ECOLOGY AND MANAGEMENT OF SMALL POPULATIONS OF SMALL POPULATIONS L. Scott Mills 1, J. Michael Scott 2, Katherine M. Strickler 2, and Stanley A. Temple 3 1 Wildlife Biology Program, University of Montana 2 Department of Fisheries and Wildlife, University of Idaho 2 Department of Fisheries and Wildlife, University of Idaho 3 Department of Forest and Wildlife Ecology, University of Wisconsin

What is a Small Population? ► ► Small population size is a relative concept   Based on comparisons with historical population sizes, conspecifics, management standards ► ► Real issue is not absolute size of the population, but its vulnerability to dropping below some threshold:   Extinction risk for imperiled species   Minimum harvestable levels for exploited species

Categorization of Small Populations International Union for Conservation of Nature (IUCN) Red List criteria for defining small populations Red list categoryVery small population Small population and declining Critically endangered <50 mature individuals <250 mature individuals, population declining Endangered <250 mature individuals <2,500 mature individuals, population declining Vulnerable <1,000 mature individuals <10,000 mature individuals, population declining

Categorization of Small Populations Natural Heritage Program conservation status rank definitions Rank RankDefinition 1 Critically imperiled. Typically 5 or fewer occurrences or 1,000 or fewer individuals 2 Imperiled. Typically 6 to 20 occurrences or 1,000 to 3,000 individuals 3 Vulnerable. Rare; typically 21 to 100 occurrences or 3,000 to 10,000 individuals 4 Apparently secure. Uncommon, but not rare; some cause for long-term concern; usually more than 100 occurrences and 10,000 individuals 5 Secure. Common; widespread and abundant

Naturally Small vs. Declining Populations ► ► Naturally small populations   Geographically constrained (islands, naturally isolated patches)   Life-history characteristics associated with small populations (trophic level, body size, home range size)   Naturally stable at small sizes in absence of threats ► ► Declining populations   Restricted to small, isolated populations following a reduction in population size   Increased risk of extinction   Management may be needed to reduce threats

Legal Mandates for Managing Small Populations ► Endangered Species Act (United States)  Provides for protection and recovery of species  USFWS and NOAA Fisheries responsible for listing or delisting recommendations ► Species at Risk Act (Canada)  Listing recommendations by Committee on the Status of Endangered Wildlife in Canada (COSEWIC) Wildlife in Canada (COSEWIC) ► CITES (Convention on International Trade in Endangered Species)  Restricts international transportation and trade of listed species ► 53 federal statutes in the United States  Provide for conservation of species and habitats

Ecological Characters Predicting Risk: ► Smaller populations ► Restricted range / narrow endemics ► High variance in population growth rate (high environmental stochasticity, var(r) > 2r) ► Large body size

The Extinction Vortex: In Chapter From Mills (2007) after Soule and Mills (1998) Stochastic and Deterministic Drivers

Demographic Stochasticity (caribou example: true survival is 0.74) (from Mills 2007)

Genetic Stochasticity (As populations become small & isolated genetic drift causes random loss of alleles and increase in homozygosity) From: Mills and Tallmon 2009

We are often concerned with populations that are small or declining (or both) -We are often concerned with populations that are small or declining (or both) -We’ve gone over various ways to measure abundance, population dynamics, survival, reproduction, etc. -But what do we do with this information? -How do we quantify: a) a population’s viability b) What to do! (the impact of different management options) Wildlife Population Risk Assessment

Population Viability Analysis

1. Viability: Not extinct, not below quasi-extinction threshold 2.Time: Predictions are less reliable farther into the future (like the weather!) So we aim to make short-term predictions couched within long-term goals. 3.Likelihood: Embrace uncertainty! Persistence is measured as a probability, often displayed as a cumulative distribution function. Components of a PVA

PVA ► Allows comparison of a range of options ► Strong biological basis (all that stuff on pop. growth, survival, fecundity, environmental stochasticity, genetics, etc.) ► Social components (how secure do we want the population to be? What are management goals?)

Types of PVAs 1.Count-based (unstructured population): uses time- series of abundance or density series of abundance or density 2.Demographically explicit PVA (structured population): uses matrix models, individual-based simulations, uses matrix models, individual-based simulations, sensitivity analysis, etc. sensitivity analysis, etc. -Metapopulation (structured, unstructured, or presence/absence) -Spatially-explicit (powerful but incredibly data hungry)

From Mills 2007 Time-series PVA based on gray whales off the coast of California. (a) Abundance estimates over time, with SE bars representing sample variance (data from Rugh et al. 2005; see also Wade 2002a). (b) The cumulative distribution function (and its 90% confidence interval) of the density-independent quasi-extinction probability of decline from the 2001 abundance of 18,178 to a quasi-extinction threshold of 10,000 or fewer whales. © Quasi-extinction probability, as a (b) except that a logistic growth model of negative density dependence is assumed. Time-series PVA

Demographically Explicit PVA Incorporates Age structure Age-specific vital rates Can also include: Sex-specific vital rates Variance and covariance in vital rates Density dependence Inbreeding depression Environmental stochasticity Demographic stochasticity Animal behavior Multiple populations Multiple populations

Can investigate specific mechanisms potentially affecting viability. Can compare the effects of alternative management options. Can compare the effects of alternative management options. But data hungry! Demographically Explicit PVA

Other Approaches to Assess Viability ► Expert opinion ► Rules-of-thumb  E.g., IUCN Red List  25,000 species worldwide assessed for conservation status! The IUCN population assessment procedures (Gärdenfors et al. 2001, IUCN 2001).

Using PVA, we can be explicit about threats to populations, can get non-intuitive results, and can identify gaps in our knowledge 1. Remain aware of data quantity and quality (“garbage in, garbage out”) 2. View viability metrics as relative, not absolute estimates of when a population will go extinct 3. Consider a range of possibilities whenever there is doubt about a process, functional relationship, or measured parameter (embrace uncertainty) Closing Thoughts About PVA

4. Don’t try to project too far into the future— relatively short predictions are more likely to fall into the realm of reality and work within political and economic constraints 5. Keep it simple, but keep in mind the things you left out 6. Consider PVAs a work in progress, not the final work

Management of Small Populations ► ► Must address proximate and ultimate causes of population declines   Improvement of birth and death rates   Population augmentation   Habitat protection   Reduction or removal of other threat factors (e.g., harvest, invasive species, contaminants) ► ► Monitoring

Conservation-Reliant Species ► For many endangered species, full recovery is not attainable ► Recovery should be viewed as a continuum

Conservation-Reliant Species ► ► “Conservation-reliant species” can maintain self-sustaining populations with ongoing management ► ► Ongoing management ensures that necessary conservation actions will continue following delisting ► ► Cooperative relationships between state governments, federal agencies, and private landowners will be required

Special Considerations for Managing Listed Species ► Permits are required for any hands-on management or research ► Take permits, listing or delisting decisions, and critical habitat designation are subject to public review ► Management programs for listed species tend to receive intense public scrutiny

SUMMARY Most effective recovery programs incorporate field data and and quantitative tools. Each species and situation presents unique challenges. No one-size-fits-all approach. Modeling framework (PVA) can help assess management scenarios and account for uncertainty. Management of small populations needs to begin early. Management of small populations needs to begin early.