Paving the way for habitat disturbance: Different road types have distinct ecological impacts on annual plant communities and gall-forming insects ...
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Paving the way for habitat disturbance:
Different road types have distinct ecological impacts on annual plant
communities and gall-forming insects in the Mojave Desert
Maria Carias1, Emily Hascall2, Kelsey Pennington1, Jasmine Summers-Evans 2
1
University of California, Los Angeles; 2University of California, Santa Barbara
Road disturbance can facilitate the establishment and growth of invasive plant species
and herbivorous insects in adjacent ecosystems. The hydrological and atmospheric
impacts of roads are especially important to understand in arid desert environments that
are highly sensitive to water stress and dust pollution. In this study, we looked at how
impacts of road disturbance change among different road types within the Mojave
Desert. Specifically, we observed how three types of roads (paved road, dirt road, and
hiking trail) and proximity to the roads impact both the abundance and size of germinating
annual plants and the density of galls on creosote bushes (Larrea tridentata). Additionally,
we tested the effect of dust pollution on the growth of Erodium cicutarium and Bromus
spp. Different types of roads produced distinct ecological impacts on the surrounding
plant and insect communities. Paved roads had the highest gall density and largest
Bromus spp., while dirt roads had smaller Erodium cicutarium. Our findings contribute to
the growing knowledgebase of how road disturbances uniquely impact plant invasion
patterns and herbivorous insects.
Keywords: road disturbance, Mojave Desert, Larrea tridentata, Erodium cicutarium,
Bromus spp.
INTRODUCTION landscapes by altering factors such as
hydrology, dust pollution, microclimate
Anthropogenic transportation corridors conditions, soil erosion, and animal
create habitat disturbances and mortality. Transportation corridors can also
fragmentation in surrounding environments serve as conduits for plant and animal
(Coffin 2007). In 1998, the term “road movement and are often cited as major
ecology” emerged to refer to the science of causal factors in the successful invasion of
how transportation corridors such as non-native flora and fauna to adjacent
highways and roads affect ecosystem landscapes (Coffin 2007). Species that
components, processes, and structures benefit from corridor facilitation are
(Coffin 2007). Since then, the subject of typically generalist species that are able to
transportation infrastructure and its effects exploit highly variable environmental
on ecosystems has become more widely conditions (Forman and Alexander 1998).
studied. Roads have been shown to affect The construction and maintenance of roads
both the abiotic and biotic components of can also remove native species and their
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seed banks, making it difficult for them to roadside plants (Farmer 1993). Such impact
reestablish after disturbance (Holzapfel and can lead to less favorable conditions for
Schmidt 1990). Understanding how native plant communities, compounding the
disturbances caused by roads impact the negative effects of road-induced
ecology of surrounding biotic communities is hydrological changes. At the same time, less
crucial for better informing transportation vigorous plants are also less desirable to
land-use decisions (Dix et al. 2010). herbivorous insects (Cornelissen et al. 2008).
While ecosystem-level impacts of These two disturbance factors can uniquely
transportation corridors on adjacent alter surrounding plant and insect
environments are well understood, less is communities.
known about how non-native plant and The hydrological and atmospheric impacts
herbivorous insect communities respond to of transportation corridors are especially
different road designs. Different types of important to understand in arid desert
roads vary in their level of environmental ecosystems that are highly sensitive to water
disturbance (Holzapfel and Schmidt 1990). stress and dust pollution. Plant succession
Paved roads pose the most ecological impact and insect responses following disturbance
on nearby ecosystems, followed by unpaved are less understood in arid landscapes
roads and walking trails (Holzapfel and relative to temperate regions (Abella 2010).
Schmidt 1990). Paved roads are designed to In the Mojave Desert, the spread of non-
drain water from their edges, which can native annual plants and the distribution of
increase the moisture available to adjacent herbivorous insects are of particular
plant communities (Holzapfel and Schmidt concern. Previous studies in the Mojave
1990). Higher water availability coupled with Desert have found the richness and biomass
additional disturbances can increase an of non-native annual plants increase with
environment’s vulnerability to non-native dirt road density (Brooks and Berry 2006).
plant invasions and insect parasitism. The invasion of non-native plant species is a
According to the Plant Vigor Hypothesis, primary cause of biodiversity decline and
insect herbivores prefer to oviposit on large, threatens the existence of native plants and
vigorously growing plants (Price 1991). their associated organisms (Barbosa et al.
