The Influence of Forest Cover on Ground Dwelling Arthropods in the
Central Cascade Range of Oregon
Samuel Koss
Ecoplexity Course, Portland State University, July 2007
ABSTRACT The impacts of canopy closure on the species richness, diversity, and
community similarity of three different trophic levels of terrestrial arthropods
(predators, herbivores, and fungivores) was evaluated. Specimens were collected using
pitfall traps in adjacent surveys areas, an early to mid seral closed-canopy Douglas fir
stand and a disturbed forest opening. My hypothesis was that the forest opening would
have a higher diversity of herbivores and predators due to a more productive plant
community. Herbivores would show low community similarity due to obligation to
differing plant prey between sites. Predators would show high community similarity due
to high mobility. Fungivores would show higher species richness and diversity in the
forested stand due to reduced exposure of the upper layers of topsoil, but higher
community similarity due to large woody debris presence on their food base. Results
showed no significant differences in species richness for all three groups. The herbivore
group showed more diversity in the open site, while the other two groups showed no
significant difference between sites. Community similarity was highest for the fungivore
group.
Canopy removal results in a cascade of microhabitat changes. These changes include
immediate significant changes in water availability due to reduced transpiration and
increased evaporation, immediate changes in potential photosynthetic productivity, as
well as long-term changes in the decomposition patterns of both the littoral and tree root
zones. These changes may have dramatic impacts on the terrestrial arthropod community.
Forest removal results in the rapid influx of a heliophilic herb layer. This layer serves as a
relatively rich and diverse source of potential food sources for herbivorous arthropods,
especially in comparison to the relatively sparse herb layer found in mid seral Douglas fir
forests in Oregon. Classic trophic modeling would indicate that an increase in
productivity and diversity of the plant community would result in an increase in
herbivorous arthropods (Hoonbok, et al, 2005). An increase in biomass and variety of
herbivores should lead to an increase in the numbers and variety of predators.
Impacts of canopy removal on the fungivorous arthropod community seem to be
propelled by a complex of opposing forces. The fungal community is largely influenced
by the availability of water. Forest openings typically show a significant increase of water
availability at the rooting level due to reduced transpiration (tree mortality), but there is
also a significant decrease in water availability at the littoral / humus layer during the
warmest months due to increased evaporation (Moldenke et al., 2002). Water availability
can be further impacted by the presence of large downed woody debris, which can
produce extreme heterogeneity in water availability across a forest opening. Further
complicating the fungal (and fungivore) community is the extreme seasonality in
precipitation on the west slope of the Oregon Cascades.
There tends to be an initial increase in fungi (and fungivores) following canopy removal
associated with a loading of sloughed organic material during the process of harvesting
(needles, twigs, bark). This littoral / humus layer is eventually reduced to much less than
the original forest litter layer due to continued decomposition of accumulated organic
material and cessation of litter production (Moldenke et al., 2002). This action would
seem to reduce available resources for fungi, and, indeed, it has been demonstrated that
mycorrhizal fungi have significant negative reactions to deforestation; however, the dead
root masses of harvested trees serve as a food bank for degradative fungi, which will
eventually serve to provide a food base for fungivorous arthropods long after the humus
layer has been reduced.
As a taxonomic group, terrestrial arthropods occupy the widest diversity of microhabitats,
niches, and trophic roles, and they fill many essential ecological roles (Kremen et al.,
1993). Their large populations, species variation, range of body sizes, range of
distributional characteristics, propensity for rapid population growth, essential roles in
ecosystem maintenance, and ease of collection make them ideal for use as indicators of
habitat changes. Given the landscape-altering impacts of our historic forestry practices,
studying the effects of canopy removal on terrestrial arthropod communities could be
very informative regarding the ecological impacts of our approaches to ecosystem
management.
The objective of this study was to determine the long-term impacts of canopy removal on
three different trophic levels of the terrestrial arthropod community: predators,
herbivores, and fungivores. What are the long term impacts of canopy removal in species
diversity within functional groups, and how does the assemblage of each functional group
compare between immediately adjacent habitat types, young Douglas fir forest with
heavy canopy closure and a maintained forest opening (“meadow”). My hypothesis is
multifaceted, and addresses each functional group separately.
Herbivores: My hypothesis was that the forest opening would show an increase in both
species richness and diversity in herbivores. Canopy removal results in a general increase
in plant diversity and productivity, providing an increased number of potential niches for
herbivores. I predicted that the herbivores would be more closely tied to food sources and
that differences in plant communities would heavily impact their makeup, magnifying the
differences in the herbivore communities between the two habitat types. I hypothesized
that the populations of herbivores between the two habitat types would show the greatest
amount of difference of the three considered functional groups.
