deciduous watersheds
may differ in structure and function, and that anticipated hemlock mortality
may impact the region’s stream ecology.
Introduction
Tsuga canadensis (L.) Carrière (Eastern Hemlock) is a highly shadetolerant
conifer that is dominant throughout much of central New England and
often forms largely mono-specific stands (Rogers 1978). Hemlock forests in
eastern North America have recently been invaded by Adelges tsugae Annand
(Hemlock Woolly Adelgid) (Hemiptera: Adelgidae), an introduced insect that
kills both saplings and adult trees (McClure 1991). Eastern Hemlocks do not
regenerate following infestation, and stands killed by the adelgid are replaced
by deciduous species (Orwig 2002, Orwig and Foster 1998). Forecasted
hemlock mortality may impact the physical and biological characteristics of
coupled aquatic ecosystems, such as headwater streams and wetlands (Ellison
et al. 2005). For example, Eastern Hemlock evapotranspires less water
than most deciduous trees and create a cool, moist, and dark forest understory
(Hadley 2000), thus providing hydrologic and thermal stability to adjacent
headwater streams (Snyder et al. 2002). In addition, hemlock stands may constrain
food resources in streams by shading periphyton communities (Rowell
and Sobczak 2008) and providing low-quality leaf litter for stream consumers
(Maloney and Lamberti 1995). Hence, the loss of Eastern Hemlock in New
1Department of Environmental and Forest Biology, State University of New York
College of Environmental Science and Forestry, Syracuse, NY 13210. 2Current address
- Department of Biological Sciences, University of Alaska Anchorage, Anchorage,
AK 99508. 3Biology Department, Holy Cross College, Worcester, MA 01610.
4Harvard Forest, Harvard University, Petersham, MA 01366. *Corresponding author
- jjwillac@gmail.com.
102 Northeastern Naturalist Vol. 16, No. 1
England may result in significant changes in the hydrology and energy flow
of the region’s headwater streams. These changes may in turn impact the biota
of headwater streams including populations of coldwater fishes, threatened
stream salamanders, and aquatic insects.
Headwater streams drain more than 80% of the landscape and are
important sources of organic carbon, inorganic nutrients, and organisms to
downstream ecosystems (Lowe and Likens 2005, Nadeau and Rains 2007,
Wipfli et al. 2007). Food resources in headwater streams draining forested watersheds
are frequently dominated by allochthonous materials such as woody
debris and leaves (Bilby and Likens 1980, Vannote et al. 1980), although
periphyton can be significant seasonally and important for some headwater
stream taxa (Mayer and Likens 1987). Decomposition rates, palatability to
consumers, and the nutritional value of these terrestrially derived inputs vary
with tree species (Maloney and Lamberti 1995, Webster and Benfield 1986).
Stream macroinvertebrates are primary consumers of these allochthonous
materials, and their distribution and productivity are tightly coupled to forest
composition and the supply of bio-available organic matter (Cross et al. 2006,
Cummins and Klug 1979, Eggert and Wallace 2003, Wallace et al. 1999).
The objective of this study was to identify differences between the macroinvertebrate
communities of two streams. Specifically, we compared two
adjacent streams with many comparable watershed attributes but contrasting
riparian forest composition in regards to hemlock abundance. We predicted
that the deciduous stream’s macroinvertebrate community would have higher
abundances, richness, and diversity than the hemlock stream’s community
due to greater food resources. In addition, we predicted that the deciduous
stream would be dominated by leaf-shredding macroinvertebrates. While our
comparative approach has been defended by some ecologists (e.g., Oksanen
2001) and employed in numerous stream-manipulation experiments (e.g.,
Wallace et al. 1999), we recognize this approach has also been criticized for
a lack of replication at the scale of stream ecosystem (Hurlbert 2004). The
ultimate goal of the study was to provide preliminary information that will
help forecast how regional declines in Eastern Hemlock may impact New
England’s stream ecosystems.
Methods
Study site
We selected two adjacent streams draining the extensively studied
Prospect Hill Tract of Harvard Forest in north central Massachusetts. The
340-ha Prospect Hill Tract is located in the towns of Petersham and Phillipston
at an elevation of 270 to 420 m (as described in Motzkin et al. 1999).
Soils are primarily acidic sandy loams and glacial tills overlying schist and
gneiss bedrock. Variability in the relief, soil depth, and the presence of a
hardpan all result in erratic patterns of soil drainage (Foster et al. 1992).
