Gopher Tortoise Minimum Viable Population and Minimum Reserve

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Gopher Tortoise Minimum Viable Population and Minimum Reserve Size
Working Group Report
Prepared by:
The Gopher Tortoise Council
24 July 2013
A workshop was held on 13-14 March 2013, to define the minimum viable population (MVP) for
the gopher tortoise (Gopherus polyphemus) using the best available scientific information. In this
case, the purpose of establishing MVP parameters is to provide acceptable benchmarks for
conservation and recovery efforts and is not to determine absolute minimum thresholds that if
not met will result in certain population demise.
Invited attendees included gopher tortoise biologists, land managers, and experts in
demographic modeling (Table 1).
Background:
The U.S. Fish and Wildlife Service’s 12-month finding for listing the gopher tortoise as
Threatened under the Endangered Species Act (ESA) has stimulated additional interest in
conservation of the species. A Candidate Conservation Agreement and a comprehensive
conservation strategy have been developed for the gopher tortoise
(http://www.fws.gov/southeast/candidateconservation/examples.html). It will be necessary to
define a minimum viable population size, the number of viable populations needed, and the ideal
geographic distribution of these populations in order to successfully conserve this species.
The scientific literature provides mixed guidance about what constitutes a minimum viable
gopher tortoise population. Results from existing population viability models vary from 25 to
250 individuals under ideal habitat conditions (Table 2). In addition, there is no consistency in
the scientific literature regarding how large a parcel of habitat must be to support a viable
population. Therefore, there was a clear need to convene experts in gopher tortoise biology,
ecology, management, and demographic modeling to reach consensus regarding the following
objectives:
(1) Define a minimum viable gopher tortoise population size based on the best scientific
information available.
(2) Identify the minimum reserve size needed to support a viable gopher tortoise population.
(3) Identify the number and distribution of viable gopher tortoise populations necessary to ensure
the long-term viability of the species.
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Workshop Format:
The workshop was held at the Charlie Elliott Wildlife Center in Mansfield, Georgia. John Jensen
with the Georgia Department of Natural Resources gave the welcome address and the workshop
facilitators (Brett Boston and Vern Herr of Group Solutions, Inc.) delivered the statement of
purpose for the meeting. This was followed by a series of brief presentations. Rachael Sulkers
presented a review of the definition of an MVP, population viability analysis (PVA), and
demographic parameters used to develop a PVA. Joan Berish presented a comprehensive
overview of previous gopher tortoise PVA’s (Table 2).
Following the formal presentations, there was considerable discussion regarding what constitutes
an appropriate MVP for gopher tortoises and how many known populations (by state) met this
threshold. Because there is a lack of information about an ideal population structure for the
species due to difficulty in locating immature tortoises, the group agreed that in application the
focus should be on counts of adults rather than individuals of all age classes. Based on these
discussions and findings by Miller et al. (2001) and Tuberville et al. (2009), the group agreed
that using a count of 250 adults as an approximation of a MVP was appropriate for the gopher
tortoise.
Populations as small as 50 individuals that are intensively managed were deemed important, in
that, they play a support role in conservation of the species. In addition, small non-viable
populations may be important on a local scale (i.e., for educational purposes).
Finally, the influence of habitat size, quality, and management on the viability of gopher tortoise
populations was discussed. The group concluded that the minimum reserve size to support a
viable tortoise population was 100 hectares (247 acres), if that site is of superior quality and will
be maintained at that quality. Future discussions will focus on determining the number and
distribution of viable gopher tortoise populations necessary to ensure the long-term survival of
the species.
