Effects of Red Cedar on Invertebrate Abundance and Morphospecies Richness

1
Effects of Red Cedar on Invertebrate Abundance
and Morphospecies Richness Jenna Noble, Todd
Tracy, Jesse Nieuwenhuis, and Jonathan
Gesink Northwestern College, Orange City, IA
ABSTRACT Although native to the eastern US, the
Eastern red cedar Juniperus virginiana is taking
over grasslands, oak savannas, and natural
prairies in the Midwest. In this study, we used
pitfall traps to examine the effects of the
presence and removal of red cedar on invertebrate
abundance and species richness, with particular
focus on ants and carabid beetles.  In September
2007 we performed 24-hour trapping sessions in
a) cedar-invaded, cedar-removed, and cedarless
areas within a riparian mixed-grass prairie
habitat along the Niobrara River at the Nature
Conservancys Niobrara Valley Preserve in
north-central Nebraska, and b) cedar-invaded and
cedarless areas within an upland remnant
tallgrass prairie at Oak Grove County Park near
Hawarden, Iowa. We found greater invertebrate
abundance and carabid beetle abundance in
non-cedar plot than traps in the cedar-infested
plot, and more ants in the cedar-removed plot
than either non-cedar or the cedar-infested plot.
Conversely, we found higher invertebrate
richness in cedar-infested areas of Oak Grove
than non-cedar areas, and we found no differences
in abundance between cedar and non-cedar areas.
We tentatively conclude that red cedar negatively
impacts invertebrate communities and discuss
possible reasons for the differences in results
between study sites.
Fig. 1. Niobrara Valley Preserve study site.
Non-cedar area is in foreground, with
cedar-removed area in the middle (yellow arrow)
and cedar-infested area in background (red arrow).
Fig. 2. Oak Grove study site.
INTRODUCTION Eastern red cedar Juniperus
virginiana is generally classified as an invasive
species in the central U.S., despite the fact
that it is a native tree.  This species has
become problematic in that it is spreading (via
birds passing seeds through their digestive
tracts) across the grasslands and rangelands of
the central U.S., areas that historically have
burned frequently enough and/or been sufficiently
trampled by buffalo to prevent woody species from
taking hold.  Until recently, red cedars range
was limited to river bottoms and cliff faces
where trampling and fires were less likely to
have occurred.  The facilitation of the spread of
the trees into upland areas has been greatly
aided by farmers, ranchers, and state Departments
of Transportation planting the species as a
windbreak.  Although the species is useful for
windbreaks and building material, economic losses
from lost rangeland and lost hunting leases,
decreased stream water quality, and lowered water
tables appear to outweigh the benefits of the
species in many areas (Drake Todd 2002). 
        The effects these cedars have on native
plant diversity is fairly easily seen, but what
is not so apparent are the effects on upland
invertebrates communities.  Studies done in
Nebraska on the endangered American Burying
Beetle Nicrophorus americanus (family Silphidae)
showed that the presence of cedar affected where
the insect ranged. Mark/recapture experiments
showed that although the beetles did not
completely avoid the invasive, there were
significantly fewer beetles ranging under the
cedar trees (Walker and Hobak 2007). While
providing some insight into the potential effects
of cedar on invertebrates, this study focused on
only one species, and thus we still lack a
broader understanding of the effects of these
cedars and their removal on invertebrates
communities in general.   In our
study, we hypothesized that the presence of red
cedar influences the overall species richness and
abundance of invertebrates in an area, and we
predicted that invertebrate abundance and species
richness would be lower in cedar-infested areas
than cedar-free areas, with areas recently
cleared of cedars showing intermediate values.
RESULTS At the Niobrara Preserve, samples from
the non-cedar area exhibited significantly higher
species abundance than either the cedar-infested
samples (p0.007) or the cedar-removed samples
(p0.046), and cedar-removed areas showed
significantly higher species abundance than
cedar-infested areas (p0.0005 Fig. 3a).  No
significant differences in species richness were
found among the Niobrara sites (ANOVA p0.086),
although there was marginally significantly
greater richness in cedar-removed samples than
cedar-infested samples (p0.059 Fig. 4a) .
Conversely, in our Oak Grove data we found
significantly higher overall richness in
cedar-infested areas than non-cedar areas
(p0.015 Fig. 4b), but no differences in species
abundance, although the difference in overall
abundance was almost significant (p0.092 Fig.
