EXTINCTION&

THE BIODIVERSITY CRISIS

Biodiversity: All the variety of life, at every level of organization...

Genetic diversity

Species diversity

Ecosystem diversity

• According to the U.S. Endangered Species Act:

– An endangered species is “in danger of becoming extinct throughout all or a significant portion of its range”

– A threatened species is “likely to become endangered in the foreseeable future”

Extinction:The death of the last survivingindividual of a species, population, or gene, locally or globally.

The Biodiversity Synthesis Report

KEY FINDINGS:

• Decline in biodiversity due to human activities more rapid

in past 50 yrs than at any time in human history

• Over last 100 yrs, human-caused species extinctions have

multiplied ~ 1,000 times

• 12 % of birds, 23% of mammals, and 32% of amphibians

are threatened with extinction

Current estimates of species loss rangebetween 10-50% over next 20-50 years.

This current rapid decline in biodiversityis known as the “Biodiversity Crisis”

What makes a species vulnerableto extinction?

• Vulnerability to introduced exotics

• Overexploitation

• Rarity

• Habitat loss / Fragmentation

Introduced Exotics• Species that humans have moved

from native locations to new geographic regions

• In absence of their native competitors, predators, parasites, and pathogens, introduced species may spread rapidly

• Introduced species that become established in a new habitat usually disrupt the community

European Starling

Fig. 56-8

(a) Brown tree snake (b) Kudzu

Brown tree snake introduced accidentally to Guam has caused extinction of 12 species of birds and 6 species of lizards.

Asian plant kudzu introduced by USDA to control erosion; now choking out native plant species.

Overexploitation• Human harvesting of wild plants or animals at rates

exceeding ability of populations of those species to rebound

• Large organisms with low reproductive rates are especially vulnerable

• Overexploitation by the fishing industry has greatly reduced populations of some game fish, such as blue-fin tuna

• World’s fish stocks have been reduced by 90% since the start of industrial fishing

• Rare species are the most vulnerable to extinction, though common species can also become extinct.

Three ways a species can be rare...

Rarity

1.

Cave Salamander

2.

3.

Maned Wolf

4.

Proboscis Monkey

5.

Hawaiian Hawk

6.

Dwarf Naupaka

• A small population is prone to positive-feedback loops that draw it down an extinction vortex

• The key factor driving the extinction vortex is loss of the genetic variation necessary to enable evolutionary responses to environmental change

Fig. 56-10

Inbreeding

Smallpopulation

Geneticdrift

Lowerreproduction

Highermortality

Smallerpopulation

Reduction inindividualfitness andpopulation

adaptability

Loss ofgenetic

variability

Case Study: The Greater Prairie Chicken and the Extinction Vortex

• Populations fragmented by agriculture and later found to exhibit decreased fertility

• Scientists imported genetic variation by transplanting birds from larger populations

• Population rebounded, confirming that low genetic variation had been causing an extinction vortex

Estimates of current rates of extinction worldwide are based primarily on species-area relationships and rates of tropical deforestation.

Habitat Loss

• The fragmentation of habitats into patches that are too small to support populations is a major cause of extinction...

Habitat Fragmentation

• edge effects

• small area

• isolation

Problems due to fragmentation:

Fig. 56-14

(a) Natural edges

(b) Edges created by human activity

Edge Effects

• edge effects

• small area

• isolation

Problems due to fragmentation:

Island Biogeography: Species richness on islandsdepends on island size and distance from mainland.

Equilibrium Model

Explains variation in species richness on differentislands by considering effects of isolation and areaon rates of immigration and extinction.

Removal Experiment - Mangrove Islands

Arthropods removed fromislands of equal size, nearand far from mainland.

Number of species oneach island at end of exptmatched number at start(at least on near island),though the species differed.

Today, the equilibrium model ofisland biogeography is being appliedto conservation issues…

The equilibrium species number is dynamic:species composition constantly changes dueto immigration and extinction.

Smaller fragments have:

• fewer habitat types

• fewer species

• smaller populations

• higher extinction rates

Area-sensitive species with large home rangesare especially vulnerable to small area effects...

Spider Monkey

White-plumed Antbird

Jaguar

Isolation• Rates of extinction are much higher on

islands than on the mainland...

...problematic if species are not immigratingdue to isolation.

Bogor Botanical Garden, Java

Habitat fragments are habitat “islands”...

Habitat (Movement) Corridors:

-links between fragmented habitats

-allow species movement between habitats

A possible solution?

Fig. 56-15

Advantages of Habitat Corridors

- increased species richness

- “rescue effect”

- maintain genetic variation

- altitudinal migration

Three-wattled Bellbird - altitudinal migrant

Disadvantages of Habitat Corridors

- increased disease transmission

- increase predation risk

- increased spread of fire

- reduced genetic variation between fragments

Biodiversity ‘Hot Spot’:A relatively small ecosystem with a high concentration of species, many of which are endemics.

Equator

Terrestrial biodiversityhot spots

Marine biodiversityhot spots

Hotspots of biodiversity are also hotspots of extinction…

Video Topics

Population Ecology: life history strategies parental care / fecundity

Community Ecology: biodiversity mutualisms, competition, predation

Ecosystem Ecology: primary producers (plants) primary, secondary, tertiary consumers water cycle

Conservation Biology: hot spot, endemic species, rarity, habitat fragmentation, extinction