Food chains and food webs--
The pathways whereby the energy and matter produced by plants are distributed among other organisms in a habitat are known as food webs. Food webs are made up of a network of food chains each of which connects an autotroph, at the lowest trophic (i.e., feeding) level, to the herbivores (i.e., primary consumers) and then to the various carnivores (i.e., secondary consumers, tertiary consumers, and so on).

The efficiency with which energy is transferred from one level to the next determines the length of food chains. Each step in the food chain involves loss of efficiency due to the costs of capturing and digesting food. As a rough approximation, the efficiency of conversion from one level to another is 10%. (The exact value depends on the P/R ratio and can be quite variable. It is generally lower in vertebrates than invertebrates, and lower in birds and mammals than in fishes.) This means that it takes about 100 grams of plants to create ten grams of herbivore; when eaten, that ten grams of herbivore will result in one gram of carnivore. Because of the inefficiency of energy transfer (due to the expenditure of energy on metabolism), food chains tend to be limited in length; that, with three levels of carnivores, is likely to be the longest food chain sustainable in many habitats.

In some circumstances, much of the plant material is not eaten by herbivores. This may be because it cannot be eaten (most herbivores cannot digest wood), or because the herbivore population is kept low by secondary consumers. In that case the plant (or its parts) dies, and is colonized by the microorganisms responsible for decay forming detritus (i.e., particulate organic material derived from the decomposition of plant and animal remains). Detritus provides food for a range of consumers (called detritivores) and a food chain results. In most habitats, the detrital food chains and those chains based on herbivores eating live plants combine to produce a food web in which the animals at higher levels eat elements from both types of chain. These webs can be very complex, such as in the Mekong River where a large number of interacting species are present.

The origins of organic material--
The relative amounts of energy passing through the autotroph to herbivore pathway and through the detritus to detritivore pathway will vary among habitats or different parts of a river. Some indication of their relative importance can be estimated from if we know the origin of organic material in the environment. Autochthonous material originates from within the aquatic ecosystem; for example, primary production by algae.

Allochthonous material originates outside and is transported into the aquatic ecosystem; for example, tree leaves that fall into rivers.

In general, most of the organic material in small streams is allochthonous, originating from the overhanging terrestrial vegetation or washed in from the surrounding forest floor. There is some autochthonous primary production by periphyton (see Section 6) but shading by riparian vegetation may limit the light needed for photosynthesis. Macrophytes are scarce. In general, therefore, autochthonous primary production is estimated to be of relatively minor importance (for details, see Cummins 1974)

The amount of allochthonous input varies greatly in space and time, but inputs of around 1 kg/m²/yr in the upstream course of shaded rivers can be expected. The timing of inputs will depend on the schedule of leaf loss and replacement of terrestrial plants. There is less seasonality in this regard in Southeast Asia than in the deciduous forests of Northwest Europe and North America.

The amounts of autochthonous production increase downstream as the channel widens and shading is reduced (see next section); in larger rivers, phytoplankton may be present in the main channel and floodplain pools (see also Section 6). The biomass of detritus is typically less than upstream, but some decomposing material carried downstream by the current will be present, regardless of whether there are substantial direct inputs from the riverbank. The particle size and quality of this transported detritus may differ considerably from that entering the river from the surrounding land.

There is some difficulty in interpreting the origin of the material on floodplains because of the alternation of aquatic and terrestrial phases over the flood cycle. Terrestrial material (nutrients and energy) can stimulate aquatic production by plants and animals during the flood phase. This is described in more detail under Section on 'Flood Pulse').

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