Today, the most destructive force in native forests - besides land development - is feral pigs. Although pigs were brought to the Hawaiian Islands by Polynesians as early as the fourth century A.D., the current severe environmental damage inflicted by pigs apparently began much more recently and seems to have resulted entirely from release of domestic, non-Polynesian genotypes. (Polynesian pigs were much smaller, more docile, and less prone to taking up a feral existence than those introduced at later times.)
Today's wild pigs probably descended from runaway domestic breeds in lowland areas. Perhaps aided by a seasonally abundant and expanding carbohydrate source-the invading nonindigenous strawberry guava-and by an enhanced protein source from abundant nonindigenous earthworms, truly feral pig populations developed and spread into adjacent pristine forests.
European domestic pigs were among the earliest introduced species to arrive in Hawai`i. Without predators or herbivore competitors, these animals adapted well to life in the wet forest and rapidly established large feral populations. Their populations multiply unchecked, (a single pair and their offspring theoretically can produce 15,000 pigs in five years!).
Feral pigs have endangered many of the native plant species by overgrazing in the rain forest. They churn up the forest floor in search of earthworms and fleshy plant roots and destroy vulnerable native plants such as mints and orchids. The starchy core of native tree ferns is also among the pigs' favorite foods. The pigs gnaw at the trees' bark and devour valuable nutrients from the forest floor. Opportunistic plant species, often nonindigenous, occupy the habitats remaining after feral pigs have eliminated native species. Seeds of nonindigenous plants are carried on pigs' coats or in their digestive tracts, and they thrive upon germination on the forest floor where pigs have exposed mineral soil. Once these aggressive plant invaders have obtained a new foothold in the forest, they spread aided by pigs and nonindigenous birds.
Measures to control pigs
In the early 1900's, the damage caused by feral European pigs in native rain forests was recognized; the Hawaii Territorial Board of Agriculture and Forestry subsequently started a feral pig eradication project that lasted until 1958 and removed 170,000 pigs from forests statewide.
The Hawaiian government has encouraged hunters to stalk the feral pigs. All legally hunted game animals in Hawaii are introduced species.
Park crews build goat and pig-proof fences around portions of parks that possess special ecological values. They eliminate pigs and offensive alien plants within these areas and the forest begins to heal with the return of native plants and animals. The fencing costs more than $40 thousand dollars per mile. The costs can be much higher in remote, rugged terrain. Haleakala National Park has spent about $5 million dollars to protect the crater area. Once you get the fence installed, the hard work begins. You've got to get rid of all the animals inside the fence. The first ones are pretty easy but it gets much harder as you progress. You've got to get into very inaccessible areas. About 10,000 acres have already been enclosed with pig-free fencing and are showing significant signs of recovery.
When you eliminate these feral animals, it doesn't make you popular. Some people would rather have the alien animals than the native plants. (The group that has come under the most criticism for its alien animal control program is The Nature Conservancy who used snares to trap pigs on Molokai, so animal-rights groups have accused the conservancy of being in-humane.)
An ecosystem is a balanced interaction of plants, animals and the environment in which they live. The balance is easily upset by "alien" interference. Look at how the alien pig is part of a chain of events that leads to disaster for native birds.
Myrica faya, an alien plant, begins to grow in a native forest.
Earthworms are attracted to the roots of the plant.
Pigs turn up the ground in search for the tasty earthworms.
The disturbed ground collects rainwater and puddles form.
The puddles provide a comfortable wallow for the pigs.
Culex quinquefasciatus, an alien
mosquito, is attracted to the standing water in the puddles and wallows.
The mosquitoes lay their eggs there and the population grows.
Plasmodium relictum is an alien parasite found in many birds, though it has very little affect on introduced birds.
Mosquitoes become parasite carriers once they bite an infected bird.
They then spread the parasite and alien viruses to other birds in the same manner.
Native honeycreepers have no resistance to the disease.
avian malaria or bird pox, which shortens their lives and depletes their populations.