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Forest Concepts, LLC
The Role of Wood in the Life Cycle of
Western Pond Turtles
(Clemmys marmorata)
Jeffrey S. Bash
Forest Concepts, LLC
1911 SW Campus Dr. #655
Federal Way, WA 98023-6441
Phone: (253) 838-4759
Fax: (253) 815-9900
Email: [email protected]
The Role of Wood in the Life Cycle of Western Pond Turtles1
(Clemmys marmorata)
By Jeffrey S. Bash
ABSTRACT
The western pond turtle (Clemmys marmorata) is a species of concern for natural
resource managers in Washington, Oregon, and California. This once abundant species is
at risk due to a combination of overharvesting, habitat loss, and predation by non-native
species. Current efforts to enhance western pond turtle populations have included 1)
captive breeding of juveniles to sizes large enough to discourage predators; 2) protection
of nesting sites with exclosures; and 3) habitat enhancement, including the addition of
wood for basking sites. In addition to providing basking space, wood structure in ponds,
lakes or rivers provides underwater refugia for turtles fleeing from predators. On land,
turtles have burrowed under logs both to aestivate and overwinter. Future opportunities
exist for designing wood structures that create both basking sites and underwater refuges
for escaping turtles. More study needs to be undertaken to determine if a structure can be
built that will significantly enhance western pond turtle populations.
I.
INTRODUCTION
The western pond turtle (Clemmys marmorata) (Baird and Girard 1852) is currently a
species of concern for natural resource managers in the states of Washington, Oregon,
and California. This once abundant species has been in decline since the early part of the
century, primarily through a combination of overharvesting, habitat loss, and predation
by non-native species. The species has nearly been extirpated in Washington State.
In addition to general concern over the possible extirpation of the species in the northern
end of its range, the western pond turtle historically occupied habitat types such as ponds,
wetlands, and marshes that are dwindling in the northwest due to urban and agricultural
growth. The habitat they require is vital to a number of aquatic species. Western pond
turtles are currently state-listed as endangered in Washington, critical in Oregon, and as a
species of special concern in California (Hays et al. 1999).
1
A report to ELWd Systems, a division of Forest Concepts LLC by Jeffrey S. Bash Consulting, 122 N.W.
40th St. Seattle, WA 98103. E-mail: [email protected]
A number of efforts are underway to restore viable populations of western pond turtles
across the historic range. These efforts have focused primarily on captive breeding, with
the goal of producing juveniles physically large enough to withstand predation. Other
components of recovery programs have included habitat enhancement and predator
control.
The purpose of this paper is to 1) examine the major factors currently limiting western
pond turtle population; 2) identify the role of wood in the life cycle of the species; and 3)
describe current efforts to enhance western pond turtle populations. Review of this
material will help to determine if development of engineered wood structures can assist
in western pond turtle recovery efforts.
ELWd Systems, a division of Forest Concepts LLC, is a developer and manufacturer of
all-wood habitat structures for aquatic and upland habitat enhancement. ELWd Systems’
structures are designed from the perspective of biological functionality and ease of
installation in remote locations.
II.
DECLINE OF THE WESTERN POND TURTLE
The Western pond turtle was once fairly common throughout its range, from Baja
California to western Washington State. There are two subspecies of Western pond
turtles. Clemmys marmorata marmorata (Baird and Girard, 1852), known as the
northwestern pond turtle, occupies a range extending from San Francisco Bay north to
Washington State (Ernst et al. 1989). Clemmys marmorata pallida (Seeliger 1945) can
be found from northern Baja California to south of San Francisco Bay in California. In
California, the turtle is found chiefly west of the Sierra crest, with the exception of the
Mojave River in California, and the Truckee and Carson Rivers in Nevada (Stebbins
1972).
Three main factors are responsible for western pond turtle decline: 1) overharvesting; 2)
predation; and 3) habitat loss. Population declines in Oregon and Washington have been
of specific concern. In Washington State, only two known natural populations remain,
both in the Columbia River basin. Only twelve individuals have been counted in the
Puget Sound area since the mid-eighties, where populations where once abundant (Storm
et al. 1995). In Oregon, the number of turtles present is estimated to be less than 10% of
historical populations. Only small populations are known to exist in and north of the
Willamette Valley (Army Corps of Engineers 1999).
