Explanatory notes for ponds foodweb


	Herbivores
Individual species of primary consumers (herbivores) in the pond tend to feed on only one of the 3 categories of plant form set out in the foodweb, rather than on all of them. This is because of differences in the type of mouthparts which they possess and the microhabitats which they occupy within the pond. For example, Daphnia swims jerkily through the water column, combing out microscopic plants from the water with feathery appendages, while snails crawl over plants and solid surfaces covered in attached algae, rasping away at the surface plant film.
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	Water Fleas
To give an idea of how simplified the illustrated foodweb is, in relation to the habitat, there are approximately 90 different species of water fleas in Britain. Obviously, not all of them will be found in one pond! However, the tiny dots visible in a tray when one has been pond dipping, may include several different species, all with subtly different requirements.

Dragonfly Larvae

The dragonfly larva actually illustrated is a Southern Hawker. Emperor Dragonfly larvae, which are of a similar appearance, eat snails. It is not certain that Southern Hawkers do the same.

Size and Trophic Levels

Size is a great determining factor in deciding what eats what in a pond. Generally, a larger carnivore will eat any smaller carnivores in its vicinity which it can catch.

Insects in the larval stage of their lifecycle, will pass through several different moults (known as instars) before becoming adult. They increase in size with each moult of their restricting external skeleton. They therefore grow in jumps, rather than continuously as we do when young. Thus, a large Great Diving Beetle larva which is close to becoming an adult, may eat a small dragonfly larva. On another occasion, the tables may be turned and a large dragonfly larva may eat a small Great Diving Beetle larva. Equally, fully grown sticklebacks may eat tiny dragonfly larvae, but a small stickleback may also be captured and eaten by a large dragonfly larva.

There is often cannibalism within the species, with individuals being eaten by other more aggressive ones of the same species. In general, smaller individuals will be eaten by larger ones, but this is not always the case. In this excellent movie clip (1.4Mb), a Great Diving Beetle larva can be seen eating another of a similar size.

For simplicity, the lower level carnivores in the pond have all been grouped into a single trophic level in the foodweb. However, because of the size factor, this third level may actually develop into several different additional levels. For example, a very small Greater Waterboatman which has just eaten a first instar Mayfly larva, may then be eaten in turn by a late instar Great Diving Beetle larva.

Plants > Mayfly larva > Greater Waterboatman > Great Diving Beetle Larva

This is therefore a food chain with 4 trophic levels rather than 3.

Detritivores (animals which feed on decaying organic material)

There are many more detritivores in the pond than the snails and Lesser Waterboatmen illustrated in the foodweb. Other detritivores include Flatworms of various different species and Water Lice. These detritivores fulfill just as vital a role as their counterparts on dry land. Dead and decaying plant and animal material in the pond is all recycled, with the help of detritivores, into reusable nutrients for uptake by water plants.

The Pond Skater in the foodweb feeds largely off other invertebrates which fall into the water and drown. Adult insects, such as Mayflies, emerging from the aquatic stage of their life history, may also be unsuccessful and drown. The Pond Skater is a carnivore rather than a detritivore because it is drawn to the frantic struggles of drowning insects and is usually consuming live food rather than dead and decaying bodies.

Foodweb Complexity

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The feeding interactions which go on in a habitat such as a pond are incredibly complex. A foodweb is an attempt to simplify these interactions to an extent where we are able to reach some understanding of what is going on. However, it is worth remembering that it is just that - an extreme simplification.

The feeding preferences of individual animals alter as they grow. They may also seek different food choices depending on what is currently available, the particular environmental conditions at any one time, the microhabitats which are available in a particular pond, as well as the time of day (a food web drawn just for the night period would look substantially different). Individuals of different sexes within the same species may even make different food choices.