Due to Forest Park's significant size and its continuous, unfragmented habitat, it boasts an impressive diversity of wildlife for an urban park.

Mammals of Forest Park

As many as 62 mammal species use the forest habitat of Forest Park. Some of the more common species include the northern flying squirrel, Townsend's chipmunk, blacktail deer, mountain beaver, bobcat, coyote and long-tailed weasel. Tracks and droppings of black bear and cougar were also observed in recent years.

Many of the species found in Forest Park are also found in the natural areas of Oregon's Coast Range. In addition to its habitat functions, the forested hills extending northwest from Portland toward the coast serve as a travel corridor for wildlife. This helps to facilitate the seasonal and long-term dispersal of individuals, helping to maintain genetic and biological diversity.

Birds of Forest Park

More than 100 species of birds reside in and/or use the forest habitat of Forest Park. Common residents include: Winter Wren, Dark-eyed junco, Spotted Towhee, Black-capped Chickadee, Chestnut-backed Chickadee, Golden-crowned Kinglet, Pileated Woodpecker, and Steller's Jay.

A number of long range migrants can also be spotted in Forest Park. Common examples include: Rufous Hummingbird, Olive-sided Flycatcher, Pacific-slope Flycatcher, Swainson's Thrush, and the Western Tanager.

Studies of relationships between composition of bird communities and habitat structure show that habitat fragmentation is a key factor in determining avian diversity. Studies indicate that as the degree of habitat fragmentation increases, avian species diversity decreases.

Currently, a tenuous corridor of natural vegetation  connects the park's northwestern end to less developed areas and, ultimately, Oregon's Coast Range.

Fish in Forest Park

Only two streams in Forest Park are known to contain fish: Balch Creek and Miller Creek. The Balch basin is twice the size of the Miller basin and a substantial portion of Balch Creek flows year-round whereas only the lowest reaches of Miller Creek are perennial.

Balch Creek supports a substantial year-round but isolated population of 2,000 to 4,000 cutthroat trout (1987 estimate). The construction of a storm sewer in 1921 isolated the population from the Willamette River. As a result, the population may present unique genetic values.

The main stem of Miller Creek supports sea-run cutthroat trout which move freely to and from the Multnomah Channel. Coho salmon, cutthroat trout and steelhead have also been identified in the lower basin of Miller Creek. Miller is one of a small and diminishing number of free-flowing streams which provide spawning habitat for these anadromous fish.

Amphibians and Reptiles in Forest Park

The most recent data (1987) indicates that eight amphibians and two reptiles can be found in Forest Park. Common species include: Northern red-legged frogs, Pacific giant salamander, Dunn's salamander, and Pacific treefrogs