The Alligator Snapping Turtle

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The Alligator Snapping Turtle
[Macrochelys (Macroclemys) temminckii]:
A review of ecology, life history, and conservation,
with demographic analyses of the sustainability
of take from wild populations
a report to:
Division of Scientific Authority
United States Fish and Wildlife Service
prepared by:
Robert N. Reed, PhD
Justin Congdon, PhD
J. Whitfield Gibbons, PhD
Savannah River Ecology Laboratory
Drawer E
Aiken SC 29802 USA
...Table of Contents...
Executive summary ................................................................................................................................ 1
Introduction ........................................................................................................................................... 2
Taxonomy, distribution, and systematics................................................................................................. 2
Ecology and Natural History ................................................................................................................... 3
Size and growth rates ............................................................................................................... 3
Longevity and age at first reproduction ..................................................................................... 3
Reproductive biology ................................................................................................................ 4
Survivorship ............................................................................................................................. 4
Diet .......................................................................................................................................... 5
Habitat use and movement ....................................................................................................... 5
Historical exploitation, conservation, and commercialization ................................................................ 5
Current Legal Status ............................................................................................................................... 7
Demographic analyses ........................................................................................................................... 8
Results and discussion ........................................................................................................................... 9
Overall conclusion ................................................................................................................................. 13
Literature Cited ...................................................................................................................................... 14
Preparation of this report was made possible by an award from the U.S. Department of the Interior
(account 10-21-RR267-154, “Cooperative “esearch on status and trends of U.S. freshwater turtles in
the wild with an assessment of the impact of collection”) to The University of Georgia’s Savannah
River Ecology Laboratory. Report preparation was also aided by contract DE-FC09-96SR18546 between
the U.S. Department of Energy and The Unversity of Georgia Research Foundation.
Photos courtesy of David E. Scott, SREL. Layout and design by Laura L. Janecek, SREL.
...Executive Summary...
The alligator snapping turtle (Macrochelys temminckii) is the largest freshwater turtle in North America.
It is a highly aquatic and somewhat secretive turtle, such that some aspects of its biology remain poorly
known. The historical distribution of the species spans 14 states, and includes the Mississippi River and
most other rivers draining into the Gulf of Mexico. While only one species is recognized in the genus, there
is marked genetic structuring evident among populations from different drainages. A literature review
revealed that body size, diet, and reproductive biology of M. temminckii have been adequately documented,
but data on survivorship, longevity, recruitment, and habitat use remain scarce. Many alligator snapping
turtle populations were decimated by increased levels of commercial harvest in the 1960’s and 1970’s, and
the species is now protected from commercial harvest in all states except Louisiana.
We analyzed the demography of this turtle using standard techniques. Results indicate that Macrochelys
populations are characterized by delayed maturation and high adult survivorship, even among turtles.
These factors make this species extremely sensitive to harvest of adults. Annual harvest of less than 2% of
adult females is unsustainable and results in population declines. We found no evidence that harvest is
sustainable.
1
...Introduction...
severe declines due to habitat degradation and exploitation,
and these factors remain as threatening factors in many parts
of the range of M. temminckii.
The alligator snapping turtle is the largest freshwater turtle
in the United States. The species has a moderately wide
distribution in the greater Mississippi drainage and numerous
Gulf drainages. Despite this broad distribution, however, the
alligator snapper is poorly known ecologically. It is a secretive
and almost entirely aquatic turtle, such that individuals are
rarely seen by the public. Other than one book-length
summary (Pritchard 1989), published ecological studies of
M. temminckii are few. No long-term studies of the
survivorship and demography of any population have been
published, and most life history data have come from
examination and dissection of individuals harvested for the
meat trade. Most populations appear to have undergone
The goals of this report are two-fold. First, we summarize
available knowledge of the ecology, life history, and status of
the alligator snapper, with special reference to the literature
published since Pritchard’s (1989) book. Second, we use
available data to construct a stable life table for this turtle,
and estimate sensitivity of a population to changes in life
history variables, including the effects of simulated
anthropogenic take of adults.
...Taxonomy, Distribution, and Systematics...
Kansas, Kentucky, Louisiana, Mississippi, Missouri,
Oklahoma, Tennessee, and Texas. Within this range,
populations in Illinois, Indiana, Kansas, Kentucky, Oklahoma,
and Tennessee are thought to be generally small or extirpated.
These small population sizes may be due to past exploitation
or because these states are on the edges of the geographic
range of Macrochelys. A detailed distribution map and
specimen records are given in Pritchard (1989) and Lovich
(1993).
The alligator snapping turtle is a member of the family
Chelydridae, in the order Testudines, class Reptilia. This North
American family contains two species in two monotypic
genera; the alligator snapping turtle (M. temminckii), and
the common snapping turtle (Chelydra serpentina). The
Chinese big-headed turtle (Platysternon megacephala) was
formerly considered a member of this family, but is now
placed in the monotypic family Platysternidae.
