A leaf suspended in the surface film is a thing of simple beauty, poised to fulfill an essential role in the life of the river. When it was still attached to its branch, it supplied energy to the tree throughout spring and summer. In the fall, leaves falling straight to the ground in time will rot, releasing various elements and organic molecules to the recycling process, benefiting next year’s tree growth. By falling into the river, especially those of the freestone type, the leaf enters into another ecosystem, and now supplies a portion of the energy needed by countless macroinvertebrates. Without the input of leaves in the fall, stream insects could not complete their life cycles, and without these essential life forms, there of course would be far few trout. Leaves from all varieties of hardwood trees play a similar role, but each species rots at a different rate than all the others. The entire ensemble of forest leaves acts like a king-sized time-release capsule of energy when the shredders begin the process of energy flow by reducing them into tiny bits due to their own unique feeding mechanisms. It may take a total of over 200 days for the river’s assemblages of macroinvertebrates to completely consume all the leaves that fell into the river the year before.
The process of converting leaves to usable energy starts the moment they enter the water and become trapped under rocks, or collect in massive underwater leaf piles in quiet back eddies. It is at this point that leaves begin to give up their soluble nutrients through leaching. Soluble components such as sugars, plant pigments and other organic compounds provide an initial burst of food for in-stream microbes (i.e., bacteria and fungi), allowing them to multiply and colonize the surface of the leaf. Hyphomycetes (water molds) are early settlers on the leaf surface, followed by a succession of distinct but related fungal communities, which attach to the leaf’s surface and digest a portion of it. After several days to weeks, the leaf becomes palatable to macroinvertebrate communities (Figure 41) that are as fussy about what they eat as we are about preparing our burgers. A plain burger just won't do; we require special sauce, pickles, cheese, and the sesame seed bun, too.
Once the microbes have “conditioned” the leaf, shredder insects (second trophic level) begin their work in earnest of reducing the particle size of rotting leaves, now referred to as detritus. Fortunately, shredders are sloppy eaters, and a significant amount of processed material flows downstream. Filter feeding species, including most net-spinning caddis larvae and many burrowing mayfly species, now feast on this “mana” from up-stream (Figure 42). Each fecal pellet produced by shredders is composed of about 50% undigested detritus, contributing another source of energy to filter feeding communities downstream from their feeding stations. The third trophic level of the aquatic food web is composed of some species of insects, most notably stonefly nymphs, which feed on both the shredders and filterers. Finally, the trout (fourth trophic level) feed on all macroinvertebrates (see: Galleries: Macroinvertebrates). Scraper insects eat mostly algal growth on the undersurface of in-stream rocks (Figure 8). They, too, become fodder for the fourth trophic level feeders.
What happens when this exquisitely balanced food web is disturbed; in particular when trees are removed from the stream bank? Taking leaves away from the river is analogous to siphoning fuel from a car. No one expects to drive as far afterward, so what happens to the river’s bio-productivity when its energy source is depleted by over-foresting? The answer is simple; the river gets less “mileage” out of its “gallon of gas”.
Remediation can be as simple as planting trees where they once stood tall along the banks. Granted, this is a long-term solution. It is one that needs to begin with the concept that maintaining the intactness of a river’s ecosystem is everyone’s responsibility. If adopted as an environmental ethic for our generation, we have the rare opportunity to leave something truly extraordinary behind for the next one to appreciate and care for. So, plant a tree and save a trout!