Summary

An ecosystem represents the integration of the biotic community and the abiotic environment. The capacity of the community to modify its environment depends on its structure and the degree to which it controls energy flow, biogeochemical cycling, and climatic conditions.

Ecosystem structure reflects the organization of various abiotic and biotic pools that exchange energy and matter. Abiotic pools are the atmosphere, oceans, and sediments that represent the sources of energy and matter for biotic use. Biotic pools are the various organisms (individuals, species populations, functional groups, or trophic levels) in the community. Autotrophs (or primary producers) are those organisms that can acquire resources from abiotic pools. Heterotrophs (or secondary producers) are those organisms that must acquire their resources from other organisms. Energy and matter storage in these pools can be represented as pyramids of productivity, numbers, or biomass.

Energy available to ecosystems comes primarily from solar radiation, captured and stored in carbohydrates by primary producers (autotrophs) through the process of photosynthesis. The total rate at which energy is captured (GPP) depends on exposure to sunlight, availability of water, and biomass. Some of the energy from gross primary production is expended through plant respiration. The remaining net primary production is stored as plant biomass and is the source of energy and matter for heterotrophs. Primary heterotrophs (herbivores) feed on autotrophs, whereas secondary heterotrophs (predators) feed on other hetero-trophs. Consumption transfers the energy stored in consumed biomass to the higher trophic level, with some lost as egestion and consumer respiration. Generally, <10% of the energy available at each trophic level is converted into biomass at the next higher trophic level, although predators generally have a higher efficiency of conversion than do herbivores. Energy remaining in organisms at the time of death becomes available to decomposers that release the remaining energy through respiration.

Energy is the currency with which organisms acquire and concentrate material resources necessary for growth and reproduction. Material resources are often available in limited supply, favoring mechanisms that facilitate retention and reuse within the ecosystem. Biogeochemical cycling represents the processes whereby material resources, including water, carbon, nitrogen, and mineral elements, are acquired from abiotic pools and recycled among trophic levels, with eventual return to abiotic pools. The efficiency with which these materials are recycled and conserved, rather than lost to abiotic pools, buffers an ecosystem against resource depletion and reduced productivity. Hence, ecosystems become organized in ways that maximize the capture and storage of resources among organisms. Resources egested or excreted during trophic transfers, as well as dead organisms, become available to decomposers that rapidly acquire and store the nutrients from organic matter. Nutrients released by decomposers become available for exchange among soil and aquatic organisms and for plant uptake. Microorganisms are particularly instrumental in making nitrogen available for plant uptake, with different specialists fixing atmospheric nitrogen as ammonia and converting ammonia to nitrate and organic nitrogen to ammonia. Volatilization by fire and denitrification by anaerobic bacteria complete the cycle by returning elemental nitrogen to the atmosphere.

Ecosystems also modify local and regional climatic conditions. The degree to which vegetation reduces soil warming, evaporation, erosion, and wind speed depends on density and vertical architecture. Insects and other organisms affect vegetation structure and hence canopy-atmosphere interactions. Tall, multi-canopied forests are most effective at modifying surface temperatures, relative humidities, and wind speed, thereby ameliorating local and regional fluctuations in temperature, wind speed, and precipitation.

Models have become important tools for synthesizing complex, and often incomplete, data for prediction of ecosystem responses to, and effects on, global environmental changes. Ecosystem models differ in structure and degree of simplification. Effects of insects on a variety of ecosystem parameters have been largely ignored in ecosystem models.

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