Extinction Processes
Chapter 7
Small populations are more at risk for extinction
What makes species rare?
 Species may be restricted to an uncommon habitat
Desert pupfish
Species may be geographically restricted
Species occur at low density
 Size of organism
 Food may be in low density
 Intraspecific competition may be intense
Rarity can lead to extinction
Specialists more prone to extinction than generalists
Skewed sex ratio
Rarity ≠ extinction
Limited adaptability and resilience
 Low reproduction
 Limited dispersal
 Limited habitat
 Etc.
Limited dispersal and habitat specificity
Human attention
Aye aye
Ecological overlap
Large home range
Species are composed of populations
Defining species with genetic diversity
 Use partitioning of heterozygosity
Ht = Hs + Dst
 If Dst > 5%, the two groups are separate populations

Metapopulations
 Patchy populations lead to a type of population structure called
metapopulation population structure
 Metapopulation is a population of populations (called subpopulations or
local populations)
 Metapopulations are distinct demographically, but not genetically
Metapopulations
 A population that consists of several subpopulations linked together by
immigration and emigration
 Not all patchy populations are metapopulations
 When gene flow is high
 Source-sink dynamics – A type of metapopulation structure whereby some
habitats have higher quality than others
Vernal Ponds
Metapopulations
Metapopulation size
 Metapopulation size is the number of occupied patches
 Turnover is the loss and gain of subpopulations – local extinctions
Extinction and Colonization
 E = probability that an occupied patch will go extinct.
 C = probability that an unoccupied patch will be colonized
Metapopulation model
Metapopulation model
Metapopulation model
Metapopulations
 Migration between patches can help species persistence
 Some species require multiple patches of habitat
 Multiple patches can spread the risk of stochastic extinction
 Empty patches are important
 Certain links are very important
 Patchy habitats are increasing!
Source-sink metapopulations
 Habitat patches vary in quality
Rescue effect
 Because sink populations go extinct without dispersal, migration from sources
“rescues” sinks
 Perennial sinks are called “ecological traps”
Peregrine falcon
Population viability analysis
 Used in part to estimate Minimum Viable Population size (MVP)
 Study of how factors interact to determine extinction probabilities for individual
species
 Demographic stochasticity
 Environmental stochasticity
 Natural catastrophes
 Genetic stochasticity
Demographic stochasticity
 Temporal variation in reproduction success driven by chance variation in fates
of individuals
 Dusky seaside sparrow
Allee effect
Environmental stochasticity
 Temporal variation in reproductive success driven by unpredictable changes in
the biotic or abiotic environment
Catastrophes
 Dramatic environmental effects
Genetic stochasticity
 Random variation in gene frequencies due to genetic drift, bottlenecks,
inbreeding, etc.
 Least effect on MVP estimates
Factors interact to form extinction vortex
PVA for turtles
Turtle PVA
 Sources of turtle mortality:
 Beaches (nesting)
 Ocean (juveniles and adults)
 Where should conservation for this species be focused?
Lights and turtles