Therefore, insect herbivores are generally 2010). Non-native annual grasses can also
drawn to less water-stressed plants, which increase the fire load of an environment,
are often found along roadsides. making ecosystems like the Mojave Desert
In addition to changes in hydrology, roads increasingly flammable and vulnerable to
can alter atmospheric composition in wildfires (Kalwij et al. 2008). Since roads can
adjacent microhabitats (Coffin 2007). Air function as corridors for parasitism and
tends to be dustier near roads, especially at disease spread as well, herbivorous insects
unpaved dirt roads. As roads are constructed such as gall-forming insects can serve as key
and surrounding soils are eroded, dust indicators of parasitic responses to
pollution is more easily spread to nearby disturbance (Lightfoot and Whitford 1991).
plants. The coating of dust can impede Galls are abnormal plant tissue growth
vegetation’s ability to photosynthesize, produced in response to injected chemical
which can hinder the growth and vigor of stimuli of invading organisms (Russo 2006).
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Though the complex chemical relationships large variations among host plant species
between host plants and gall-forming insects that are still widely unknown (Russo 2006).
vary greatly by species, there is growing We hypothesized that areas closer to
evidence that galls redirect nutrients from roads would have a higher abundance of
their host plant’s roots and leaves into gall non-native annual plant species, larger non-
tissues, forming nutrient sinks for the host native annual plants, fewer native annual
plants. The management of non-native plants, and a higher density of galls. We also
plants and parasitic insect species is hypothesized that the paved road would
hampered by a lack of information on desert display the largest non-native plants and the
ecosystems, making the Mojave Desert an highest density of galls compared to the dirt
ideal location to further investigate the road and trail. Since paved roads typically
negative effects of road disturbances funnel higher amounts of water to
(Brooks and Berry 2006). surrounding landscapes (Holzapfel and
In this study, we explored how annual Schmidt 1990), we expected surrounding
plant species composition and regional gall non-native plant and pest communities to
density differ in areas near and far from benefit the most near the paved road
different types of desert roads. We studied compared to the other road types. We
three road types: a paved one-lane expected dust pollution to be higher on the
expressway, a dirt vehicle road, and a dirt dirt road than on the trail and paved road,
hiking trail. We focused on two dominant causing plant abundance and gall density to
non-native plants in the Mojave Desert— be lowest at that site. Lastly, we expected
Erodium cicutarium and Bromus spp.—due native plant species abundance to be highest
to their pervasiveness in the region (Brooks at the trail road since it is the least disturbed
and Berry 2006). We asked how road type area. We hoped answering these questions
and proximity to a road would affect the would provide ecologists and those involved
abundance and size of Erodium cicutarium in road construction management with vital
and Bromus plants, the abundance of native information on how impacts of road
annual plants, and the density of gall disturbance on the surrounding plant and
parasitism. To assess gall density, we studied insect communities change between road
creosote bushes (Larrea tridentata), which designs.
are critical host plants for gall-forming midge
flies (Cecidomyiidae) and their parasites. METHODS
Creosote bushes are hosts to at least 16
species of gall midges, all from the genus 2.1 Study System
Asphondylia (Russo 2006). Roadside
We conducted our research between
creosote shrubs tend to be larger and have
February 24 and February 28, 2021, during
denser foliage, as well as higher arthropod
the germination period of annual plants in
density (Lightfoot and Whitford 1991). As
the Mojave Desert. Research was done at
predicted by the Plant Vigor Hypothesis, the
the Sweeney Granite Mountains Desert
quantity of water availability has shown to
Research Center (hereafter referred to as
be a large determinant in the ability of a host
“the Granites”), a field station owned by the
plant to support gall midges, yet there are
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University of California, Riverside. The near zone included plants within three
Granites is located in the East Mojave Desert meters of the edge of the road and the far
in San Bernardino County and lies within the zone included plants at least 10 meters away
Mojave National Preserve. Near the center’s from the edge of the road. Within each zone
eastern boundary, the topography descends at each site, we conducted multiple surveys
from high ridges into bajadas and washes regarding the surrounding annual plant and
densely vegetated by multiple habitat types gall communities.