Predators: Due to the increase in herbivore presence and diversity in the forest opening,
I hypothesized that the predator component would follow this trend. Predators are
typically more mobile than the other two functional groups, enabling them to more easily
move between the adjacent sites. Due to their mobility and the tendency for many
predators to be generalists, I hypothesized that the arthropod predator component of each
of the habitat types would be similar.
Fungivores: Hypothesizing on the impacts of canopy removal on the fungivore
population was more complicated. I assumed that the species richness and diversity of
fungivorous arthropods would be greater under the forest canopy due to the moisture-
mediating effects of the canopy on the humus layer. I also hypothesized that the
fungivore components of the adjacent habitat types would be similar. Given the potential
size of fungal hyphae nets and the presence of large downed woody debris in the forest
opening, I assumed that the fungi in each site may have significant overlap, enabling the
fungivores to live in either habitat, though most likely in differing numbers due to
inconsistent moisture availability in the humus layer.
Methods
1) Site Selection
The study site was located on the grounds of the H.J. Andrews Experimental Forest on
the Willamette National Forest, one of 24 LTER (Long Term Ecological Research ) sites
funded by the National Science Foundation. The study site was located just off an access
road and included a cleared area (meadow) with scattered young Douglas firs, an early to
mid seral stand dominated by Douglas fir with heavy canopy closure, and the transitional
area between them. The “meadow” community is a long term roadside forest opening
maintained by continual disturbance. This site was selected because of its potential to
reflect the arthropod communities in immediately adjacent habitat types and their ecotone
as well as its ease of access.
2) Arthropod Collection
Specimens were collected using pitfall traps. Traps consisted of one-gallon buckets
buried so that the rims were flush with the ground surface. Smaller plastic cups were
placed inside each bucket containing propylene glycol as a preserving agent. A thin sheet
metal funnel was placed inside the mouth of each bucket in order to funnel unfortunate
arthropods into the preservative. Each bucket was covered with thin sheet metal in order
to prevent rain from entering the trap.
Traps were set in three trap lines of seven traps. Each trap line began in the meadow and
ran into the adjacent forested stand with the middle trap resting within the ecotone. Trap
lines were spaced five meters apart. Individual traps within each line were also set five
meters apart. A total of nine traps were set in each habitat type and three were set along
the ecotone. The traps remained in place for six days.
Preserved specimen cups were labeled upon collection. Sorting and identification took
place on site with the oversight of resident expert entomologist, Dr. Andrew Moldenke
(Oregon State University).
3) Data Analysis
For the purposes of this study, specimens collected along the ecotone were omitted.
Although traps were set in lines running through both meadow and forested habitats, each
set of nine traps was considered representative of a distinct community or site. The
specimens collected from the adjacent meadow (m) and forested (f) communities were
divided into three groups based on trophic function: predators, herbivores, and
fungivores.
Species richness was compared between communities by comparing the mean number of
species per trap for each functional group in each habitat type. Means were expressed
with a 95% confidence level.
Biodiversity was compared between sites for each functional group. Simpson’s Diversity
Index, D=1/Pi2, where Pi is the proportion of the number of specimens of each species
to the total number of specimens found, was calculated for each trap. Mean diversity
values of each functional group were compared for both community types using a 95%
confidence level.
Community similarity was calculated for each functional group using Sorrenson’s
Quotient of Similarity, 2J/(A+B), where A is the total number of species found in the
meadow site, B is the total number of species found in the forested site, and J is the
number of species common to both sites.
Results
Average Number of Species Per
Trap
Species Richness
10
9
8
7
6
m
5
f
4
3
2
1
0
predator
herbivore
fungivore
Functional Groups
Figure 1.A comparison of species richness of each functional group for both meadow (m)
and forest (f) communities by showing mean number of species per trap with a 95%
confidence level.
Biodiversity Comparison
0.9
Sinmpson's Diversity index
0.8
0.7
0.6
0.5
m
0.4
f
0.3
0.2
0.1
0
predator
herbivore
fungivore
Functional Groups
Figure 2. Biodiversity. Displaying a comparison of the means (95% confidence level) of the
Simpson’s Diversity Index values for each trap by functional group and community type.
Community Similarity
Index
Increasing
Commonality
0.6
0.5
0.4
0.3
0.2
0.1
0
pred
herb
fung
Functional Groups
Figure 3. A comparison of the similarities between each site for each
functional group using Sorenson’s Similarity Index.
Increasing Diversity
Biodiversity of Total Specimens Found in Each
Functional Group in Meadow Versus Forest
Habitat (Simpson)
1
0.8
0.6
Meadow
0.4
Forest
0.2
0
Predator
Herbivore
Fungivore
Functional Groups
Figure 4. Simpson’s Biodiversity index calculated for total specimens found in each
functional group.
