Ecology is the study of how organisms interact with their environment.
The environment consists of biotic and abiotic components that are interrelated in an ecosystem. All ecosystems must have a constant input of energy from the Sun. Producer organisms are capable of trapping the Sun's
energy and converting it into biomass. Herbivores feed on producers and
are in turn eaten by carnivores, which may be eaten by other carnivores.
Each level in the food chain is known as a trophic level. Other kinds of organisms involved in food chains are omnivores, which eat both plant and
animal food, and decomposers, which break down dead organic matter
and waste products. All ecosystems have a large producer base with successively smaller amounts of energy at the herbivore, primary carnivore,
and secondary carnivore trophic levels. This is because each time energy
passes from one trophic level to the next, about 90 percent of the energy is
lost from the ecosystem. A community consists of the interacting populations of organisms in an area. The organisms are interrelated in many ways
in food chains that interlock to create food webs. Because of this interlocking,
changes in one part of the community can have effects elsewhere.
Major land-based regional ecosystems are known as biomes. The
temperate deciduous forest, northern coniferous forest, temperate rainforest,
tropical rainforest, desert, savanna, and tundra are examples of biomes.
Aquatic communities can be divided into marine, freshwater, and estuarine
communities.
Each organism in a community occupies a specific space known as
its habitat and has a specific functional role to play known as its niche. An
organism's habitat is usually described in terms of some conspicuous element
of its surroundings. The niche is difficult to describe because it involves so many interactions with the physical environment and other living things.Interactions between organisms fit into several categories. Predation
is one organism benefiting (predator) at the expense of the organism
killed and eaten (prey). Parasitism is one organism benefiting (parasite)
by living in or on another organism (host) and deriving nourishment
from it. Commensal relationships exist when one organism is helped but
the other is not affected. Mutualistic relationships benefit both organisms.
Competition causes harm to both of the organisms involved, although
one may be harmed more than the other and may become extinct, evolve into a different niche, or be forced to migrate.
Many atoms are cycled through ecosystems. The carbon atoms of living
things are trapped by photosynthesis, passed from organism to organism
as food, and released to the atmosphere by respiration. Nitrogen
originates in the atmosphere, is trapped by nitrogen-fixing bacteria,
passes through a series of organisms, and is ultimately released to the atmosphere
by denitrifying bacteria. Phosphorus originates in rock and is used by organisms and eventually deposited as sediments.
A population is a group of organisms of the same species in a particular
place at a particular time. Populations differ from one another in
gene frequency, age distribution, sex ratio, and population density. A typical
population growth curve consists of a lag phase in which population
rises very slowly, followed by an exponential growth phase in which the
population increases at an accelerating rate, followed by a leveling off of
the population in a stable equilibrium phase as the carrying capacity of the
environment is reached.
Humans as a species have the same limits and influences that other organisms do. However, humans can reason and predict, thus offering the possibility of population control through conscious population limitation. Human activities have had many impacts on the global ecosystem. These include: habitat destruction, release of pollutants, climate change,
and reduced biodiversity.
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