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 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 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. 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 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