Shrub encroachment creates a dynamic landscape of fear for desert lagomorphs via multiple pathways

Dryland state transitions alter trophic interactions in a predator-prey system

Environmental change is expected to alter trophic interactions and food web dynamics with consequences for ecosystem structure, function and stability. However, the mechanisms by which environmental change influences top-down and bottom-up processes are poorly documented. Here, we examined how environmental change caused by shrub encroachment affects trophic interactions in a dryland. The predator-prey system included an apex canid predator (coyote; Canis latrans), an intermediate canid predator (kit fox; Vulpes macrotis), and two herbivorous lagomorph prey (black-tailed jackrabbit, Lepus californicus; and desert cottontail, Sylvilagus audubonii) in the Chihuahuan Desert of New Mexico, USA. We evaluated alternative hypotheses for how shrub encroachment could affect habitat use and trophic interactions, including (i) modifying bottom-up processes by reducing herbaceous forage, (ii) modifying top-down processes by changing canid space use or the landscape of fear experienced by lagomorph prey and (iii) altering intraguild interactions between the dominant coyote and the intermediate kit fox. We used 7 years of camera trap data collected across grassland-to-shrubland gradients under variable precipitation to test our a priori hypotheses within a structural equation modelling framework. Lagomorph prey responded strongly to bottom-up pulses during years of high summer precipitation, but only at sites with moderate to high shrub cover. This outcome is inconsistent with the hypothesis that bottom-up effects should be strongest in grasslands because of greater herbaceous food resources. Instead, this interaction likely reflects changes in the landscape of fear because perceived predation risk in lagomorphs is reduced in shrub-dominated habitats. Shrub encroachment did not directly affect predation pressure on lagomorphs by changing canid site use intensity. However, site use intensity of both canid species was positively associated with jackrabbits, indicating additional bottom-up effects. Finally, we detected interactions between predators in which coyotes restricted space use of kit foxes, but these intraguild interactions also depended on shrub encroachment. Our findings demonstrate how environmental change can affect trophic interactions beyond traditional top-down and bottom-up processes by altering perceived predation risk in prey. These results have implications for understanding spatial patterns of herbivory and the feedbacks that reinforce shrubland states in drylands worldwide.

Effects of multiple mammalian herbivores and climate on grassland-shrubland transitions in the Chihuahuan Desert

The replacement of grasses by shrubs or bare ground (xerification) is a primary form of landscape change in drylands globally with consequences for ecosystem services. The potential for wild herbivores to trigger or reinforce shrubland states may be underappreciated, however, and comparative analyses across herbivore taxa are sparse. We sought to clarify the relative effects of domestic cattle, native rodents, native lagomorphs, and exotic African oryx (Oryx gazella) on a Chihuahuan Desert grassland undergoing shrub encroachment. We then asked whether drought periods, wet season precipitation, or interspecific grass-shrub competition modified herbivore effects to alter plant cover, species diversity, or community composition. We established a long-term experiment with hierarchical herbivore exclosure treatments and surveyed plant foliar cover over 25 years. Cover of honey mesquite (Prosopis glandulosa) proliferated, responding primarily to climate, and was unaffected by herbivore treatments. Surprisingly, cattle and African oryx exclusion had only marginal effects on perennial grass cover at their current densities. Native lagomorphs interacted with climate to limit perennial grass cover during wet periods. Native rodents strongly decreased plant diversity, decreased evenness, and altered community composition. Overall, we found no evidence of mammalian herbivores facilitating or inhibiting shrub encroachment, but native small mammals interacting with climate drove dynamics of herbaceous plant communities. Ongoing monitoring will determine whether increased perennial grass cover from exclusion of native lagomorphs and rodents slows the transition to a dense shrubland.

Forager-mediated cascading effects on food resource species diversity

Perceived predation risk varies in space and time. Foraging in this landscape of fear alters forager-resource interactions via cascading nonconsumptive effects. Estimating these indirect effects is difficult in natural systems. Here, we applied a novel measure to quantify the diversity at giving-up density that allows to test how spatial variation in perceived predation risk modifies the diversity of multispecies resources at local and regional spatial levels. Furthermore, we evaluated whether the nonconsumptive effects on resource species diversity can be explained by the preferences of foragers for specific functional traits and by the forager species richness. We exposed rodents of a natural community to artificial food patches, each containing an initial multispecies resource community of eight species (10 items each) mixed in sand. We sampled 35 landscapes, each containing seven patches in a spatial array, to disentangle effects at local (patch) and landscape levels. We used vegetation height as a proxy for perceived predation risk. After a period of three nights, we counted how many and which resource species were left in each patch to measure giving-up density and resource diversity at the local level (alpha diversity) and the regional level (gamma diversity and beta diversity). Furthermore, we used wildlife cameras to identify foragers and assess their species richness. With increasing vegetation height, i.e., decreasing perceived predation risk, giving-up density, and local alpha and regional gamma diversity decreased, and patches became less similar within a landscape (beta diversity increased). Foragers consumed more of the bigger and most caloric resources. The higher the forager species richness, the lower the giving-up density, and alpha and gamma diversity. Overall, spatial variation of perceived predation risk of foragers had measurable cascading effects on local and regional resource species biodiversity, independent of the forager species. Thus, nonconsumptive predation effects modify forager-resource interactions and might act as an equalizing mechanism for species coexistence.