Key Threatening Process Nomination Form For adding a threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (epbc act)




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5.Impacts of herbivory by feral deer

Decline of deer-preferred species


As selective feeders, deer can modify the relative abundance of species and alter the composition and dynamics of plant communities (Cote et al. 2004). Native plants in foraged habitats will fall into one of three categories defined by Forsyth et al. (2003): (1) preferred, (2) neither preferred nor avoided, (3) avoided. Those species that are preferred components of deer diet are at greatest risk of decline, particularly if they are rare. They may be at risk even if deer densities are low as ‘even low levels of foraging could have impacts on the regeneration, growth and abundance of rare species’ (Davis et al. 2008). As Cote et al. (2004) caution, it should not be assumed that deer impacts are simply proportional to deer density across sites; extirpations may accelerate once plant populations grow sparse (warranting a precautionary focus on potentially susceptible rare species).
Although it can be difficult to prove links between deer herbivory and species declines, particularly when there are co-existing herbivores, in some cases the link is obvious. (The height of browsing often indicates deer herbivory as large deer can reach much higher than native herbivores – up to 2.5 m for Sambar (Peel et al. 2005.) In East Gippsland Peel et al. (2005) recorded severe browsing pressure by Sambar and decline of several species (they list about 50 species ‘severely and adversely affected’ by browsing in East Gippsland). For example, saplings of the rainforest canopy species Black Wattle (Acacia mearnsii), Blackwood (Acacia melanoxylon), Lily Pily (Acmena smithii), Yellowwood (Acronychia oblongifolia), Sweet Pittosporum (Pittosporum undulatum) and Muttonwood (Rapania howittania) are being browsed to death, preventing regeneration, opening up rainforest margins and increasing the risk of fire entering rainforest. Browsing damage attributed to deer was recorded on 73-100% of the tree ferns Cyathea australis and Dicksonia antarctica in the Yarra Ranges (Forsyth 2007, citing Bennett 2002). XXXX XXXX (pers. comm.) characterised Gynatrix macrophylla, a rare Victorian species now critically endangered according to IUCN criteria, as ‘ice-cream’ for Sambar (see under Q5). Similarly, Keith and Pellow (2005) concluded that Syzygium paniculatum was a particularly palatable species for Rusa Deer after finding that more than 75% of foliage and branchlets were consumed on most of the 93 individuals to which deer had access for 3 months (see under Q9).
Plants likely to be least tolerant of browsing are those that grow slowly (such as understorey plants in shady forests) and small ephemeral herbs such as orchirds that lose all their leaves or flowers in a single bite (Cote et al. 2004). Species browsed or grazed by deer that are not typically eaten by native herbivores could be highly susceptible to herbivory (as anti-herbivore defenses are costly for plants) (Gomez and Zamora 2002). The dietary overlap between Rusa Deer and Swamp Wallabies (Wallabia bicolor) in Royal NP ranged from only 24% in autumn in a heath habitat to 60% in winter in a cleared/mosaic habitat (Moriarty 2004b). Deer tended to consume more trees, shrubs and introduced grasses while wallabies tended to consume more herbs, orchids, sedges, grasses, rushes and fungi. In contrast there was substantial overlap in the diet of Hog Deer and Swamp Wallabies foraging in Coastal Grassy Woodland at Yanakie Isthmus, Wilsons Promontory (Davis et al. 2008).
Selective herbivory can strongly affect competitive relationships among plants, facilitating increase in browse-tolerant and unpalatable species. Although not a definitive example, because other factors are likely to be involved, the encroachment of Leptospermum laevigatum at Yanakie Isthmus may be assisted because it is mostly avoided by herbivores, exotic and native (Davis et al. 2008).

Loss of plant biomass


As medium to large herbivores, feral deer eat large volumes of plant matter. The sheer volume of herbivory can have substantial impacts on ecosystem structure and processes, with implications for birds and other wildlife that depend on particular vegetation structure. This is the case whether herbivores are exotic or native, as overabundant deer populations in their native ranges in North America and Europe have demonstrated (Cote et al. 2004).
Assessing volumes in the rumen of Rusa Deer in Royal National Park, Moriarty (2004b) found they ate an ‘alarming’ amount of native vegetation, likely to have ‘profound consequences’ for the NP, a relatively small area with high habitat diversity. He estimated the deer population (from 2500-2900 in 1999-2001) ate a total of 47 million litres of material per year, of which about three-quarters was native. Deer in different habitats consumed different amounts, with an average deer in:

  • cleared/mosaic areas consuming ~ 280 litres of food/season (160 litres native)

  • forest/rainforest ~ 430 litres (210 litres native)

  • woodland ~ 380 litres (346 litres native)

  • heath ~ 244 litres (220 litres native).

