The National Park Service is interested in finding an indicator group
in order to efficiently measure the ecological health of various meadows
in the Eastern Sierra Nevada. In addition, the National Park Service
in interested in the basic community structure and movement patterns
of invertebrates in meadows since very little is known about montane
meadows in general. The primary objective of the study was to determine
if trails through the heavily visited meadows in Devil's Postpile National
Monument, CA are having a measurable effect on the meadow fauna as well
as collecting community structure data for invertebrates in the meadow
using pitfall traps. No significant affects on meadow invertebrates
could be linked specifically to trails through Soda Springs Meadow.
However, the results seem to suggest a correlation between vegetation
type and community structure. There is a need for further studies in
heavily used montane meadows.
In the course of human inhabitation, natural habitat
is inevitably artificially fragmented. Fragmentation has proven to be
significantly detrimental for many species of flora and fauna (Forman
1995) and is thus an extremely important subject of study. Studies
on habitat fragmentation are not only of interest to the ecological
scientist, but to those that manage natural areas for the use of human
recreational activities as well.
Fragmentation can come in many shapes, sizes and degrees.
Natural boundaries can be created, for example, by a river traversing
a meadow or a forest fire that burns a large patch of trees. In contrast
to areas of human created fragmentation, natural fragments are rarely
permanent and actively maintained. A common source of fragmentation
caused by humans is the creation roads and railways for transportation
(Forman 1995). Disturbances
from roads and railways that cause fragmentation are usually in place
for an extended period of time (Forman
1995). In any case of fragmentation, a number of ecological effects
could be occurring.
Besides the obvious shrinkage of natural habitat for
native flora and fauna to simply exist in as a result of fragmentation,
other ecological effects have been shown to occur. Previous studies
have shown that landscape boundaries can have an effect on ecological
flows (Wiens 1992). Nutrient
flow, genetic flow between populations, and individual movement, among
others, can be altered depending on the nature of the fragmentation
boundary (Wiens 1992). Genetic
flow between populations and simple movement of individuals can be a
serious issue when considering metapopulation dynamics and the recolonization
of habitat patches (Hansson,
Fahrig, and Merriam 1995). In a landscape mosaic considering metapopulation
movement, the actual distances between patches are not straight-line
measurements, but rather a complex function of boundary permeabilities
to moving organisms (Wiens 1997).
When considering the management of significantly reduced
natural habitat, a category so many of the national and state reserves
in the United States find themselves in, most question whether it is
possible to maintain a high population survival probability of organisms
(Hansson et al. 1995). In
most cases, habitat fragmentation cannot be qualified using one measure;
it involves the simultaneous loss of habitat, reduction of patch size
and increase in patch isolation (Hansson
et al. 1995). As a result of difficulties in measuring the affect
of habitat fragmentation on biotic communities, there is no simple strategy
for landscape managers trying to protect reserves. In considering the
plight of landscape managers, it is then particularly important to do
fine scale ecological studies in areas under active wildlife management
in order to develop a strategy to reduce the threat of habitat fragmentation
on the survival of organisms.
Many studies on habitat fragmentation have shown that
paved roads can be detrimental to the movement patterns of fauna (Trombulak
and Frissell 2000). Interestingly enough, very few studies have
focused on smaller disturbances that segment habitats like trails and
footpaths. Although a dirt or gravel path may not seem as obviously
adverse to faunal movements as a paved highway with a high traffic volume,
research has shown that even grassy field-tracks can pose difficulties
for some animals (Mader, Schell,
and Kornacker 1990). In fragile ecosystems, such as those of mountain
meadows, the effects of fragmentation might be magnified. In effect,
a grassy path could possibly act just as though a paved highway were
placed through the meadow.
Devil's Postpile National Monument, part of the Sierra
Nevada Network parks, is home to many ecologically important features,
including montane meadows. The park is also a popular tourist destination
with more than 100,000 recreational visitors per year (National
Park Service Survey 2003). Hence, the trails through the park meadows
are in frequent use. The meadows in the monument provide habitats for
a variety of plants and animals in addition to capturing sediment and
buffering stream flows (Hagberg
1995). Furthermore, the meadows are highly valued for scenic vistas
and colorful flora, causing a disproportionate amount of public use
The National Park Service is interested in finding
an indicator group in order to efficiently measure the ecological health
of various meadows in the Eastern Sierra Nevada. In addition, the National
Park Service in interested in the basic community structure and movement
patterns of invertebrates in meadows. Insects have the potential to
be excellent indicators for the status of montane meadows. They are
diverse, easy to sample, and sensitive to certain microclimates (Rosenberg,
Danks, and Lehmkuhl 1986). Insects constitute a significant portion
of terrestrial biomass and play a significant role in ecosystem functioning
(McGeoch 1998). Terrestrial
invertebrates have the potential to be of excellent use to the National
Park Service and are thus the subject of this study.
