Population fluctuations and cycles Populations tend to fluctuate through time Localized populations fluctuate more than entire populations

What could be causing these fluctuations?

Northern Bobwhite populations Timing of density measurement affects population size estimate and degree of fluctuation detected Northern Bobwhite populations

Resilience Rate at which population springs back to equilibrium after jumping above or dipping below it due to a “disturbance” Body size and lifespan effects Smaller, shorter-lived species show wider swings in population size. Why? Smaller, shorter-lived animals show greater resilience. Why?

Oscillating populations appear to have regular cycles

Key hypotheses for why populations oscillate Time lags Predator-prey relationships Links to environmental cycles Example: El Niño and kelp forests Life cycles of parasites Example: Malaria-carrying mosquitos Changes in gene frequencies Behavior...

Snowshoe hare cycles: Evidence of decline is evident just prior to peak High winter weight loss Decreased juvenile growth rate Decreased juvenile overwinter survival

What makes a population vulnerable to extinction? Low density Difficult to find mates (Queen conchs in Florida) Vulnerable to hybridization (less choosy) Isolation Restricted gene flow/cut off from metapopulation Why a problem? Small size Could lose a higher proportion of individuals in a disaster Smaller subset of genotypes; additional genetic drift

Mass extinctions In which geologic period, and approximately how many millions of year ago, did the largest extinction occur? Permian (250 mya) When did the dinosaur extinction occur? Cretaceous (60 mya) Was this extinction restricted to dinosaurs or to land?

Process of extinction Role of localized extinctions Habitat destruction or other cause  Marginalization of individuals ( survival/ reproduction) Fragmentation of habitat and populations  Continuous populations  metapopulations Reduction of gene flow Inbreeding and genetic drift  Local population wiped out by disaster  Many localized extinctions  Species itself is threatened or eliminated

Deterministic causes of extinction Occurs due to force or change with continued effects No escape for population or species over the long term Asteroid theory of Cretaceous extinction: species unable to escape climate change Overhunting of passenger pigeon and great auk

Great auk: What led to its extinction? Initial distribution: widespread across the North Atlantic Localized killing for food and bait  fragmentation For-profit killing for fat and feathers  extinction of local populations When rare, high-paid trade in skin and eggs… Last pair and one egg taken in Iceland, 1844 Image courtesy of the Canadian Museum of Nature

Stochastic causes of extinction Examples? How might determinstic and stochastic causes work together to cause extinction Heath hen example… (also in text)