Download Biodiversity Crisis

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 surviving
individual 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 range
between 10-50% over next 20-50 years.
This current rapid decline in biodiversity
is known as the “Biodiversity Crisis”
What makes a species vulnerable
to 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
Brown tree snake introduced accidentally to Guam has caused
extinction of 12 species of birds and 6 species of lizards.
(a) Brown tree snake
(b) Kudzu
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
Rarity
• Rare species are the most vulnerable to
extinction, though common species can also
become extinct.
Three ways a species can be rare...
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
Small
population
Inbreeding
Genetic
drift
Lower
reproduction
Higher
mortality
Reduction in
individual
fitness and
population
adaptability
Smaller
population
Loss of
genetic
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
Habitat Loss
Estimates of current
rates of extinction
worldwide are based
primarily on speciesarea relationships
and rates of tropical
deforestation.
Habitat Fragmentation
• The fragmentation of habitats into patches
that are too small to support populations is a
major cause of extinction...
Problems due to fragmentation:
• edge effects
• small area
• isolation
Fig. 56-14
(a) Natural edges
(b) Edges created by human activity
Edge Effects
Problems due to fragmentation:
• edge effects
• small area
• isolation
Island Biogeography: Species richness on islands
depends on island size and distance from mainland.
Equilibrium Model
Explains variation in species richness on different
islands by considering effects of isolation and area
on rates of immigration and extinction.
Removal Experiment - Mangrove Islands
Arthropods removed from
islands of equal size, near
and far from mainland.
Number of species on
each island at end of expt
matched number at start
(at least on near island),
though the species differed.
The equilibrium species number is dynamic:
species composition constantly changes due
to immigration and extinction.
Today, the equilibrium model of
island biogeography is being applied
to conservation issues…
Smaller fragments have:
• fewer habitat types
• fewer species
• smaller populations
• higher extinction rates
Area-sensitive species with large home ranges
are especially vulnerable to small area effects...
Jaguar
Spider Monkey
White-plumed Antbird
Isolation
• Rates of extinction are much higher on
islands than on the mainland...
...problematic if species are not immigrating
due to isolation.
Habitat fragments are habitat “islands”...
Bogor Botanical Garden, Java
A possible solution?
Habitat (Movement) Corridors:
-links between fragmented habitats
-allow species movement between habitats
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.
Terrestrial biodiversity
hot spots
Marine biodiversity
hot spots
Equator
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