Herbivores: Herbivore species were more numerous in the meadow site, with 13 species
captured, than in the forest site, with only 6 species captured (Total of 18 species
captured). Species richness, calculated using the means of individual trap contents,
showed no significant difference between the forest and meadow site (Fig.1).
Biodiversity of herbivores was found to be higher in the meadow than the forest (Fig.2).
The similarity index (Fig.3) for the herbivore populations of the meadow and forest was
relatively low (0.32), indicating that the similarity of the species makeup of the two sites
was relatively low.
Predators: This group had the highest number of species collected of the functional
groups, with a total of 52 species collected in the survey. 29 species were found in the
meadow and 27 species were found in the forest. Species richness showed no significant
difference between the forest and meadow site (Fig.1). The comparison of biodiversity
indices for each site also showed no significant difference. The community similarity of
the predator populations between the two sites was the lowest of the three groups (Fig.3).
Fungivores: The overall number of fungivore species collected was the lowest of the
three functional groups (17). Fungivore species richness (Fig.1) and diversity (Fig.2)
showed no significant differences between the two sites. The community similarity of
fungivores between the two sites was the highest of the three functional groups
compared.
Discussion
Herbivores: Contrary to my hypothesis, this group was not extensively represented in
our specimen collection. Pitfall traps are designed to catch arthropods moving along the
ground surface. Perhaps this is not an efficient way of catching herbivorous arthropods
that spend most of their time on or in plants, potentially producing data that
underestimates the numbers and species in this group. Despite the small numbers
collected, several more species of herbivores were captured in the meadow site (13) than
in the forested site (6). These numbers could signify a significant difference in the species
composition of these habitat types that could be substantiated with alternative sampling
techniques (i.e. branch beating, light traps).
This was the only functional group that showed a significant difference in diversity
between the two sites. As was hypothesized, the diversity of herbivores was found to be
higher in the meadow than the forest (Fig.2); however, the average biodiversity indices
used in the graph were affected by low turnout of herbivores in the traps. The forest site
only had two traps that contained any herbivorous arthropods. These low numbers made
it impossible to calculate a confidence level for the forest herbivores.
The similarity index (Fig.3) for the herbivore populations of the meadow and forest was
relatively low (0.32), indicating that species makeup of the two sites was not very similar,
not surprising considering the obvious differences in the plant communities at the two
sites. This finding supports my original hypothesis as well.
Predators: This group had the highest number of total species collected of our functional
groups. Considering their tendency for rapid and more extensive mobility, predators
would tend to fall more frequently into pitfall traps than the other functional groups.
Potentially complicating the predator results even further, predators generally tend to
concentrate in higher densities at edges or transitions between habitat types, whereas
animals from other trophic areas have more random distribution (Ferguson, 2004). Our
survey sites were located adjacent to one another, at the edges of their respective habitats.
Perhaps our entire trap array could be considered to be influenced by an edge, or ecotone,
between two major habitat types. In my original hypothesis I had assumed that we would
find significantly more species richness in the meadow than the forest. There was not a
significant difference in species richness between the two sites, although the meadow site
did have slightly more species than the forest.
Contrary to my hypothesis, the data shows that the two sites did not have any significant
differences in biodiversity (Fig.2); however, when biodiversity is calculated using total
numbers of species and individuals within each functional group captured at each site
(Fig.4) it becomes more clear that the forested site had a higher overall biodiversity than
the meadow site. Simpson’s biodiversity calculations rely on species richness combined
with the evenness of their collected numbers. Upon closer analysis of the raw data, the
diversity index for the predators at the meadow appears to be deflated due to the great
numbers of ants found in a few traps at that site (Myrmica sp., Formica fusca, F.
neorufibarbis, and F. sanguinipes). Though clearly an important component of the
predator community of the meadow, it is possible that the social behavior of the ants is
responsible for their unusually high numbers in some of the meadow traps. In contrast,
the forested site showed no high numbers of predator species, but did have a number of
predators that were found only in the forested site: two native harvestmen (Leuronychus
parvulus and Hesperognamastna sp.), the native predatory snail (Haplotrema sp.), the
folding trapdoor spider (Antrodiaetus sp.), and the large snail-eating carabid (Cychrus
tuberoses). Though beyond the scope of this study, these species may be candidates for
indicators of closed canopy forest conditions.
Also contrary to my original hypothesis, the community similarity of the predator
populations between the two sites was the lowest of the three groups (Fig.3). I had
assumed that this group would show a high degree of similarity due their increased
mobility allowing them to pass between the adjacent sites with relative ease. Again,
however, the extremely disproportional ant data appears to have skewed the results.
