Macroinvertebrates, particularly insects, are of vital importance to the production of trout, and thus to the flow of energy of the trout stream. The mayflies and caddis flies are two of the numerous examples of macroinvertebrates 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 macroinvertebrates common to most cold water rivers throughout the world. Immature forms of other groups of invertebrates - various diptera (e.g., deer fly and blackfly larvae), aquatic beetles, snails and crayfish - live there, too.
Many of these species function as shredders, reducing the size of plant material (e.g., leaves, grasses, branches, trees; referred as crude particulate matter or CPOM) into pieces (fine particulate matter or FPOM) suitable for consumption by filter-feeding communities. While most macroinvertebrates prefer detritus for their source of energy, 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 rainforests.
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 gauntlet 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 immediately sink to the bottom and stick to the submerged rocks, maturing through their nymphal stages over the following year, thus completing their life cycle. But in fact, even some egg masses are eaten by fish, as well.
A trout stream's productivity (usually expressed as the annual amount of kilograms of trout per kilometer of river) depends heavily on a robust production of in-stream macroinvertebrates. Many things can detract from the production of macroinvertebrates - deforestation, pesticides, heavy metals, siltation, stream channelization (Figure 44), 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 took its toll. 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 macroinvertebrates 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