Invasive species are major drivers of ecosystem degradation globally. How invasive herbivore impacts differ from native herbivore impacts remains understudied. We examined the relationships between herbivore sign and vegetation height, foliage density, cover of forbs, weeds, bare ground, and soil compaction across environmental and herbivore activity gradients in the mainland Australian Alps. We detected native and exotic herbivore sign at 32.8% and 94.0% of sites, respectively. Total herbivore activity was primarily attributed to exotic herbivores and was associated with elevation and grassland type. Greater horse (exotic) activity was associated with lower vegetation height, lower foliage density, higher forb cover, and higher soil compaction. Greater rabbit and hare (exotic) activity was associated with lower vegetation height, lower foliage density, and a higher cover of bare ground. Greater total herbivore activity was associated with greater weed cover. Neither deer (exotic) nor kangaroo and wallaby (native) activity was related to response variables. We demonstrate that exotic herbivores dominate mammalian herbivory in these grasslands, which evolved without analogous hooved species. Given the restricted distribution and high endemism of these ecosystems, and associations between exotic herbivores and characteristics of degraded grasslands, we recommend landscape-scale exotic herbivore management, focusing on maintaining ground cover and vegetation structure.

We derived landscape elements from publicly available spatial datasets. These included elevation (m a.s.l.), landscape position (Topographic Wetness Index), distance from the nearest waterbody (m), distance from the nearest road or track (m), and distance from the nearest woodland (m) (see Supporting Information S2 for source information). We manually corrected a small number of distance measures where elements were present on-ground but not captured by the spatial classification.

Along each 50 m transect, we recorded the overlapping percent cover of nongrass graminoids, shrubs, forbs (native and exotic combined), grasses, weeds (exotic flora, all forms) and bare ground within circular plots of 2 m radius at 10 m intervals. We calculated the mean groundstorey vegetation height (cm) and foliage density (number of foliage intercepts in 10 cm bands of a structure pole up to 50 cm), excluding flowering parts, for each site. We measured soil compaction in kilopascals (kPa) at 20 points along the transect using a handheld penetrometer according to manufacturer specifications (ST 315, Prospectors, Australia). As soil moisture can affect soil compaction, we classified the soil as wet or dry at the time of measurement according to whether soil stuck to our fingers upon touch.

We considered a “herbivore” to be any mammal with a diet consisting primarily of plant material. We used a relative index of evidence of herbivory, quantified by the frequency of herbivore sign at each site. We calculated herbivore activity by recording the presence or absence of herbivore sign, such as scats and diggings, within each half of a 2 m radius plot at 5 m intervals along the transect to give a frequency measure (out of 20) per sign type (Forsyth et al., 2007; Hone, 2002). We moved elevated vegetation but did not disturb the litter layer or dense ground cover to uncover sign. We classified sign types to family according to Triggs (1996) and Claridge (2016b), as species-level differences between sign cannot always be reliably distinguished. We classified worn paths, consumed vegetation, and pulled vegetation as “generic". We did not include pellets with evidence of decay, nor did we convert indices to animal densities (Forsyth et al., 2007). We conducted all surveys within a 4-week period, in February and March 2020, to minimize seasonal and weather variability between sites. At this time, snow cover was absent and mammal populations were likely to be occupying their maximal elevational ranges within the bounds of their current distribution (Foster et al., 2021).

Soil compaction data are missing from two sites and height data are missing from one site.