The nomenclatural history of the alligator snapping turtle is
complex and apparently still evolving. The species was first
described as Testudo planitia in 1789, but Gray erected the
genus Macroclemys in 1855. Some subsequent authors
referred to the genus as Macrochelys, but this was refuted
by Smith (1955). More recently, Webb (1995) showed that
Macrochelys has precedence over Macroclemys, and the
Society for the Study of Amphibians and Reptiles has adopted
this revision (Crother et al. 2000). In this report, therefore,
we refer to the alligator snapping turtle as Macrochelys
temminckii.
The genus Macrochelys is considered to be monotypic.
However, many populations are distributed allopatrically,
raising the question of whether independent evolutionary
lineages exist. This topic was explored by Roman et al.
(1999), who sequenced mitochondrial DNA from 158
individuals from 12 drainages throughout the range of
Macrochelys. Their results indicate strong phylogeographic
structuring, indicating that major genetic divergence has
occurred among river drainages. Within this overall structure,
three major evolutionarily significant units were found. These
roughly correspond to the greater Mississippi drainage, the
Gulf drainages to the east of the Mississippi, and the Suwannee
River. In contrast to the common snapping turtle, Chelydra
Macrochelys temminckii is historically known from
Alabama, Arkansas, Florida, Georgia, Illinois, Indiana,
2
serpentina, which undertakes significant overland
movements and has little mtDNA variation over its
range, the aquatic habits of Macrochelys have resulted
in strong divergence among populations. Roman et
al. (1999) also concluded that population-specific
genetic markers should allow geographic
identification of individuals in the meat or pet trade,
allowing a means of monitoring interstate transport
of illegally collected animals.
Range of the alligator snapping turtle, Macrochelys
temminckii.
...Ecology and Natural History...
Because most available data are from harvested individuals,
estimates of growth rates in the wild are few. Among 12
recaptured subadults (3 male, 9 female) in a Louisiana
bayou, annual CL growth rates were 5.3% among males and
5.2% among females (Harrel et al. 1997). Weight gain in
these turtles averaged 4.1% among males and 10.6% among
females. Dobie (1971) gave growth curves calculated from
harvested turtles; these data indicated that growth rates were
variable both within and between individuals, especially
before attainment of sexual maturity.
Size and growth rates: Contrary to common perception
in the United States, the alligator snapping turtle is not the
world’s largest freshwater turtle. The Asiatic softshell turtles
of the genera Chitra and Pelochelys may exceed 200 kg,
much larger than Macrochelys (Pritchard 2001). However,
Macrochelys is the largest freshwater turtle in the New World
and reaches impressive sizes. In a Louisiana population, adult
males averaged 49.46 ± 7.04 cm in carapace length (CL;
n=11, range 36-57.1), whereas adult females averaged 43.08
± 4.32 cm CL (n=53, range 35-50.9; Sloan et al. 1996).
Nesting females along the Apalachicola River in Florida were
larger, averaging 45 cm CL (range 41.8-51; Ewert and Jackson
1994). Males attain larger sizes than do females, and the
largest known male was 80 cm CL (Lovich 1993). However,
Tucker and Sloan (1997) estimated an asymptotic size in
Louisiana of 39.9 cm CL for females and 50.3 cm CL for
males; the latter estimate is smaller than the maximal size of
Lovich (1993) or the 57.6 cm CL given by Dobie (1971).
Immature females harvested in Louisiana averaged 33.8 cm
CL, whereas males averaged 34.5 cm CL (Tucker and Sloan
1997). Hatchlings are 4.1-4.3 cm CL in Louisiana (Dobie
1971) and 3.4-3.8 cm CL in Florida (Powders 1978).
Longevity and age at first reproduction: Alligator
snapping turtles are long-lived organisms. Using ventral scute
annuli as indicators of annual growth, Sloan et al. (1996)
estimated average ages of 25.24 ± 11.37 years among adult
males (n=11, range 11–45) and 22.23 ± 5.93 years among
adult females (n=53, range 15–37) in Louisiana. Also in
Louisiana, Dobie (1971) stated that the oldest animal that
he could reliably age was a 36-year-old male. Tucker and
Sloan (1997) stated that immature individuals averaged 16
years (females) and 16.8 years (males), whereas mature
individuals averaged 21.4 years (females) and 29.3 years
(males). The oldest individuals of known age in the latter
3
study were 39 years (females) and 45 years (males). Alligator
snapping turtles have lived longer than 70 years in captivity
(Snider and Bowler 1992).
adult females (38.5 cm CL) may lay as few as 9 eggs in GA
(Powders 1978). In Louisiana, a series of 13 harvested
females had clutch sizes averaging 23.8 (range 16–38), and
egg sizes were 34.0–51.8 mm at their greatest diameter
(Dobie 1971). Reproductive output is positively correlated
with female body size, but this relationship is characterized
by high variability among females (Tucker and Sloan 1997).
Two Louisiana populations (Dobie 1971, Tucker and Sloan
1997) had lower clutch sizes and smaller female sizes than
were observed in a Florida population (Ewert and Jackson
1994). However, this observation is likely an artifact of sizespecific fecundity and differential exploitation. Long-term
exploitation of Louisiana populations may have resulted in
fewer large, fecund females relative to the Florida population
which has experienced lower levels of exploitation.