Fungivores: The overall number of fungivore species collected was the lowest of the
three functional groups. Their lack of representation in pitfall trap samples is likely due to
their tendency to reside beneath the litter layer rather than moving along the surface.
Contrary to my hypothesis, both fungivore richness and diversity showed no significant
differences between the two sites. Supporting my hypothesis, the community similarity of
fungivores between the two sites was the highest of the three functional groups
compared. The presence of unusually large numbers of springtails (both Entomobrya sp.
and Hypogastrura sp.) in a few of the meadow traps may have impacted the data for this
group.
The presence of large downed logs and decaying stumps within the meadow site may
have served to reduce any differences between the adjacent sites. Ferguson (2004)
demonstrated that several groups of arthropods tend to move closer to moist edges, such
as those produced by large decaying logs, especially during times of precipitation
limitation. Course woody debris found in forest openings has been established to provide
critical habitat components for a variety of invertebrates (and vertebrates), increasing soil
moisture levels, moderating surface temperatures, reducing erosion, and providing a
source of nutrients and organic material. It has also been established that the
decomposition rates of logs in forest openings are not significantly different from those
under the forest canopy.(Edmonds et al.,2000)
Overall, the results of this study have demonstrated the limitations of designing a field
study and predicting outcomes without considering the multiple variables that may
influence the data collection. For example, the comparison of immediately adjacent
habitat types initially seemed like it would reduce the confounding variables that plague
the comparisons of isolated sites, such as differences in elevation, slope, aspect, soil
types, and plant associations. However, in this design, all of the traps were placed near
the ecotone between the two sites, producing a potentially powerful influence on the
biological community that was not accounted for in the data analysis. Placement of traps
closer to the center of each adjacent habitat type could produce data that is more
representative of each habitat type.
Another unaccounted for variable that may have severely influenced the data was the
presence of large coarse woody debris in the open canopy site. The presence of large
decaying woody debris may have a more significant impact on the terrestrial arthropod
community than the presence or absence of a canopy. Ferguson (2004) has demonstrated
that edges, such as those produced by logs, have significant impacts on key components
of the terrestrial arthropod community, especially in precipitation-limited environments
such as those found in the littoral layer of open habitats during the summer months in
Western Oregon. Edmonds et al. (2000) found that springtails, mainly fungivores, have
been found in significantly higher densities in clearcuts containing course woody debris
than in forests with similar components. Collections made in open canopy habitat without
a large woody component may produce very different results than were gathered here.
Seasonality may also influence data collection. This study was conducted at the end of
June, at the end of the rainy season in Western Oregon. Numbers and species of
terrestrial arthropods, especially littoral fungivores and detritivores would likely be
significantly impacted by seasonality. Hoonbok and Moldenke (2005) conducted a
terrestrial arthropod study in similar habitat that included both a wet season and dry
season collection in order to get a more complete picture of the community over time.
Conducting only a wet-season survey of terrestrial arthropods may have inflated the
numbers of particular species such as springtails.
The use of Simpson’s index to calculate biodiversity may also be limiting. Simpson’s
biodiversity measure takes into account only species richness and evenness, ignoring
differences in the sizes of organisms, seasonality, degree of mobility, degree of niche
specialization, microhabitat restrictions, and degree of endemism. Without taking into
consideration individual species ecology and behavior, species are reduced to equivalent
numbers, potentially creating misrepresentation of the community in collected data, such
as what appears to have occurred in this study with the presence of large numbers of
Formica spp. and springtails in particular traps within the meadow site. Further studies of
the impacts of canopy removal should incorporate other measures, such as using indicator
species to assess the qualities of the sites being compared.
The small sample size, inherent variability of microhabitats at individual trap locations,
and the systematic design of the trap array used in this study were also limiting.
Increasing the sample size by including multiple study sites and / or a series of annual
data collections should reduce the influences of microhabitat variability within both
canopied and open habitat surveys. Efforts to introduce randomization into the trap
positioning within survey areas should also be made.
References
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Harvesting on Soil Organisms and Decomposition in Western Washington. USDA Forest
Service Gen. Tech. Rep. PSW-GTR-178.
Ferguson, Steven H. 2004. Does Predation or Moisture Explain Distances to Edge
Distribution of Soil Arthropods? Am. Midl. Nat. 152:75-87.
Hoonbok, Yi and Moldenke, Andrew. 2005. Response of Ground Dwelling Arthropods to
Different Thinning Intensities in Young Douglas Fir Forests of Western Oregon.
Environmental Entomology. 34(5):1071-1080.
Kremen, C., Colwell, R.K., Erwin, T.L., Murphy, D.D.., Noss, R.F., and Sanjayan, M.A.
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Landscape Pattern of Arthropod Diversity in Headwater Riparian Zones. OSU, Dept. of
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