3b).         With regard to ant abundance, we
found significant differences among the three
areas at the Niobrara site (ANOVA p0.006 Fig.
3a), with the cedar-removed area showing
significantly greater ant abundance than both the
cedar-infested (p0.003) and non-cedar areas
(p0.002). No difference was found between the
non-cedar and cedar-infested areas (p0.33), and
no differences in ant abundance were seen in the
Oak Grove data (p 0.90).         To determine
whether the differences in abundance among the
three Niobrara plots could be attributed solely
to differences in ant numbers, we ran t-tests
comparing the mean abundance of invertebrates at
both sites, excluding the ants (Fig. 3a).  We
found no differences in non-ant abundance between
non-cedar and cedar-removed plots (p0.14) but
found that both non-cedar and cedar-removed plots
showed significantly greater non-ant abundance
than the cedar-infested plot (p0.0001 and 0.022,
respectively). We also examined the abundance
of carabid (Fig. 5ab) and staph (Fig. 6ab)
beetles at both study sites and found that the
Niobrara data showed a significantly higher
abundance of carabid beetles in the non-cedar
area than the cedar-infested area (p0.01 Fig.
5a) and marginally significantly more staph
beetles in the cedar-infested area than
cedar-removed area (p0.057 Fig. 6a). In Oak
Grove no significant differences in carabid (Fig.
5b) and staph (Fig. 6b) beetle abundance were
found, although we did find marginally
significantly more staph beetles in
cedar-infested samples than non-cedar samples
(p0.092 Fig. 6b).
A
B
A
B
Fig. 3. Invertebrate abundance at A) Niobrara,
and B) Oak Grove. Overall abundance, ant
abundance, and non-ant abundance are shown for
Niobrara, while only overall abundance is shown
for Oak Grove. Bars connect means that are
significantly different at alpha 0.05.
Fig. 4. Invertebrate richness at A) Niobrara,
and B) Oak Grove. Differences were not
significant at Niobrara but were significant at
Oak Grove. Bar connects means that are
significantly different at alpha 0.05.
A
B
A
B
MATERIALS AND METHODS Our pitfall-trapping in
Nebraska took place on September 14 -15, 2007,
at the Nature Conservancys Niobrara Preserve in
north-central Nebraska, in a riparian zone along
the Niobrara River. The research area was
relatively flat with sandy soil and typical
mixed-grass prairie grasses and forbs. Our test
plots were the following cedar-present,
cedar-removed (in a controlled burn the previous
winter), and non-cedar (area never invaded). We
marked and flagged 10 separate sample points a
minimum of 15m apart within each of the three
test plots. Pitfall cups were then placed at
ground level beside each flag, and cups were
filled with 2cm of DeathJuice?, a combination of
70 ethanol and dish soap, used to trap and kill
the insects that fall into the cup. We collected
the cups 24 hours later and poured their contents
into individually labeled bags, which were then
taken back to Northwestern College, where
identification of samples to the morphospecies
level was performed using dissecting microscopes.
Our pitfall-trapping in Iowa took place on
September 19-20, 2007, in an upland oak savanna
at Oak Grove County Park, where tallgrass prairie
species are found interspersed with woody plants
and shrubs, including eastern red cedars (Fig.
2). The same trapping method was used, but with
only infested and non-cedar areas being tested,
as no cedar-removed areas were available. We
paired cedar and non-cedar traps by placing one
pitfall cup under the outer branches of a cedar
tree and the corresponding non-cedar cup anywhere
from 5 to 10m away from the tree in a clear,
grassy area. Ten pairs of cups were placed.
These were harvested 24 hours later, and their
contents were poured into labeled baggies and
taken to the lab for analysis. Invertebrate
abundance and richness values were determined for
each sample point at both sites. For the
Niobrara Preserve data, we performed an ANOVA and
pairwise t-tests comparing both the abundance and
the richness among treatment plots. For the Oak
Grove data, we performed t-tests comparing
abundance and richness between cedar and
non-cedar samples. Due to the large number of
ants found in the cups of the Niobrara Preserve,
we also ran the ANOVA and individual T-tests
using the ant data for each of the three areas.
After subtracting the number of ants from the
total abundance of invertebrates, we reran the
comparative abundance ANOVA and T-tests in order
to determine whether differences in abundance
could simply be attributes to differences
observed in ant abundance among the three
treatments. We also performed an ANOVA and
t-tests on staphylinid (staph) and carabid
beetles in each of our test sites, as carabid
beetle abundance is often used as an indicator of
larger community structure (e.g., Jennings and
Tallamy 2006).
