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Chapter 8

ENVS 137 Chapter Notes - Chapter 8: National Park Service Organic Act, Wilderness Act, Ecosystem Services

Environmental Science
Course Code
ENVS 137

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8.5: Threats to Biodiversity
Human activities → increased rate of extinction
Destruction of habitat
Development → habitat fragmentation
Population growth → exploited plant/animal populations
Relocation of species → humans introduce non-native competitors, predators, and
Air and water pollution
Altered patterns of disturbance
Global climate change
Habitat Loss and Degradation
“Worldwide, the single greatest threat to biodiversity is habitat loss”
By 1900 over 95% of North America’s deciduous forests had been heavily
logged/cleared for agriculture
Agriculture replaces grasslands → only 3% of North America’s mixed and tall-grass
prairies remain, and those that remain only in small isolated patches
Only 1% of longleaf pine savannas left → provide habitat for diverse endemic flora and
fauna → now endangered
Dwindling tropical forests → contain A LOT of the world’s species → today half these
forests are gone
Tropical forests being cleared at a rate of ½ million square miles every decade
Effects on biodiversity:
Current global extinction rate in tropical forests → 1,000 species per year
Species suffer from isolated populations in fragmented habitats
Marine ecosystems → habitat loss
Coastal development and boating → destroy sea grass beds in shallow coastal
waters (provide habitat for shellfish, source of food for many endangered
20% of Earth’s coral reefs (most diverse marine habitats) destroyed, another
20% severely damaged
Habitat Fragmentation
“Human development and land use is dividing landscapes into even smaller and more
disconnected habitat islands”
Urban development, roads, agriculture → divides large habitats into patches/”islands”
Populations within habitat patches smaller and more vulnerable to local extinction
Recolonization of patches difficult
→ decline in diversity
Food and shelter needed by animals may be separated
Larger animals separated from breeding territories
Blocked migration pathways
Sound of traffic confuses wildlife

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Loss of species in aquatic ecosystems
Ex. dams divide fish populations, create barriers for fish to migrate
up/downstream for reproductive cycles
Species with large bodies, slow population growth rates, and flocking behavior are
particularly vulnerable to human exploitation
Species in greatest danger → those that humans value as sources of food or medicine
and those prized by collectors
Ex. species of whales driven near extinction for whaling oil and meat
Fisheries → UN Food and Agriculture Organization (FAO) → majority of the world’s
marine fish species being fully exploited, 30% of fish species being overfished
Physical and behavioral characteristics → vulnerable to overharvesting
Large animals:
vulnerable to human hunting
Slow growth rates → slow to recover if diminished
Feed at higher trophic levels → smaller populations
Animals living in herds/flocks/schools
Easier to capture and kill in large numbers
Ex. Nassau grouper
Part of solution → strictly enforced fishing laws prohibiting harvest during breeding
months, marine preserves
Non-Native Invasive Species
Newly introduced species may be invasive because of their own biological traits or
because of the absence of predators or competitors
Commerce and travel → redistribution of species
Sometimes intentional → ex. Plants on farms and gardens
Consequences for individual species and biodiversity:
Some introduced species have traits that allow them to out-compete native
species → invasive → invader consumes most available resources → native
species driven towards extinction
Ex. garlic mustard (from Europe) poisons other plants (in US) and takes
Lack of competitors for introduced species
Ex. brown tree snake (from Indonesia) → Guam has no native snakes to
compete, birds have no native defenses → devastated bird and small
mammal populations
Few predators or diseases in the new community
No natural enemies → alien population grows quickly → consumes
available resources → drives native population towards extinction
Natural selection builds genetically based resistance to common diseases
10% of the non-native species are able to become established when enter foreign area
10% of these species become invasive
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