Article and photos by John Sparks
Three species of frog, one of them a predatory interloper, call Oaks Bottom home. Two native frogs, the well-known Pacific chorus frog and the red-legged frog, seem to be increasing in numbers. The “interloper” is the American bullfrog, which – alas – is the amphibian you are most likely to see on a casual stroll around the wildlife refuge during the day.

Tadpole Pond in the summer
During the first decade of the 21st century, Portland Parks constructed seasonal ponds to provide breeding habitat for amphibians in Oaks Bottom’s North Meadow, an area closed to the public. In addition, Tadpole Pond was built to provide an educational site where visitors can encounter amphibians, their tadpoles and egg masses.
Red-legged frog (Rana aurora)
This large frog is endemic to the Pacific Northwest and is the species most at risk of the amphibians found at Oaks Bottom. Red-legged frogs have a low tolerance for disturbed habitats. They need cool, shaded water for breeding and adults will stay close to water, preferably where there is vegetation cover, like cattails or rushes. Red-legged frogs at Oaks Bottom breed in the vernal pools of the North Meadow, some created especially for them, and also Tadpole Pond. The tadpoles have metamorphosed by mid-summer.

Red-legged frog nestled in a meadow
Red-legged frog numbers are fairly low at Oaks Bottom. A study by Chauncy Anderson et al. of the U.S. Geological Survey indicates that the presence of the fungus-based disease chytridiomycosis at Oaks Bottom may be a contributing factor. Chytridiomycosis is responsible for major declines and extinctions of amphibians around the world. It seems, however, that red-legged frogs have spread their breeding locations to all the ponds in the North Meadow in recent years.
Pacific chorus frog (Pseudacris regilla)
This pretty little frog, sometimes called a tree frog, is perhaps the best known of the amphibians at Oaks Bottom. Although the smallest of our frogs, the males have the loudest and most persistent calls and have become uncredited background assets in many a Hollywood motion picture, whether it’s set in the Amazon, Africa, or the Indian jungle! They can be found far from water due to a coating on their skin which allows them to hold in moisture. The adults have a black mask but otherwise can range from bright green to a dull brown in color.

Chorus frog at Oaks Bottom
You will begin to hear the chorus frogs in early spring. Their most favored breeding spot at Oaks Bottom is Tadpole Pond in late March/early April. Up to 20,000 chorus frog eggs may be deposited there in any given year. You can see their tadpoles there from April through June. A favorite hangout of juvenile Pacific chorus frogs is the pool of water that collects at the joint of the leaf and the stem of non-native teasel plants (Dipsacus fullonum), a refuge where they can patiently await an unsuspecting insect. While chorus frogs readily migrate to Wapato Lake, they do not favor breeding there, preferring the vernal pools in the meadows.

Chorus frogs may also be brown colored
Bullfrog (Lithobates catesbeianus)
An irony surrounds the introduction of this big East Coast species to the Pacific Northwest. Their large, beefy thighs ($10 – $30 per lb.) are considered a delicacy by some, but the ‘tables’ have turned and millions of bullfrogs have avoided a chef’s cleaver. Instead, as a bully larger than any of our native frogs, they wreak predatory havoc on Oregon’s wetland ecosystems. Bullfrogs seem to dominate the shallows of Wapato Lake as well as the ash swales at Oaks Bottom, and you may hear their splashes as they dive into the water at your approach. If you look carefully, you may notice their bulging eyes following you as they float on the surface.

Bullfrog lurking in the shallows
Bullfrogs seem to consume anything that matches their size, and this includes newly hatched ducklings, baby turtles, fish, and other bullfrogs. They are quite tolerant of environmental damage and have become a major competitor of the red-legged frog. Being warm water lovers, bullfrogs lay their eggs in the summer, and like most invasive species, they are prolific, with some females laying up to 20,000 eggs!

Juvenile bullfrog
Principal references:
Chestnut, Tara E., Chauncey W. Anderson, Radu Popa, Andrew R. Blaustein, Mary Voytek, Deanna H. Olson, Julie Kirshtein. Heterogeneous occupancy and density estimates of the pathogenic fungus Batrachochytrium dendrobatidis in waters of North America. Oregon Water Science Center, 2014.
Corkran, Charlotte C. and Chris Thoms. Amphibians of Oregon, Washington and British Columbia. Lone Pine Publishing, 1996.
Guderyahn, Laura. In-house report on the status of breeding amphibians at Oaks Bottom Wildlife Refuge. City of Portland: Bureau of Parks and Recreation, February 2019.
Holzer, Katie. Two-year study of amphibians in Oaks Bottom Wildlife Refuge. City of Portland: Bureau of Parks and Recreation, August 2009. https://www.portlandoregon.gov/bes/article/273024