(James Andre, Sweeney Granite Mountains
Desert Research Center). The washes 2.2 Plant Surveys
support creosote bush scrub and mixed
desert scrub communities (Simpson et al. In order to determine how different types
1997). The landscape cover also supports an of roads distinctively serve as corridors for
abundant annual plant community, invasive species, we surveyed annual plant
including both native and non-native species abundance and size within a 0.5 m² quadrat.
such as Erodium cicutarium and Bromus We quantified the most common non-native
spp.—which likely include a mix of Bromus annual species, Erodium cicutarium and
madritensis and Bromus tectorum. Bromus, as well as all native annual plants
We collected data from paved, dirt, and we encountered. For abundance, every
trail roads. The paved road site was located Erodium cicutarium, Bromus, and native
at Kelbaker Road, a one-lane expressway annual was counted. For size, five Erodium
west of the Granites research facility. cicutarium and five Bromus were randomly
Kelbaker Road was first developed as a dirt selected for measurement. To measure
road in the 1930s. The road was paved in Erodium cicutarium, we used the length
1988 and repaved in 2018 (Pers. Comm. between the two longest leaves. For
James Andre). The dirt road site was located Bromus, we pulled the single grass strand
along a 1.1-mile-long vehicle road stretching straight and measured from base to tip.
between Kelbaker Road and the Granites To account for possible differences in size
research facility. The road was first of germinating annuals based on their
developed in the 1950s but has seen a location either inside or outside the canopy
dramatic increase in use over the past 15 cover of surrounding bushes (Holzapfel and
years, as it provides access to campsites Mahall 1999), we replicated all plant surveys
alongside the road. The trail site was located both inside and outside the canopy cover of
along the Al A. Allanson trail within the creosote. To do so, we placed the quadrat
boundary of the Granites research facility. inside the canopy cover, with the edge of the
The trail was built in 2001, but the first 100 quadrat aligning with the bush’s longest
meters of the trail dates back to the 1930s as branch. We then flipped the quadrat so it lay
a portion of a previous two-track road outside the canopy cover with the same
historically used by ranchers. edge still aligning with the longest branch of
To disentangle road disturbance from the bush.
other numerous geographical or biological
factors, we looked at two zones within each
road site: near and far from the road. The
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2.2 Gall Survey control group and was left undusted. Plot 2
was dusted only once at 10:00 A.M. on
To assess how different types of roads February 24. Plot 3 was dusted every
distinctively serve as corridors for insects, morning at 10:00 A.M. for each of the five
we conducted surveys on gall density in days of our experiment. An equal amount of
creosote bushes. Creosote bushes were dust was administered between the two
systematically selected by surveying the next experimental plots, leaving the number of
available bush within our designated near times dusted as our only variable. To
and far zones at each of our three road standardize the amount of dust used within
types. To account for possible geographical each plot, every Erodium cicutarium
differences between the left and right sides replicate was sprinkled with dust until it was
of the roads, we surveyed bushes on both visibly completely covered. Every Bromus
sides. To account for size differences was sprinkled with 1 tablespoon of dust. The
between creosote bushes, we took dust used in the study was collected dirt
measurements for the volume of an ovoid – found directly outside the plot (void of any
as it best represented the shape of an rocks, debris, or other plant material).
average creosote within our survey Each plant replicate was initially measured
parameters. A transect tape was used to at the start of our experiment on February
measure the bush’s longest radius, shortest 24 at 9:00 A.M. and remeasured at the end
radius, and height of tallest branch. Galls of the experiment on February 28 at 5:00
were counted within each bush, identifying P.M. Initial measurements were subtracted
and enumerating between species types. from final measurements to calculate the
Binoculars aided in counting galls on tall, growth of every plant replicate across the
unreachable branches. five days.