Estimates of the age at first reproduction are important in
demographic analyses. Dobie (1971) estimated that sexual
maturity is attained at 11–13 years for both males and females
(Dobie 1971). Based on laprascopic examination of the
reproductive tracts of free-ranging subadults, however, J.B.
Harrel (personal communication, 24 January 2002) found
no reproductive females under 15 years old, and no
reproductive males under 17 years old. Similarly, Tucker and
Sloan (1997) found that immature females averaged 16.0
years and immature males 16.8 years for a sample of
harvested Louisiana turtles. These more recent data may
indicate that Dobie (1971) underestimated minimum age at
maturity.
Survivorship: The main roadblock to rigorous
demographic analyses for M. temminckii is an almost
While ventral scute annuli may be reliable indicators of
annual growth in some cases, they may be occasionally
misleading. For instance, Powders (1978) aged a
reproductive female at 28–31 years old using scute annuli,
but stated that its size would place it in the 11–14 year old
age class using Dobie’s (1971) growth curves. Without longterm population data, it is not possible to determine whether
this female was an exceptionally slow-growing individual or
whether scute annuli are not reliable indicators of growth at
older ages.
Reproductive biology: In captivity, mating has been
observed from February to October (Allen and Neill 1950,
Grimpe 1987), but geographic variation in mating season is
poorly understood. Males apparently are capable of sperm
production year round (Dobie 1971). Females ovulate in
the spring, and most nesting occurs in May through July
(Ewert 1976; Powders 1978; Grimpe 1987). Females appear
to breed annually, but may skip a year if they have poor
foraging success (Tucker and Sloan 1997). Production of
multiple clutches in a single year has not been observed in
the wild.
Somewhat surprisingly considering their larger size, M.
temminckii has a lower average reproductive output than
C. serpentina and has not been observed to lay more than
one clutch per year (Dobie 1971). Clutch sizes are 31-40
(n=17) in the Apalachicola River (FL; Ewert 1976), but some
Alligator snapping turtles frequent slow-moving
rivers and streams such as the one pictured here.
4
overland movements appear to be undertaken only by nesting
females and juveniles moving from the nest to water. Females
have been observed to nest up to 72 m from the nearest
water, although nests averaged 12.2 m from the nearest water
(Ewert 1976). Radiotelemetry has been used to study the
aquatic movements of subadults (Harrel et al., 1996) and
adults (Sloan and Taylor 1987) in Louisiana. These results
indicate that adults are capable of moving >1 km/day, and
mean daily distances traveled ranged 27.8–115.5 m/day.
Home range sizes were variable and ranged 18-247 ha. Both
subadults and adults used cypress-lined river channels
heavily. In Arkansas, mark-recapture of Macrochelys revealed
that individuals moved both upstream and downstream, with
a maximum distance between capture locations of 1.8 km
(Trauth et al. 1998). Pritchard (1989) proposed that some
individuals may move up river continuously during their
lifetimes, thus accounting for large isolated individuals in
stream headwaters. This poorly supported hypothesis (based
on literature records of long-distance upstream movements
of two turtles) was disputed by Ernst et al. (1994).
complete absence of data on survivorship of various age
classes. Like most turtles (Congdon et al. 1994), it appears
that Macrochelys has high adult survivorship under natural
conditions (i.e., absent anthropogenic mortality). Average
age of adults is greater than that of many other turtles,
suggesting that adult survivorship of Macrochelys is especially
important demographically.
Diet: Alligator snapping turtles are very nearly omnivorous
and consume a wide variety of plant and animal matter. They
possess a unique lingual lure used to catch fish, and fish are
a dietary mainstay (Redmond 1979, Harrel and Stringer
1997). However, dietary items other than fish are common
in turtle stomachs; these include plant matter (oak acorns,
tupelo fruits, wild grapes, roots, palmetto fruits, hickory nuts,
persimmons, etc.) and animal matter (salamanders, crayfish,
mussels, snakes, alligators, turtles, clams, mammals, snails,
etc; Redmond 1979, George 1987, Spindel et al. 1987,
Shipman et al. 1991, Sloan et al. 1996).
Habitat use and movement: Alligator snapping turtles
are among the most aquatic of freshwater testudines, and
...Historical Exploitation, Conservation, and Commercialization...
Alligator snapping turtles have long been harvested by
humans as a food source in the southeastern United States.
However, the size of this industry increased dramatically in
the late 1960’s through the 1970’s. During this period
Campbell’s Soup Company produced frozen turtle soup, with
meat largely obtained from Chelydra and Macrochelys
(Pritchard 1989). Numerous New Orleans seafood
restaurants and dealers also purchased large numbers of
Macrochelys from trappers in southeastern states. The
appetite of the consumer was such that the industry seasonally
took three to four tons of alligator snapping turtles from
Georgia’s Flint River every day during the early 1970’s to
meet demand, until population sizes decreased below
commercial viability (Al Redmond, cited in Pritchard 1989).
Populations in Louisiana, Florida, and Alabama, were also
depleted by commercial harvest (Pritchard 1989).
Large-scale harvesting of reproductive adults caused swift
declines in most populations of Macrochelys. However, an
absence of data on historical population sizes of these turtles
meant that declines were usually documented by interviews
with turtle trappers and others who exploited the resource.