.
Overharvesting
From the late 1800’s to the 1920’s and 30’s, thousands of pond turtles were taken from
lakes in central California and shipped to food markets and restaurants in San Francisco
(Storm et al. 1995) where they were sold for $3-$6 a dozen (Ernst et al. 1989). Turtles
were also harvested for food in the Puget Sound Region (Hays et al. 1999). The sale of
turtles as a commodity contributed to a major decline of the species throughout its range.
The selling of western pond turtles is now illegal, but the resulting reduction in
population due to commercial exploitation made the species susceptible to further decline
from loss of habitat and introduced predators (Hays et al. 1999).
Predation
Predation is considered to be the primary limiting factor to establishment of viable
western pond turtle populations in Washington State (K. McAllister, pers. comm. 1999).
Juvenile turtles are at most risk of predation. Non-native species such as bullfrogs and
largemouth bass in particular are known to feed on pond turtle young (Storm et al. 1995).
Primary predation pressure is upon juveniles less than five grams in weight (F. Slavens,
pers. comm. 1999).
The western pond turtle is currently at a critical point because the existing base
populations are not large enough to withstand predation pressure. Western pond turtles
become sexually mature at 8-12 years old and lay 3-13 eggs a year. Generally, only 2
hatchlings survive per year, making it difficult to maintain a viable natural population.
(F. Slavens, pers. comm. 1999). As the western pond turtle has a lifespan of over forty
years, the presence of adults may not be a good indicator of the true health of the
population (Garrison, 1998).
In addition to predation by bullfrogs and warm-water fishes, mammals such as raccoons,
skunks, otters, gray fox, and dogs have been known to feed on the turtles or eggs. In one
instance, black bear and coyotes wiped out an entire population in California. Various
raptors and snakes are also suspected of preying upon western pond turtles. (Hays, 1999)
Habitat Loss
Habitat loss has occurred primarily through urbanization, the drainage of wetlands for
agriculture or development, and the alteration of watercourses. It has been estimated that
33-50% of wetlands present prior to European settlement have been lost in Washington.
The losses are greater for counties adjacent to Puget Sound (Hays et al. 1999). Dams and
diversion schemes have both eliminated habitat and isolated populations. The alteration
of riparian vegetation through fire suppression, water diversion projects and grazing may
have created habitat less suitable for juvenile pond turtles. Fire suppression may also
have resulted in the shading of traditionally open nesting areas (Hays et al. 1999).
Other Population Pressures
In 1990, an unknown respiratory virus killed at least 36 turtles in Washington State. It
was suspected that non-native turtle species may have introduced pathogenic agents to
which the western pond turtles had never been exposed (Hays et al. 1999). Drought may
impact western pond turtle populations by eliminating habitat or the prey base required
for survival. If the turtles have not received adequate fat reserves, they may be unable to
survive overwintering (Hays et al. 1999). Human interference at both ponds and nesting
sites may impact the ability of females to lay eggs, or may reduce the amount of time
turtles spend basking, which may be important for egg maturation (Hays et al. 1999).
Turtles in habitats near roads have been killed by automobile traffic (F. Slavens, pers.
comm. 1999).
III.
PHYSICAL DESCRIPTION
The genus Clemmys contains four species of small to medium size turtles in the United
States and Canada. Turtles in this genus include the wood turtle (terrestrial, northeastern
U.S./Canada), the spotted and bog turtles (eastern U.S., semi-aquatic), and the western
pond turtle, which is an aquatic species (Nussbaum et al. 1983).