2.2 Experimental Plot STATISTICAL ANALYSES
To assess the effect of dust pollution on All statistical analyses were done using
the growth rate of germinating non-native JMP Pro 15 statistical software, (JMP®,
annuals, we conducted a sub-experiment on Version < Pro 15>. SAS Institute Inc., Cary,
both Erodium cicutarium and Bromus from NC, 1989-2019).
February 24–28, 2021, that measured how
growth rate differed given different 3.1 Plant Survey Analyses
frequencies of dusting. The experiment
included three different 1 m² plots located at A Two-Way ANOVA was used for all plant
the Granites research facilities away from surveys. To account for variation among
any road disturbances. In each of the three individual creosote bushes, creosote ID
plots, 25 Erodium cicutarium and 25 Bromus number was nested as a random effect. To
were randomly selected to receive the see how Erodium cicutarium and Bromus
dusting treatments. spp. abundance differed, we analyzed
We dusted each plot’s plant replicates at abundance across the three different roads,
different frequencies. Plot 1 served as our the distance from the road, and the
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interaction between road type and road RESULTS
distance. To assess how Erodium cicutarium
and Bromus sizes differed, we analyzed size 4.1 Plant Surveys
across each road type, the distance from the
road, and the interaction between road type Erodium cicutarium abundance did not
and road distance. differ among road types (N= 102, F= 1.87, p=
0.16, Fig. 1), between distance from road (N=
3.2 Gall Survey Analyses 68, F= 0.017, p=0.90, Fig. 1), and there was
no interaction effect between road type and
To account for size differences between distance from road (N= 34, F= 2.80, p=
creosote bushes when looking at galls, we 0.065). Bromus spp. abundance was lowest
used gall abundance to calculate calculated at dirt roads (N= 68, F= 6.42, p= 0.0024, Fig.
gall density within each cubic meter of a 2). Bromus did not differ in abundance
creosote bush. First, we calculated the between near and far zones (N= 102, F= 1.73,
volume of each bush replicate by using the p= 0.19, Fig. 2), and there was no interaction
formula for volume of an ovoid, (4/3 * between road type and road distance (N= 34,
longest radius * shortest radius * height). F= 2.87, p= 0.062). Native annual abundance
Creosote volume data was log transformed was higher near trails (N= 68, F= 12.65, p=
to meet the constraint of normality. Gall
and far zones (N= 102, F= 1.73, p= 0.19). There was no
interaction between road type and road distance (N=
34, F= 2.87, p= 0.062).
Figure 1. The effect of road type on Erodium
cicutarium abundance. We counted Erodium
cicutarium abundance near creosote bushes by three
different types of roads in the Mojave Desert—a
paved one-lane expressway, a dirt vehicle road, and a Figure 3. The effect of road type on annual native
hiking trail. Surveys were conducted both near plant abundance. To assess how road types affect
(within 3 m of the road) and far (at least 10 m away) native plant establishment, we collected native plant
from the roads. Erodium cicutarium abundance did abundance across three different road types- a paved
not differ among road types (N= 102, F= 1.87, p= 0.16) one-way express lane, a dirt vehicle road, and a hiking
or by distance from road (N= 68, F= 0.017, p=0.90). trail. Surveys were conducted both near (within 3 m
There was no interaction effect between road type of the road) and far (at least 10 m away) from the
and distance from road (N= 34, F= 2.80, p= 0.065) roads. The abundance of native annuals was higher
near trails (N= 68, F= 12.65, p=Surveys were conducted both near (within 3 m of the paved one-lane expressway, a dirt vehicle road, and a road) and far (at least 10 m away) from the roads. The hiking trail. Gall abundance surveys were done both size of Erodium cicutarium was smallest at dirt roads near the roads (within 3 m of the road), and far (at (N= 340, F= 8.92, p= 0.0003, Figure 1), larger near least 10 m away) from the roads. Gall density was roads (N= 510, F= 191.64, p=
Table 1. Total number of galls of each species found 4.3 Experimental Plots
per road type. Within 144 creosote bushes, we
counted 2,101 galls belonging to four main species of gall Erodium cicutarium grew the most in the
midges. The most abundant species was Asphondylia
resinosa, accounting for 49.8% of total galls.
plot that was dusted only once (N = 25, F =
17.36, p =potentially allowing Bromus to grow in was consistent with our hypothesis that road
greater abundance near paved roads disturbance facilitates a higher density of
compared to dirt roads and trails. gall-forming insects.