The property is approximately 90% forested, primarily second growth, and
characterized as the transition hardwood forest type with common species
including: Quercus rubra L. (Northern Red Oak), Acer rubrum L. (Red
Maple), Betula lenta L. (Sweet Birch), B. papyrifera Marsh. (Paper Birch),
2009 J.J. Willacker, Jr., W.V. Sobczak, and E.A. Colburn 103
Fraxinus americana L. (White Ash), Pinus strobus L. (Eastern White Pine),
and Eastern Hemlock (Westveld 1956).
The two comparison streams are both first-order tributaries of Bigelow
Brook, a tributary of the Swift River that drains into the Quabbin Reservoir. The
two watersheds are adjacent to one another and comparable in gradient, total
area, underlying geology, and water chemistry (Table 1); however, one stream’s
riparian area is dominated by Eastern Hemlock, while the other’s is composed
primarily of deciduous species (Red Maple, Paper Birch, and Northern Red
Oak) with diffuse patches of Eastern Hemlock. The hemlock stream had slightly
lower pH than the deciduous stream; however, this difference is likely a function
of hemlock abundance. Both streams have small springs as their source,
consistent summer flow, and mean summer water temperatures between 10
and 12 °C. Inorganic N and P concentrations are low in both streams (Table 1),
and high discharge events dominate fluvial losses of dissolved C, N, and P in
the hemlock-dominated stream. Summer photosynthetically active radiation
values are ≈3-fold greater in the deciduous stream (Rowell and Sobczak 2008).
The hemlock stream’s hydrology, chemistry, and macroinvertebrate community
have been previously characterized (Collins et al. 2007).
Field collection
Invertebrate samples were taken from four randomly chosen locations in
each stream during a two-week period in late July and early August of 2005.
At each location, one sample was taken from the nearest riffle area and one
from the nearest depositional area; thus, eight samples were taken from each
stream. Moss was sampled when present on the substratum. Peak emergence
of insects usually occurs in early to late spring; hence, it is likely that our
sampling regime underestimated the abundance, richness, and diversity of
the streams’ macroinvertebrate communities. Samples were taken by thoroughly
disturbing the substrate within a 0.25-m2 quadrat for a 30-second
period and collecting all dislodged material in standard D-frame kick nets
(250-μm mesh) placed downstream. Samples were live-picked, with care
being taken to collect all sizes of individuals. Live-picking has been used
extensively in rapid biological assessments and environmental monitoring
with favorable results (Chessman and Robinson 1987, Courtemanch 1996,
Table 1. Physical and chemical characteristics of the two study streams. Riparian hemlock was
estimated using half-meter resolution satellite imagery within a 10-m buffer on either side of
the streams. Temperature was measured during sample collection. Photosynthetically active
radiation (PAR) data is from Rowell and Sobczak (2008). Water chemistry data represent the
mean values from two sampling dates in July of 2007.
Characteristic Hemlock Deciduous
Watershed area (ha) 24 27
Riparian hemlock (%) 88 36
Mean summer temperature (°C) 10.8 11.2
pH 5.2 5.4
Dissolved organic carbon (mg/L) 1.4 1.5
Nitrate (mg/L) 0.018 0.013
Phosphate (μg/L) 0.9 1.0
PAR (μmol m-2 s-1) 22 64
104 Northeastern Naturalist Vol. 16, No. 1
Growns et al. 1997, Marchant et al. 2006, Metzeling et al. 2003). While
live-picking has traditionally been assumed to underestimate small and
cryptic taxa (Humphrey et al. 2000), comparisons between live-picking and
laboratory sorting have shown that this method is effective at detecting even
relatively fine-scale differences between watersheds (Growns et al. 1997,
Metzeling et al. 2003). In this study, each sample was live-picked for three
hours rather than the 30 minutes to one hour generally used in rapid assessments
of stream communities; thus, we believe that our samples accurately
portray any differences between streams. All invertebrates were preserved in
70% ethanol and brought back to the laboratory.
Laboratory methods and analysis
In the laboratory, invertebrates were sorted and identified to the genus level
(with the exception of Chironomidae, which were identified to the subfamily/
tribe level) using dissecting and compound microscopes. Taxa were then segregated
among the grazer, shredder, collector-gatherer, collector-filterer, and
predator functional feeding groups based on the ecological information known
for each taxon (Merritt and Cummins 1996, Stewart et al. 1993, Wiggins 2000).
In addition, taxa unique to each stream were identified. We calculated mean
abundance (number of individuals/m2), richness (number of taxa/sample), and
Shannon’s diversity index of macroinvertebrates in each stream and compared
them using a Student’s t-test. In addition, the composition of the two streams
was compared at the order level and among functional feeding groups.