Fig. 5. Carabid beetle abundance at A) Niobrara,
and B) Oak Grove. Bar connects means that are
significantly different at alpha 0.05.
Fig. 6. Staphylinidae (staph) beetle abundance
at A) Niobrara, and B) Oak Grove. No significant
differences were detected.
DISCUSSION In this study, we examined the effects
of invasive eastern red cedar on invertebrate
communities by pitfall-trapping in a riparian
area along the Niobrara River at the Nature
Conservancys Niobrara Valley Preserve in
north-central Nebraska and in an upland savanna
at Oak Grove Park near Hawarden, Iowa. Our
Niobrara data showed that the presence of red
cedar had an adverse effect on ant abundance and
overall invertebrate abundance (both including
and excluding ants) but not invertebrate
richness. We also found that ants preferred the
cedar-removed plot to either the cedar-infested
plot or the non-cedar plot and that carabid
beetles, often used in studies as an indicator of
larger community structure, were found in
significantly greater numbers in non-cedar areas
than cedar-infested areas. Conversely, at Oak
Grove we found significantly greater invertebrate
richness in cedar-infested than non-cedar areas
but no significant difference in overall, ant,
carabid, or staph beetle abundance. Results
from our Niobrara site support our hypothesis
that invertebrate abundance is adversely affected
by the presence of red cedar, while results from
Oak Grove suggest otherwise. Differences between
the two sites can be accounted for perhaps by the
different terrain, substrate, and slope exposure,
as our Nebraska site was sandy and flat, while
our Iowa site was grassy and sloped.
Furthermore, our Iowa site is heavily invaded by
downy brome Bromus tectorum and leafy spurge
Euphorbia esula, two invasives not directly
studied in this experiment. Most of our
non-cedar pitfall traps at Oak Grove were placed
in the ground where one or both of these species
were present, and thus an apparent preference for
cedar-infested areas over non-cedar areas at Oak
Grove may be an avoidance of these two invasives
rather than an actual preference for cedar.
Neither of these two invasives was found at the
Niobrara site, and no other invasives were
apparent in any of the Niobrara plots, and thus
results at Niobrara are less likely to reflect an
avoidance of something other than cedar. While
the presence of cedars clearly relates to a
decreased abundance of invertebrates at our
Nebraska site, cedars do not seem to impact
morphospecies richness. In general, all
morphospecies collected at the Niobrara site were
found in greater numbers in non-cedar areas than
infested areas, but species still used
cedar-infested areas in low numbers, which
supports the conclusions of Walker and Hoback
(2007) in their Nebraska beetle study. More
sampling dates and sampling over multiple years
are necessary to fully understand the effects of
cedar and its eradication on invertebrate
communities. For example, long-term monitoring
will elucidate the effects of the restoration
process. The vast numbers of ants in the
cedar-removed area may be the result of the
burning of cedars in the cedar-removed plot, and
repeated sampling of the area over a period of
several years will reveal whether the
invertebrate community gradually becomes more
like the non-cedar area. Moreover, we know from
our reeserach of the European buckthorn at
Northwestern Colleges Alton forest and at Oak
Grove that patterns in invertebrate abundance and
richness between areas of high and low buckthorn
density vary from one season to the next, and
without repeated sampling in our present study we
can only draw tentative conclusions about the
effects of cedar in the fall. The results of our
present study can help us more fully understand
the impacts of red cedar on its surroundings.
Not only can cedars displace native plants, alter
soil properties, lower water tables, and decrease
the quality of pastureland, they also appear to
displace many of the invertebrates living in
invaded areas. One encouraging finding in our
Niobrara study is that within a year of the
removal of cedars in an attempt to restore an
area heavily invaded by cedars, the invertebrate
populations have already shown signs of recovery
in the restoration area.
REFERENCES Drake, B., and Todd, P.  2002.  A
strategy for control and utilization of invasive
juniper species in Oklahoma Final report of the
redcedar taskforce.  Oklahoma Department of
Agriculture, Food, and Forestry.  Jennings, V.,
and Tallamy, D. 2006. Composition and abundance
of ground-dwelling coleoptera in a fragmnented
and continuous forest. Environmental Entomology
351550-1560. Walker, T., and Hoback, W. 2007.
Effects of invasive eastern redcedar on capture
rates of Nicrophorus americanus and other
Silphidae. Environmental Entomology 297-307.