Summarized Workshop Findings:
Objective (1)
a. The consensus MVP for the gopher tortoise is 250 adults with a density of no less than
0.4 tortoises per hectare (approximately 0.16 tortoise per acre) with:
x
A male-female ratio of 1:1
x
Evidence of recruitment into the population
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x
Variability in size and age class; the smallest size and age classes should be
monitored over the long-term (i.e., every 5-10 years) because recruitment and
detectability within any given year can vary considerably.
x
The landscape should not have major constraints to gopher tortoise movement (i.e.,
major rivers or highways).
b. Populations less than 250 adults and as small as 50 individuals, of any age class (i.e.,
“support” populations), may persist for long periods of time and, although vulnerable to
stochastic events, they play an important role in supporting the overall recovery of the
species. Support populations should be candidates for intensive management to attain
MVP levels, where feasible.
c. Small non-viable populations (<50 individuals) can be used for education, community
interest, translocation, or augmentation purposes.
Objective (2)
a) Minimum reserve size:
x
An MVP of gopher tortoises (>250 adults) can persist on a reserve that is at least 100
ha in size, provided the site receives intensive management.
x
High quality soils are necessary to support a viable tortoise population on a reserve of
this size.
x
Some assessment of land management effectiveness must take place every 1-2 years
with appropriate management action taken as needed.
Objective (3)
a) Number and distribution of viable populations:
x
The number of MVPs should be high enough that stochastic events do not
result in long term decline of the species and the number and distribution of
the populations represent the historic range of the species.
x
Each state must first summarize current population sizes and age class
structure within their state to meet this objective.
x
Additional data are needed to determine the genetic viability within and
among gopher tortoise populations.
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Table 1. Participants at the Gopher Tortoise Minimum Viable Population and Minimum Reserve
Size Workshop at the Charlie Elliott Wildlife Center in Mansfield, Georgia, 13-14 March 2013.
PARTICIPANTS
Joan Berish
Becky Bolt
Doug Bruggeman
Deborah Burr
Kyla Cheynet
Matt Elliott
Jeff Gainey
Steve Godley
Jim Godwin
Jeff Goessling
Craig Guyer
Mike Harris
Sharon Hermann
Matt Hinderliter
Jen Howze
John Jensen
Keri Landry
Jess McGuire
Janet Mizzi
Clint Moore
Erin Rivenbark
Lora Smith
Jodie Smithem
Rachael Sulkers
Mark Thornton
AFFILIATION
Florida Fish and Wildlife Conservation Commission
InoMedic Health Applications, Kennedy Space Center
Ecological Services and Markets, Inc.
Florida Fish and Wildlife Conservation Commission
Plum Creek
Georgia Department of Natural Resources
United States Forest Service
Cardno Entrix
Auburn University
Auburn University
Auburn University
Georgia Department of Natural Resources
Auburn University
United States Fish and Wildlife Service
Joseph W. Jones Ecological Research Center
Georgia Department of Natural Resources
Louisiana Department of Wildlife and Fisheries
University of Georgia
United States Fish and Wildlife Service
University of Georgia
United States Fish and Wildlife Service
Joseph W. Jones Ecological Research Center
United States Fish and Wildlife Service
Environmental Services, Inc
Ft. Benning – Conservation Branch
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Ran baseline models for gopher tortoise populations in Florida on
(1) all potential habitat and (2) managed habitat only.
Model was range-wide, but varied demographic parameters for the
south, central, and periphery of the range. For each scenario, 100
simulations were run for both 100 and 200 years.
Root, K.V., and J. Barnes. 2006. Risk assessment
of a focal set of rare and imperiled wildlife in
Florida—Final report FWC Contract No. 03111.
Florida Fish and Wildlife Conservation
Commission, Tallahassee, FL. 17pp.
Tuberville, T.D., J. W. Gibbons, H. E. Balbach.
2009. Estimating the viability of gopher tortoise
populations. U.S. Army Corps of Engineers
ERDC/CERL-TR-09.
5
Used initial population numbers ranging from 50-20,000 for three
regions in Florida (Panhandle, North Central, and South). Northern
populations (N= 50 and 250) showed as much as 60% reduction
after 100 years if URTD was included in the model.
