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 (chemosyn-thesis), 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).
Food webs are often very complex and it is easier to appreciate the overall biotic interactions in a forest by breaking the food web into three main subsystems as in Fig. 1.9. The plant subsystem is utilized by the herbivore subsystem, which in forests commences with the living tissues of plants eaten by primary consumers (including directly herbivorous animals - grazing the whole plant or browsing parts such as shoots, twigs and leaves - exudate feeders, parasitic plants and fungi) and eaten in turn by secondary and tertiary consumers. The decomposition (or detritus) subsystem involves the consumption of dead organic materials. The herbivores within this subsystem are detritivores and decomposer organisms (see Section 1.5 and Chapter 7).
Certain carnivores, which consume both herbivores and detritivores, link the herbivore and decomposition subsystems. There are many variations on these basic themes. Omnivorous forest bears eat plants, hunt live animals and eat carrion. Similarly some fungi are parasites (living on their hosts but not killing them), others are pathogens (rapidly killing their hosts) and others are saprotrophs (decomposing dead hosts), and yet others are combinations of these. A fungus might invade a tree without causing undue harm, but if the tree is weakened in a drought year, it may rapidly succumb to the same fungus which can then use the dead tree as a food supply while exploring for another host. Eventually the dead remains of autotrophs and heterotrophs, together with faeces, are mineralized and the whole process, which began with the exploitation of mineral nutrients in the soil, can start again.
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