A2GEOGRAPHYREVISIONTROPICAL ENVIRONMENTS7.2 TROPICALECOSYSTEMS

KEYTERMSANDDEFINITIONSOxisols are infertile, acidic, deeply weathered soils, which contain clays of iron and aluminium.Ultisols are deeply weathered tropical and subtropical soils with clay accumulations in the B horizon.Vertisols are clay soils which expand when wet and crack when dry. They develop in climates with marked wet and dry seasons.Soil catena refers to the changes in soil characteristics down a slope.Succession is the spatial and temporal changes in a plant community as they move towards a seral climax.Climax community is the group of species that are best suited to exploit the prevailing environmental conditions.

KEYTERMSANDDEFINITIONSBiome is a large-scale global ecosystem such as the tropical rain forest or savanna grassland.Plagioclimax is a plant community affected by human activity.Ferralitic refers to leached soils containing iron and aluminium.Xerophytic refers to plants adapted to drought.Pyrophytic refers to plants adapted to fire.Trophic levels are feeding levels in an ecosystem such as producers (plants), primary consumers (herbivores), secondary consumers (carnivores), and so on.

KEYTERMSANDDEFINITIONSGerschmel diagrams illustrate the relative proportion of nutrients in stores (biomass, litter and soil) and transfers between them and to and from other sources.

KEYTERMSANDDEFINITIONSNutrient cycling is the transfer of nutrients from one part of an ecosystem to another. Trees, for example, take up nutrients such as nitrogen from the soil through their root systems and store them in leaves. When the leaves fall they are decomposed by bacteria and the nutrients are released back into the soil where they become available for root uptake again.

KEYTERMSANDDEFINITIONSSavannas are grassland regions with a light to dense scattering of drought-resistant trees that occur in the tropics. They have seasonal climates, with the length of the dry period generally increasing with latitude.

TOPICSUMMARYSuccession refers to the spatial and temporal changes in plant communities as they move towards a seral climax.

● A plagioclimax refers to a plant community permanently influenced by human activity.● Volcanic activity has destroyed much of the tropical rainforest in Montserrat.● The tropical rainforest is the most diverse ecosystem or biome in the world, yet it is also the most fragile.

TOPICSUMMARYThe net primary productivity (NPP) of this ecosystem is 2,200 g/m2/yr compared with 900 g/m2/yr for the savanna.

● The hot, humid climate of tropical rainforests give ideal conditions for plant growth and there are no real seasonal changes.● The soils of these areas are usually heavily leached and ferralitic, with accumulations of residual insoluble minerals containing iron, aluminium and manganese.● Rainforest soils are inherently infertile.● Tropical rainforests are disappearing at an alarming rate and ‘Green Jungles’ are being changed into to ‘Red Deserts’.

TOPICSUMMARYForest clearance in the tropics will continue in order to satisfy the demands of the growing population.

● All nutrient cycles involve interaction between soil and the atmosphere and involve many food chains.● Savannas are areas of tropical grasslands that can occur with or without trees and shrub.● Savannas cover about one-quarter of the world’s land surface and are found between the tropical rain forests and the subtropical high-pressure belts that produce the world’s great deserts.● The climate that characterises savanna areas is a tropical seasonally humid climate.● The link between climate and soil could hardly be closer.

TOPICSUMMARYThere is also a relation between soil and vegetation – on the thicker clay-based soils there is frequently woodland whereas on the leached sandy soils, with poor water retention, grassland predominates.

● Savanna vegetation is xerophytic (adapted to drought), and therefore adapted to the savanna’s dry season and pyrophytic (adapted to fire).● The nutrient cycle also illustrates the relationship between climate, soils and vegetation.

ADDITIONALWORK1. Describe the process of succession. Comment on the succession on Krakatoa following the eruption of the volcano.2. Compare and contrast the nutrient cycle of tropical rainforests with that of savannas.3. Outline the characteristics of vegetation in a the tropical rainforest and b the savanna. In what ways are they adapted to their environment?4. Comment on the role of human activities in modifying the ecosystem of either tropical rainforests or savannas.5. Describe and explain the main characteristics of soils in rainforests and in the savanna.6. Describe and explain the main characteristics of savanna ecosystems.7. Compare the impact of human activities on the nutrient cycles of rainforests or savanna ecosystems.

SUGGESTEDWEBSITEShttp://postconflict.unep.ch/sudanreport/sudan_website/doccatcher/data/documents/Lateritic%20Soils%20in%20Distinct%20Tropical%20Environments.pdf for lateritic soils in distinct tropical environments.www.terc.csiro.au/research.asp for the CSIRO Tropical Ecosystems Research Centre.http://catdir.loc.gov/catdir/samples/cam032/99047853.pdf for a chapter from Tropical Ecosystems and Ecological Concepts.www.eurekalert.org/images/release_graphics/pdf/Posa.pdf for an article entitled ‘Hope for threatened tropical biodiversity: Lessons from the Philippines’.www.globalcanopy.org for the Global Canopy Programme for tropical rainforests.