Estimated viability of gopher tortoise populations in Florida
impacted by development. Simulations were run under harsh (littleto-no management activity), moderate, and favourable conditions
(greater attention to management) for 200 years. They defined a
viable population as the population size needed to provide a >90%
probability of surviving for at least 200 years. Defined demographic
characteristics of the initial population (10% juveniles, 30%
subadults, 60% adults).
The model population consisted of a spatially defined group of
gopher tortoise colonies. Ran simulations under 3 scenarios: burn
probability= 0.25, 0.50, and 0.75, with a starting population size of
600 individuals.
Description
Miller, P.S. 2001. Preliminary population viability
assessment for the gopher tortoise (Gopherus
polyphemus) in Florida. IUCN/SSC Conservation
Breeding Specialist Group and Participants in
PVA Workshop, Tallahassee, FL. 45 pp.
Cox, J., D. Inkley, and R. Kautz. 1987. Ecology
and habitat protection needs of gopher tortoise
(Gopherus polyphemus) populations found on
lands slated for large-scale development in
Florida. Nongame Wildlife Program Technical
Report No. 4. Florida Game and Fresh Water Fish
Commission, Tallahassee, FL. 69 pp.
Abercrombie, C.L. 1981. A simulation model for
the management of the gopher tortoise (Gopherus
polyphemus) and gopher tortoise habitat.
Unpublished report.
Reference
Table 2. Summary of the findings of gopher tortoise population viability models.
Models were most sensitive to
uncertainty in juvenile mortality (age
class 0-1) and adult female mortality.
Initial baseline analysis showed lower
growth rates in northern populations, but
no risk of extinction at 100 years. Same
was true when they restricted carrying
capacity.
For baseline models, the probability of
extinction was zero over the next 100
years. Adult survival (>15 years old) was
the most influential parameter on
population growth. Only the largest
populations (those with >27 adult
females) were likely to be occupied after
100 years; smaller populations were not
likely to persist 100 years without
occasional dispersal.
Under all scenarios, gopher tortoise
populations exhibited gradual declines.
Only initial populations of at least 250
individuals were able to persist for 200
(1) Under harsh conditions even
relatively large populations had little
chance of surviving for extended time
periods, (2) Under moderate conditions
populations >130-150 individuals were
needed to survive to 200 years, (3) under
favourable conditions populations of
approximately 40-50 individuals met
criteria for a viable population.
Under the burn probability of 75%, >90%
of tortoises were alive after 100 years.
Findings
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6
years. Demographic sensitivity analysis
revealed that hatchling survivorship was
the most critical life history stage.
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References
Abercrombie, C.L. 1981. A simulation model for the management of the gopher tortoise (Gopherus
polyphemus) and gopher tortoise habitat. Unpublished report.Department of Interior, Fish and Wildlife
Service. 2011. Endangered and Threatened Wildlife and Plants; 12-Month Finding on a Petition To List
the Gopher Tortoise as Threatened in the Eastern Portion of Its Range. Federal Register 76(144):
45130-45162.
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19
20
Cox, J., D. Inkley, and R. Kautz. 1987. Ecology and habitat protection needs of gopher tortoise
(Gopherus polyphemus) populations found on lands slated for large-scale development in Florida.
Nongame Wildlife Program Technical Report No. 4. Florida Game and Fresh Water Fish Commission,
Tallahassee, FL. 69 pp.
21
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Miller, P.S. 2001. Preliminary population viability assessment for the gopher tortoise (Gopherus
polyphemus) in Florida. IUCN/SSC Conservation Breeding Specialist Group and Participants in PVA
Workshop, Tallahassee, FL. 45 pp.
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Root, K.V., and J. Barnes. 2006. Risk assessment of a focal set of rare and imperiled wildlife in
Florida—Final report FWC Contract No. 03111. Florida Fish and Wildlife Conservation Commission,
Tallahassee, FL. 17pp.
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Tuberville, T.D., J. W. Gibbons, H. E. Balbach. 2009. Estimating the viability of gopher tortoise
populations. U.S. Army Corps of Engineers ERDC/CERL-TR-09.
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