As large animals with hard hoofs and antlers and damaging (from a plant perspective) breeding rituals deer can have a physically destructive impact on the environment.
Deer hoofs can damage delicate plants, sensitive environments such as wetlands and mossbeds, and nests (including Malleefowl mounds). Keith and Pellow (2005) found that the margin of wetlands used by Rusa Deer in Royal NP were ‘exposed, compacted and deformed’ by deer footprints. Three sites subject to high deer use were denuded of vegetation and had lost up to 0.6 m of topsoil. Tree and shrub roots were exposed and broken. Peel et al (2005) observed that erosion is becoming a significantly greater issue as Sambar gradually move from higher country into lower elevation country and begin to graze wetlands, sometimes along with Hog Deer. Being very large Sambar can wade further out into wetlands and damage vegetation in mud or deeper water, altering vegetative and hydrological structures and destroying fish and crustacean habitat. While fencing around some wetlands has helped to restrict access by domestic stock grazing, Sambar deer are capable of jumping standard stock fences (Peel et al. 2005).
Deer are attracted to raised patches of bare dirt in the form of Malleefowl mounds in the South East region of South Australia, leading sometimes to their collapse and abandonment (James Darling pers. comm.; see under Q9). Deer trampling of plants is a particular concern where rare plants occupy a very small area frequented by deer, such as the threatened Summer Leek-orchid (Prasophyllum canaliculatum) occupying only about 0.5 ha in the South East Forests NP (XXXX XXXX pers. Comm..)
Some deer create bare scrapes for rutting purposes, trampling the ground and removing vegetation. Sambar rutting areas have been observed along floodplains of small creeks in East Gippsland, where vegetation over an area up 15 x 7 m has been completely cleared (Peel et al. 2005). Such areas become sites for weed invasion. Sambar also create wallows in areas with shallow water and a muddy base – in swamp scrub, warm temperate rainforest, salt marsh and estuarine wetlands. Physical damage to plants in the vicinity can be severe, and wallows are vulnerable to gully erosion (Peel et al. 2005). Sambar wallows (and trampling) have been observed in Alpine mossbeds, which are particularly sensitive to damage (Tolsma 2009; see under Q9). Sambar signs have recently been observed at a substantial number of breeding sites for the critically endangered Northern Corroboree Frog (Pseudophryne pengilleyi) (XXXX XXXX pers. comm).
Deer also create ‘rub trees’, scraping away the surface bark with their antlers. This is a major problem for the critically endangered (based on IUCN criteria) Shiny Nematolepis (Nematolepis wilsonii) (see under Q7). It can ringbark and kill trees, exposes them to wood borers and fungal pathogens, reduce foliage and compromise the health of the tree (Bennett and Coulson 2011). Sambar had rubbed 61% of 92 Cherry Ballarts (Exocarpus cupressiformis) in Mount Buffalo National Park (Forsyth 2007 citing Millington 1991). Peel et al. (2005) list several species adversely affected by antler rubbing: ‘So widespread and ubiquitous is the damage that at the current rate of attrition, several species are under threat just from antler rubbing alone.’ They found over 100 rub trees of the rare Yellowwood (Acronychia oblongifolia) in one patch within East Gippsland Coastal Warm Temperate Rainforest. Sambar rub marks go as high as 2.1 m.
Thrashing of saplings during rutting is also very damaging, such as recorded for Shiny Nematolepis (Bennett and Coulson 2011).
Exotic invaders sometimes cause damage by facilitating other biotic invasions (Simberloff and Von Holle 1999) – in the case of deer by weed spread, pathogen spread, and facilitating exotic predators.
Feral deer (and other feral herbivores) can facilitate weed spread by creating gaps in vegetation for weed germination and by dispersing weed seeds, and this is recognised as part of the threatening process for one critically endangered habitat (see littoral rainforest under Q9). Davis et al. (2010) recommend that the potential for endozoochory ‘be considered more widely in studies assessing the impacts of exotic mammals on plant communities’. They assessed seed germination from the faeces of Hog Deer in Victoria’s Wilsons Promontory National Park as a case study of the role of exotic mammalian herbivores as seed dispersers in weedy ecosystems. They estimated the total number of viable seeds dispersed daily by the hog deer population (on the Greater Yanaki Isthmus) as 133,000, of which about one-quarter were of exotic species. Each Hog Deer on average was depositing 494 seeds a day. With some Hog Deer moving more than 2 km to feeding areas and occasionally undertaking much longer trips, they can facilitate weed spread across landscapes and accelerate plant invasions. They can also facilitate spread of native species. Acacia longifolia, an encroaching native shrub, germinated from Hog Deer faeces. Moriarty (2004b) found 11 weed species and several garden plant species in the rumen of Rusa Deer in Royal NP.
Sambar are implicated in the spread of the weed Himalayan Honeysuckle (Leycesteria formosa) in alpine areas (Wright et al. 2009, citing Eyles 2002). Deer are also spreading Senegal Tea (Gymnocoronis spilanthoides) and Ludwigia (Ludwigia peruviana) (NSW Scientific Committee 2004). (In some cases deer could control weeds by browsing them.) In Queensland pest plants such as Rubber Vine (Cryptostegia grandiflora), Chinee Apple (Ziziphus mauritania) and Parthenium (Parthenium hysterophorus) are flourishing in areas around Charters Towers where chital are removing pastures and native vegetation (Jesser 2005).
One of the threatening processes identified in the listing of the critically endangered Littoral Rainforest and Coastal Vine Thickets of Eastern Australia is weed invasion following vegetation damage by Sambar. One area severely affected is near Genoa River, in Victoria, where vegetation gaps created by deer have been colonised by Cape Ivy (Delairea odorata) and Madeira Winter-cherry (Solanum pseudocapsicum), contributing to the collapse of rainforest patches by smothering shrubs and young trees (EPBC Listing Advice 2008).
There is concern about spread of pathogens and parasites by deer, focused mostly on the risk to domestic livestock (Jesser 2005). One environmental concern is that deer could spread the dieback pathogen Phytophthora cinnamomi (Masters 2009), which is listed as a key threatening process.
Another example of a facilitative interaction between exotic species occurs when deer fragment and create pathways through thick vegetation that permit easy access to exotic predators. Sambar create paths through dense scrub that normally serves as refuge for ground nesting birds and small native mammals. Sambar paths ‘essentially become highways through the bush for introduced predators’ (Peel et al. 2005). Exotic predators also benefit when hunters discard deer carcasses. Peel et al. (2005) observed that recreational deer hunters who seek just a trophy head are leaving hundreds of tonnes of meat for a rapidly increasing feral dog population. The height of the hunting season corresponds with the birth and weaning of wild dogs. The peak in Sambar calving during winter provides another source of food for feral dogs when prey is likely to be limiting (ibid.). The impact on native mammals and livestock is likely to be high: Faecal pellet counts in the Upper Yarra Catchment suggest that Sambar are approximately 100 times more abundant there than Black Wallabies (Peel et al. 2005 citing Houston 2003 and Stockwell 2003), which may be due to both competition and wild dog predation on wallabies (Ibid.)