These individuals were unlikely to have kept detailed records,
and so their perceptions of massive population declines are
fairly subjective. The current inability of the Macrochelys
fishery to sustain historical numbers of turtle trappers is itself
evidence of widespread population decline. Pritchard (1989)
gives an excellent overview of the documentation of declines
in populations of Macrochelys, including multiple
corroborative reports from former turtle trappers.
The abovementioned population declines resulted in legal
protection of M. temminckii in most states where it occurs.
However, commercial harvest is still legal in Louisiana. New
5
The impact of turtle farming on wild populations of
Macrochelys has not been quantified. Breeding stock was
almost certainly obtained from natural populations. Using
Dobie’s (1971) average of 23.8 eggs per female, the export
tally of 23,870 hatchlings for the year 2000 would require
1002 adult females on the farms, plus a smaller number of
males. A persistent rumor states that Arkansas requires turtle
farming operations to release at least 150 captive-hatched
M. temminckii per year. There is no such requirement
according to the Arkansas Game and Fish Commission
herpetologist (K. Irwin, pers. comm., Feb. 2002). Those
people who have released turtles have largely released
commercially undesirable individuals, likely with low fitness
(e.g., turtles with ‘kinktail’ and other defects caused by overly
high incubation temperatures; J.B. Harrel, personal
communication 24 January 2002). Because turtle farmers
are under no obligation to keep records of the origins of
breeding stock, released hatchlings may also be of mixed
genetic provenance. Given the marked genetic structuring
of allopatric populations of Macrochelys, (Roman et al.
1999), this practice is indefensible from a conservation
standpoint.
Orleans is the center of this trade, especially for turtle soup
and Cajun restaurants. As additional examples, the mail-order
outdoor retailer Cabela’s Inc. offers turtle meat in its catalogs,
and mail-order frozen turtle meat is available on-line from a
number of New Orleans seafood houses. While much of this
meat is undoubtedly from Chelydra, Apalone, or emydid
turtles, it is unknown how many Macrochelys are exploited
for this trade.
Louisiana has recently instituted a 15 inch (38.1 cm)
carapace length minimum size limit for Macrochelys taken
by commercial operations, but still allows harvest of
unlimited numbers of individuals above this size. Although
designed as a conservation measure, Louisiana’s minimum
size limit is very likely ineffectual in stabilizing populations.
First, because of the low meat to weight ratio of small turtles,
these individuals would rarely be taken by commercial
collectors anyway. More importantly, the 15” minimum allows
continued harvest of 71% of adult females and 100% of adult
males in Louisiana (Tucker and Sloan 1997). Continued
harvest of adult females is currently not known to be
compatible with long-term population persistence of any
turtle species (Congdon et al. 1993, 1994; Close and Seigel
1997) and is likely to be especially harmful to a long-lived
species like M. temminckii.
Widespread commercialization of wild-caught alligator
snapping turtles has generally decreased, and the species is
largely protected in most states where it occurs. All else being
equal, these measures may allow eventual population
recovery in the case of populations that have not been
extirpated. Unfortunately, however, all else is not equal, as
ongoing habitat loss may ultimately be a more destructive
influence than past commercialization.
As the turtle meat trade has waned, the pet trade in alligator
snapping turtles has increased tremendously. Based on
USFWS export documentation, exports of Macrochelys have
increased from 290 individuals in 1989 to 23,780 individuals
in 2000. Virtually all of these are hatchlings from commercial
turtle ‘farming’ operations, and are bound for overseas pet
and food markets (Franke and Telecky 2001). These animals
wholesale for less than $20, and may retail for $35. Ironically,
most of these hatchlings are not legal for sale in the U.S. due
to existing laws prohibiting sale of turtles <4 inches (10.16
cm) carapace length. The geographic origin of the exported
turtles is difficult to analyze, as many individuals are exported
from states where they do not occur (primarily California).
At least 10,000 turtles are exported annually from the state
of Arkansas, which apparently has the most extensive network
of Macrochelys farms (6-8 licensed farmer/dealers operated
during 1998-2001; K. Irwin, personal communication 3/26/
02).
6
...Current Legal Status...
harvest is prohibited (commercial harvest is limited to the
genera Chelydra and Apalone; Tim Slone, personal
communication 3/26/02).
Macrochelys temminckii is currently unprotected by either
the U.S. Endangered Species Act of 1973 or the Convention
on International Trade in Endangered Species. Macrochelys
is subject to the Food and Drug Administration ban on sale
of turtles under four inches carapace length, but this applies
only to sales in the United States; exported animals currently
can be of any size (FDA 21 CFR 1240.62). Despite minimal
protection at the federal level, the species is protected by
many of the U.S. states encompassing its native geographic
range. These protections are discussed below; most of this
information was taken from Levell (1997) or SSAR (2002).
Louisiana: A fishing license is required to collect M.
temminckii for non-commercial purposes, and there is a
recreational take limit of 4 specimens (no size limit applies
to non-commercial use). Commercial take is limited to
individuals over 15 inches in carapace length, with no limit
on the number of specimens which can be taken.
