Pasture Food Web

In the two food webs which can be seen below, there are complex interactions between different trophic levels. In these food webs, we have concentrated on the smallest soil organisms. You need a microscope to see them. We focussed on the small organisms as it is the "little things that make the underworld work" and they are regarded as being the most important organisms in soil processes such as decomposition and nutrient cycling in soil. The smaller the size of a soil organism, the more numerous and the more metabolically active they are.

The two food webs still show only some of the possible inter-connections between soil organisms. The "top" predators in these food webs are the small predatory nematodes and mites. Linkages (arrows) could be drawn between each group and its decomposer groups. Remember, as each organism dies, it decays through the activities of the invertebrate and microbial decomposers. However, if we included all these inter-connecting arrows the diagram would look like an untidy bird's nest!

Food webs can be extended from a "who eats who" basis to include the amount of nutrients which flow through the various trophic (or feeding) levels. In the case of the two food webs below, different thicknesses of arrow denote the amount of nitrogen that is being mineralised or released by each different group from the organic matter base. The arrows represent net annual flows of nitrogen.

These two food webs were constructed from data obtained from two grazing systems at Armidale, NSW, Australia. One is a native, unfertilised pasture dominated by the native grasses, Themeda australis and Poa sieberiana. The other pasture is fertilised with superphosphate, and sown to the grass, Phalaris aquatica, and Trifolium repens (white clover). The clover in the introduced pasture fixes nitrogen so that both the soil and organic residues formed on the pasture are higher in nitrogen than that formed on the native pastures. Hence the thicker the arrows in the introduced pasture diagram.