The Western pond turtle is generally 88-177 mm (3.5 to 7 in) in shell length (Stebbins,
1985). The upper shell (carapace) of the Western pond turtle is olive or dark brown to
blackish. In many individuals there are radiating black and cream lines on the large dorsal
plates. The ventral shell (plastron) is yellowish tan in color, sometimes with varied dark
markings in the center of the scutes. The majority of individuals have a solid yellow
plastron. The Western pond turtle’s limbs and head are light to dark brown with scattered
black markings (Nussbaum et al. 1983, Storm et al. 1995). Males have light or pale
yellow throat, concave plastron, and the cloacal vent is at posterior edge of carapace.
Females have a brownish throat with dark flecking, a more convex plastron, and a cloacal
vent within the posterior edge of carapace (Storm et al. 1995).
Western pond turtle hatchlings measure approximately 25-31 mm (1 in) in length (Hays
et al. 1999), with a tail nearly as long as the shell. The shell is uniformly olive or brown
on the carapace, with yellow markings at edge of marginals. The shields have numerous
small tubercles. The plastron is yellow, with large irregular central black marking. The
head, limbs and tail of hatchlings are marked and pale yellow. The shell of the male is
flatter and less marked than the female (Stebbins 1985, Nussbaum et al. 1983).
IV.
HABITAT REQUIREMENTS
Typical Western pond turtle habitats include wetlands, ponds and riparian areas with
open adjacent uplands, such as oak-pine savanna, prairie, or pastures (Hays et al. 1999).
In Washington State, western pond turtles are fo und primarily in ponds. In other parts of
the range, they may be located in the slow- moving portions of streams, as well as
wetlands and ponds. The turtle usually stays close to water, with the exception of
overland travel by females for nesting and individuals escaping drought conditions
(Stebbins, 1972). In rivers and streams, western pond turtles are restricted to areas near
banks or backwaters where basking sites and underwater refugia are available (Hays et al.
1999).
Basking Habitat
Western pond turtles spend a significant amount of time basking, in order to thermoregulate body temperature. Basking may occur throughout the year, but increases in
frequency through spring to a peak in early June (Hays et al. 1999). If not basking on top
of aquatic vegetation or logs, the Western pond turtle will position themselves just below
the surface upon aquatic vegetation, where there are elevated water temperatures. The
turtles have also been known to bask on sand and mud banks. The turtles may even
climb onto tree branches that dip into the water from bank vegetation (Nussbaum et al.
1983). Other types of basking sites reported included abandoned autos, planks, and the
carcasses of large mammals (Hays et al. 1999).
The pond turtle is alert, and will rapidly dive from basking sites when approached, even
at distances greater than 50 meters (164 ft). This behavior makes it difficult to locate
pond turtles in the wild (Storm et al. 1995). Pope (1939) reported that those turtles not
basking in the middle of the day may be resting on the pool bottoms. In the morning and
evening, they may move upstream or downstream from one pool to another (Ernst et al.
1989).
Western pond turtles are reported to be aggressive when competing for basking sites.
Adults and juveniles of both sexes are equally aggressive. The aggression is manifested
through biting, pushing, and open- mouthed threats in which the bright edges of the mouth
and reddish tissues inside the mouth are exposed and may serve as warning signals
(Nussbaum et al. 1983, Storm et al. 1995). This behavior appears to maintain spacing
between basking turtles (Hays et al. 1999).
Aquatic Vegetation
Aquatic vegetation also appears to be an important habitat component for the Western
pond turtle. In the northern part of their range, arrow weed and pond lilies are often
associated with pond turtle habitat. Dense growth of woody vegetation surrounding the
edges of a watercourse is not optimal for the western pond turtle, as it reduces available
basking areas (Hays et al. 1999).
An oxbow lake in the Corvallis, OR area measuring 1.5-2.0 hectares contained an
estimated 75 pond turtles. A similar oxbow near Salem, OR contained an estimated 180
pond turtles (and 60 painted turtles), a considerably larger population at a lake of similar
size. The lake near Salem contained additional aquatic vegetation, consisting of pond
lily, pondweed, smartweek, burreed and bulrush (Nussbaum et al. 1983). This finding
suggests that aquatic vegetation is an important habitat component for the western pond
turtle.
