Lecture Twenty One - Human impacts on terrestrial ecosystems Flashcards
What are the values of forests?
Economic. Fauna and flora habitat. Microbes habitat. Clean air and water. Equate temperature. Reduce particulates. Increase ozone.
What is deforestation?
‘Temporary or permanent clearing of forest for agriculture of other purposes.’
E.g. fuel for domestic use, contraction or smelting.
- By fire or cutting.
- Began with early human settlements >7000 y BP.
Europe: major deforestation AD1050, North America in 1600s.
- Many consequences include cataclysmic disasters.
What are the causes of deforestation?
Immediate causes:
Spread of agriculture (major cause).
Domestic firewood collecting, unregulated timber harvesting.
Lead to:
Loss of soil fertility, cultivation abandoned, overgrazing, soul erosion, no forest regrowth.
Socioeconomic causes:
Unemployment, population growth, land tenure issues, lack of government regulation of industry.
Indirect:
Associated with pollution, disease etc.
Where is there now forest poor areas due to deforestation?
Forest poor - much of Africa, Asia, parts of central and South America.
What is tropical deforestation?
Since 1980:
320 x 10^6 hetares (25%) of tropical forest areas cleared.
Population in tropical countries has almost doubled.
Rapid economic growth exacerbates pressures on tropical forests.
53,000 sq miles (FAO 2002) tropical forests destroyed each year during 1980s (21,000 sq miles annually in S. America, mostly in Amazon Basin).
Deforestation rates:
1978-1986: 6200 sq miles yr^-1.
1986-1993: 4800 sq miles yr^-1.
Between 2000-2005
• 27 million hectares of rainforest area — roughly 2.4% of global rainforest cover cleared from 2000 to 2005.
- Results suggest tropical forest loss continues at rates roughly similar with those observed in the 1990s.
What is the relationship between deforestation and climate change?
Deforestation is the second largest anthropogenic source of CO2 to atmosphere, after fossil fuel combustion.
While logging of young and old-growth forests =
same long-term emissions.
Short-term emissions are 2x higher for old-growth forests.
What are frontier forests?
Frontier forests:
“Large, ecologically intact and relatively undisturbed natural forests.”
“Likely to survive indefinitely without human assistance.”
What are the effects of deforestation?
Soil degradation and erosion - loss of top soil, loss of nutrients (N and P).
Increased sediment (turbidity) in rivers and coastal waters.
Increased runoff -> flash flooding and degraded rivers.
Drought and desertification, increased fire risk.
Canopy removal in tropical rainforest reduces precipitation and evaporation. Cloud cover decreases. Surface temperatures rise.
Decrease in leaf litter -> soil degradation.
Loss of habitat, and habitat complexity.
Loss of biodiversity (plants, animals, pollinators, seed dispersers, soil organisms etc).
Invasion by non-native weedy plants & animals.
Increase, spread of plant and animal diseases.
Forest burning increases atmospheric CO2, enhancing greenhouse effect (temperature increase) – 20% of anthropogenic carbon loading.
Increased CO2 released to atmosphere from soil & decay of unburnt biomass.
Widespread loss of plant species erodes the foundations of agricultural productivity and threatens other plant-based products used by billions of people worldwide.
Many medicinal plants are at risk from over- harvesting and habitat destruction.
What are the global impacts of deforestation on the carbon cycle?
- 460-575 x 109 tonnes C held in tropical forests (plants + soil) - 180 tonnes acre^-1.
- Deforestation 1850-1990 released 122 x 109 tonnes C into atmosphere.
- Current release ~ 1.6 x 109 tonnes C yr^-1.
- Coal, oil, gas burning - ~ 6 x 109 tonnes C yr^-1.
What is desertification?
Definition (International Convention on Desertification): “Degradation of land in arid, semi-arid and sub-humid dry areas caused by climatic changes and human activities.”
• Involves loss of biological and economic productivity and complexity in croplands, pastures, and woodlands.
• 70% of the world’s drylands (excl. very arid deserts) - 3,600 million Ha - are degraded.
What are the causes of desertification?
- Climate variability.
- Unsustainable human activities e.g. over- cultivation, overgrazing, deforestation, and poor irrigation practices
- Increased fire frequency, soil salinity.
- Link between population and desertification not clear.
The primary causes of desertification: • Overgrazing. • Over-cultivation. • Increased fire frequency. • Water impoundment. • Deforestation. • Over-drafting of groundwater. • Increased soil salinity. • Global climate change.
What are the effects of desertification?
- Exposed, eroded topsoil blows away or is washed away by rain; nutrients lost; soil less productive.
- Dust storms.
- Increased evaporation and runoff.
- Vegetation damaged, roots exposed.
- Species lost (e.g. overgrazing) – impacts on biodiversity.
- Surface and ground-water depleted.
- Negative consequences for people affected (one-fifth of world population).
Global impacts:
• Carbon stored in vegetation in dry zones declines when vegetation disappears.
• C-rich soils common in the dry zones – destruction / loss of soils affects the C cycle, increases Greenhouse effect.
• Loss of biodiversity due to habitat destruction.
• Reduction of fresh water reserves can lead to salinisation.
How can desertification be solved?
The Sahel:
• Improve water supplies - large reservoirs? Deeper wells? e.g. Water Aid projects.
• Local small-scale schemes that conserve water.
eE.g. ‘Bunding’ in Burkino Faso – lines of stones placed along slopestrap water, retain topsoil.
• Sustainable farming (locally-made tools, not tractors).
• Intercropping – alternating annual crops with edible perennials.
• Tree planting schemes –> Reduce soil erosion.
• Use of drought-resistant seed.
E.g. millet (northern Nigeria).
Internationally:
• Use of GM cropshigher yields.
• International action to reduce the causes of global warming.
E.g. Kyoto Protocol.
What is salinisation?
- Dryland salinity - caused by rising water table.
• One of the greatest environmental threats in Australia (affects > 5.7 million ha) - results in gradual loss of farm & grazing land.
• Salt in soil built up over 1000s of years from weathering of rocks or sea (via rain or wind).
• Clearing native vegetation and replacing with shallow- rooted crops and pasture increases runoff, raises water table. Salt in soil dissolves and moves nearer surface. - Irrigation salinity – caused by over-irrigation/ inefficient water use – adds to water table rise.
What are the effects of salinisation?
• Current extent ~ 6 m Ha, but postulated increase to 12.5 m Ha at hydrological equilibrium in 20-50 yrs.
• Economics (estimate $270 million) - probably a conservative estimate.
- death of pastures and crops.
- loss of productive land area and agricultural production.
• Decline and death of native vegetation loss of habitats (terrestrial and wetland) and reduction in biodiversitylocal, regional or global extinction of species.
• Increased soil and wind erosion.
• Salinisation of lakes, rivers and streams, loss of water quality (drinking, irrigation).
