Download How are the borders of an ecosystem defined

Name________________________
Date__________
How are the borders of an ecosystem defined?
The borders of an ecosystem may be well defined or they may be gradual and
difficult to delineate. Freshwater streams are usually considered well defined. As such
they are often studied separately from surrounding ecosystems by researchers with
different training and using different methods. Research on streams in southeast Alaska,
in which salmon spawn, has raised questions as to the practice of studying aquatic and
terrestrial ecosystems separately.
Salmon are anadromous fish, fish that come from the ocean to spawn in
freshwater streams. Salmon are born in freshwater streams; they then migrate to the
oceans, where most of their growth occurs. They return to their home streams, spawn
and die. In a way, salmon are transporting resources from the oceans to the freshwater
ecosystems. Just by their sheer large numbers they make significant contributions to the
organic and mineral contents of the freshwater streams.
Salmon have a high lipid content compared to other fish and are a good source of
energy for animals that prey upon them. Also their decay adds nitrogen, phosphorus,
carbon, and other inorganic elements to the freshwater. The salmon carcasses provide
nourishment for algae, fungi, and bacteria. Invertebrates and other fish feed on decaying
bits of fish. Bears and other carnivores eat both live and dead salmon during their
upstream migration. Nutrients derived from salmon pass into the soil and vegetation
surrounding the streams.
Spawning fish have higher proportions of heavy isotopes of nitrogen and carbon
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( N and 13 C). These can be used to trace the relative contributions of salmon and other
anadromous fish to the carbon and nitrogen content of organisms in food webs. One
study found salmon contributed 10.9% of the nitrogen found in invertebrate predators and
17.5% in the foliage of riparian plants. While it is not surprising to find that aquatic
invertebrates which feed on salmon eggs and juveniles, with large amounts of nitrogen
from the salmon, researchers were surprised at the high levels of nitrogen in the stream
side vegetation. When terrestrial animals feed on salmon, their feces and any uneaten
salmon carcasses decay and add nutrients to the surrounding soil, where they are taken up
through the roots of plants.
In southeast Alaska, over 40 species of mammals and birds feed on salmon.
Salmon migrations attract large numbers of predators to streams and lakes. Salmon and
other anadromous fish thus appear to link the ocean, freshwater, and land ecosystems to
an extent that is only beginning to be appreciated.
APES
1
Mr. Clark
Bethpage HS
Name________________________
Date__________
Critical Thinking Questions:
1. Given the intricate connections between aquatic and terrestrial ecosystems along
salmon streams, how should we define the boundaries of the ecosystems?
2. When more adult salmon reach the spawning grounds than are needed to maintain
the population, some are considered excess. How might the research described
here affect that view?
3. Some wildlife biologists consider salmon a keystone species. Given what you
know about keystone species, would argue for or against this designation.
4. In recent years, the numbers of anadromous fish along the Pacific Coast of North
America have declined precipitously because of overfishing and habitat
destruction. What effects would you predict this might have on the ecology of
freshwater streams and their adjoining land areas?
5. What types of wildlife management decisions about fish, wildlife, and forests
would follow as a result of recognizing the connections between aquatic and
terrestrial ecosystems.
APES
2
Mr. Clark
Bethpage HS