Results
Community composition
The deciduous stream had a significantly higher mean richness (24.4
vs. 11.0, P-value < 0.001; Fig. 1A), more unique taxa (17 vs. 3; Table 2),
and more taxa (45 vs. 31; Table 2) than the hemlock stream. The deciduous
stream also had a significantly higher mean diversity (2.3 vs. 1.7, P-value =
0.008; Fig 1B). The deciduous stream had a higher mean abundance (397.3/
m2 vs. 288.5/m2) than the hemlock stream, although the difference between
the two streams was not significant (P-value = 0.223; Fig. 1C). The structure
of the streams’ macroinvertebrate communities, in terms of composition
by order, also differed (Fig. 2). The deciduous stream’s community was
dominated by Diptera and Trichoptera (46% and 43%, respectively), and the
hemlock stream’s community was composed primarily of Diptera (59%),
Trichoptera (19%), and Ephemeroptera (19%).
Functional feeding groups
The functional composition of the two streams’ macroinvertebrate communities
were very different (Fig. 3). In both streams, the collector-gatherer
feeding group dominated; however, their relative importance varied between
streams, with the collector-gatherer group comprising 62% of the hemlock
community and only 32% of the deciduous stream community. Shredder and
predator feeding groups were notably lower in the hemlock stream, comprising
only 17% and 15%, respectively, versus 28% and 22%, repectively, in the
2009 J.J. Willacker, Jr., W.V. Sobczak, and E.A. Colburn 105
Figure 1. Comparison
of three
c o m m u n i t y
structure metrics:
A) richness
(number of
taxa/sample),
B) Shannon’s
diversity index,
and C) abundance
(number
of individuals/
m2) between a
hemlock-dominated
stream
and a deciduous-
dominated
stream during
summer 2005
at Harvard Forest,
MA.
deciduous stream. In both streams, the percentage of grazers was low (≈1%
in both streams).
There were large differences in the composition and relative abundances
of the taxa comprising the predator, shredder, collector-filterer, and collector-
gatherer feeding groups (Fig. 4). In both streams, the predator feeding
106 Northeastern Naturalist Vol. 16, No. 1
Table 2. Classifications and abundances (number of individuals/m2) (± SD) of taxa found in a
hemlock-dominated stream and a deciduous-dominated stream during the summer of 2005 at
Harvard Forest, MA. FFG = functional feeding group.
Abundance ± SD
Taxa Order FFG Hemlock Deciduous
Simulium sp. Diptera Filterer - 0.3 ± 0.7
Diplectrona sp. Trichoptera Filterer - 5.8 ± 9.8
Dolophilodes sp. Trichoptera Filterer - 0.8 ± 1.5
Parapsyche sp. Trichoptera Filterer 8.0 ± 11.8 35.5 ± 58.6
Wormaldia sp. Trichoptera Filterer 0.8 ± 1.5 11.8 ± 26.2
Chelifera sp. Diptera Gatherer - 0.3 ± 0.7
Chironomini Diptera Gatherer 6.5 ± 15.3 22.3 ± 19.6
Clinocera sp. Diptera Gatherer 0.3 ± 0.7 4.3 ± 5.5
Dixa sp. Diptera Gatherer - 0.3 ± 0.7
Orthocladiinae Diptera Gatherer 11.0 ± 13.9 6.8 ± 6.1
Ptychoptera sp. Diptera Gatherer - 0.3 ± 0.7
Tanytarsini Diptera Gatherer 113.3 ± 119.0 73.3 ± 94.8
Ameletus sp. Ephemeroptera Gatherer - 0.5 ± 1.4
Eurylophella sp. Ephemeroptera Gatherer 56.8 ± 49.9 16.8 ± 20.8
Amphinemura sp. Plecoptera Gatherer 2.3 ± 4.1 -
Lype sp. Trichoptera Gatherer 0.5 ± 1.4 0.3 ± 0.7
Ectopria sp. Coleoptera Grazer - 1.5 ± 2.3
Optioservus spp. Coleoptera Grazer - 0.3 ± 0.7
Molanna sp. Trichoptera Grazer 3.5 ± 9.1 3.3 ± 5.8
Neophylax sp. Trichoptera Grazer - 0.3 ± 0.7
Agabus spp. Coleoptera Predator 0.5 ± 1.4 0.5 ± 0.9
Celina sp. Coleoptera Predator 0.3 ± 0.7 -
Cymbiodyta sp. Coleoptera Predator - 0.3 ± 0.7
Hydrobius sp. Coleoptera Predator - 2.3 ± 4.8
Dicranota sp. Diptera Predator 0.5 ± 1.4 7.5 ± 4.6
Hexatoma sp. Diptera Predator 1.3 ± 2.8 5.0 ± 4.1
Hybomitra sp. Diptera Predator 0.8 ± 1.5 4.5 ± 7.2
