7.4 TROPHIC LEVELS, CASCADES AND LIMITING FACTORS

Organisms can be grouped into trophic levels that are linked by feeding relationships into food chains and food webs. Because transfer of energy between trophic levels is inefficient there are generally no more than five or six trophic levels in a food chain or in a food web. What is the ecological significance of the number of levels in a food chain? Is there an important difference between communities with (say) four or five trophic levels?

Imagine a simple food chain consisting of an autotroph (periphytic algae) and a primary consumer (a herbivorous insect). The insects will graze the algae and increase in abundance until there is no longer enough plant food available to support further increase. In this case, algal biomass will be low because it is limited by herbivore pressure. What if another trophic level is added in the form of a predatory insect? These secondary consumers will reduce the abundance of herbivores, so decreasing grazing pressure on the algae. Freed from the influence of herbivores, algal biomass will continue to increase until nutrients limit primary production.

Hypothetical food chains for river organisms illustrating direct and indirect trophic interactions.

Now imagine a food chain with four trophic levels: an autotroph, a primary consumer and a secondary consumer as before, plus a tertiary consumer (an insectivorous fish). The fish will probably prefer to feed on larger (predatory) insects. This will reduce predation on the herbivorous insects, which will be able to increase in abundance and exert grazing pressure on the algae. Algal biomass will therefore decline. Addition of a fifth trophic level (a piscivorous fish) will reverse the effect, as the piscivore will reduce the abundance of insectivorous fish, allowing the predatory insects to increase and reduce the number of herbivores with the consequence that algal biomass accumulates.

Can you predict the consequences for algae if it were possible to add a sixth trophic level?

Two important points are illustrated by these food chains.

• The effect of upper trophic levels on lower ones may be either direct (by direct grazing or predation) or indirect (by changing the abundance of herbivores or predators feeding upon a lower trophic level). Trophic cascades occur where consumers on one trophic level indirectly influence a lower level. Three out of the four food chains (illustrated in figure above) show cascade effects (for more examples, see Power, 1990).

• Algal biomass in the food chains with two and four trophic levels is limited by grazing from a higher trophic level; there is top-down limitation. Algal biomass in food chains with three and five trophic levels is limited not by grazing but by nutrient availability in the surroundings; there is a bottom-up limitation in that case.

In more general terms, bottom up limitation occurs when a particular trophic level is limited by the supply of food (algae, detritus or prey) from the level below it. Top down limitation occurs when a particular trophic level is limited by the activities (grazing, predation) of the level above it.