3.2 Food webs
The Eucalyptus provides food for other species too, so a more realistic way of expressing the complicated interrelationships of plants and animals, is in a food web as shown in Figure 2 for the common oak.
This figure does not tell the whole story; in what way is it incomplete?
The figure does not show what happens to the organisms in the food web when they die.
There are many organisms that feed on dead organic matter: animals such as earthworms, called detritivores, and soil fungi and bacteria, termed decomposers. Note that most animals are able to use more than one species for food and are, in turn, preyed upon by a variety of other animals. This reduces the vulnerability of a species but it also makes it very difficult for ecologists to predict the outcome of an alteration in the availability of any one food source for the other populations in the food web.
All the nutrients in a food web derive from the producers (also referred to as ‘primary producers’. These use solar energy and simple chemicals to build organic molecules. The process is called photosynthesis. The net amount of organic compound accumulating in plants is called net primary production (NPP). It is measured as the productivity of an area over a specified time. (You might come across the term gross primary production. This is a measure of the total energy captured by the plants. It exceeds the NPP by a value equal to the amount of energy lost in plant respiration.) A glance at Table 1 shows the tremendous variability in productivity between different ecosystems (an ecosystem is a unit comprising several habitats and the organisms within them). This is of enormous economic importance to us. NPP is available to herbivores and, via herbivores, to carnivores as food. However, humans are unique in making further and different uses of plant material.
Table 1 Average values for net primary production (NPP) in different ecosystems.
|Ecosystem||NPP/kJ per square metre per year|
|areas over continental shelf||131500|
|temperate deciduous forest||261000|
For what purposes other than food do we use plants?
Clothing (e.g. cotton and flax); housing and furniture (e.g. timber); energy (e.g. timber and peat); medicines (e.g. witch hazel, willow and yew); transport (e.g. rubber and hemp).
Unfortunately, not all of the NPP from the primary producers is available to carnivores. As well as using energy and materials for growth, the herbivores will have used energy in various other metabolic processes, principally respiration; there is also considerable loss as heat and also in faeces. To sustain a carnivorous diet requires a much greater input of NPP than is necessary for an herbivorous diet. In terms of numbers, there are far more plants than animals and more herbivores than carnivores. This is represented in Figure 3 as a pyramid of numbers. There are many more species and individuals at lower levels than there are at higher levels. Each organism occupies a particular feeding level or trophic level. On average, the transfer of energy from one trophic level to the next is only about 10% of the energy potentially available.
Many organisms (ourselves included) can feed at more than one trophic level, and this is just one of the factors that makes unravelling the interactions within a habitat so complex. It is very easy to see how the fortunes of populations of one species can have a knock-on effect on other species, but it is not always possible to predict exactly which species will be most affected by any novel changes.
There are three factors that can check the growth of any animal population: disease, predation and limitations of the food supply. This is expressed for human populations as disease, war and famine.
Figure 4 shows the way in which the populations of the lynx and the snowshoe hare have fluctuated over 90 years. Notice that the peaks in the snowshoe hare population are always followed by peaks in the lynx population.
Why is the second peak always lower than the first peak?
This is an example of the pyramid of numbers. The prey species (snowshoe hare) is always more numerous than the predator (lynx).
Predator–prey relations are seldom as straightforward as this, because most predators do not rely on a single prey species. In the same way, the relationship between koala and Eucalyptus is unusual because most herbivores eat a varied diet. Any species with a specialized diet, or indeed any other specialized habitat requirement, is vulnerable to the specific loss of that resource. However, most ecological investigations reveal more complex interrelationships than these. Hence interference with the members of one species within an ecosystem can have unpredictable results, as we shall see in the next section.