There are many ways we degrade habitat and cause loss of biodiversity. An IUCN table of categories includes most…

There are many ways we degrade habitat and cause loss of biodiversity. An IUCN table of categories includes most…

DeforestationFrom Sisk et al. (1994) and Myers et al. (2000), we have seen that recent patterns of human population pressure (growth rate + population density) and biotic impoverishment (deforestation rate) are highest in countries with high total biodiversity and high species endemism.

While we have made progress in North America, Europe and the former USSR to replenish forests, other parts of the globe are sustaining significant losses.

Tropical moist forests are thought to contain at least half of the world's species, though they represent  only 6% of the global land area.  The forests are being destroyed at an exceptional rate, estimated at 150,000 km2 per year. 

Consider first losses across a variety of habitats without reference to place, then specific forest losses in different regions:

Med. forests, woodlands and temperate forests logged 1st, then subtropical and tropical forests; more inaccessible forests last

In addition to destruction, tropical forests and other natural habitats have been greatly modified, particularly grasslands. These lands were easily converted to cropland (e.g. bananas, coffee, grazing land), often with tremendous applications of pesticides. In Latin America this is particularly apparent.

Changes in land use from 1850 to 1985 in Latin America. Pastures and croplands increase, undisturbed forests decline

Countries currently harvesting their tropical forests most intensively include Ivory Coast, Costa Rica, Vietnam and Paraguay (pre 1990 data). Indonesia has increased logging in recent years

2 estimates of deforestation for 15 countries

Annual change in forest cover between 1977-1989 (top). Note net growth of forest in North America, Europe and Russia.

Same idea, but now we look between 1990-1997. Archard et al. 2002. Science

Compare FAO and Trees-II studies and deforestation is lower than expected

Other larger countries with very high biodiversity and endemism (Brazil, Indonesia, India) are destroying a much greater area of forest. (note there are 2 estimates for rates in the 1980s).

We already know how important population pressure is. In Brazilian Amazonia, in the state of Rondonia, the population grew from 111,000 to >1,000,000 between 1975 and 1986. The amount of forest cleared increased from 1250 km2 in 1975 to ~17,000 km2 in 1986, and was made far worse by road building. Roads provide access to areas where humans could not go previously. 

We can use the species-area relationship from island biogeography to estimate what fraction of species diversity will be left as forest area declines:

      S = cAz the ratio of current to remaining species at equilibrium involves only the ratio of future to current Az.

In Amazonia (including Brazil, but also parts of Venezuela, Colombia, Ecuador, Peru, Bolivia, Surinam, and Guyana) there are both fully protected areas and ‘Native Reserves’ that are partially protected. We’ll consider many aspects of reserve design in an upcoming lecture.

A map of Amazonia, the fully protected areas are blackened, and the more numerous ‘Native Reserves’ are outlined with dashed lines.

If only 10% of tropical forest is preserved, then only 54% of species will persist at the new equilibrium. If only currently protected areas in Amazonia remain, the loss will amount to approximately 66% of plants and a slightly higher proportion of birds and other species.

Costa Rica has become proactive in developing ecotourism and protecting what remains of its forests, but more than 99% of:

1) dry deciduous forest,

2) lowland moist forest,

3) premontane moist forest,

4) montane wet forest were destroyed prior to protective legislation.

Dan Janzen has been regrowing dry deciduous forest in NW Costa Rica in Guanacaste. It was extensively deforested because it was accessible, had extensive seasonal dry forest, and, once logged, was easily transformed into cattle pasture.

Deforestation has been nearly as acute in some regions of Madagascar, a poorly studied, high-diversity area.

Why do they log? Agricultural expansion, infrastructure growth, commercial and fuel wood extraction in Asia and Latin America, fuel wood only in Africa

Geist & Lambin, Bioscience, 2002

Forest Loss in Asia is directly proportional to human population density and gross national income

Sodhi et al. 2004, TREE

Logging in the Philippines

Forest cover in the Philippines, 1848-1987

Deforestation rate

Kumer & Turner, Bioscience 1994

Deforestation a net result of

1) migrants moving to forested areas & converting land for agric.,

2 )commercial logging

Species Endemism in 4 SE Asia Biodiversity Hotspots

Sodhi et al. 2004, TREE

blue=recorded extinctions, green = inferred extinction, yellow = minimum projected loss, red = maximum projected loss

Sodhi et al. 2004, TREE

Why is deforestation so devastating?

1. limiting dispersal and colonization, e.g. birds, mammals and insects that live in forest interior may not cross this barrier.

2. limiting foraging ability of animals; social organisms (gibbons, orangutans) that forage in intact forest may avoid open areas and typically will not descend from trees to cross the gap.

3. restricting mate availability for the same reason, resulting in fragmentation of the population.

4. changing microclimate including light, temperature, wind, and incidence of fire.  Deforestation-associated edge effects may be perceptible up to 50 m into adjacent, intact foresta. Increased T during day (and lower ones at night)b. increased wind exposure (and lower humidity)

Habitat degradation may greatly exceed the area that is deforested owing to 'edge effects'. Edge effects may include:

5. serving as a corridor for invasion by non-indigenous species incapable of establishing in intact forest.

6. enhancing seed dispersal opportunities, providing an entry for species not otherwise able to colonize forested habitat.

Fragmentation introduces new ‘edges’ and dramatically reduces the area of core forest. Effects are evident in a comparison of data from 1978 and 1988 in Amazon rain forest regions…

An excellent example of the indirect effects of forest loss and fragmentation comes from studies of the effects of cowbirds on the native forest avifauna…

Brown-headed Cowbirds (Molothrus ater) and Edge Effects:

Robinson et al. (1995) studied nest predation and parasitism for nine different forest-dwelling birds (acadian flycatcher, indigo bunting, Kentucky warbler, northern cardinal, ovenbird, red-eyed vireo, worm-eating warbler and wood thrush) in 9 different forests that ranged in size from ~2,500 to 32,500 ha in Illinois, Wisconsin, Indiana, Missouri, Minnesota and Wisconsin.

9 Forest patches studies in Great Lakes survey of cowbird effects on native bird species

Robinson et al. (1995)

Sizes of the forests studied ranged from ~2000 ha to >30,000 ha.

Cowbird parasitism was negatively correlated with the percent forest cover for all 9 species.  Some species e.g. worm-eating warblers and wood thrushes, were particularly heavily parasitized in small forest patches (near 100%).

Robinson et al. (1995)

Wood thrush nests were rarely if ever parasitized by cowbirds in the larger forest tracts.

State-by-state variation in parasitism rates was also evident for most species, suggesting local factors (e.g. vegetation type) may also be important.

wood thrush

wormeating warbler

Robinson et al. (1995)

Nest predation rates were clearly negatively related to patch size. Major nest predators included mammals (raccoons), blue jays and snakes. Birds most affected were ground- and near-ground nesting species.

Robinson et al. (1995)

Putting studies of parasitism and predation together in a meta-analysis, Paton (1994) reported that:

1) In 14 of 21 studies predation rates increased with proximity of 'edges';

2) 3 of 5 studies showed parasitism increased with proximity to 'edges’;

3) nest success increased with increasing forest area in 8 of 8 studies;

4) Most studies found that edge effects occurred within 50m of the forest edge.

How long will it take to lose species after a forest has been fragmented?

Brooks et al. (1999, Conserv. Biol) explored this with birds in 5 tropical forest fragments of known date of isolation, in Kenya

Exponential decay curves for shortest (bottom) and longest (top) half lives for the 5 forest fragments

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