Mississippi: Classifed as Nongame Species in Need of
Management. Non-commercial possession limit of four
individuals (valid hunting license required). No reptiles taken
from the wild may be purchased or sold, and reptiles may
only be imported under the authorization of a nongame
importer’s permit.
Alabama: Legally protected non-game animal; collection,
possession, and sale are prohibited. Permits are required
for any activity involving this species.
Arkansas: Wild individuals are protected from take,
including eggs. Import of Macrochelys prohibited. All captive
M. temminckii over 10” carapace length must be tagged by
an agent of the Game and Fish Commission (plans to reduce
the minimum size for tagging to five inches are in process;
K. Irwin, personal communication 3/26/02).
Missouri: Residents may possess five individuals for personal
use. Commercialization of native species is prohibited, and
commercialization of imported native species from other
states requires a Class I Wildlife Breeders Permit. Despite
these apparent rules, however, there is at least one
commercial alligator snapping turtle farm in Missouri.
Florida: Listed as a Species of Special Concern. Possession
limit of one specimen. May not be sold.
Oklahoma: Listed as a Species of Special Concern, with no
legal implication. There is a closed season on Macrochelys,
which may only be taken with a scientific collecting permit.
Georgia: Listed as Threatened, may not be taken without
scientific collection permit.
Tennessee: Listed as In Need of Management. Permits
required for all activities involving M. temminckii.
Commercialization of Macrochelys prohibited, but sale of
Chelydra permitted.
Illinois: Listed as Threatened, may not be possessed without
permit.
Indiana: Listed as Endangered, may not be taken without
permit. Imports are not restricted.
Texas: Listed as Threatened, permits required for all activities
involving M. temminckii.
Kansas: Listed as In Need of Conservation, legally protected
from take.
Kentucky: No official list of state threatened or endangered
species exists for Kentucky. The Kentucky State Nature
Preserves Commission lists M. temminckii as Threatened,
but this listing has no statutory or legal implications. Harvest
of M. temminckii for personal use is legal, but commercial
7
...Demographic Analyses...
Annual fecundity (mx): Defined as the number of eggs
produced annually, divided by two. This adjusts for
production of males by making an assumption of an equal
primary sex ratio. Annual fecundity was not adjusted for
clutch frequency; i.e., we operated under the assumption
that all adult females reproduce annually and lay no more
than one clutch annually.
Demographic processes (birth, death, and migration) are
rarely equal among age classes in a population. For most
species, certain life history stages or age classes are
characterized by increased risk of mortality, while maximal
reproductive output may be achieved by members of entirely
different age classes. Life tables are tabulations of patterns
of mortality and survivorship in a population and allow
construction of graphical representations of survivorship and
fecundity (Begon et al. 1986). Only after demographic
analysis is it possible to understand trends in population sizes,
or to predict future population sizes.
Adult survivorship (Sx Adult): Defined as the proportion of
adult females surviving every year; assumed to be constant
among all adult age classes. This assumption is validated by
long-term studies of common snapping turtles in Michigan,
among which adult survivorships are high and constant
(Congdon et al. 1994). It is also characteristic of adult turtles
in other long-term studies (Gibbons and Semlitsch 1982,
Mitchell 1988, Frazer and Gibbons 1990, Congdon et al.
1993).
Almost invariably, construction of life tables requires longterm study of a population in order to record schedules of
births and deaths. Turtles typically display a Type III
survivorship curve (Pearl 1928), with high juvenile mortality
followed by adult age classes with high annual survival
(Congdon and Gibbons 1990). Because the lifespans of adult
turtles may exceed those of turtle researchers, few studies of
turtle ecology are of suitable temporal duration for rigorous
demographic analyses.
Juvenile survivorship (S x Juvenile): Defined as the
proportion of juvenile females surviving every year after
emergence from the nest, and was also assumed to be
constant among all juvenile age classes.
Demography of alligator snapping turtles is poorly known.
Variables such as clutch size and body size have been
examined, but our knowledge of survivorship across age
classes is scanty. The most appropriate study for comparison
is that of a population of common snapping turtles in
Michigan (Congdon et al. 1994). This population is
characterized by high adult survivorship, and adult females
are the most important segment of the population in terms
of population persistence. However, this type of rigorous
analysis was only possible after 17 years of intensive field
research. Similar studies are lacking for any population of
Macrochelys. We therefore used very conservative values for
life history variables in our analyses. Wherever possible, we
erred on the side of an assumption of population stability
(discussed below). This type of conservatism means that our
results probably underestimate the importance of adult
survivorship for population stability.
Nest survivorship (Sx Nest): Defined as the proportion of
juvenile females that survived the period between oviposition
and emergence from nests.
Basic reproductive rate (Ro, also known as the
fundamental net per capita rate of increase): The mean
number of female offspring produced per original female by
the end of the cohort (i.e. death of the oldest female in the
cohort). It therefore indicates both average number of female
offspring produced by a female over the course of its life,
and the population multiplication factor which will indicate
the size of the population one generation hence. Thus,
population sizes will decrease when Ro < 1.0, and vice versa.
