Since the early
1990s concern has been expressed nearly worldwide about apparent declines in amphibian
populations. On a regional level also, casual observations suggest that amphibian
populations were down substantially (Adams, personal observations).
Factors suggested
as contributing to the decline include disease, ultraviolet radiation, climatic
factors and human impacts on the environment. The Asylum Lake Preserve
has been used extensively by humans for more than 150 years, and the activities
during
that time have
had a profound impact on the amphibian populations present.
Reptile presence
and distribution undoubtedly also has been profoundly influenced by these site-specific
human activities. Frequent disturbance, possible collecting, habitat
fragmentation, road-caused
mortality and domestic animals all could depress reptile populations.
Methods
Amphibians were
surveyed by both sight and sound throughout the inventory period. The edges of
Asylum Lake, its in- and out-flows, and the edges of Little Asylum Lake
were searched for basking amphibians. Surveyors overturned fallen trees,
rocks and other objects to find salamanders and small reptiles. Pit traps
were employed initially to capture migrating amphibians, but human interference
caused us to abandon this technique. Reptiles were documented by basking
turtle surveys, systematic surveys of potential habitat and chance sight
encounters.
Results
Thirteen species
of reptiles and amphibians were documented throughout the inventory (Table
F). The basking surveys revealed a viable population of Midland painted
turtles (Chrysemys picta marginata) in Asylum Lake. The shell of a
common musk turtle
(Sternotherus odoratus) was discovered, although a live specimen was
not. Four other species—common map turtle
(Graptemys
geographica), Blanding’s turtle (Emydoidea blandingii), common snapping
turtle (Chelydra
serpentina) and eastern box
turtle
(Terrapene carolina
carolina)—were observed,
bringing the total to six.
Amphibian surveys
documented three anuran species: green frog (Rana
clamitans melanota)—including tadpoles,
northern spring peeper (Pseudacris
crucifer crucifer) and eastern American toad
(Bufo
americanus). Despite a spirited search, no salamanders
were uncovered. Both reptiles
and amphibians were found primarily in Asylum Lake, Forest A, and Forest
B.
Three of the
species documented are listed as species of special concern by the State of
Michigan: Blanding's turtle, eastern box turtle and black rat snake. The presence
of these species on the Asylum Lake property is encouraging, although further
studies are needed to determine the size of their populations. A number of
eastern box turtles have been seen in the water and wetland systems of nearby
Parkview Hills. Managing the property using these as target species will require
managing the three basic habitat types of forest, field and water.
Blanding’s turtles
prefer shallow waters with medium densities of vegetation and muddy bottoms. They
are primarily carnivorous, eating such things as crayfish, tadpoles and carrion
while under water. Females often travel long distances in June to lay 3-21 eggs, which
hatch after two and one-half months. The young take 15-20 years to reach sexual
maturity and live to about 50 years.
Eastern box turtles
are primarily terrestrial, preferring open woods and meadows near water. They are omnivorous,
eating worms, snails, carrion, leaves and fruit. Females lay 3-8 eggs in
June which hatch three months later. The young mature slowly and can
reach an age of 100
years.
Black rat snakes
are arboreal, often basking and foraging in trees. They eat primarily
small mammals and birds. Unlike the other
snakes found, they are oviparous. The females lay 5-44 eggs
in June or July which hatch one and on-half months later.
Most Michigan turtle species
hunt or graze in the littoral (shoreline) zone, and Blanding’s turtle
is no exception. A natural shoreline with varying amounts of vegetation provides excellent
habitat for both the turtles and their prey. While some turtle species (painted
turtle and snapping turtle) tolerate organic pollution, most do not. In addition,
poor quality water cannot support an extensive prey base.
The health of
the population also depends on recruitment into the adult cohort and
genetic diversity. All
three of the species in question lay eggs and are thus vulnerable to nest predation.
Since turtles especially are long-lived, poor survival of young might
not be immediately
apparent. The biggest culprit is the raccoon, an adaptable animal that has made the
cities and suburbs its home. Unfortunately, raccoons raid turtle nests as
well as garbage cans.
Since their population is supported at artificially high levels,
this predation puts
an unsustainable pressure on any prey species whose numbers are constrained by “natural”
habitat, from turtles and snakes to frogs and birds.
Adults and young
of the three target species suffer road mortality while searching for
nest sites or dispersing
from their natal territory. Roads can act as genetic barriers, resulting in inbreeding
and lowering the fitness of the divided populations.
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