Browsing by non-native invasive sambar deer dramatically impacts forest structure

Abstract

Large herbivores introduced to novel habitats can cause major ecosystem impacts. While impacts of overabundant deer have been documented around the world, there are few examples from Australia, where deer are non-native and natural predators are absent.Here, we outline the impacts of sambar deer in wet eucalypt forest understoreys of the Yarra Ranges National Park, south-eastern Australia. Using data obtained from a full and partial (permitting native herbivores) exclusion experiment over four years, we (i) demonstrate differences between native herbivore and introduced deer browsing, (ii) identify plant species commonly browsed by deer, and (iii) provide evidence that vegetation condition can recover in the absence of deer.While species richness and composition were unaffected, the exclusion of deer led to substantial recovery of several attributes of forest understorey vegetation. Treatments excluding deer (partial and full exclosures) saw increases in the cover of understorey trees, shrubs, ferns and climbers. Within the 1–2 m height class (the primary deer browse zone), vegetation cover-abundance increased 3–4 fold over time in full and partial exclusion plots compared to non-exclusion controls.Tree ferns (Dicksonia antarctica and Cyathea australis) were particularly impacted by deer, and browsing impacts to tree ferns were dramatically reduced from ∼50% of fronds browsed to negligible levels with deer exclusion. Deer exclusion also saw substantial reductions in browsing to individual trees, shrubs and ground ferns. The growth of commonly browsed trees, e.g. Pomaderris aspera, were demonstrably higher in partial and full exclosures than unfenced control plots. The low levels of browsing on trees and shrubs that persisted in partial exclosures probably indicates the background levels of browsing pressure attributable to native herbivores in these forests.In the absence of effective deer control, commonly browsed understorey trees, shrubs and tree ferns are likely to decrease substantially in cover over coming decades, potentially leading to increased light availability, drying of the understorey, and increased susceptibility to wildfire. The potential cascading ecological effects of unabated and increasing deer impacts may be substantial and pose major challenges to forest management.Despite management culls and recreational hunting in recent years, deer populations are increasing across Australia. As sambar populations increase, ecological impacts are likely to exacerbate. It is therefore critically important to reduce sambar numbers, before negative and potentially irreversible changes occur to forest structure, composition and function.