Moriarty (2004b) concluded that Swamp Wallaby numbers in Royal NP are depressed by Rusa Deer. In most of their range, the wallabies are at densities of 8-19 animals/km2 but densities in Royal NP are an estimated 2.2-2.7 animals/km2, and the highest wallaby densities were recorded where deer densities were lowest. Modelling by Moriarty suggested that Royal NP was inhabited by almost three times the number of medium-sized herbivores than other areas along the east coast of Australia. An average Rusa Deer consumed about 3.9 times the volume of food eaten by a Swamp Wallaby but less diversity.


Bennett (2008) found that Sambar were consuming almost all available forage at a favoured feeding location in Yarra Ranges National Park. Sambar eat about 3–4 kg dry weight/day, compared with 400 g eaten by swamp wallabies (Wallabia bicolor) and 700-1450 g by common wombats (Vombatus ursinus). Bennett calculated that Sambar consumed 5.8–30.0 tonnes (dry weight) per month. Using selective exclosures that permitted differentiation between Sambar and native herbivores, Bennett (2008) recorded significantly reduced biomass in forest understorey in areas with high Sambar densities. Most browsing occurred on branches above 60 cm in height, and prevented the vertical growth of plants in the understorey. Three species were browsed to a significantly greater extent than by native herbivores: Hazel Pomaderris (Pomaderris aspera), Prickly Tea-tree (Leptospermum continentale) and Prickly Bush-pea (Pultenaea juniperina).
Large numbers of Chital around Charters Towers have caused significant damage in grazing vegetation to bare ground (Jesser 2005 citing Peterson 2004). Some sensitive or low-productivity ecosystems, such as in alpine areas where the growing season is short or forest understoreys, may be particularly sensitive to the loss of plant biomass through deer herbivory.
Most of the native herbivores currently competing with feral deer for forage are widespread and abundant so not threatened by deer. However, deer expansion into the habitat of threatened native herbivores could be a problem. Dawson and Ellis (1979) found that feral goats competed with endangered yellow-footed rock wallabies for water and food particularly during drought. Feral deer could be a problem for Malleefowl for this reason; sheep grazing reduces their breeding density by up to 90% (Benshemesh 2007).

Declines in plant diversity


In the one Australian study assessing the impacts of deer on plant diversity, Moriarty (2004b) found that Rusa Deer in high densities substantially reduced diversity in three vegetation communities in Royal NP, implicating them as ‘gross habitat changers’. Moriarty assessed species diversity in littoral rainforest (wet forest), sandstone gully forest (dry forest) and sandstone heath, each of which contains rare and threatened plant species. Using fenced exclosure plots, he compared no deer, low deer density (<5/km2) and high deer density (>20/km2). Few differences were recorded for no deer versus low deer density, probably because of the short experimental duration (3 years compared to the 10-30 years it usually takes for significant changes to be detected). But there were substantial (and significant) differences between plots with low deer density and high deer density. Plant diversity was reduced by 27 to 54% in the three habitats (see Table 3). For example, in littoral rainforest plots subject to high deer density the mean number of species was 17 compared to 37 in plots subject to low deer density.
Table 3: Percentage reduction in plant species diversity in three habitats in Royal NP in areas with high deer density compared to low deer density (recorded by Moriarty 2004b).




Saplings

Understorey spp

Ground cover spp

Plant spp total

Littoral rainforest

58%

28%

65%

54%

Sandstone gully forest

49%

28%

37%

33%

Sandstone heath

-

21%

29%

27%

Rusa eat a wide range of native plants, with Moriarty identifying material from 155 species (18 genera) in the rumen of deer in Royal NP. About one-third of species could not be identified. The majority were shrubs (43%) or trees (26%).


In opportunistic observations of bushland in Royal NP, Keith and Pellow (2005) recorded that deer ate 60-66 of 70 plant species within 1km of the Bundeena township. Shoot removal was particularly conspicuous for leguminous species, with complete defoliation of some species. Bark removed from some species may also have been eaten. Nine species, most prevalently orchids, had flowers consumed or damaged.
About 15% of the identified species in the Rusa Deer rumens were listed as threatened species (2 listed as endangered and 9 as vulnerable under the TSC Act) or regionally rare (13 species). They were present mostly in deer from low-density areas implying they had already been eradicated or almost so from high-density areas. NSW’s Scientific Committee (2004) accepted that some of these species were threatened or potentially threatened by deer herbivory. A number of species listed under Victoria’s Flora and Fauna Guarantee Act (FFG Act) and browsed by Sambar were recognised by Victoria’s Scientific Advisory Committee (SAC 2007) as threatened by Sambar.
Some mycological experts are also concerned about the impact of feral deer on fungi. Rare fungi species in the wet forests of central and southern Victoria inhabited by Sambar are no longer recorded at former known locations and are being replaced by common species (David Cameron, DSE pers. comm.). Some mosses and liverworts may also be susceptible (Peel et al. 2005).