Intrinsic rate of natural increase (r): Calculated as
ln(Ro), and indicates the change in population size per
individual per unit time.
Our methods largely followed Congdon et al. (1993, 1994).
We performed standard demographic analyses, using the
following variables:
The derivation and calculation of the abovementioned
variables are discussed in detail by Begon et al. (1986).
8
...Results and Discussion...
After construction of the stable life table, we examined the
consequences of changes in survivorships among adults,
juveniles, and nests. These are the variables most likely to
be important for management and conservation of alligator
snapping turtles, as it is unlikely that fecundity or age at
maturity can be manipulated via management. Manipulation
of survivorship allows an estimate of the future ramifications
of anthropogenic harvest of these animals, expressed as
predicted changes in population size. Rather than attempting
to model these changes by focusing on esoteric variables
such as Ro or r, we chose population doubling time as a more
intuitive measure of population size. Population doubling
time (DTime) was calculated as the number of years required
to double population size as a consequence of changes in
survivorship. Negative values for this variable indicate the
time required to halve the population.
We constructed a stable life table using the following
population parameters, which were obtained from various
sources cited above (see Ecology and Natural History
section):
Stable adult Sx = 0.98
Stable juvenile Sx = 0.687
Stable nest Sx = 0.201
Fecundity (mx) = 16 female eggs/year
The resultant stable life table for Macrochelys temminckii
resulted in a cohort generation time (Tc) of 49.07 years,
and a doubling time of the population of 8507 years. The
basic reproductive rate (Ro) was 1.004, and the intrinsic
rate of population increase (r) was 8.147 x 10-5 (Table 1).
Table 1: A summary of values used in construction of a stable
life table for Macrochelys temminckii, including stable
population parameters resulting from the life table.
Reproduction
Clutch size
32
Clutch frequency
1.00
Annual fecundity
16
Survivorships (lX)
Nest (Age 0)
0.2010
Juvenile (ages 1-12)
0.6870
Adult females
0.9800
Emigration rate
0.00 per year
Age at maturity
13-16 years
Stable population parameters
Basic reproductive rate (R0)
Intrinsic rate of population increase (r)
1.004004
8.1475 x 10-5
Cohort generation time (TC)
49.06625
Population doubling time (DTime)
8507.45
9
Results of these analyses were striking. In
order to maintain a stable population using
biologically realistic values for fecundity, age
at maturity, and survival of nests and juveniles,
annual adult survivorship of females must be
98%. Reducing adult survivorship by as little
as one quarter of one percent (to 97.75%)
will result in population size being halved in
410 years (Table 2). Reducing adult
survivorship by two percent (to 96%), which
would be equivalent to annually removing
only two adult females from a total population
size of 200 turtles (assuming even sex ratios)
will halve the population in only 50 years
(Table 2). Reducing survivorship of juvenile
females also affects long-term population
stability, but at a rate indicating that roughly
two juveniles must be removed from the
population to equal the removal of a breeding
female (Table 3, Figure 1). Finally, we
modeled the population consequences of increasing nest survivorship via anthropogenic
protection of nests and hatchlings (e.g.,
headstarting). Increasing nest survivorship
by a 5% (to 25%) still requires 145 years to
achieve a doubling of the population, and
Table 2. Effects of increased annual mortality of adult female alligator snapping
turtles on population stability. See Table 1 and text for definition of variables.
Survivorship (SX)
R0
r
TC
DTime (years)
0.980 (STABLE)
1.004004
8.1475 x 10-5
49.066
8507.45
0.9775
0.922356
-1.6875 x 10-3
47.328
-410.75
0.9750
0.8508517
-3.4522 x 10-3
45.671
-200.78
0.9725
0.7879845
-5.2125 x 10-3
44.099
-132.98
0.9700
0.7324935
-6.9680 x 10-3
42.612
-99.48
0.9675
0.6833208
-8.7186 x 10-3
41.211
-79.50
0.9650
0.6395776
-1.0464 x 10-2
39.892
-66.24
0.9625
0.6005147
-1.2204 x 10-2
38.657
-56.79
0.9600
0.5654996
-1.3938 x 10-2
37.499
-49.73
40% nest survivorship is required to double the population
in less than 50 years (Table 4).
logarithmic scales. Clearly, the relative decline of alligator
snapping turtle populations continues as smaller population
sizes result from sustained harvest. Incremental decreases
in adult or juvenile survivorship would result in even shorter
times required to halve population sizes.
The relationships between reductions in survivorship and
population halving time depicted in Figure 1 appear to be
asymptotic at about 50 years, such that the observer may be
lulled into believing populations are safe for decades even
with increased mortality from harvesting. However, this is
not the case. Figure 2 shows the same data, plotted on
Comparisons of population doubling times with population
halving times offer insight on trade-offs between increases
in nest survivorship and either decreased adult or juvenile
Table 3. Effects of increased annual mortality of juvenile alligator snapping
turtles on population stability. See Table 1 and text for definition of variables.
