Feral Deer Population Impact Calculator

Deer species

Control Work Inputs

Deer removed

Estimated % females

Month of removal

Projection period

Deer are slow breeders, so impacts build over years rather than months. Longer projections show the compounding more clearly.

Net Model: Mortality Rates

Fawn mortality (to breeding age)

Fallow pre-weaning mortality reaches ~19% (PestSmart); sambar juvenile survival was estimated at 0.81, i.e. ~19% mortality (Watter et al. 2020). Defaults add later first-year losses to breeding age.

Adult annual mortality

Adult feral deer are long-lived with few natural predators in Australia, so adult mortality is low. Sambar live to ~13 years; fallow rarely past 12.

The shaded band on the chart shows plus or minus 5% from your selected rates.

Net range (plus/minus 5%)

Impact of the prevented population

Grazing pressure

National context

$69.1M/yr

Feral deer's estimated cost to Australian agriculture in 2021, part of a $91.3M national total (McLeod 2023). No authoritative per-property or per-deer dollar figure exists, so we show none.

Prevented Population Over Time

Interpreting subsequent generations: The shaded band widens over time because small differences in survival rates compound with each generation. Deer mature slowly, so the first generation graduates only after 16 (fallow) to 22 (sambar) months, and the steep growth comes later. Treat far-out generations as indicative rather than precise.

Estimate the population growth prevented by feral deer control work, and the pasture that population would graze. Two species are modelled, fallow deer (Dama dama) and sambar deer (Rusa unicolor), because their biology differs markedly. Selecting a species loads its published defaults; every value remains adjustable.

Model structure: Monthly timestep simulation. Tracks adult females, adult males, and fawn cohorts by age. Fawns are dropped during the selected season and graduate to breeding adults at the species maturity age (fallow 16 months, sambar ~22 months). Deer are single-young breeders, so reproduction is one "fawns per breeding female per year" figure (~1) rather than litters. Offspring sex ratio assumed 50:50 (a standard biological assumption, not stated in the sources).

Net estimate model: Applies fawn mortality (to breeding age) and adult annual mortality, converted to monthly rates. The removed founders are treated as wild adults: had they not been removed they would have faced the same natural mortality, so they decay at the adult rate along with their descendants. This keeps the estimate a realistic counterfactual rather than assuming every removed animal survives and breeds for the full projection. Both rates are adjustable by plus or minus 5% (the shaded chart band).

Fallow deer defaults: matures at 16 months; single fawn dropped Nov to Dec after the autumn rut; 25% fawn mortality to breeding age (PestSmart puts pre-weaning mortality at ~19%); 10% adult annual mortality. These produce a ~19%/yr rate of increase, a moderate figure below the National Feral Deer Action Plan's stated 34 to 50%/yr under favourable conditions. Fallow adults weigh 35 to 55 kg (females) and 50 to 97 kg (males).

Sambar deer defaults: mean age of first reproduction 1.8 years (~22 months); calving year-round with ~80% of hinds calving April to August; juvenile survival estimated at 0.81 (~19% mortality); 10% adult annual mortality. These reproduce the 15 to 24%/yr rate of increase measured for wild sambar in Victoria by Watter et al. (2020). Sambar are Australia's largest deer, males to 300 kg and females to 230 kg, and the most rapidly expanding species.

Mortality is partly calibrated. The sambar fawn-mortality default comes straight from Watter et al.'s measured 0.81 juvenile survival. The fallow fawn figure and both adult figures are calibrations, not directly cited percentages: they are chosen so the model reproduces each species' published rate of increase rather than importing an overseas number. All are adjustable.

Grazing pressure: the prevented population is converted to dry-sheep-equivalents (DSE), one DSE being the grazing pressure of a 50 kg wether, so the figure is also the number of sheep whose feed the deer displace. The National Feral Deer Action Plan states a fallow deer "eats 1.8 to 2.3 times as much as a sheep"; Davis, Forsyth & Bengsen (2023) measured fallow deer at 1.0 (juvenile) to 1.85 (adult) DSE and found a high-density fallow population (~37 deer/km squared) cut potential domestic stocking rate by ~50% on the NSW properties studied. The fallow default (1.8) sits at the conservative end of that evidence. No sambar-specific DSE has been published; although sambar are larger than red/rusa (which the NFDAP puts at 2.8 to 3.6 times a sheep) and body mass alone would imply a higher figure, the sambar default (3.5) is set at the top of the red/rusa range rather than extrapolated upward, and should be treated as provisional. The subtext converts DSE to annual pasture dry matter at ~1 kg per DSE per day (Meat & Livestock Australia).

On dollar costs: we deliberately show no per-deer or per-property dollar figure, because none is authoritative. Only the national total is: feral deer cost Australian primary producers about $69.1 million in 2021, part of a $91.3 million national total that also includes control and research ($17.8M), vehicle collisions ($3.3M) and rail impacts ($1.2M) (McLeod 2023, for the Centre for Invasive Species Solutions). Deer also cause impacts this tool does not attempt to value: fence and infrastructure damage, browsing of native vegetation and forestry, erosion and waterway fouling, and vehicle and train collisions.

Sources (all Australian):

NSW DPIRD, Feral deer: Species Information: species, breeding seasons, distribution and impacts.
PestSmart, Identifying fallow deer and Identifying sambar deer: body size, breeding, diet.
National Feral Deer Action Plan, The Growing Problem: population estimate, growth rate, DSE grazing multiples.
Watter, Thomas, White, Finch & Murray (2020), Animal Reproduction Science 223:106630: sambar age of first reproduction, calving season, juvenile survival, and the 15 to 24%/yr rate of increase.
Davis, Forsyth & Bengsen (2023), Wildlife Research 50(9):701-715: fallow deer diet, DSE values, and stocking-rate reduction on NSW pastoral properties.
McLeod (2023), Annual Costs of Feral Deer in Australia, Centre for Invasive Species Solutions: national agricultural, control, vehicle and rail cost breakdown.