Compromised regeneration


Deer herbivory is known to compromise multiple stages of plant life cycles – consumption of seedlings preventing establishment, consumption of shoots of juvenile plants reducing survival and growth, consumption of shoots of adult plants reducing seed production, and consumption of seeds preventing regeneration. Keith and Pellow (2005) propose that the greatest influence of deer on population viability of species is in preventing the establishment of seedlings and reducing seed production.
Destruction of seedlings: Deer often target seedlings and saplings and tree species are most vulnerable at this stage. Peel et al. (2005) list 13 tree and shrub species in East Gippsland for which regeneration is prevented by saplings being ‘browsed to death’. Deer herbivory may be of particular concern after fire: ‘bushfires expose more seedlings to browsing by deer because they release seeds of many species from dormancy or canopy storages’ (Keith and Pellow 2005). Deer densities in Royal NP escalated after a fire burned 90% of the park in 1994, rising from <500 just after the fire to 2500 by 1999 (Keith and Pellow 2005). Peel et al. (2005) observed that Sambar can devastate regrowth after fire or logging.
Deer can also prevent seedling establishment by destroying thickets that act as nursery sites or regeneration refuges, as Peel et al. (2005) documented for Sambar. Regeneration refuges provide physical barriers against other browsers, such as Black Wallabies in Victorian rainforests, within which palatable species can survive. Barriers may consist of fallen trees, thorny or stinging species such as Bursaria spinosa and Urtica incisa and plants unpalatable to most herbivores but they do not deter Sambar. They also eat vines that would normally protect regenerating plants around tree falls and remove protective branches of fallen trees. Destruction of these refuges exposes seedlings and saplings to grazing/browsing by wallabies, rabbits and Hog Deer.
Deer browsing on seedlings can also compromise revegetation projects. Browsing by Fallow Deer necessitated the erection of deer-proof fencing around areas revegetated in Yellingbo Nature Conservation Reserve in Victoria’s Central Highlands (Wright et al. 2009). In South Australia, deer are compromising regeneration of Allocasuarina verticillata, the main foodplant of an endangered subspecies of Glossy Black-cockatoos (see under Q9).
Reduced seed production and seed consumption: Peel et al. (2005) observed that Sambar reduced the reproductive output of species such as Yellow Milk Vine (Marsdenia flavescens), Prickly Currant-bush (Coprosma quadrifida) and Muttonwood (Rapania howittana) in East Gippsland by consuming flowers and fruits as well as seeds and seedlings. Deer may consume all reproductive material of orchids or lilies in a single visit (Keith and Pellow 2005). In Queensland, Red Deer have recently started consuming the seeds of Bunya Nuts leading to concerns about the future viability of Araucaria bidwillii (see box). Experimental caches of Telopea speciossima seeds were eaten by Rusa Deer in Royal NP (Keith and Pellow 2005, citing Auld and Denham).

Red Deer and Araucaria bidwillii (Bunya Pine)

Bunya Pines occur in two widely disjunct and relictual populations in north and south Queensland, with a gap of ~1000 km. There is ‘extreme’ genetic differentiation between the northern and southern populations and also between some of the sub-populations in southern Queensland (Pye and Gadek 2004).


Red Deer have recently been observed eating the seeds of Bunya Pines, leading to concerns that deer expansion in Queensland could compromise regeneration and genetic diversity of this species (Smith et al. in prep.). Deer predation of Bunya Pine seeds was observed initially just around the Barambah Education Centre in Wratten State Forest (the Jimna population of Bunya Pines). As the deer have spread – over an approximate 3 km radius out from the Education Centre during the past 3 years – they have eaten most of the bunya cones before they open on the ground. 
The dry rainforest areas containing A. bidwillii extend between Yarraman, through Jimna, Wratten and the Mary River. Much of the area was cleared for plantations, and only steep areas and firebreaks retained (Smith and Butler, 2009). Although southern populations are considered secure, its long-term future may be compromised because of poor seed dispersal (it is possible that its former animal dispersers are now extinct). Deer predation of cones add to predation by rats and further reduces recruitment potential (Smith et al. in prep.).

 

Based on observations of the extent of seed predation in the area currently occupied by Red Deer, if their expansion into forests with A. bidwillii continues, they could eventually threaten the species by preventing regeneration and reducing genetic variation  (XXXX XXXX pers. comm.)






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