1. Ch. 7 Extinction Processes

2. 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.

3. 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

4. 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.

5. 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

6. 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”

7. 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

8. Metapopulation dynamics
Turnover ~ Populations appear and disappear, winking in and out of existence
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

9. Population Viability Analysis (PVA)
Answer questions about population size, persistence, and management
Model
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)

10. Stochasticities of Extinction
Demographic stochasticity results from random variation in reproduction and survival.
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

11. 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
drift
bottlenecks
inbreeding
not important for short-term PVAs

12. 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