Predator Avoidance
Western pond turtles will seek safety from predators in the water beyond reach, or by
burying themselves in waterlogged leaves and brush. Turtles tend to dive to the same
underwater refugia when disturbed (F. Slavens, pers. comm. 1999). Shallow water
habitat preferred by western pond turtles such as logs, rocks, and open banks also happen
to be good habitat for the bullfrog, a major predator of juveniles (Hays et al. 1999).
Over-Wintering and Aest ivation Habitat
In the southern portion of their range, western pond turtles are active all year. In the
north, hibernation generally occurs from November to February (Stebbins, 1985). These
turtles usually hibernate in bottom mud, but have occasionally been seen basking during
the winter months in the Willamette Valley (Nussbaum et al. 1983). In Washington,
approximately half of turtles overwinter in upland habitats (in leaf litter) and half
overwinter in the mud bottoms of lakes and ponds (F. Slavens, pers. comm. 1999).
During warm weather, western pond turtles may aestivate, or achieve an inactive state.
This behavior occurs during drought conditions, when normal functions may not be
achieved at the pond site. Pond turtles may bury themselves in leaf litter or under logs
during aestivation (Hays et al. 1999).
Nesting Habitat
Western pond turtles require 8-12 years to attain sexual maturity, depending on their
location within the range (less time to maturity in southern portions of range) (Storm et
al. 1995, Wildlife Society 1999). Little information is available on courtship and mating
(Nussbaum et al. 1983).
Western pond turtles nest May to August, primarily June to mid-July, time varying with
locality (Stebbins, 1985). Clutch sizes may contain from 3-13 eggs (Nussbaum et al.,
Storm et al. 1995). The incubation period of eggs in nature is unknown. Artificially
incubated eggs hatched between 73-80 days (Nussbaum et al. 1983). Some individuals
may deposit eggs every other year, while others may produce two clutches in a year
(Storm et al. 1995). Eggs are white, elliptical in shape, and average 34 mm (1.34 in) in
length and 21 mm (.83 in) in diameter (Hays et al. 1999).
The female usually leaves the water in the evening to lay her eggs, and may wander a
great deal before selecting a nesting site. The female nests in dry, well-drained soils, in
open areas with grass, herbaceous vegetation, with shrubs and trees in proximity (Hays et
al. 1999). Full sunlight seems to be a requirement for site selection. (Ernst et al. 1989).
The female may travel of distances up to 187 meters (614 ft) away from water to nest.
The average nesting distance is approximately 100m (328 ft) from the water (Hays et al.
1999).
The nest is flask-shaped, with a 4.5 cm (2 in) opening (Storm et al. 1995). The mother
expends considerable energy concealing the nest with soil and adjacent low vegetation,
making the nest difficult to locate unless it has been disturbed by a predator (Storm et al.
1995). Nests are highly susceptible to predators as well as trampling by cattle or people
(Garrison, 1998).
It appears that some hatchlings overwinter in the nest. According to Storm and Leonard,
this phenomenon is more prevalent in the northern reaches of the range. It was suggested
that winter rains might be necessary to loosen the soil in order that the female may dig
the nest. The young appear in the spring following the year the eggs were first laid.
Individual growth rates in the wild are low. Adults may survive more than 30 years in
the wild (Storm et al. 1995).
Foraging Habitat
Western pond turtles are omnivorous, although it has been reported the species exhibits a
strong preference for animal food (Ernst et al. 1989). Their diet consists of aquatic
plants, insects, fish, invertebrates, and carrion (Stebbins 1985). Crayfish, fishes,
tadpoles, and frogs are consumed occasionally. Plant food includes lily pods, tule, cattail
roots, and filamentous algae (Storm, et al. 1995). Feeding behavior is solitary under
normal conditions. Western pond turtles need water in order to swallow their food. The
turtles will forage at the pond or stream bottom, searching through submerged leaf litter
and other detritus (Hays et al. 1999). Post-partum females appear to eat more plant
material than other individuals, occasionally ingesting quantities of cattail or bulrush
(Hays et al. 1995).
V.
