Energy Considerations: The Flow Of Life Through
The River
Macro Invertebrates
Macro
invertebrates, particularly insects, are vital to the production
of trout, creating the opportunity for the flow of energy from
plants to insects, and then to the fish. The mayflies
and caddis
flies are two of the numerous examples of macro invertebrates
that carry out most of their lives under the rocks and sand of
most freestone rivers, and in the vegetation within limestone
streams. Together with the stoneflies,
they constitute the majority of macro invertebrates common to
most coldwater rivers throughout the world. Immature forms of
other groups of invertebrates - various diptera
(e.g., deer fly and black fly larvae), aquatic beetles, snails
and crayfish - live there, too.
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Many of these species function as shredders,
reducing the size of plant material (e.g., leaves, grasses, branches,
trees; referred to as crude particulate matter or CPOM) into pieces
(fine particulate matter or FPOM) suitable for filter feeding
communities to consume. While most macro invertebrates prefer
detritus
for their dinner, a few species of stoneflies and caddis flies
eat the nymphal and larval forms of many species of aquatic insects,
thus diverting the flow of energy away from the trout. Taken as
a whole, these diverse invertebrate groups interact to form
food webs in every way as complex and inter-dependent as those
found in coral reefs or rain forests.
When the nymphal and larval forms mature and transform into winged
adults, the energy they represent leaves the river and enters
the adjacent stream-side ecotones,
supplying food for frogs, birds, insects, spiders and other insectivorous
predators. Adult female insects that survive the onslaught of
that gantlet give back some of the lost nutrients to the river
when they return to it to lay their eggs. The females of most
species of mayflies, for example, float downstream after depositing
their eggs on the water and become an easy meal for a fish. The
eggs they have laid sink to the bottom, separate, and then stick
to the submerged rocks, maturing through their nymphal stages
over the following year, completing their life cycle. But in fact,
even some egg masses are eaten by fish, as well.
A trout stream’s bio-productivity
(usually expressed as an annual amount of kilograms of trout
per kilometer of river) depends heavily on a robust production
of in-stream macro invertebrates. As already emphasized, the amount
of invertebrate life produced in rivers that run through hardwood
forests depends diectly upon the abundance of leaves and related
plant material that falls into the river throughout the year.
Many things detract from the production of macro
invertebrates - deforestation, pesticides, heavy
metals, siltation,
stream channelization,
and thermal
pollution - disconnecting the underwater life forms from their
food webs. Without the smooth flow of energy upward through
the four trophic
levels, the number and size of the trout that survive will
be far less than before the disturbance. These insults can largely
be reversed by initiation of good ecosystem
management practices, or can be prevented altogether. Rivers
and streams cleanse themselves if returned to a reasonable level
of intactness. All that is needed is the desire, and sometimes
the money, to do so.
Millions of people depend upon a constant source
of clean water the origin of which, in many cases, is a dammed
up portion of a trout stream. This underscores the importance
for maintaining them as functional habitats. When aquatic ecosystems
break down, it threatens all who depend upon it. If the macro
invertebrates and trout disappear because of adverse changes in
the environment, then we not only lose a natural resource, we
eventually may lose ourselves in the bargain. We are all connected.
That is why the expression “We all live downstream”
rings so true.