The primary objective of the study was to determine
if trails through the meadows in Devil's Postpile National Monument
are having a measurable effect on the meadow fauna. In order to accomplish
this task, a matrix of pitfall traps was employed in collecting terrestrial
invertebrates adjacent to two trails that traverse the meadow. In addition
to the primary objective, a comparison of terrestrial invertebrate sampling
methods was accomplished using the community structure data gained from
this study as well as of that from Holmquist (unpublished data 2004).
Materials and Methods
Portions of this study have been modeled after prior
studies on the movement patterns of Coleopterans, namely, the 1990 study
published in Biological Conservation, "Linear Barriers to Arthropod
Movements in a Landscape" by Mader, Schell and Kornacker, and the
1984 study published in Biological Conservation, "Animal Habitat
Isolation by Roads and Agricultural Fields" by Mader. Modifications
have been made, specifically in the spacing of the pitfall traps, to
fit the scale of this meadow study.
As an initial step to determine scale for this project,
two days and one night of pitfall traps were set in Soda Springs Meadow
in Devil’s Postpile National Monument. Initially, a matrix of
3 traps by 4 traps was placed in a North-South orientation approximately
1.0m apart. The traps were set for 6 hours in an area of relatively
low vegetative cover. The number and order of invertebrates captured
was recorded for each trap. An average of 4 common black ground-dwelling
beetles per trap was found. Relatively few other invertebrates were
found including spiders (Araneae) and mites (Acari). The traps were
then emptied and replaced for the night trial. No invertebrates were
found in the traps after the 10-hour night trial. After the night trial,
the traps were taken out and the soil cores replaced. The trap matrix
was then moved to another portion of the meadow further west with more
vegetative cover for comparison to the sparsely vegetated previous site.
During this second day trial, the traps were placed in a matrix of 3
traps by 4 traps in an East-West orientation 2m apart. The traps were
set for 5 hours and an average of 2 of the indicator beetles per trap
was found. Almost no other invertebrates were found including ants (Hymenoptera),
spiders (Araneae) and mites (Acari). After the fauna were classified
to order, they were released and the soil cores replaced.
In light of the results from the pilot study, the trap
matrix at 1.0m apart was feasible for a mark-release-recapture study
in this particular meadow. In addition, two ground-dwelling beetles
seemed abundant throughout the meadow so as to be easily measured as
indicators of ecological health.
During the months of July and August 2004, the commonly employed pitfall
trap method (Mader et al. 1990,
Mader 1984) was used to survey the community structure of terrestrial
invertebrates in relation to two trails through the meadows of Devil’s
Postpile National Monument, CA.
The study site was adjacent to the Ranger Station in
Devil’s Postpile National Monument, located in Mammoth Lakes,
CA. The two trails chosen for this study, a 2.6m wide gravel path delineated
with large stones and a 1.0m wide packed dirt trail with no hard edges,
originate from the Ranger Station area and traverse the meadow in a
general North direction. The site was chosen because the trails through
the meadow receive a large amount of foot traffic each year from visitors
on their way to the Devil’s Postpile geological formation, Rainbow
Falls, or other popular landmarks. In addition, very little is known
about the invertebrate population in the Devil’s Postpile National
Monument meadows and the park service is eager for any type of census
survey to be conducted.
Two common terrestrial beetles (Family Carabidae) were
used as an indicator of invertebrate movements in relation to the two
trails that traverse the meadow. A mark-release-recapture study using
the pitfall traps placed alongside the trails to capture the beetles
was conducted. Pitfall traps are useful, and often utilized, for mark-recapture
studies involving small vertebrates and invertebrates (Holland
and Smith 1999, Mader et al. 1990, Mader 1984, Perner and Schueler 2004).
Pitfall traps for this study consisted of clear plastic cups, approximately
7.6cm in diameter and 7.6cm deep. The cups were placed in corresponding
holes of the same dimensions, cut out of the meadow with a hand trowel,
so that the lip of the plastic container was level with the ground.
The cores of soil and plant matter from the holes were placed in a protected
area, out of the sun and wind. At the conclusion of the study the soil
cores were replaced in the exact location they were taken from so as
to minimize any impacts on the meadow.
At each of the two trails, grids of pitfall traps were
oriented so that three rows of traps extend into the meadow from each
side of the trail (see Figure 1). The first row
of pitfall traps was located 1.0m from the edge of the trails and each
successive row was located 1.0m from the previous row. Each trail held
two replicates; each replicate consisted of 30 traps, 15 on each side
of the trail (see Figure 1). Thus, 120 pitfall
traps were in use during the study period.