Previous studies have shown Erodium
cicutarium is a generalist species, meaning it 5.3 Future Research
is resistant to drought and low levels of
disturbance (Kimball et al. 2014). The Due to the timing of our study, we were
resilient nature of Erodium cicutarium only able to observe annual plants that had
supports our finding that abundance did not begun germinating a few weeks prior. A
change across different road types. Though longer-term observational study on annual
its ability to establish was not influenced by plants could provide additional insight into
roads, our finding that its size was higher in factors that affect non-native annual plant
the zones near paved roads and trails, as well as abundance and growth near desert roads. A
in the lightly dusted experimental plots, leads previous study conducted on the effects of
us to believe Erodium cicutarium was able to predation and competition on survivorship
thrive under conditions of light disturbance. and community structure of desert annuals
High levels of disturbance can be in the Sonoran Desert found that
detrimental to native species and destroy competition among annual plants reduced
seed banks (Holzapfel and Schmidt 1990). As species’ biomass and growth rates (Inouye et
expected, the road type with the lowest level al. 1980). As the density of Erodium spp.
of disturbance supported the highest decreased due to rodent predation, other
abundance of native annual species. This annual species were able to increase in
finding shows how different road types not abundance across the landscape.
only influence non-native plant species but Specifically, it would be interesting to assess
also impact the success of native species. if Erodium cicutarium and Bromus
experience interspecific competition that
5.2 Gall Density alters their distribution and survivorship
over the course of their lifespan.
Higher water availability from runoff can According to the Plant Vigor Hypothesis,
produce more vigorous creosote bushes that insect herbivores such as gall-forming
would attract more gall-forming insects midges may prefer plants that are less water
(Ludwig and Blanche 2001). Creosote bushes stressed (Price 1991). Thus, creosote bushes
have been observed to have faster already parasitized by other species known
vegetative growth rates and higher leaf to increase water stress on plants
production when provided additional water (Sangüesa-Barreda et al. 2013)—such as
(Sharifi et al. 1988). We observed larger, mistletoe (Viscum album)—could exhibit
more vigorous creosote bushes near our less gall density compared to creosote
road sites, which aligns with prior research bushes not already parasitized by mistletoe,
on the correlation between water especially when mistletoe increases water
availability and creosote health. Our findings stress on plants. Within our gall survey, we
that gall density was highest near roads in observed three different occurrences of
general and especially high near paved roads mistletoe, but there were not enough
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CEC Research https://doi.org/10.21973/N3KX0W Winter 2021 Vol. 5 Issue 2 10/12replicates to see any trends in regard to how communities, especially when making
mistletoe parasitism impacted gall decisions on road design.
abundance. Based on our study results, we
speculate creosote bushes parasitized by ACKNOWLEDGMENTS
mistletoe may not be as impacted from road
disturbance as non-mistletoe parasitized This work was performed at the University
creosote would be, and that mistletoe of California’s Sweeney Granite Mountains
parasitism on creosote bushes may produce Desert Research Center, doi:
the greatest impact along paved roads 10.21973/N3S942. Thank you to reserve
compared to other road types. This would be directors, Jim André and Tasha La Doux, for
important to further investigate how roads welcoming us to the Granites and sharing
impact the surrounding plant communities your knowledge of the area with us. Thank
within the Mojave Desert. you to our instructors, Tim Miller and Sarah
Kingston, for the continuous support,
5.4 Conclusion guidance, and enthusiasm throughout our
research. Additional thanks to Jake
Overall, this study contributes to the Hernandez, MC Moazed, and Elyse
growing literature on how different road Fitzsimons for mentoring and advising us
designs impact the ecology of surrounding along the way.
plant and animal communities within the
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