Frogs of the Willamette Valley,
--by Karen Wegner
approximately 5,000 described species of amphibians (Class Amphibia) worldwide. Frogs
and toads, Order Anura, make up the
majority of these species. The
Order Caudata, otherwise known as salamanders, is the next most common
amphibian group. The lesser-known caecilians, a wormlike amphibian known only
in tropical environments, make up the Order Gymnophiona.
During recent decades,
much has been published regarding declines in amphibian populations.
Declines are better documented in some species and regions than in
others, but concern for these trends has increased in many areas of the globe.
Many potential causes are currently being studied, including: disease,
environmental factors, introduced species, habitat loss including degradation,
modification, and the conversion of temporary water to permanent water.
All probably have important roles in the population declines depending
on species and location.
In the Willamette
Valley, habitat loss through development, and the conversion of temporary
water to permanent water are probably the strongest stressors on native
amphibian populations. Temporary
water, also known as ephemeral water, seasonal ponds, or vernal pools, is
usually a shallow depression, or pond where water collects during the rainy
season and persists for a short period of time.
Most temporary ponds are dry by the middle of summer. The benefit of temporary water to Willamette Valley
amphibians is the impermanence, as it prevents fish from coexisting within the
same pond as amphibians. Willamette
Valley amphibians evolved in an environment generally free from predatory
fish. When the depth of a
temporary pond is increased it can become habitat to non-native fishes such as
bluegill, sunfish, bass, and carp. Non-native
fish have been shown to reduce native amphibian populations through direct
predatory actions, or indirectly by increasing the survival rates of bullfrog
tadpoles which may out-compete native tadpoles.
Only 3% of amphibian
species live in North
America. In Oregon, we
have 12 native species of frogs and toads, and one introduced species.
The Willamette Valley is currently the home for two native species of
frogs, and the one introduced species.
Although the amphibians of Oregon occur in a wide variety of habitats
including cold, torrential streams, warm ponds, ephemeral pools, and high
mountain lakes, Willamette Valley frogs have similar habitat requirements and
often co-occur. There are historical records of a third native frog (Oregon
Spotted Frogs, Rana pretiosa) in the Willamette Valley, but this
species appears to be lost from the region.
Amphibian Life History
word amphibian is derived from the Greek work, amphibious, meaning
living a double life. Frogs lead
a “double life” since adults may spend most of their time on land, but
their eggs must be laid in water. Newly
hatched frogs, commonly called tadpoles or pollywogs, remain in the water
until metamorphosis occurs.
masses laid by Willamette Valley frog species differ from each other greatly
and are easily distinguishable (see descriptions below in species accounts).
Commonly, egg masses include a varying number of dark embryos
individually encased in a clear capsule.
All the individual embryos are then encased as a whole within a clear
frog eggs are laid in masses in the water by a female frog. The eggs hatch
can also be called pollywogs. This stage hatches from the egg. The tadpole
spends its time swimming in the water, eating, and growing. Tadpoles breathe
using gills and have a tail.
3–Tadpole with Legs
this stage, the tadpole sprouts legs (and then arms), has a longer body, and
has a more distinct head. It still breathes using gills and has a tail.
this stage, the almost mature frog breathes with lungs and still has some of
adult frog breathes with lungs and has no tail because the body has absorbed
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Pacific Treefrog (Hyla regilla)
treefrogs are the most common frog species found in the Willamette Valley and
throughout most of the west coast. There are two calls commonly associated
with male treefrogs, one heard during the breeding season while males are in
water, and another often referred to as a “dry-land” call that comes from
males while some distance from water. Their
diet consists mostly of small, terrestrial invertebrates.
are small frogs often under 50mm in length.
They have three distinctive features: a dark stripe across each eye
extending to the shoulder, a “Y” shaped mark on the top of the head, and
rounded toe pads. Treefrogs occur
in a variety of colorations, including bright green, brown, gray, and red.
Coloration generally relates to geographic area, and the frogs in the
Willamette Valley are most often green or brown.
are found in both permanent and temporary ponds during
the breeding season. The rest of
the year they occupy upland areas, and in moist areas they may be found on
the Willamette Valley, male treefrogs move to breeding ponds and begin to
vocalize in January. Females lay
a small egg mass about half the size of a tennis ball, which averages 25 eggs
attached to submerged aquatic vegetation.
The eggs hatch in 3-5 weeks, and metamorphosis follows in 8-10 weeks.
