Parts of an Ecosystem
This diagram presents a simplified community
of interacting organisms, known as an ecosystem. Decomposers, producers, and
consumers are connected to one another according to the food they provide or
the food they eat. Click on the labels to learn about the parts of an
ecosystem.
Food
Web, set of interconnected food chains by which energy and
materials circulate within an ecosystem (see Ecology). The food web is
divided into two broad categories: the grazing web, which typically begins with
green plants, algae, or photosynthesizing plankton, and the detrital web, which
begins with organic debris. These webs are made up of individual food chains.
In a grazing web, materials typically pass from plants to plant eaters
(herbivores) to flesh eaters (carnivores). In a detrital web, materials pass
from plant and animal matter to bacteria and fungi (decomposers), then to
detrital feeders (detritivores), and then to their predators (carnivores).
Generally, many interconnections
exist within food webs. For example, the fungi that decompose matter in a
detrital web may sprout mushrooms that are consumed by squirrels, mice, and
deer in a grazing web. Robins are omnivores, that is, consumers of both plants
and animals, and thus are in both detrital and grazing webs. Robins typically
feed on earthworms, which are detritivores that feed upon decaying leaves.
TROPHIC LEVELS
|
Marine Food Pyramid
The food web can be viewed
not only as a network of chains but also as a series of trophic (nutritional)
levels. Green plants, the primary producers of food in most terrestrial food
webs, belong to the first trophic level. Herbivores, consumers of green plants,
belong to the second trophic level. Carnivores, predators feeding upon the
herbivores, belong to the third. Omnivores, consumers of both plants and
animals, belong to the second and third. Secondary carnivores, which are
predators that feed on predators, belong to the fourth trophic level. As the
trophic levels rise, the predators become fewer, larger, fiercer, and more
agile. At the second and higher levels, decomposers of the available materials
function as herbivores or carnivores depending on whether their food is plant
or animal material.
ENERGY FLOW
|
Through these series of
steps of eating and being eaten, energy flows from one trophic level to
another. Green plants or other photosynthesizing organisms use light energy
from the sun to manufacture carbohydrates for their own needs. Most of this
chemical energy is processed in metabolism and dissipated as heat in
respiration. Plants convert the remaining energy to biomass, both above ground
as woody and herbaceous tissue and below ground as roots. Ultimately, this
material, which is stored energy, is transferred to the second trophic level,
which comprises grazing herbivores, decomposers, and detrital feeders. Most of
the energy assimilated at the second trophic level is again lost as heat in
respiration; a fraction becomes new biomass. Organisms in each trophic level
pass on as biomass much less energy than they receive. Thus, the more steps
between producer and final consumer, the less energy remains available. Seldom
are there more than four links, or five levels, in a food web. Eventually, all
energy flowing through the trophic levels is dissipated as heat. The process
whereby energy loses its capacity to do work is called entropy.