USE OF WOOD BY WESTERN POND TURTLES
Western pond turtles appear to be more abundant in habitats that have basking sites
(Holland and Bur y 1998). However, there is no evidence that additional basking habitat
actually increases pond turtle habitat. It does, however, make the turtles more visible to
observers. At one site in Oregon, western pond turtles were discovered in a pond with
very few basking structures. Vegetation such as eurasian millfoil allowed the turtles to
bask in shallow, warm water close to the surface (K. Beal, pers. comm. 1999).
When basking structures are added, however, the turtles tend to use them. This might be
an indication that turtles prefer sites out of the water, where they may gain higher body
temperatures, especially if the water is cold. Basking in higher temperatures might also
be linked to reproductive functions of females. A population of western pond turtles
living in a river (Coast Fork of the Willamette) were found to be physically smaller than
turtles in pond populations. This might indicate that the ability to achieve higher body
temperatures is linked to growth rate and reproductive success (K. Beal, pers. comm.
1999).
While the lack of downed logs has been considered a limiting factor in some locations,
this has not been considered as important as the reduction of predation upon juvenile
turtles. As the populations of western pond turtles increase, availability of basking sites
probably might become more of a limiting factor. In addition to basking, wood also
appears to play a role as a refuge from predators and weather extremes, both in the water
and on land.
Basking Sites
The prominent role of wood in the western pond turtle life cycle is for basking. Western
pond turtles favor habitats with large amounts of emergent logs or boulders, where they
aggregate to bask (Storm et al. 1995). They have also been known to bask on aquatic
vegetation (Hays et al. 1999). Western pond turtles are aggressive in defending basking
space. Little is known about the dimensions of “personal space” required by basking
turtles of different ages and sexes.
While the turtles naturally bask on whatever surfaces they can find, their preference is to
bask away from the shore. It is more difficult for predators such as bullfrogs to reach the
turtles if they have swim across water (F. Slavens, pers. comm. 1999). McAllister (pers.
comm. 1999) suggested that wood might be preferable as a basking site for this reason, as
it is more common for wood to be located away from shore, helping the turtles to avoid
predators. Protruding rocks used as basking sites would tend to be in shallower water,
offering easier access for predators. In addition, it might be easier for turtles to gain a
clawhold on wood versus rock to pull itself out of water. Further studies on this matter
need to be undertaken in order to verify if a preference exists between wood and other
basking surfaces (K. McAllister, pers. comm. 1999).
The relationship of the basking site to the sun is important. The value of basking sites
vary during the day with the angle of the sun. Western pond turtles are fairly predictable
about using basking areas based on sunlight availability (F. Slavens, pers. comm. 1999).
It is important to note that basking sites are constantly changing, based upon changes in
the level of the water during drought and flood cycles. It is difficult to predict where and
how wood will settle in a pond (F. Slavens, pers. comm. 1999).
Refugia – Overwintering, Aestivation, and Predator Avoidance
Western pond turtles use logs to avoid predation by diving into the interstitial spaces
formed underwater by woody debris. The turtles tend to dive quickly if disturbed, and
retreat to hiding places close to their basking sites. Apparently, they will frequently dive
to the same location to hide (F. Slavens, pers. comm. 1999). Within the pond, an ideal
log for predation avoidance would be located away from the shoreline and would have
crevices or interstitial spaces beneath it for the turtle to retreat to in a dangerous situation.
The presence of aquatic vegetation is also important, but it is not clear if it is more
important than wood structures for hiding places. Turtles may also sleep under woody
debris, especially in crevices under logs (F. Slavens, pers. comm. 1999). Other hiding
locations include rocks, submerged vegetation, or holes along the bank (Hays et al.
1999).
When leaving the water, turtles may use logs as hiding or resting sites (Hays et al. 1999).
Individuals occasionally use logs as shelters for aestivation and overwintering (Hays et
al. 1999). Aestivating turtles appear to primarily burrow under leaves or the litter layer
(Hays et al. 1999). While most turtles overwintering on land appear to find sites in leaf
litter near to trees and shrubs, in some instances turtles actually use logs during
hibernation. In one of the Washington State pond turtle sites, two turtles were found dug
in under logs. Thirteen were found under small shrubs (Hays et al. 1999). A review of
the literature suggests that logs are not necessarily vital for aestivation or overwintering.
