Lecture 23 Ricklefs Reading: The Biodiversity Crisis

From Chapter 26: Biodiversity, Extinction, and Conservation
pp. 555-561

*Human activities have accelerated the rate of extinction
- Populations disappear when deaths > births over a prolonged period.
- Extinction may result from a variety of mechanisms that influence birth and death
- Extinction as a failure to adapt to changing conditions either because: 1) changes occur to rapidly, or 2) population is unable to respond to them

- IUCN (International Union for the Conservation of Nature) contains a Red List that ranks levels of extinction risk:

  • Critically endangered >3000 species: species that has decrease or is expected to decrease by 80% within three generations.
  • Endangered ~5000 species: species at risk of extinction due to small populations, threatened critical habitats or imminent risk from predation or disease.
  • Vulnerable >8000 species: likely to become endangered unless factors threatening their survival and reproduction change.

*Primary Causes of population declines of endangered species in the United States:

1) habitat reduction and modification
- causes extinction by wiping out suitable places for species to live
- fragmentation of habitat threatens some kinds of wildlife
> coastal forests of Brazil have been reduced, critically endangering endemic birds and mammals such as the golden lion tamarin
> in North America, tallgrass prairie now only exists in few isolated refuges and many populations of prairie plants and aninmals are locally extinct
—> U.S. national parks have lost many of their mammal species within the past 50 years suggesting that these preserves are too small to maintain viable poplulations

2) small population size
- due to stochastic tendencies of small populations, probability of extinction is higher due to nature of small population
- prone to reduced genetic variation through inbreeding and bottlenecking, reducing capacity to respond to rapid change in the environment
- exceptions due to conservation efforts: there are several cases of species that have been reduced to near extinction and have lost much of their genetic variation, yet have recovered when protected

3) overexploitation
- weapons and harvesting tools (such as fishing technology) result in more efficient hunting leading to species being hunted to extinction
—> this is not an old phenomenon: 50,000 years ago, flightless birds and a tortoise disappeared from Australia; arrival of human inhabitants on island of Madagascar resulted in demise of two dozen species, and many more examples
- most extinctions of large mammals and birds occurred before the years of modern science

4) species introductions
- introductions of predators, competitors, or pathogens decrease habitat quality
- occur accidentally or as deliberate introductions for use of crops, ornamentals, game species or biological control agents
- aquatic ecosystems are particularly vulnerable due to 1) dominance of top-down control by predators and 2) high rate at which energy and nutrients are processed in aquatic food webs.

Climate change affect on species lost is unclear at this point. Society has mostly focussed on assigning a specific amount of lands for conservation efforts. Changes in climate could permanently change the species that will remain in each area and possibly adverse the ecosystem.

Human made pollution is a large contributor to species extinction.

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