MODERN CAUSES OF SPECIES EXTINCTIONS: HABITAT DESTRUCTION

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 7: pages 163-174 & Chapter 9: pages 197-229 (Skip Box 13)

1. Introduction

   Habitat destruction is an important cause of known extinctions. As deforestation proceeds in tropical forests, this promises to become THE cause of mass extinctions caused by human activity.

   All species have specific food and habitat needs. The more specific these needs and localized the habitat, the greater the vulnerability of species to loss of habitat to agricultural land, livestock, roads and cities. In the future, the only species that survive are likely to be those whose habitats are highly protected, or whose habitat corresponds to the degraded state associated with human activity (human commensals).

2. Destruction, degradation, fragmentation – elements of habitat loss

   Habitat is damaged in many ways. Putting the prairie under the plow eliminated much prairie dog habitat, and made the blackfooted ferret very rare. Increased sediment inputs to rivers may destroy habitat, or degrade it. Converting large continuous habitat areas into isolated, small pieces fragments habitat, with subtle and far-reaching consequences. 

3. Deforestation in Perspective

   Habitat damage, especially the conversion of forested land to agriculture (and, often, subsequent abandonment as marginal land), has a long human history. Began in China ~ 4,000 yrs BP, completed in Europe by about 400 yrs BP, swept across USA over ~150 years, now mopping up last forests of Pacific Northwest. Lowland, seasonal, deciduous forest began to disappear after 1500; forbidding tropical humid forests mainly in 20^th C.

4. Why are the tropical forests so important?

They harbor 50% (?90%) of world’s biodiversity, and are declining. Original extent of tropical rain forests was 15 million km2; now about 7.5-8 million km2, so half is gone. Current rate of loss is estimated at near 2% annually (100,000 km2 destroyed, another 100,000 km2 degraded). Uncertainty regarding rate, and what it will be in future. Likelihood is reduction to 10-25% of their original extent by late 21^st C.

How many species will be lost? We know that small land areas support fewer species than large land areas. We can calculate the number of extinctions if we estimate (a) the number of species that live in the rain forests (~10 million), (b) the rate of forest loss (~1.8% annually), and (c) the slope of the S-A relationship (0.15-0.35). The prediction: 27,000 species/yr, 3/hr.

Habitat destruction is especially serious when it threatens hotspots (areas of high species richness) and regions with many endemics.

5. North Vs. South America: Similarities And Differences

Why should we worry about deforestation in the tropics, since deforestation in North America caused few species to go extinct? Some differences: (a) deforestation proceeded more gradually in NA, with large areas remaining intact. Caused changes in abundance more than outright extinction. (b) prairies of NA suffered more than forests – safety valve. (c) tropical species apparently more specialized, more restricted in distribution, so don’t readily disperse. (d) tropics contain many more species.

Transparencies: 1. Rain forest priorities, 2. Estimates of deforestation rates, 3. From old-growth to young-growth, 4. Forest fragmentation in Wisconsin, 5. Willamette River habitat loss, 6. Dams and river fragmentation, North America, 7. US forests - 1760 and 1960, 8. Costa Rica deforestation, 9. Species-area relationship, 10. hotspots

Links:

• Rainforest Action Network: this website for the Rainforest Action Network (RAN) is full of information about tropical habitat destruction. Really cool website!
• Analysis of International Environmental Problems: a look at the human view of deforestation--who are the people who cut down the forests, why do they continue to do it, and their effects on wildlife habitat.
• Deforestation: How does deforestation happen and its consequences.