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Who Eats What In a Stream

Who is eating what in a stream is an enormously complex question: it is probably nearly impossible to make a completely accurate foodweb showing all possible relationships, because:

     1) the animals you'll see change depending on the region of the country you're in;
     2) what each animal eats changes as it grows larger;
     3) many animals in a stream have adapted to eat many different kinds of food;
     4) the different genera within each order of insects have adapted to eat different kinds of food.

The foodweb below, therefore, should be viewed as just one blurry look at a foodweb. It describes a small river in the Pacific Northwest of the United States. Although you can see that is quite complex, still a number of animals have been left out of it.

PNW foodweb

In foodwebs like this one, an arrow is drawn from "dinner" to the "diner." That is, whoever the arrow is pointing at is eating the animal or plant that is pointing the arrow. So you can see, plants, fungus, and bacteria are on the bottom of the web, and the "top predators," whom no one else eats, are at the top (like bears and bald eagles).

Here is another very simplified foodweb, this one of a medium-sized Southwestern U.S. river.

SW foodweb

The term aufwuchs (pronounce: OWF-vooks) that you see in the above diagram is used to describe the fuzzy, sort of furry-looking, slimy green coating that you see on objects like plant stems below water. It consists not only of algae like Chlorophyta, but also diatoms, protozoans, bacteria, and fungi. An animal may be so tiny it can only graze on aufwuchs--but it contains plenty of protein and other nutrients.

Even here, we leave out an important consumer: the scavenger! When a bear dies, it may be picked apart by vultures. When the vulture dies, it is eaten by insects, fungus, and bacteria, and eventually the ultra-processed "nutrients" may be washed back into the stream, for other life-forms to use. Some stream residents are scavengers as well, including catfish and crayfish. Other important parts of the foodweb are feces--the waste products of other animals--as well as scales that have been shed by fish, and the body parts, exuviae (outer shell, or exoskeleton, that is shed by an insect nymph during metamorphosis), and pupa cases of insects. Finally, parasites infiltrate the foodweb at every level.

There is so much diversity in eating habits within the orders of immature insects, like mayflies, caddisflies, stoneflies, and true flies, that it makes more sense to lump them into guilds. One particular group of insects, with representatives from several different orders--or maybe all of them!--will be called Scrapers, for example. These insects all have mouthparts that allow them to scrape algae and diatoms off of rocks and wood, and whether they are mayflies or true flies, we'll still call them Scrapers. In the foodweb above, you'll see that some of the words are in boldface type, with asterisks: Scrapers, Predators, Collectors, Shredders, and Filterers. These are some of the guilds to which we might assign insect larvae and nymphs.

A lot of the food that is eaten grows right in the stream, like algae, diatoms, nymphs and larvae, and fish. This food that originates from within the stream is called autochthonous (ah-TAHK-the-nuss). Most food in a stream, however, comes from outside the stream. Leaves fall from bushes and trees. Worms drown in floods and get washed in. Leafhoppers and caterpillars fall from trees. Adult mayflies and other insects mate above the stream, lay their eggs in it, and then die in it. All of this food from outside the stream is called allochthonous (al-AHK-the-nuss).

Insects that have fallen in are ready-to-eat, and may join exuviae, copepods, dead and dying animals, rotifers, bacteria, and dislodged algae and immature insects in their float downstream to a waiting hungry mouth. This swarm of edible foodstuffs that travels downstream is an important source of food to trout and juvenile salmon, and is called (reasonably enough) drift.

Leaves that fall in are not ready to eat. They must be processed. A host of microorganisms takes over, covering each leaf with a slimy coating, and these begin the process of decay. The tiny organisms include bacteria, fungi (especially Hyphomycetes), and protozoa. They essentially cause the leaf to break down, to decay into smaller and more easily digested fragments. A crayfish comes along, and eats the leaves: it is not looking for plant material, but rather the organisms that coat the leaves. It digests its food, excretes it, and the leaf floats on downstream in the form of smaller particles. A big stonefly may get the next particle and shred it (we would call this stonefly a Shredder). It gets what it wants and sends even smaller particles downstream, to a waiting mayfly, who collects it (we call the mayfly a Collector), and sends even tinier fragments downstream. These tiny fragments may be filtered out of the water by a true fly larva, and we'll call that larva a Filterer.

Conveniently, we have names for these different sizes of food particles: The big pieces that have to be shredded are CPOM (coarse particulate organic matter). The tiny particles that can be collected or filtered are FPOM (fine particulate organic matter).

So now we have a convenient way to describe the insect part of the food web:

Reading listTo learn more about stream ecology, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).

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Resource Partitioning

When a group of animals that appear to be very alike have special ways that allow them to use resources that their compatriots can't, it gives them an advantage. If there are too many of your kind eating algae and you can't get enough of it because of that, you might just decide to filter whatever you can out of the water--or you could eat your compatriots! To filter water, you might grow long hairs on your head to use as a net, or you might even learn to spin nets from silk. To eat your compatriots, you might try developing some viciously sharp mouthparts. In orders of stream insects, it is common to see different families within each order, and even different genera within each family, eating entirely different kinds of food--or even the same food, but from different locations. This is a kind of survival strategy, called resource partitioning.

Salmon as Nutrients

Pacific salmon leave the ocean and migrate up rivers and streams until they get to shallow, gravelly areas, and extrude and fertilize their eggs there. Most of them die almost immediately after accomplishing this; it's a very long and hard journey up a river. They must climb fish ladders, if there are dams in the way. They must leap high obstacles like small waterfalls and logjams. They must swim against strong currents, and find their way up rapids. They must maintain a constant state of alertness, to avoid the many predators who come to feast on them during their journey upriver. And finally they'll swim into waters that are too warm for their comfort. They spent a year or two in the ocean, just to get big and strong and fat enough to accomplish all of this--but once they have begun their upriver journey, they won't eat again. Weakened by starvation, injury, and stress, and physically battered, they become infested with parasites, bacteria, and fungi: they begin to decay while still alive. Once they've reproduced, they'll die--the decay speeds up, and their "nutrients" now flow downstream. The nutrients then fertilize the river, providing food for the salmon fry when they emerge.

Trophic Groups

Of the trophic groups that R. W. Merritt and K. W. Cummins (1978) have identified for aquatic insects, only 5 are likely to be found in a stream using typical collection and sorting methods. These are listed below.

Aquatic Insect Trophic Groups

Shredders Collectors Scrapers Piercers Predators
Have strong, sharp mouthparts that allow them to shred and chew live plants or decomposing fragments. These are common among true flies, caddisflies, and stoneflies. Gather the very finest suspended matter in the water. To do this, they often sieve the water through rows of tiny hairs. These sieves of hairs may be displayed in fans on their heads (blackfly larvae) or on their forelegs (some mayflies). Some caddisflies and midges spin nets, and catch their food in them as the water flows through. Scrape the algae and diatoms off of surfaces of rocks and debris, using their mouthparts. Many mayflies, caddisflies, and true flies eat this way. These herbivores pierce plant tissues or cells and suck the fluids out. Some caddisflies do this, as well as many true bugs, or Hemiptera. You may have watched a weevil sucking plant juices through a tubular mouthpiece--a weevil is a true bug. Predators eat other living creatures. Some of these are engulfers; that is, they eat their prey whole or in parts. This is very common in stoneflies and dragonflies, as well as caddisflies. Others are piercers, which are like the herbivorous piercers except that they are eating live animal tissues.