Limnophila spp. Diptera Predator - 0.5 ± 0.9
Palpomyia sp. Diptera Predator 3.3 ± 7.6 3.0 ± 3.2
Pedicia sp. Diptera Predator 1.0 ± 1.5 0.5 ± 1.4
Psuedolimnophila sp. Diptera Predator - 0.5 ± 1.5
Tanypodinae Diptera Predator 24.3 ± 26.1 48.5 ± 32.3
Gerris sp. Hemiptera Predator - 0.5 ± 1.4
Microvelia sp. Hemiptera Predator 0.8 ± 2.1 1.0 ± 1.5
Sialis sp. Megaloptera Predator 1.3 ± 1.8 2.0 ± 3.2
Cordulegaster sp. Odonata Predator 0.3 ± 0.7 2.0 ± 3.9
Lanthus sp. Odonata Predator 0.3 ± 0.7 0.8 ± 1.0
Sweltsa spp. Plecoptera Predator - 1.3 ± 2.3
Oligostomis sp. Trichoptera Predator 0.5 ± 0.9 6.3 ± 8.8
Rhyacophilia spp. Trichoptera Predator 2.3 ± 2.7 2.5 ± 3.2
Haliplus sp. Coleoptera Shredder - 0.3 ± 0.7
Tipula spp. Diptera Shredder 0.5 ± 0.9 2.8 ± 2.6
Pyralidae Lepidoptera Shredder 0.3 ± 0.7 0.3 ± 0.7
Leuctra sp. Plecoptera Shredder 2.3 ± 2.7 4.5 ± 3.2
Ironoquia sp. Trichoptera Shredder 0.3 ± 0.7 -
Lepidostoma sp. Trichoptera Shredder 20.3 ± 21.6 73.8 ± 58.3
Psilotreta sp. Trichoptera Shredder 13.5 ± 21.7 14.0 ± 9.3
Pycnopsyche sp. Trichoptera Shredder 5.3 ± 9.9 13.0 ± 22.1
2009 J.J. Willacker, Jr., W.V. Sobczak, and E.A. Colburn 107
group was dominated by the chironomid subfamily Tanypodinae; however,
in the deciduous stream, there were also large tipulid populations (Dicranota
spp. and Hexatoma sp.). Shredder abundance was ≈3-fold greater in the
deciduous stream. In both streams, the primary shredders (by abundance)
were the Trichopterans, Lepidostoma sp. and Pycnopsyche sp.; however, the
abundances of both these taxa were higher in the deciduous stream. In the
collector-gatherer feeding group, the two streams had the same dominant taxa,
Figure 2. Relative abundance (% total) by order of macroinvertebrate communities in
headwater streams draining hemlock and deciduous forests at Harvard Forest, MA.
Figure 3. Relative abundance (% total) by functional feeding group of macroinvertebrate
communities in headwater streams draining hemlock and deciduous forests
at Harvard Forest, MA.
108 Northeastern Naturalist Vol. 16, No. 1
the chironomid tribe Tanytarsini, but with rather substantial differences in
their abundance and in the composition of the remaining taxa. The hemlock
stream had more Tanytarsini along with a greatly increased abundance of the
Ephemeropteran genus Eurylophella.
Discussion
We found ecologically important differences between the macroinvertebrate
communities of adjacent deciduous and hemlock streams. We believe
that these differences are a function of differences in the streams’ riparian
vegetation; however, we recognize that our study was spatially and temporally
limited.
Figure 4. Number of individuals sampled in four functional feeding groups: A)
collector-filterer, B) collector-gatherer, C) shredder, and D) predator in headwater
streams draining hemlock and deciduous forests at Harvard Forest, MA. Taxa with
less than 5% relative abundance were pooled together and designated “other.”
2009 J.J. Willacker, Jr., W.V. Sobczak, and E.A. Colburn 109
The deciduous stream supported higher richness and abundance of
macroinvertebrates, and had more unique taxa than the hemlock stream. In
addition, we found the composition of the communities in the two streams
differed, both at the taxonomic and functional levels. These findings support
and geographically extend many of the findings in Snyder et al.’s (2002)
benchmark study on macroinvertebrates in numerous streams draining
deciduous and hemlock-dominated watersheds in the Delaware Water Gap
National Recreation Area; however, our findings differ in some regards. Importantly,
Snyder et al. (2002) found that hemlock streams supported more
taxa of macroinvertebrates relative to deciduous streams.
