The role of a species in the community, like the job and social
function of a person, is its _niche_. Similar species of animals
have different niches, thus lessening competition for food and
living space. Thrushes hunt close to the ground; vireos and kinglets
hunt among the branches; flycatchers snap up airborne insects. The
flicker feeds upon insects, excavates nesting holes that are later
occupied by other species such as squirrels and owls, and is preyed
upon by the great horned owl; its niche is _insect exterminator /
food for carnivores / homebuilder_. The great horned owl, hunting
mammals, birds, and reptiles by night, preys on species different
from those hunted by the goshawk, and thus occupies a parallel
niche. When it dies, its remains, like those of other animals, are
decomposed and return to the soil.]
Canopy
Great Horned Owl
Yellow-bellied Sapsucker
Understory
Flying Squirrel
Shrub Layer
Ruffed Grouse
Herb Layer
Red Squirrel
Western Toad
Forest Floor
Shorttail Weasel
Scavenging Insects
Deer Mouse
Garter Snake
Soil Layer
Ground Squirrel
Earthworm
Masked Shrew
[Illustration: Sun, Green Plants, and Animals
The sun is the source of energy for any plant-and-animal community.
Green plants draw nitrogen and minerals from the soil, and in a
process called photosynthesis use sunlight to convert raw materials
(carbon dioxide and water) into carbohydrates (sugar, starch,
cellulose), giving off oxygen as a by-product. Besides burning
oxygen, animals depend on plants for food.
Green Plants, trees and shrubs, grasses and sedges, wildflowers,
ferns, mosses, algae and lichens—are fed upon by animals, which are
unable to manufacture their own food.
The Redback Vole, like other rodents, pikas and hares, seed-eating
birds, grazing and browsing hoofed animals, and herbivorous insects,
derives its energy from the seeds and other parts of green plants
that it eats.
The Garter Snake, feeding upon the vole, is dependent upon plants
even though it does not eat them.
The Great Horned Owl, preying upon the garter snake, is one more
step removed from the green plants—but still dependent on them.
Scavengers such as carrion beetles feed upon the carcass of the owl;
the remains are then attacked by Decomposers, primarily bacteria,
that break down the animal tissues into basic organic compounds.
The Soil, enriched by the minerals and carbon and nitrogen compounds
added to it by the decomposers (and by other processes such as fire)
supports new green plant growth.
Thus energy derived from the sun flows through the ecosystem in a
food chain. A plant-and-animal community is a complex, interlocking
web of such food chains.]
Sun
Green Plants
Redback Vole
Garter Snake
Great Horned Owl
Scavengers, Decomposers
Soil
[Illustration: A Pyramid of Numbers
Necessarily, the number of plants in an ecosystem far exceeds the
number of plant eaters, and the number of prey species must exceed
the number of predators. During its lifetime, a golden eagle will
consume a vast number of lesser animals. The combined mass of prey
animals necessary to sustain an eagle greatly outweighs the eagle
itself. Ecologists refer to this proportional relationship of mass
between each link in the food chain as the _pyramid of numbers_.
The diagram represents a numbers pyramid for the alpine zone.
Because of its limiting environment, the alpine zone supports a
lesser plant mass than the forest zone. As a result, the carrying
capacity of the alpine is less than that of the forest.