Producers and consumers

Primary production is undertaken by autotrophs, which in forests are green plants (photoautotrophs) that produce complex compounds from simple raw materials using the energy of light in the process of photosynthesis. Chemoautotrophs do this using the energy of chemical reactions (chemosynthesis), but do not play an important role in woodlands. Heterotrophs, by contrast, consume other organisms and so are dependent on the uptake of energy in organic materials synthesized by these other organisms. Herbivores, carnivores, parasites and decomposers (saprotrophs) are all heterotrophs; they vary in size from microorganisms and insect larvae to elephants and all play important roles in woodland, forest and related ecosystems. The increase in biomass of heterotrophs is known as secondary production. Heterotrophs that exploit autotrophs directly are called herbivores or primary consumers. These are consumed by secondary consumers, the carnivores, and some of these may in turn be eaten by tertiary consumers to form food chains. It is rare for an animal to feed on just one other species, so in reality food chains become a food web, a network of interconnected food chains (see Fig. 1.10). Many of the consumers forming this plant-dependent web influence green plants adversely, often by feeding or trampling. Others are positive, acting as pollinators and dispersers of fruits and seeds, and even more significantly, promoting nutrient cycling (see Section 8.3).