Figure 1 – Pictorial representation
of pitfall trap grid
around a trail in Devil’s Postpile National Monument.
The traps were in use for 6 daylight hours during
weekdays. When not in use, the traps were closed by removing the plastic
container from its hole. After each day of trapping, the number and
order of all invertebrates in each individual trap was noted. Considering
the results of the pilot study, an average of three beetles per trap
was expected. Thus, an average trap day would yield 360 beetles. Beetles
captured in the traps were individually marked on their elytrae using
enamel paint applied with a blunted hypodermic needle while being held
still with a light vacuum. Using a set of nine enamel paints (red, orange,
yellow, green, blue, silver, gold, brown and white), captured beetles
were given a unique color code that did not change throughout the study
period. For example, the first beetle caught was marked with a singular
red dot. Single dots continued until all colors had been exhausted;
at this point, two-color combinations were utilized. An example of the
two-color system would be a beetle with a red and an orange dot (see
attached Table 2 for color combinations).
Once all invertebrates had been counted and the paint
was dry on the beetles, the invertebrates were released 50.0cm from
their place of capture. The position of release with respect to the
pitfall trap of capture was determined using a random number table (see
attached Table 3 for random numbers). In this
study, the area within a 50.0cm radius of the trap was divided into
twelve sections, similar to a clock face, with “noon” at
magnetic North. The random numbers range from 1 to 12. For releases,
12 corresponded to “noon” or magnetic North, and all other
numbers corresponded to their respective “clock face” direction.
The pitfall trap location was noted for any recaptures and the recaptured
beetles were released using the method as described above to the most
recent place of capture.
At the conclusion of the study, an analysis was conducted
on the community structure of invertebrates adjacent to the trails that
included the difference in general abundance of terrestrial invertebrates
between the gravel trail and the dirt trail, the abundance of different
orders of invertebrates in relation to distance from a trail, and regression
analyses. Community structure data was analyzed using JMP 4.0 software
(2001). Unfortunately, it was impossible to conduct an analysis on the
movement patterns of the indicator beetle both between traps on the
same side of the trail and across the trail, for not enough data was collected.
As mentioned previously, the initial premise for this
study was to focus on two particular species of beetle. Unfortunately,
only one recapture occurred out of 69 marked beetles. The beetle was
initially captured in the matrix surrounding the hard-packed dirt trail.
Two days after the initial release, the beetle was found in a trap 3.0m
South from the original trap. The beetle did not cross the trail.
Fortunately, however, the method for capturing the
two beetle species, outlined as the major experiment, allowed for extensive
terrestrial invertebrate community structure data to be collected at
both trails. A two-tailed t-test analysis (n = 4, p = .05) of the average
number of each order of invertebrate found at each site showed that
there was significantly fewer (p = .005) invertebrates found that fell
into the “other” category including true bugs (Hemiptera),
flies (Diptera), caterpillars (Lepidoptera), leafhoppers (Homoptera),
centipedes (Scolopendrida), and grasshoppers (Orthoptera). There was
no statistical difference found between the rest of the orders. However,
there were generally more spiders (Araneae) found at the hard-packed
dirt trail and generally more ants (Hymenoptera) and mites (Acari) at
the gravel trail.
In addition to a difference in the community structure
between trail types, a difference was found in abundance among most
invertebrate orders in the overall study area. Although the abundance
of mites and the abundance of ants were found not to be significantly
different from one another (n = 2, p = .52), they were both generally
more abundant than all other orders of invertebrates. Ants constituted
50% of all invertebrates collected. Mites (33%), spiders (11%), beetles
(6%), and other invertebrates (1%) comprised the rest of the invertebrates
captured (Figure 2). The abundances of each order
varied markedly throughout the study period. In late July 2004, all
orders showed a general decline in abundance at the study site (Figure
The invertebrate community data also showed some general
trends in order abundance in relation to distance from the trails. The
hard-packed dirt trail showed an increase in the number of beetles captured
with an increase in distance from the trail (Figure
4d). Similarly, an increase in distance from the gravel trail showed
a decrease in the number of ants captured in the pitfall traps (Figure
4a). Not much difference was found in the other orders of invertebrates
(Figure 4b,c,e). Although there were general trends in the abundance
of invertebrates at varying distances from the trails, no statistical
difference (n = 4, p = .05) was found (Table 1).
The decrease in invertebrate abundance with an increase in distance
from the object of disturbance is markedly different from the findings
of J. Holmquist (unpublished data, 2004) using the vacuum sampling technique.
Holmquist found that an increase in distance from trails showed an increase
in the number of invertebrates captured.