Survivorship (SX)
R0
r
TC
DTime (years)
0.6870 (STABLE)
1.004004
8.1475 x 10-5
49.066
8507.45
0.6845
0.9575465
-8.7861 x 10-4
49.066
-788.91
0.6820
0.9130802
-1.8292 x 10-3
49.065
-378.92
0.6795
0.8705269
-2.7705 x 10-3
49.065
-250.18
0.6770
0.8298107
-3.7027 x 10-3
49.064
-187.20
0.6745
0.7908589
-4.6260 x 10-3
49.063
-149.84
0.6720
0.7356009
-5.5405 x 10-3
49.062
-125.10
0.6695
0.6513252
-8.2342 x 10-2
49.061
-84.18
0.6670
0.6838982
-7.3444 x 10-2
49.061
-94.38
10
Population halving time (years)
900
800
700
600
500
400
300
200
100
0
0
0.005
0.01
0.015
0.02
0.025
Reduction in Sx (annual survivorship)
Figure 1. Relationship between reductions in annual survivorship (Sx) and time
required to halve population size among alligator snapping turtles (Macrochelys
temminckii). Solid line and diamonds indicate results for adult females, dashed
line and squares indicate results for juveniles. Stable population survivorships
were 0.98 for adult females and 0.687 for juvenile females. Figure thus depicts
the population-level effects of reducing survivorship from 0.0025 to 0.02, such
that the lowest survivorship depicted is 0.96 for adults and 0.667 for juveniles.
survivorship (Tables 2, 3, 4). For example, the population
halving time of 133 years resulting from a .075% decrease
in adult survivorship is roughly temporally equal to the
population doubling time of 145 years resulting from a 5%
increase in nest survivorship, indicating that a large jump in
the latter is necessary to counteract the former. A more careful
examination of the numbers reveals that, in order to maintain
population sizes, a reduction of adult survivorship by 2.0%
requires an increase in nest survivorship of 16%. Among
juveniles, reduction of survivorship by 2.0% requires an
increase in nest survivorship of 8.5%. These results
unambiguously point towards the overwhelming importance
of adult survivorship for population stability.
Attempts at managing alligator snapping turtle populations
for the purposes of maintaining a sustainable commercial
harvest would likely hinge on two schemes. The first is
controlling the rate of adult harvest, while the second is
increasing survivorship of nests (via headstarting or similar
programs). Our analyses indicate that controlling the former
Table 4. Effects of increased nest survivorship of age class 0 alligator snapping
turtles on population stability. See Table 1 and text for definition of variables.
Survivorship (SX)
R0
r
TC
DTime (years)
0.201 (STABLE)
1.004004
8.1475 x 10 -5
49.066
8507.45
0.251
1.253757
4.7656 x 10 -3
49.066
145.44
0.301
1.503509
8.8356 x 10 -3
49.066
78.45
0.351
1.753261
1.2461 x 10 -2
49.066
55.63
0.401
2.003013
1.5745 x 10 -2
49.066
44.02
11
Log10 [Population halving time (years)]
3.0
2.75
2.5
2.25
2.0
1.75
1.5
-2.75
-2.25
-1.75
Log 10 [Reduction in Sx (annual survivorship)]
Figure 2. Relationship between reductions in annual survivorship (Sx) and
time required to halve population size among alligator snapping turtles
(Macrochelys temminckii). However, axes are logarithmically transformed in
this graph (log10).
is unlikely to prevent population declines, as increasing
mortality of adult females by as little as 1% results in decline.
Currently the state of Louisiana is alone in permitting
commercial harvest of Macrochelys, subject to a 15”
minimum carapace length size limit. Unfortunately, this size
limit allows legal harvest of virtually all adults, while imposing
no statewide quota on the numbers of legal-sized turtles taken
from wild populations. To rectify these problems, the state
of Louisiana could perhaps impose an annual quota on
harvest of adults, or harvest could be limited to adult males
only. The former solution, however, does not guarantee a
geographically balanced harvest of individuals from
metapopulations or demes. In other words, intensive local
population exploitation could satisfy statewide quota
requirements but result in severe local population crashes
requiring decades of recovery time. The option of limiting
harvest to adult males may be difficult for commercial
trappers to implement because it would require a working
knowledge of anatomy and mathematics, as adult males are
chiefly distinguished by the ratio of carapace length to
precloacal tail length (Ernst et al. 1994). Pronounced malebiased sexual size dimorphism within the species means that
alligator snappers over a certain body size are almost certainly
males, but these animals are also the oldest in a population
and will likely be rapidly depleted by sustained harvest without
replacement occurring at a rate that could sustain
commercial harvest operations.
