Figure 25.26: An exotic species, the brown tree snake, hitch-hiked on an aircraft to the
Pacific Islands, causing the extinctions of many bird and mammal species which had evolved
in the absence of predators. ( 24 )
Causes of Habitat Destruction
Clearing some habitats of vegetation for purposes of agriculture and development is a major
cause of habitat destruction or loss. Within the past 100 years, the area of cultivated
land worldwide has increased 74%. Land for the grazing of cattle has increased 113%!
Agriculture, alone, has cost the United States 50% of itswetlands (Figure 25.27) and
99% of itstallgrass prairies(Figure25.28). Native prairie ecosystems, with their thick
fertile soils, deep-rooted grasses, diversity of colorful flowers, burrowing prairie dogs and
burrowing owls, herds of bison and pronghorn antelope, and other animals, are virtually
extinct (Figure25.29).
Another habitat that is being rapidly destroyed is forests, most significantly tropical rain-
forests, one of the two major ecosystems (or biomes) with the highest biodiversity on earth.
The largest cause of deforestation today isslash-and-burn agriculture(Figure25.25),
practiced by over 200 million people in tropical forests throughout the world. Depletion of
the thin and nutrient-poor soil (even so, biodiversity here is high – can you guess why?)
often results in people abandoning the forest within a few years, and subsequent erosion can
lead todesertification(a process leading to production of a desert of formerly productive
land [usually at least semi-arid]).
Half of the earth’s mature tropical forests are gone. At current rates of deforestation, all
tropical forests will be gone by 2090. Poverty, inequitable land distribution, and overpopu-
lation combine in many developing countries to add pressure to habitats which are already
stressed. Use of firewood, charcoal and manure for cooking and other energy needs, and
waste of crops further degrade environments, threatening biodiversity through habitat loss.