Biotic resistance and disturbance: rodent consumers regulate post‐fire plant invasions and increase plant community diversity

Abstract

Biotic resistance and disturbance are fundamental processes influencing plant invasion outcomes; however, the role of consumers in regulating the establishment and spread of plant invaders and how disturbance modifies biotic resistance by consumers is unclear. We document that fire in combination with experimental exclusion of rodent consumers shifted a native desert shrubland to a low-diversity, invasive annual grassland dominated by Bromus tectorum (cheatgrass). In contrast, burned plots with rodents present suppressed invasion by cheatgrass and developed into a more diverse forb community. Rodents created strong biotic resistance to the establishment of aggressive plant invaders likely through seed and seedling predation, which had cascading effects on plant competition and plant community diversity. Fire mediated its positive effects on plant invaders through native plant removal and by decreasing the abundance and diversity of the rodent community. The experimental disruption of plant and consumer-mediated biotic resistance of plant invaders using fire and rodent exclusion treatments provides strong evidence that native plants and rodents are important regulators of plant invasion dynamics and plant biodiversity in our study system. While rodents conferred strong resistance to invasion in our study system, fluctuations in rodent populations due to disturbance and climatic events may provide windows of opportunity for exotic plant species to escape biotic resistance by rodent consumers and initiate invasions.

Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

“…We therefore expected to see top-down effects of rodents on plant communities translate to shifts in invertebrate community composition and structure. Our results suggest that invasive plant cover strongly affects ant diversity (Tables 7 and 8; Figure 3), and while rodents may alter which types of invasive plants dominate in burned areas (St. Clair et al, 2016), they seem to have less effect on the percent cover of invasive plants in burned plots (Tables 7 and 8; Figure 3). Our results suggest that invasive plant cover strongly affects ant diversity (Tables 7 and 8; Figure 3), and while rodents may alter which types of invasive plants dominate in burned areas (St. Clair et al, 2016), they seem to have less effect on the percent cover of invasive plants in burned plots (Tables 7 and 8; Figure 3).…”

“…Previous research in our Great Basin plots shows that rodent exclusion in burned areas dramatically increased the cover of cheatgrass leading to loss of plant biodiversity (St. Clair et al, 2016). We therefore expected to see top-down effects of rodents on plant communities translate to shifts in invertebrate community composition and structure.…”

“…Each block was split into four equal (30 × 30 m) subplots, which were assigned to one of four factorial treatment combinations: burned or unburned, and rodent access or rodent exclusion (St. Clair et al, 2016 fire between shrubs at the Great Basin site, we placed 300 g/m −2 of wheat straw in the shrub interspaces in our burn plots (St. Clair et al, 2016). Each block was split into four equal (30 × 30 m) subplots, which were assigned to one of four factorial treatment combinations: burned or unburned, and rodent access or rodent exclusion (St. Clair et al, 2016 fire between shrubs at the Great Basin site, we placed 300 g/m −2 of wheat straw in the shrub interspaces in our burn plots (St. Clair et al, 2016).…”

“…However, we observed no significant main effects of rodent exclusion on ant community richness and diversity or invertebrate community abundance (Tables 5 and 6). This suggests that the loss of native shrubs and their replacement by invasive annuals following fire has a larger impact on invertebrate communities than differences in the composition of invasive annual communities (annual grasses vs. annual forbs) created by rodents (St. Clair et al, 2016). For example, invasive plant cover in burned-rodent access (BRA) and burned-rodent exclusion (BRX) plots were nearly identical between sites (68% and 67% in BRA plots in the Mojave and Great Basin, respectively, and 72% in BRX plots at both sites).…”

“…Many rodent species include insects as part of their diet, and small mammal insectivory has been shown to have strong effects on grassland invertebrate communities (Churchfield, Hollier, & Brown, 1991). A previous study at our Great Basin site determined that rodents can suppress cheatgrass invasion (St. Clair, O'Connor, Gill, & McMillan, 2016); rodent exclusion produced a plant community dominated by invasive grasses, and where rodents had access, the plant community was a much more diverse annual forb community. A previous study at our Great Basin site determined that rodents can suppress cheatgrass invasion (St. Clair, O'Connor, Gill, & McMillan, 2016); rodent exclusion produced a plant community dominated by invasive grasses, and where rodents had access, the plant community was a much more diverse annual forb community.…”