Download Extinction Processes

Ch. 7 Extinction
Processes
Why are some species rare?
• Restricted to uncommon habitat
• specialists ~ cave dwelling species
• habitat is rare ~ vernal pools
• poor competitors, can only persist in habitat with few competitors
• Limited to small geographic range
• geographic barriers ~ islands, mountain ranges, lakes
• locally adapted ~ specific soil type
• Occur at low population densities
• big things need more space
• require scarce or dispersed resources ~ apex predators
• fitness increases at lower densities
• Check out the Venn diagram in your book, page 132
• if you are restricted to uncommon habitat, in a small geographic
range, and you need a lot of space, you’ll be really rare.
How rarity increases vulnerability
• More vulnerable to stochastic environmental changes
• a single major change can encompass a narrowly distributed
species’ entire range
• 75% of extinctions since 1600 were island species
• More vulnerable to habitat changes
• habitat specialists more vulnerable than generalists, even in the
same habitat
• can’t seek refuge elsewhere or be “rescued” by neighboring
populations
• More vulnerable to genetic problems
• skewed sex ratios
• genetic drift, inbreeding, bottlenecks
• reduced ability to adapt to changes
What makes species vulnerable
to human-induced change?
• Limited adaptability and resilience
• low reproductive capacity
• limited dispersal capability
• stringent habitat requirements
• Human attention
• tasty
• unpopular and persecuted
• Ecological Overlap
• competing with humans for habitat ~ grasslands, rivers, coasts
• Large Home-ranges
• bound to run into trouble somewhere
• sensitive to fragmentation
• tend to be large and disliked by people
• Evolutionarily unprepared for introduced predators, exotics,
diseases, or over exploitation ~ especially true for island spp.
Population Structure
• Population ~ group of individuals of the same species occupying a
given area at the same time
• arbitrarily ~ political boundary
• demographically ~ age structure and demographic parameters differ
• genetically ~ different alleles, little gene flow
Metapopulations
• A population of populations
• Species with patchy distributions
• Each patch of habitat is a different population
• Occasionally, individuals move between patches, but this is
infrequent because non-habitat presents a barrier
• species that will move through non-habitat, or have general
requirements don’t show metapopulation structure because gene
flow too high ~ “patchily distributed populations”
Types of subpopulations
• Sources
• produce a net surplus of individuals
• emigrants disperse to other patches
• Sinks
• do not produce enough individuals to maintain themselves
• rely on dispersers from other patches
• Rescue effect: Sources rescue sink populations from going
extinct
• Patch may be a source some years and a sink others
• Ecological traps: Attractive sinks
• Individuals prefer these patches even though they will lose fitness
there
• Associated with humans manipulating perception of habitat
quality
Metapopulation dynamics
• Turnover ~ Populations appear and disappear, winking in and
out of existence
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colonization ~ an appearance
local extinction ~ a disappearance
may be fast or slow
may be patch wide (disturbance or succession) or vary by species
• core subpopulations ~ persist a long time
• generally large patches/populations
• generally sources
• satellite subpopulations ~ those that wink in and out
• generally smaller
• generally sinks
• Particularly useful concept in fragmented areas
Population Viability Analysis (PVA)
• Answer questions about population size, persistence, and
management
• Model
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dependent variables ~ population size
independent variables ~ habitat measures, weather, harvest/take
parameters ~ birth, death, and reproductive rates
accounts for variation in time and space
• model randomly selects a value for each parameter from the range of
probable values
• useful for small populations where chance matters more
• Steps
• Single projection for a specified period
• Repeat 500+ times (each time, different parameter values)
• Calculate proportion of projections that included threshold you are
interested in (75% of the projections predict population of 100
individuals in 10 years)
Stochasticities of Extinction
• Demographic stochasticity results from random variation in
reproduction and survival.
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small populations
skewed sex ratios
Allee effects
Population size, sex ratio, age structure, birth and death rates for
sexes and ages
• Environmental stochasticity results from random variation in
habitat quality
• climate and water
• species interactions
• more difficult to model, usually limited to a few key variables
Stochasticities of Extinction
• Catastrophes
• occur randomly
• discrete events
• modeled by killing a proportion of the population outright, rather
than continuously effecting birth/death rates
• Genetic stochasticity results from random variation in gene
frequencies
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drift
bottlenecks
inbreeding
not important for short-term PVAs
Be Mindful
• Demographic, environmental, catastrophic, and genetic
stochasticities are interactive
• one may multiply the effects of the other
• feedback loops, or snowballing -> extinction vortex
• Slow, incremental losses can lead to global extinction