Feral Pig Population Impact Calculator

Estimate the population growth prevented by feral pig control work, and the lamb predation that population represents. Every figure is drawn from an Australian source; where a source states an impact only qualitatively, the model is calibrated to a quantified Australian population figure rather than assuming a number.

Model structure: Monthly timestep simulation. Tracks adult females (sows), adult males (boars), and piglet cohorts by age. Breeding occurs during the selected window, which defaults to year-round (NSW DPI: feral pigs "can breed throughout the year"). Annual litters are distributed evenly across the breeding months. Offspring sex ratio assumed 50:50 (a standard biological assumption; not stated in the sources). Piglets graduate to breeding adults at 8 months (NSW DPI: females reach sexual maturity "between 7 and 12 months of age").

Reproductive defaults: 2 litters/year at 6 piglets (12 piglets/sow/year). NSW DPI states litters are "4 - 10 piglets"; PestSmart states sows "can potentially produce two litters ... in a year" in good conditions; the National Feral Pig Action Plan uses "up to six" piglets. These are the good-conditions figures the tool is designed to illustrate, and all are adjustable.

Net estimate model: Applies piglet mortality 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.

Mortality: calibrated, not cited. No Australian source gives a piglet- or adult-mortality percentage; both are stated only qualitatively (NSW DPI: "most piglets will never reach one year of age"; "few feral pigs survive longer than five years"). Rather than import overseas percentages, the defaults are chosen so the model reproduces Australia's quantified population figure. NSW DPI: "when conditions are good, feral pig populations can increase by up to 86% in one year" (also Choquenot et al. 1996; the National Feral Pig Action Plan cites Bengsen et al. 2014 that 55-70% of a population must be culled annually just to hold it steady). The resulting defaults are 72% piglet mortality to breeding age and 40% adult annual mortality (the latter implying ~8% of adults reach five years, matching "few survive past five years"); together they yield ~84%/year growth. Both are adjustable by ±5% for the shaded band.

Lamb predation (shown as a range): the prevented population is converted to a density (pigs/km²) over your property area, and that density drives a lamb-loss rate. We show a range between two Australian estimates rather than a single figure: the lower bound follows Gong et al. (2009) ("4% at low, 7% at medium, 9% at high pig densities"), and the upper bound follows Choquenot et al. (1996) ("5% loss at 1 pig/km², up to 20% at 6 pigs/km²"), both cited in the National Feral Pig Action Plan. The upper bound is capped at 20%, since Choquenot's study densities only reached ~6.4 pigs/km², so the line is not extrapolated above that. Treat even the upper figure as an upper bound on prevented loss: feral pigs scavenge stillborn and non-viable lambs, so not all measured loss is additive. Australian studies that isolated predation of viable lambs report about 2% (across all predators). The number also depends heavily on your lambs-at-risk figure: extensive rangeland (where pig predation is worst) carries only ~10-30 lambs/km², so a high flock density on a small area will overstate losses. Applies to sheep enterprises; set lambs-at-risk to 0 otherwise.

On other impacts: feral pigs also uproot pasture, foul water, damage crops and fences, spread disease and prey on native species (NSW DPI). We deliberately show no per-pig figure for ground rooting (the National Feral Pig Action Plan notes "limited studies have investigated the influence [of rooting] on soil properties", and no Australian per-pig rate exists), nor a per-property dollar cost, since only the national total is authoritative: feral pigs cost Australian agriculture about $106.5 million per year (McLeod 2004, Counting the Cost).

Sources (all Australian):

• NSW DPI (2023), Feral pigs - Biology, ecology and impacts: year-round breeding, litter size, sexual maturity, "up to 86% in one year", lifespan, impacts.
• PestSmart (Centre for Invasive Species Solutions), Feral pig biology, ecology and behaviour: two litters per year, sexual maturity, reproductive rate "akin to rabbits".
• National Feral Pig Action Plan (2020), background report: lamb predation (Choquenot et al. 1996; Pavlov et al. 1981; Plant et al. 1978), sustainable cull rate (Bengsen et al. 2014), economic cost (McLeod 2004).
• Primary studies cited above: Choquenot, McIlroy & Korn (1996), Managing Vertebrate Pests: Feral Pigs (Bureau of Resource Sciences); Gong, Sinden, Braysher & Jones (2009), The economic impacts of vertebrate pests in Australia (Invasive Animals CRC); McLeod (2004), Counting the Cost: Impact of Invasive Animals in Australia; Bengsen, Gentle, Mitchell, Pearson & Saunders (2014); Pavlov et al. (1981); Plant et al. (1978).