In regard to nesting, it appears that turtles prefer open sunny sites generally not
associated with wood. It is possible that the wood may be more important outside the
pond to turtles at the Columbia River sites, as wood and vegetation is generally in short
supply compared to the site in Pierce County (McAllister, 1999).
Foraging
Little information exists on the role of wood as a source of food for western pond turtles
(for example, as a site for colonies of benthic macroinvertebrates), but it is likely that
more food would associated with woody debris than rocks.
VI.
EXAMPLES OF WESTERN POND TURTLE REHAB PROJECTS
Two main recovery programs for the western pond turtle have been implemented in the
Northwest. These programs are primarily financed and operated by government entities,
in concert with universities or non-profit organizations.
Washington State Recovery Plan for the Western Pond Turtle
To support the revival of western pond turtle populations in western Washington, the
state and the Woodland Park Zoo developed a recovery program in 1990. At the time,
there were only two known populations of western pond turtles in the state, in Skamania
and Klickitat Counties on the Columbia River. The turtle had been considered extirpated
from the Puget Sound, as only twelve individuals had been sighted since 1986 (Slavens,
1999).
The main goal of the program is to create viable western pond turtle populations that can
survive via natural recruitment of juveniles (Hays et al. 1999). The main method of
achieving this aim has been development of a “Head Start” program, in which western
pond turtles are raised in captivity for the first year. The turtles grow to a size where they
are too large to be eaten by non- native predators such as bullfrogs and warm-water fishes
(F. Slavens, pers. comm. 1999).
In addition to bolstering the existing Columbia River populations, the WDFW established
a population of western pond turtles at a state-owned pond complex in Pierce County. Of
the 12 turtles that had been found in the Puget lowlands, four had been kept at the zoo for
captive breeding, to keep the Puget Sound gene pool active. Hatchlings raised at the zoo
that were introduced at this site appear to be doing well and may have a survival rate of
up to 90% (Slavens, 1999). The current number of western pond turtles in Washington is
estimated at 250-350 individuals, with approximately half from the Head-Start program
at the Woodland Park Zoo (Hays et al. 1999).
Biologists working with these sites have tried to remove predators prior to the reintroduction of turtles in some ponds. In one removal effort, a lake was drained and over
200 kg (450 lbs) of fish were removed. Bullfrog predation has been addressed by
removing egg masses from some of the restoration sites. Exclosures have been built
around turtle nesting sites to protect the eggs from predation (F. Slavens, pers. comm.
1999).
Although the main focus of the recovery project has been captive breeding of juvenile
western pond turtles, habitat enhancement measures have been undertaken at both the
Columbia River and Pierce County locations. The task of enhancing, restoring, and
managing habitat for western pond turtles is considered secondary to controlling the
effect of predation on the population. The Washington State recovery plan does call for
the addition of logs or rafts for emergent basking sites where needed (Hays et al. 1999).
The Washington State pond turtle recovery management team designed basking
platforms for placement in ponds which were considered deficient in basking locations.
The basking platforms were made from three or four 2”x12” pieces of wood in a triangle
or square, with each corner overlapping. One corner of the structure was anchored to a
milk carton filled with cement. The opposite corner(s) protrude from the water. During
an initial survey for western pond turtles in 1991 and 1992, 45 artificial rafts were
distributed at 31 sites in five counties (Hays et al. 1999). The structures have been
effective as emergent basking sites; based upon visual inspection, they were heavily used
by turtles (Slavens 1999).
At the restoration site in Pierce County, project managers deposited oak logs into the
pond. The logs were salvaged from a land development site in Dupont, Washington. The
logs were heavy and large enough that they did not have to be anchored, but simply
dropped in the water. Some of the logs were installed in troughs dug at pond edge,
extending out into the water. A backhoe was used for this operation. The logs were 15”24” in diameter (38-61 cm), with 3”-6” (8-15 cm) branches, and were spaced at regular
intervals around the pond. The logs have been effective in providing basking habitat
locations throughout the pond and are heavily used by the pond turtles (K. McAllister,
pers. comm. 1999).
