Although specific records are lacking, the historic bottomland and upslope forested areas likely provided habitat for species typically found throughout the region including migratory and resident birds, amphibians, reptiles, mammals, and invertebrates.
Habitat fragmentation can cause major impacts to wildlife and even lead to local extinctions (Johnson and O'Neil, 2001). Large, contiguous tracts of habitat generally support more native species than small, isolated patches. Balch subwatershed provides significant wildlife habitat links to the south with the northern portions of Forest Park. Forest Park, in turn, provides a habitat link for the Tualatin River Valley, the Coast Mountain range, the Willamette River, Sauvie Island, the Columbia River, and the Vancouver Lowlands in Washington.
Sixty-two mammal species occur in Forest Park including northern flying squirrel, Townsend’s chipmunk, blacktailed deer, mountain beaver, bobcat, coyote, wandering shrew, Trowbridge shrew, shrew mole, deer mouse, Pacific jumping mouse, western pocket gopher, beechey ground squirrel, little brown bat, long-eared bat, Roosevelt elk, and both short tailed and long tailed weasel. Evidence of black bear and cougar has also been noted(City of Portland Bureau of Planning, 1995 and 2001b).
Bird species observed or known to occur in Forest Park include rufous hummingbird, olive-sided flycatcher, Pacific slope flycatcher, western wood pewee, house wren, Swainson’s thrush, western tanager, blackheaded grosbeak, great horned owl, pileated woodpecker, pygmy owl, red-tailed hawk, ruby-crowned kinglet, winter wren, and bald eagle (City of Portland Bureau of Planning, 1995 and 2001b).
Amphibian species observed in the Audubon Preserve pond include rough-skinned newts, Pacific treefrogs and their egg masses, and northwest salamanders (City of Portland Bureau of Planning, 1995).
Loss of species diversity, the decrease in large mammal species and subsequent increase of small mammal species, habitat loss, pollution, hunting pressure, and predation from introduced species have impacted wildlife communities. Exotic species negatively impact native species through predation, displacement, or modification of habitat(Johnson and O'Neil, 2001). Increased population and urban development have led to the extirpation or reduction of large mammalian predators, such as gray wolves, bears, and wild cats. The numbers of smaller predators (weasels, coyote, raccoons, and opossums) and secondary consumers (deer and rodents) increase in the absence of large predators.
Roads cut-off or severely hamper diurnal movements and seasonal migrations of vertebrates. Many animals die or sustain injuries while crossing roads.
Invasive plant species have reduced the structural complexity of habitat and generally support fewer native insect species (Johnson and O’Neil, 2001). The predominance of English ivy, a highly invasive plant species, has simplified the forest understory and degraded the habitat quality for native food-source plant species. During mid-1990 field surveys in Forest Park, no amphibians or reptiles were detected in areas overgrown with English ivy (City of Portland Bureau of Planning, 1995). This is likely due to a reduction in native insect food (i.e. springtails and other small invertebrates commonly eaten by native salamanders), and an increase in unpalatable insect prey (e.g. pillbugs and sowbugs) (City of Portland, Bureau of Planning, 1995).