Their biggest impact is generated by the fact that they tend to feed on the roots of the plants rather than leaves, leaving no chance of recovery, even for the plant species that are able to resprout after grazing . Feral pigs have had direct impacts on native forest birds by destroying understory vegetation, spreading alien weeds, and creating mosquito breeding areas from their rooting and wallowing in wet forests. The photo above demonstrates how pigs and other introduced game animals help spread alien plants in the Hawaiian environment. On the right side of the fence, an alien plant species, milkweed, has taken over. The soil disturbance caused by rooting pigs is a boon to highly competitive alien species, helping them gain a foothold from which to expand their range into native ecosystems.
Plants compete for living space just like animals do.
With introduced grazing and rooting animals in the mix, the Hawaiian plants are at a disadvantage because they evolved without the presence of such animals. With its small numbers, however, few of these bird songs are still sung today. Native bog without pigs, showing healthy tussocks of a Hawaiian sedge, Oreobolus furcatus . Three years after pigs found the bog. Seven years after the bog was fenced.