ВУЗ:
Составители:
Рубрика:
tained in their tissues, after passing through the decomposer food web, are ultimately released by bacterial and
fungal decomposition, a process that reduces complex organic compounds into simple inorganic compounds
available for reuse by plants.
Within an ecosystem, nutrients are cycled internally. But there are leakages or outputs, and these must be
balanced by inputs, or the ecosystem will fail to function. Nutrient inputs to the system come from weathering
of rocks, from windblown dust, and from precipitation, which can carry material great distances. Varying quan-
tities of nutrients are carried from terrestrial ecosystems by the movement of water and deposited in aquatic
ecosystems and associated lowlands. Erosion and the harvesting of timber and crops remove considerable quan-
tities of nutrients that must be replaced. The failure to do so results in an impoverishment of the ecosystem.
This is why agricultural lands must be fertilized.
If inputs of any nutrient greatly exceed outputs, the nutrient cycle in the ecosystem becomes stressed or
overloaded, resulting in pollution. Pollution can be considered an input of nutrients exceeding the capability
of the ecosystem to process them. Nutrients eroded and leached from agricultural lands, along with sewage
and industrial wastes accumulated from urban areas, all drain into streams, rivers, lakes, and estuaries. These
pollutants destroy plants and animals that cannot tolerate their presence or the changed environmental condi-
tions caused by them; at the same time, they favor a few organisms more tolerant to changed conditions.
Thus, precipitation filled with sulfur dioxide and oxides of nitrogen from industrial areas converts to weak
sulfuric and nitric acids, known as acid rain, and falls on large areas of terrestrial and aquatic ecosystems.
This upsets acid-base relations in some ecosystems, killing fish and aquatic invertebrates, and increasing soil
acidity, which reduces forest growth in northern and other ecosystems that lack limestone to neutralize the
acid.
The functional units of an ecosystem are the populations of organisms through which energy and nutrients
move. A population is a group of interbreeding organisms of the same kind living in the same place at the same
time. Groups of populations within an ecosystem interact in various ways. These interdependent populations of
plants and animals make up the community, which encompasses the biotic portion of the ecosystem.
The community has certain attributes, among them dominance and species diversity. Dominance results
when one or several species control the environmental conditions that influence associated species. In a forest,
for example, the dominant species may be one or more species of trees, such as oak or spruce; in a marine
community, the dominant organisms frequently are animals such as mussels or oysters. Dominance can influ-
ence diversity of species in a community because diversity involves not only the number of species in a com-
munity, but also how numbers of individual species are apportioned.
The physical nature of a community is evidenced by layering, or stratification. In terrestrial communities,
stratification is influenced by the growth form of the plants. Simple communities such as grasslands, with little
vertical stratification, usually consist of two layers, the ground layer and the herbaceous layer. A forest has up
to six layers: ground, herbaceous, low shrub, low tree and high shrub, lower canopy, and upper canopy. These
strata influence the physical environment and diversity of habitats for wildlife. Vertical stratification of life in
aquatic communities, by contrast, is influenced mostly by physical conditions: depth, light, temperature, pres-
sure, salinity, oxygen, and carbon dioxide.
The community provides the habitat – the place where particular plants or animals live. Within the habitat,
organisms occupy different niches. A niche is the functional role of a species in a community – that is, its occu-
pation, or how it earns its living. For example, the scarlet tanager lives in a deciduous forest habitat. Its niche,
in part, is gleaning insects from the canopy foliage. The more a community is stratified, the more finely the
habitat is divided into additional niches.
2. ENVIRONMENT
Environment comprises all of the external factors affecting an organism. These factors may be other living
organisms (biotic factors) or nonliving variables (abiotic factors), such as temperature, rainfall, day length,
wind, and ocean currents. The interactions of organisms with biotic and abiotic factors form an ecosystem.
Even minute changes in any one factor in an ecosystem can influence whether or not a particular plant or ani-
mal species will be successful in its environment.
Organisms and their environment constantly interact, and both are changed by this interaction. Like all
other living creatures, humans have clearly changed their environment, but they have done so generally on a
Страницы
- « первая
- ‹ предыдущая
- …
- 75
- 76
- 77
- 78
- 79
- …
- следующая ›
- последняя »
