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INHABITANTS OF ECOSYSTEMS
THE INHABITANTS OF ECOSYSTEMS
Ecosystems often
contain an astonishing assortment of organisms that interact with each
other and are interdependent in a variety of ways. Consider for a
moment a salt marsh in the Chesapeake Bay on the east coast of the
United States. This hay is the world's richest estuary (a semi-enclosed
body of water where fresh water drains into the ocean). Biological
diversity and productivity abound wherever fresh water and salt water
form a salinity gradient (a gradual change from unsalted fresh water to
.salty ocean water), as they do in the Chesapeake Bay. The salinity
gradient in the bay results in three distinct marsh communities:
freshwater marshes at the head of the bay, brackish (moderately salty)
marshes in the middle bay region, and salt marshes on the ocean side of
the bay. Each community has its own characteristic organisms. Sail to
one of the salt marsh islands in the Chesapeake Bay, such as South Marsh
Island, and you can explore a salt marsh community that is relatively
unaffected by humans. A salt marsh presents a monotonous view—miles and
miles of flooded meadows of cordgrass (Spartina). High salinity
(although not as high as that of ocean
water) and twice-daily tidal inundations create a challenging
environment to which only a few plants have adapted.
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Nutrients
are such as nitrates and phosphates, rapid growth of both cordgrass
and microscopic algae. These organisms are eaten directly by some
animals, and when they die, their remains (called detritus) provide
food for many inhabitants of both the salt marsh and the bay.
A casual
visitor to a salt marsh would observe two different types of animal
life, insects and birds. Insect pests such as salt marsh mosquitoes
and horseflies number in the millions. Birds nesting in the salt
marsh include seaside sparrows, laughing gulls, and clapper rails.
Migratory birds spend time in the salt marsh as well.
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Study the salt
marsh carefully and you'll find it has numerous other species. Large
numbers of invertebrates seek refuge in the water surrounding the
cordgrass. Here they eat, hide to avoid being eaten, and reproduce. Many
of them gather in the inter-tidal zone because food (detritus, algae,
protozoa, and worms) is abundant there. A variety of crusta
ceans live in
the sale marsh. The marsh crab, for
grass and small animals as well as detritus. Mollusks include the marsh
periwinkle, a snail that moves along the cordgrass skimming off attached
algae for its food. Marsh periwinkles climb up the cordgrass to avoid
becoming prey for larger marsh animals such as terrapins.
Almost no
amphibians inhabit salt marshes, because the salty water dries out their
skin, but a few reptiles have adapted—the northern diamond-back
terrapin, for example. It spends its time basking in the sun or
swimming in the water searching for food—snails, crabs, worms, insects,
and fish. Although a variety of snakes abound in the dry areas adjacent
to salt marshes, only the northern water snake (which preys on fish) is
adapted to brackish water-
Mammals are
represented in the salt marsh by the meadow vole, a small rodent that
constructs its nest of cordgrass on the ground above the high-tide
scamper about the salt marsh day and night.
Their diet consists mainly of insects and cordgrass.
The Chesapeake
Bay marshes are an important trout,
Atlantic croaker, striped bass, and bluefish, to name just a few. Other
fish, such as bay anchovies, bull minnows, and tidewater silversides,
never leave the estuary, spending their summers in the salt marsh
shallows and their winters burrowed in the mud or swimming in the deeper
waters of the bay.
Add to all these
visible plant and animal organisms the unseen microscopic world of the
salt
Protozoa, fungi,
and bacteria, and you can begin to appreciate the complexity of a salt
marsh community
Ecosystems such
as the Chesapeake Bay salt marsh teem with life. Where do these
organisms get the energy to live? And how do they harness this energy?
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