Sus scrofa

Boar Coexistence Plan

The Zero-Population Growth (ZPG) Management Program was first designed for the Mid-Peninsula Open Space Agency, CA

Humans are responsible for introducing the vast majority of invasive non-native species that currently exist in many ecosystems throughout the world, where they become a destructive presence.

Domestic pigs, wild boar, or currently, feral pigs (Sus scrofa) are not native to the Americas. The first domestic pigs were brought to the United States as livestock by early European settlers. Much later, Eurasian wild boar were introduced into parts of the US for recreational hunting. In areas where the two species were found together, crossbreeding occurred. Today, many hybrid populations exist throughout the wild boar’s range. They have been reported in at least 45 states.

The popularity of wild boars as a game species has played a significant role in their range expansion. The sudden appearance of boars in an area is often a result of:

  1. Escapes of stocked animals from “fenced-in” hunting preserves.
  2. Illegal transport and release of boars into areas for hunting.

In countries where wild boars are native, they provide ecological services that sustain forests, and serve as food for larger predators. For example, in the Caledonian forest in Scotland, the rooting activities of wild boar create ideal conditions for the germination of the Scots pine (Pinus sylvestris) seedlings, as they thrive best in exposed mineral soil. Other plant species have benefited from the boar “planting” seeds as they push through the soil with their snouts. However, the wild boar was extirpated and absent from the Forest for at least the past four centuries. Many attribute the overall decline of the forest to the absence of their essential ecological role. (read our blog posting “When an extirpated native species should not be reintroduced”)

Feral Pig Control

Research shows that at least 80 percent of the feral pig population must be cropped annually to result in a reduced population the following year, which is impossible to accomplish. Therefore, it would be a waste of time and money trying to eliminate or even substantially reduce pig damage by the extensive killing of pigs.

A host of factors in wild pig biology make their elimination simply a pipe dream.  Therefore, local pig control, not elimination, is the only reasonable alternative.  This implies that the agency pursuing the control of feral pigs must decide what is an acceptable amount of pig impact, where on their lands the impact will be tolerated, and to recognize that all impact cannot, categorically, be considered damage.

It is true that destroying pigs that are damaging particular sites would remove some of the offending animals. However, the remaining animals would compensate for this drop in population by having larger litters. Sooner or later other pigs would move into the resulting vacuum at the sites, causing additional damage, and requiring more killing.

Using a lethal solution this cycle of killing and re-population would be repeated indefinitely. Additionally, pigs would disperse to avoid areas where they had been previously trapped, spreading damage into areas where there had been none.

The Zero Population Growth (ZPG) sterilization program offers long-term results. The primary advantage of the ZPG program is that the population dynamic of feral pigs remain constant in an area, eliminating the stimuli for compensatory birthing, immigration and emigration.

The ZPG program encourages cooperative efforts with bordering landowners, so as not to be working at cross-purposes. The Program also provides proactive conflict resolution strategies to mitigate for pig damage that may be incurred by the altered population in biologically sensitive areas, until the population becomes functionally extinct through natural attrition.


To find out more information about this Plan, please contact us at: Back to Model Programs