Red-Legged Frog (Rana aurora aurora)
Northern Red-legged frog ranges from Mendocino County, California, to British
Columbia, Canada. The closely
related California Red-legged Frog (Rana aurora draytonii) occurs
farther south in California, and is listed as threatened under the Endangered
Species Act. Red-legged frog
diets consist primarily of aquatic and terrestrial invertebrates, and
occasionally, other amphibians.
frogs are 30-100mm in length. They
are generally brown or reddish brown with black flecks, with a light lip line
on the upper jaw, and may have a dark facemask.
There are strong dorsolateral folds
extending down the sides of the back. The underside of the hind legs is red,
the groin is mottled, and the belly is cream to gray.
The red may continue onto the sides and belly where it will appear
blotchy with the predominant belly color.
frogs may be found in cool, shady coniferous or deciduous upland areas some
distance away from water in the non-breeding season.
Breeding may occur in permanent, or temporary ponds. On warm rainy
nights, adults may be seen on roads. Aquatic vegetation is needed to anchor
the egg mass.
frogs usually begin breeding in February. Males vocalize under water. Females
deposit 500 -1,000 eggs in an egg mass approximately the size of a cantaloupe
on submerged aquatic vegetation. Time
to hatching depends on water temperature, but is about three weeks. There are
documented hatch times of 83 days in very cool water, and 6.6 days in very
warm water. Time to metamorphosis
is around 14 weeks.
are not native to Oregon. Their
native range, east of the Mississippi, has been extended to include most the
entire Untied States except high elevations, and parts of the northern plains. Bullfrogs appear to have been introduced in Oregon in the
early 1900s, and were probably brought here as a food item. Males have a booming call; juveniles give a chirping call
when startled—often as they retreat into water.
Because of their size, bullfrogs can ingest a remarkable variety of
prey. Turtle hatchlings, rodents,
other amphibians, and birds have all been documented as prey.
These types of prey items are likely the exception with aquatic and
terrestrial invertebrates forming the basic diet.
The dietary habits of the bullfrog may make them a threat to native
amphibians, although recent research suggests that introduced warm water fish
may pose an even greater threat.
are the largest North American frogs. They range in color from light green to
deep olive, and often have black splotches.
Dorsolateral folds bend around the tympanum down to the shoulder,
instead of running down the back as in other ranid species in which folds are
present. Females may be as
large as 200mm, and males may achieve 180mm.
Males have a very large tympanum that is
equivalent in size to twice the diameter of the eye, a yellow throat, and
during breeding season the nuptial pads on the back
of their thumbs will be enlarged and dark.
are usually pond dwelling amphibians, although they may be found at least
temporarily in streams and temporary ponds.
In the West, bullfrogs usually require at least one winter before
metamorphosis can occur, so breeding is concentrated in permanent ponds.
occurs in the Willamette Valley from June to August. Female bullfrogs can lay more than 10,000 at a time.
The egg masses may be 2ft. in diameter.
Clutch size is correlated to female size and age, the larger the female
the larger the clutch size. Tadpoles
hatch within a week, depending on latitude; metamorphosis usually begins to
occur after one winter.
Adams, MJ., Pearl, C.A., and R.B. Bury. 2003.
Indirect facilitation of an anuran invasion by non-native fishes.
Ecology Letters (6): 343-351.
Corkran. C.C, and C.T. Thoms. 1996. Amphibians
of Oregon, Washington and British Columbia.
Lone Pine Publishing. Renton, Washington.
Duellman, W.E., and L. Trueb, 1994.
Biology of Amphibians. Johns
Hopkins University Press. Baltimore, Maryland.
Nussbaum, R.A., Brodie Jr., E.D., and R.M. Storm.
Amphibians & Reptiles of the Pacific Northwest. University of Idaho
Press. Moscow, Idaho.
Leonard, W.P., Brown, H.A, Jones, L. L. C., McAllister, K.R., Storm, R. M. 1993. Amphibians of Washington and Oregon. Seattle Audubon Society. Seattle, Washington.
Swedish scientist Carolus Linnaeus (1707-1778) developed the two-name, binomial
nomenclature, (Genus and species) system for identifying and classifying living
things. This system is based on a simple hierarchical structure in which
organisms are sorted using the nomenclature listed below. Organisms belonging to
the same kingdom are not necessarily very similar; organisms belonging to the
same species are very similar and can produce offspring.
For example, the Northern Red-legged Frog would be:
Folds—Glandular ridges that usually run down both sides of a frog's
back. Not all frogs have them.
physical, biochemical, and behavioral change that usually differentiates the
larval and adult stages in amphibians.
Pads—A thickening of tissue on the thumbs of some male frogs.
Aids in helping the male hold onto the female during Amplexus.
circular membrane, which functions as an eardrum, located externally behind the
study of amphibians and reptiles.
Brodeur, Cedric Cooney, Audrey Hatch, Steve Morey for their comments, and to
Christopher Pearl for his comments and photos.