The vacuum sampling method employed by Holmquist in
his invertebrate study in Devil’s Postpile National Monument illustrated
a difference in the numbers and kinds of orders captured from that of
the pitfall traps used in this study. Not surprisingly, the vacuum sampling
method managed to capture more flying invertebrates like flies and leafhoppers
(Figure 5). The pitfall trap method was useful
for capturing ants and mites (Figure 5).
Table 1 - Statistical
difference between the abundance of invertebrates at a Dirt Trail and
a Gravel Trail by distance from trail.
– No statistical difference in overall order abundance was found
in community structure near trails in Devil’s Postpile National
3 – Ants (a) and mites (b) were similarly more abundant than
spiders (c), beetles (d), and all other invertebrates (e) captured over
the study period in Devil’s Postpile National Monument. All orders
showed a decrease in abundance during the course of the study. Error
bars represent one standard error from the mean of 4 replicate areas.
4 – The hard-packed dirt trail showed an increase in the number
of beetles (d) captured with an increase in distance from the trail.
The gravel trail showed a decrease in the number of ants (a) captured
with an increase in distance from the trail in Devil’s Postpile
National Monument. Not much difference was found in the mites (b), spiders
(c), or other (e) orders of invertebrates.
5 – By vacuum net sampling and pitfall trapping terrestrial
invertebrates in Devil’s Postpile National Monument two completely
different community assemblages are produced. Vacuum sampling captures
many more flying insects than pitfall trapping.
The purpose of this study was to determine if trails
through meadows in Devil’s Postpile National Monument are having
a measurable effect on the local fauna, specifically, terrestrial invertebrates.
Unfortunately, the results of this study are quite inconclusive.
The lack of recapture data from the mark-release-recapture
study on the ground-active beetles was highly detrimental to the purpose
of the study. It is possible that the paint used to mark the elytra
of the beetles caused the invertebrates to be subject to increased predation
or other sources of mortality. In addition, few beetles overall were
captured, possibly caused by a natural decrease in abundance due to
the lateness of the season.
The results of the supplemental study in community
structure did show a difference between the two trails in invertebrate
assemblages. The difference found in the abundance of spiders at the
hard-packed dirt trail and the abundance of ants at the gravel trail
most likely were dependent on the vegetation type surrounding the trail.
The vegetation surrounding the dirt trail was predominated by an abundance
of willow (Salix sp.) whereas grasses and sedges dominated the vegetation
surrounding the gravel trail. In addition, the availability of moisture
may have had an effect on the invertebrate community structure. The
dirt trail followed alongside the San Joaquin River while the gravel
trail was much drier. Since there are many compounding factors that
cause variation in the relationship between invertebrates and vegetation
type, it is difficult to make an absolute conclusion (Strong,
Lawton, and Southwood 1984).
It is surprising that there was no statistical difference
in the abundance of invertebrates in relation to distance from the trail.
It is possible, since measurements were only taken 3.0m into the meadow,
that a difference would not have been detectible until further out into
the meadow. Holmquist and Schmidt-Gengenbach (Sierra
Nature Notes, 2003) found detectable differences in invertebrate
abundance at distances up to 10m from trails in meadows in Yosemite
The two different sampling methods, vacuum netting
and pitfall trapping, provided very different snapshots of the community
structure in the Devil’s Postpile meadows. It is impossible and
also unnecessary to determine which sampling method is ultimately more
useful than the other. Ideally, using both methods in conjunction would
give the most complete representation of invertebrate community structure.
Both methods have their drawbacks. The vacuum method cannot be used
in wilderness areas due to the mechanized parts, though using pitfall
traps involves digging a matrix of holes and thus disturbing wild habitat.
Depending on what needs to be sampled, a variety of non-mechanized sampling
methods could be used (Jervis
and Kidd 1996). For example, a combination of sweep netting and
pitfall trapping would be useful in wilderness areas, although both
methods are not useful for collecting quantitative data.
Overall, more data is needed to make any conclusions
concerning the degree of impact that trails are having on the meadows
in Devil’s Postpile National Monument. In addition, further comparisons
should be made between vacuum sampling and pitfall trapping data to
determine the degree of biases associated with each method.
I would like to thank my mentor Dr. Jeff Holmquist
and Jutta Schmidt-Gengenbach for advice and assistance throughout this
study. I would also like to thank the National Science Foundation, the
University of California at San Diego, and the White Mountain Research
Station for funding and the use of facilities for this ecological research.
Table 2 –
Color combination chart used for marking captured
beetles in Devil’s Postpile National Monument.
Table 3 –
Random numbers for use in releasing captured beetles from pitfall traps
in Devil’s Postpile National Monument.