Rear-edge populations of an endemic alpine daisy are most impacted by invasive sambar deer, even with targeted feral animal control

Abstract

Management of endangered species requires that threats to their persistence are identified and then mitigated. We document how an invasive ungulate that has recently and rapidly colonised Australian high mountain vegetation, sambar deer (Rusa unicolor), impacts a highly palatable endemic alpine daisy (Celmisia sericophylla, silky snow daisy). Further, we assess the effectiveness that lethal control conducted across the landscape, as part of a program for reducing deer numbers, has had on minimising impacts to the threatened daisy. We quantified the impact of deer at 57 Celmisia populations in relation to landscape context and herbivore control intensity by the land manager. Despite sustained lethal control being conducted across the landscape, we detected spatial differences in ongoing sambar deer impacts on the threatened species. Evidence of sambar deer were detected at 46 % of Celmisia populations sampled, and we found that rear-edge populations - nearest the treeline - were at highest risk of being occupied relative to populations further from the treeline. Activity of sambar deer at occupied Celmisia populations ranged from low level browsing of foliage and/or flowers (at 25 % of sites) to severe trampling and browsing; 5 % of sites). The probability of deer occupancy was higher with an increasing number of deer controlled with 1 km of Celmisia populations. This points to an ongoing and potentially predictable dispersal of deer into the alpine zone from subalpine forests, with such movements negatively affecting Celmisia populations nearest the invasion pathways, despite ongoing lethal control. Given the limited capacity of the broader deer control program to eliminate activity nearest the rear-edge populations, we recommend that landscape-scale sambar deer management needs to ramp-up in frequency and/or intensity, targeting alpine treelines, where most occupancy of Celmisia populations are currently being observed.