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Gopher Tortoise Minimum Viable Population and Minimum Reserve Size Workshop 13-14 March 2013

Gopher Tortoise Minimum Viable Population and Minimum Reserve Size Workshop 13-14 March 2013. Background Participants Process Findings. Photo by Dirk J. Stevenson. Minimum Viable Population.

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Gopher Tortoise Minimum Viable Population and Minimum Reserve Size Workshop 13-14 March 2013

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  1. Gopher Tortoise Minimum Viable Population and Minimum Reserve Size Workshop13-14 March 2013 • Background • Participants • Process • Findings Photo by Dirk J. Stevenson

  2. Minimum Viable Population • the smallest isolated population size that can persist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity. • usually estimated as the population size necessary to ensure high probability of persistence over a certain length of time.

  3. Population Viability Analysis (PVA) • Comprehensive analyses (models) of environmental and demographic factors that affect survival of a population • A tool to simulate the relative impact of various factors on persistence of a population • PVAs require a lot of demographic data

  4. Demographic Parameters Needed for PVA • Age structure • Sex ratio • Age at first reproduction • Longevity • Reproductive output • Age-specific survival (from egg to adults)

  5. Existing PVAs for the Gopher Tortoise

  6. Existing PVAs

  7. Workshop Objectives 1. Define a minimum viable gopher tortoise population based on the best scientific information available. 2. Identify the minimum reserve size needed to support a viable gopher tortoise population. 3. Determine the number and distribution of viable gopher tortoise populations necessary to ensure the long-term viability of the species.

  8. Objective 1- MVP • 250 adult tortoises; density ≥0.4 tortoises per hectare (~1 tortoise per acre) with: • A sex ratio of 1:1 • Evidence of recruitment; variability in size/age classes • No major constraints to gopher tortoise movement (i.e., major rivers or highways)

  9. Objective 1-Additional considerations • Populations between 50 and 250 individuals of any age (i.e., “support” populations) should be candidates for intensive management to attain MVP levels, where feasible. • Populations with <50 individuals can be used for education, community interest, and considered for translocation.

  10. Objective 2- Minimum Reserve Size • An MVP of gopher tortoises (>250 adults) can persist on a tract that is at least 100 ha (~250 ac) in size, provided the site receives intensive management. • High quality soils are necessary to support a viable tortoise population on a reserve of this size. • Some assessment of land management effectiveness must take place every 1-2 years with appropriate management action taken as needed.

  11. Objective 3- Number and Distribution of MVPs (in progress) • The number of MVPs should be high enough that stochastic events do not result in long term decline of the species. • The number and distribution of the populations represent the historic range of the species. • Additional data are needed to determine the genetic viability within and among populations. • Each state will summarize current population sizes and age class structure to meet this objective.

  12. Questions?

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