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Paving the Way for Habitat Disturbance 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 1University 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 CEC Research | https://doi.org/10.21973/N3KX0W Winter 2021 Vol. 5 Issue 2 1/12 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). CEC Research | https://doi.org/10.21973/N3KX0W Winter 2021 Vol. 5 Issue 2 2/12 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
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