It is conceivable that continued harvest of adult alligator
snapping turtles could be offset by artificially raising nest
survivorship. Similar efforts have been undertaken on a
massive scale for sea turtles via collection and incubation of
eggs from the wild, followed by release of hatchling or
juvenile turtles (headstarting; Donnelly 1994). The return
on this investment of time and money is hotly debated
(Wibbels et al. 1989, Frazer 1992, Heppell et al 1996), and
headstarting may not offset continued take of adults. A recent
review of headstarting concluded that, “…under the most
favorable of conditions, headstarting must be judged an
interesting experiment, not a conservation technique; it
should never be used as an excuse to weaken protection for
adult wild turtles or for the nests and juveniles of wild
populations. When conditions are less than favorable,
headstarting is not even an interesting experiment and may
do serious harm…” (Meylan and Ehrenfeld 2000). Thus
headstarting may be useful for re-establishing locally
12
extirpated populations or increasing the size of depleted
populations, but is unlikely to be an effective countermeasure
for harvest of adults. Commercial turtle farmers have shown
that production of thousands of Macrochelys hatchlings is
possible so long as an economic stimulus exists. If
headstarting for conservation purposes is attempted, however,
the participating agencies must also consider the drainagespecific genetic identities of existing populations in order to
avoid loss of overall genetic diversity.
alligator snapping turtles. Our wish was to forestall criticism
that our analyses were driven by a conservation-minded
agenda rather than by objective science. As an example, our
estimate of 98% adult survivorship is extremely high,
exceeding known survivorship rates of virtually all turtle
populations studied thus far (Congdon et al 1993, 1994,
Frazer and Gibbons 1990, Gibbons and Semlitsch 1982). If
actual survivorship of adults is lower, then these adults
become even more important to the population, and harvest
of adults will reduce population size at a greater rate than
our calculations indicate. Similarly, our estimate of 20% nest
survivorship may be higher than that experienced by wild
populations. Based on observation of Macrochelys nests in
Louisiana, J.B. Harrel (pers. comm., Feb. 2002) has
estimated nest survivorship of 5% or less. Increased nest
mortality may be due to recent collapses in the fur market
and elimination of large-bodied predators across the
southeastern U.S., which in turn has resulted in booming
populations of meso-predators such as raccoons and foxes
(Congdon et al. 1993). Finally, we utilized a conservative
estimate of fecundity by assuming that 100% of females
produce clutches annually. The sister taxon of Macrochelys
is the common snapping turtle (Chelydra serpentina), which
should be the most appropriate organism for comparisons
of reproductive frequency. Among Chelydra in Michigan, only
85% of females reproduce annually, thus reducing average
fecundity by 15%. A reduction of this magnitude would result
in fecundity of 13.6 eggs per year for M. temminckii rather
than 16 eggs per year. Thus, taken in aggregate, our results
depict a demography of alligator snapping turtles that is more
stable than can be reasonably expected in wild populations.
While sustained harvest of adults is clearly precluded by the
demographic characters of alligator snapping turtles, in situ
measures may help long-term population recovery efforts
without invoking expensive headstarting techniques.
Reducing levels of nest predation by subsidized predators
may allow greater numbers of eggs to survive, boosting the
numbers of individuals in early age classes. Subsidized
predators are those which occur at unnaturally high levels
because of subsidies provided by humans (Mitchell and
Klemens 2000). Among subsidized turtle nest predators, the
raccoon (Procyon lotor) has been identified as the single
most important predator of turtles in North America (Stancyk
1982, Mitchell and Klemens 2000), and the reduction of
raccoon populations results in increased nest survival among
turtles (Christiansen and Gallaway 1984, Smith 1997).
Despite the various options for increasing the numbers of
juveniles into declining populations, however, the continued
removal of adults will ultimately result in extirpation.
As discussed earlier, our analyses were marked by extreme
conservatism. This conservatism was mandated by the
uncertainty associated with some demographic aspects of
...Overall Conclusion...
Our demographic analyses provide no support for the
sustainability of harvest of adult alligator snapping turtles.
Increasing annual mortality of adult females by less than 1%
will result in long-term population declines, although these
declines may occur on timescales longer than human
lifetimes. Additionally, the life history of Macrochelys
temminckii is characterized by a suite of demographic traits
which imply that exploited populations will take decades to
recover even after cessation of exploitation.
13
...Acknowledgments...
This report was improved by critical comments from the following individuals:
John Jensen, Wildlife Biologist (State Herpetologist).
Georgia Department of Natural Resources, NongameEndangered Wildlife Program, 116 Rum Creek Drive,
Forsyth, GA 31029, P: (478) 994-1438, E:
[email protected]
Kelly Irwin, Arkansas State Herpetologist. Arkansas Game
and Fish Commission. 915 E. Sevier St., Benton AR
72015-3811. P: (877) 847-2690 ext. 16, E:
[email protected]
Paul E. Moler, Biologist. Florida Fish and Wildlife
Conservation Commission. Wildlife Research Laboratory,
4005 S. Main St., Gainesville FL 32601-9099. P: (352)
955-2230, E: [email protected]
Kurt Buhlmann, Amphibian and Chelonian Conservation
Coordinator. Conservation International, 1919 M Street
NW, Suite 600, Washington DC 20036. P: (803) 7255293, E: [email protected]
J. Brent Harrel, Fish & Wildlife Biologist and Private
Lands Coordinator/Kentucky. U.S. Fish & Wildlife
Service/Kentucky Field Office, 3761 Georgetown Rd.,
Frankfort, KY 40601. P: (502) 695-0468, E:
[email protected]
Jeffrey Lovich, Research Manager. Western Ecological
Research Center, Biological Resources Division, US
Geological Survey. 7801 Folsom Blvd, Suite 101
Sacramento, CA 95826. P: (916) 379-3742, E:
[email protected]
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