Recently, the Washington Department of Fish and Wildlife produced a revised draft
recovery plan for the western pond turtle (August 1999). The state recovery plan calls for
the establishment of at least five populations of 200+ Western pond turtles, with no more
than 70% adults which “occupy habitat that is secure from development or major
disturbance” (Hays et al. 1999). Other objectives of the plan are to monitor the progress
of the turtles and maintain sighting records. The state will undertake scientific
investigations to facilitate and enhance recovery, develop a Washington identification
guide, and evaluate regulations concerning the release of exotic pets into wetlands, which
threaten Western pond turtle survival (Hays et al. 1999).
In addition to maintaining the existing populations in the Columbia River Gorge and
Pierce County, the program is currently preparing to release a new population of western
pond turtles into the Pierce National Wildlife Refuge, near Beacon Rock in the Columbia
River (F. Slavens, pers. comm. 1999).
Army Corps of Engineers
The Army Corps of Engineers controls numerous water projects in the Willamette Valley
of Oregon. This area formerly contained abundant Western pond turtle populations.
Population surveys were conducted in 1993. Populations were identified in multiple
reservoirs, ponds and wetlands (Army Corps of Engineers 1999). Habitat enhancement
and protection of existing sites are the main goals of the Army Corps of Engineer team
working with western pond turtles. Populations in Oregon have not reached the low
levels that exist in Washington State (K. Beal, pers. comm. 1999).
The addition of wood for basking has been part of the Corps’ habitat enhancement
activities. Where wood has been added, pond turtles readily use it for basking. As pond
turtles prefer to bask away from shore, the Corps has used floating basking platforms
(cans, which provide a natural roundness) that are anchored or cabled to masonry blocks.
In some cases, these platforms have had to be buoyed with bags of foam. They have also
fallen trees along shorelines. The Corps has considered installing rootwads into ponds,
which have both basking habitat and interstitial spaces for hiding, but have not yet
implemented this plan (K. Beal, pers. comm. 1999).
Much of the Corps’ management efforts have been focused on the Fern Ridge reservoir,
which holds the largest pond turtle population in the Willamette Valley, numbering over
200 individuals. Nesting areas are monitored daily during the nesting season, and
exclosures are placed over fresh nests to protect them from raccoons. Twenty-two to
twenty-eight nests have been located each year since 1992. Fern Ridge’s proximity to the
West Eugene wetlands and connection to the Amazon canal make it a logical place for
recovery efforts to maintain populations in the southern Willamette. The Corps is
working with the Oregon Department of Fish and Wildlife (ODFW) to manage this and
other reservoir populations (Army Corps of Engineers, 1999). This partnership also
includes captive breeding of juvenile pond turtles (K. Beal, pers. comm. 1999).
VI.
SUMMARY
While predation is clearly the major limiting factor for western pond turtle survival in
Washington State, wood might be limiting in some environments where turtles must
compete for optimum basking space, such as with the populations present in the
Willamette Valley. While it is unknown whether the addition of wood would actually
result in increased western pond turtle populations, it might create additional optimum
habitat for basking. In addition to basking, the interstitial spaces formed by woody debris
underwater serves as a refuge from predators. Wood on land may be used as a refugia
from predators during overland travel. It may also serve as shelter for turtles
overwintering or aestivating.
The addition of wood to western pond turtle habitat has been considered a success in that
basking turtles are often found on the new structures. Future opportunities exist for
designing wood structures that create both basking sites and underwater refuges for
escaping turtles. More study needs to be undertaken to determine if a structure can be
built that will significantly enhance western pond turtle populations.
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Beal, Kat. Wildlife Biologist. U.S. Army Corps of Engineers, Willamette Valley Project Office. Personal
communication. September 20, 1999.
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