Global processes  (Page 3/9)

Different vegetation types and topographies have varying surface roughness    , which change the flow of winds in the lower atmosphere and inturn influences climate. Lower surface roughness also tends to reduce surface moisture and increase evaporation. Farmersapply this knowledge when they plant trees to create windbreaks ( Johnson et al . 2003 ). Windbreaks reduce wind speed and change the microclimate, increase surface roughness, reducesoil erosion, and modify temperature and humidity. For many field crops, windbreaks increase yields and productionefficiency. They also minimize stress on livestock from cold winds.

Soil and water conservation

Biodiversity is also important for global soil and water protection. Terrestrial vegetation in forestsand other upland habitats maintain water quality and quantity, and controls soil erosion.

In watersheds    where vegetation has been removed, flooding prevails in the wet season and drought in thedry season. Soil erosion is also more intense and rapid, causing a double effect: removing nutrient-rich topsoil and leading tosiltation in downstream riverine and ultimately oceanic environments. This siltation harms riverine and coastalfisheries as well as damaging coral reefs ( Turner and Rabalais 1994 ; van Katwijk et al . 1993 ).

One of the most productive ecosystems on earth, wetlands    have water present at or near the surface of the soil or within the root zone, allyear or for a period of time during the year, and the vegetation there is adapted to these conditions. Wetlands are instrumentalfor the maintenance of clean water and erosion control. Microbes and plants in wetlands absorb nutrients and in the processfilter and purify water of pollutants before they can enter coastal or other aquatic ecosystems.

Wetlands also reduce flood, wave, and wind damage. They retard the flow of floodwaters and accumulatesediments that would otherwise be carried downstream or into coastal areas. Wetlands also serve as breeding grounds andnurseries for fish and support thousands of bird and other animal species.

Nutrient cycling

Nutrient cycling is yet another critical service provided by biodiversity -- particularly bymicroorganisms. Fungi and other microorganisms in soil help break down dead plants and animals, eventually converting thisorganic matter into nutrients that enrich the soil ( Pimentel et al . 1995 ).

Nitrogen is essential for plant growth, and an insufficient quantity of it limits plant production in bothnatural and agricultural ecosystems. While nitrogen is abundant in the atmosphere, only a few organisms (commonly known asnitrogen-fixing bacteria) can use it in this form. Nitrogen-fixing bacteria extract nitrogen from the air,and transform it into ammonia, then other bacteria further break down this ammonia into nitrogenous compounds that can beabsorbed and used by most plants. In addition to their role in decomposition and hence nutrient cycling, microorganisms alsohelp detoxify waste, changing waste products into forms less harmful to humans.