L13 - Extirpation Flashcards
What is extinction?
Death of the last individual of the species
How do we demonstrate extinction?
- If there are no longer individuals found where previously there had been, we can classify this species as extinct
- A “thorough search” is conducted to classify a species as extinct, but different agencies disagree as to what constitutes a “thorough search”
What is the last stage before full extinction?
- Extinct in the wild
- A species can be extinct in its habitat, but still found in captivity
- Ex: Tasmanian tiger, Passenger pigeon, California condor (1987)
Why won’t species survive in captivity?
- Do not breed well
- Prone to disease
- Low genetic diversity
What is functionally extinct?
- A species which still has members present in the environment
- Population is greatly reduced compared to the ancestral population
- Have decreased below their minimum viable population
- Have obvious factors in the environment preventing populations from recovering
- Are no longer performing their role in the ecosystem
Define minimum viable population
- Genetic diversity may be too low for healthy breeding over the long term.
- Greater likelihood that “chance events” could wipe out your entire population
What is an example species that is functionally extinct?
American Chestnut
- Once the most common nut tree in Eastern North America
- Many different animal species ate the nuts, making it a primary producer for forest ecosystems
- Early 1900s: fungal disease killed ~4B trees
- Today: few thousand trees remain in the wild but this species is no longer an important contributor to the ecosystem
Why do species go extinct?
- Global extinction is a result of local extirpations
What is extirpation?
- Local extinction of a population from a
geographical range. - Other populations of the species survive
elsewhere.
Why do populations get extirpated?
- Population dynamics: changes in population size (N) and composition over time
- Extirpation: N=0
What determines the number of individuals (N) in a population over time?
- Numbers in the population in previous time, Nt-1
- Intrinsic rate of population increase, r
- Nt= Nt-1*(1+r)
Extirpation occurs when Nt=0
** Can also add “stochasticity”
What are density-dependent factors?
- Factors affecting population numbers
- An environment only has enough resources (food, space…) to support a specific number of individuals in a population
What is the carrying capacity K?
The number of individuals an environment can support before resources run out or the environment begins to degrade.
Other factors that affect population numbers?
- Human impacts (harvesting, animal husbandry)
- Extreme events (natural catastrophes, spread and impact of disease)
- Pollution (chemicals, changes in environment)
- Land-use change (changing environments, loss of habitat)
What are the consequences of low population size?
- Increased effect of stochasticity (random chance)
- Population processes (Allee effects)
Allee effects: what are the possible side effects of a loss in genetic diversity?
- Greater susceptibility to disease
- Greater chance of deleterious alleles (detrimental to survival) becoming prominent in the population
What are metapopulations?
Networks of spatially isolated populations, connected by some exchange of individuals (or pollen, gametes) over time.
How does immigration affect population numbers?
- Allows for gene flow between isolated populations
What is gene flow?
- The movement of alleles between two geographically separated populations
- Gene flow can be caused by a single individual (immigration/emigration) moving between populations
Population viability: role of chance?
- In probability of natural catastrophes and population demography…
- Stochasticity in life history parameters and in environmental conditions that influence survival and reproduction
- In metapopulation dynamics…
- Degree of movement among adjacent populations and environmental conditions that influence emigration/immigration
- In effects of small population size
- Detrimental influence on interactions among individuals (Allee effect)
- Interbreeding depression
- Genetic bottlenecks (loss of alleles)
What are our past and current species numbers?
- ~ 5B species have evolved on Earth
- ~8.7 Million species currently
- Extinction is the rule for species, not the exception
What are the human mechanisms that have caused species extinctions?
- Direct predation by humans
Secondary effects: - Land burning
- Habitat change
- Introduced species
- Cascade effects
Did humans cause extinctions?
- Lines of evidence: Overkill hypothesis
- Extinctions fall at 10 000 BP, kill sites lead us to believe that humans caused extinctions of large mammals
- But also
- There are extinctions of species not caused by humans
- Some vulnerable species survived
- Extinctions occurred where there were no humans present
- Kill sites are not that abundance and there is a lack of evidence in the fossil recor
Climate caused extinctions?
- Last glaciation coincides with extinctions
- Changing vegetation, food source, competition
- May explain impacts on species that weren’t hunted
BUT
- May explain impacts on species that weren’t hunted
- No extinctions through dozens of past glacial advances and retreats
- extinctions may be too rapid