Snyder et al. (2002) attributed higher species richness of streams draining
hemlock forests to increased stability of daily and seasonal temperature
and flow regimes. We believe the hydrologic stability that hemlock stands
provide may allow increased abundance of taxa well adapted to the stream’s
environmental conditions; however, increased environmental stability may
also lead to a reduction in the heterogeneity of habitats, which is an important
determinate of diversity (Death and Winterbourn 1995, Miller and Stout
1989, Power et al. 1988). We propose that in some streams with riparian
areas dominated by deciduous species, localized and seasonal variation in
riparian litter inputs, light levels, and water temperatures may augment the
diversity of microhabitats relative to hemlock-dominated streams, thus allowing
increased niche partitioning. Environmental conditions in streams
flowing through hemlock stands may be optimal for some taxa, but not for
many taxa with the potential to colonize hemlock-dominated streams. Thus,
the conversion from hemlock to deciduous forest may result in an increase
in the diversity of aquatic macroinvertebrate communities; however, this
change may be accompanied by the loss of some hemlock-adapted taxa.
Hemlock and deciduous forests differ in the quality, quantity, and diversity
of food resources they provide to stream biota. Streams draining
deciduous forests receive more light annually than those draining hemlock
forests because of reduced shading during leaf-off, and reduced canopy density
during leaf-out (Hadley 2000, Rowell and Sobczak 2008). Light is often
a limiting resource for primary production in headwater streams (Hill and
Knight 1988); thus, increases in light availability may stimulate in-stream
primary production, increase autochthonous food resources, and provide a
broader food base for macroinvertebrates.
Eastern Hemlock also influences allochthonous food resources by contributing
litter of poorer quality relative to many deciduous forest species.
Maloney and Lamberti (1995) found that hemlock needles decayed more
slowly and supported fewer macroinvertebrates than leaves of most deciduous
riparian plant species. Based on studies with other conifer species, primarily
of the genus Pinus, it appears that conifer needles are a nutritionally poor food
resource, and are generally avoided by shredders (Webster and Benfield 1986,
Whiles and Wallace 1997). Because allochthonous inputs are a critical energy
source for forested headwater streams (Eggert and Wallace 2003, Wallace et
al. 1999), it is likely that a transition from the low-quality inputs from Eastern
Hemlock to the relatively high-quality inputs from deciduous species will
result in changes in headwater stream macroinvertebrate communities.
110 Northeastern Naturalist Vol. 16, No. 1
We predicted that differences in the riparian vegetation would result in differences
in the functional composition of the two streams, particularly in
regards to the shredder feeding group. Our findings and those of Snyder et al.
(2002) support this prediction. In this study, the difference can primarily be
attributed to greater abundances of shredding trichopteran taxa, particularly
Lepidostoma sp. Lepidostoma is often associated with leaf packs and debris
dams (Wiggins 2000). We found that predators comprised a greater proportion
of the deciduous stream’s community; this finding differs from those of Snyder
et al. (2002). Both streams’ collector-gatherer communities were dominated by
the chironomid tribe Tanytarsini; however, in the hemlock stream,
Eurylophella mayflies also comprised a large proportion. Several species of
Eurylophella have been shown to inhabit aquatic mosses (Funk and Sweeney
1994) which are prevalent along many hemlock-dominated streams. Snyder et
al. (2002) also found Eurylophella weakly associated with hemlock stands.
Overall, our findings suggest that macroinvertebrate communities in
streams flowing through hemlock-dominated and deciduous-dominated riparian
zones differ in both structure and function, and that anticipated hemlock
mortality may impact central New England’s stream ecology. An emerging
body of literature suggests that headwater streams can influence the ecology
and biogeochemistry of connected downstream ecosystems (Lowe and Likens
2005, Nadeau and Rains 2007, Wipfli et al. 2007). Further research over broader
geographic and longer temporal scales is needed to understand more precisely
how potential alterations in the macroinvertebrate communities of New England’s
headwater streams may influence downstream ecosystems.
Acknowledgments
We would like to thank Matt Kaufman and Grace Wu for their help in the field, numerous
colleagues for their editorial assistance, Roy Norton for providing lab space,
and Kelly Walton for her tremendous help with many aspects of this study. Susan Eggert,
Craig Snyder, and an anonymous reviewer provided extremely helpful comments
on a previous version of the manuscript. This research was made possible by the NSF’s
Research Experience for Undergraduates (REU) program at Harvard Forest. Additional
funding was provided by the State University of New York College of Environmental
Science and Forestry’s Department of Environmental and Forest Biology.
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