Figure 5.1 gives an insight into the relationships between producers and consumers. It shows a scheme predicting the changes in abundance of trophic (mineral nutrient) elements along a gradient from low to high plant productivity. At very low productivity (a) where the vegetation consists entirely of stress-tolerators (see Section 4.1), defences against generalist herbivores work so well that the herbivores are restricted to a small biomass of specialist feeders. The relations of eucalypts (Section 1.3.1) with the koala approximates to this model. Predators are also likely to be extremely sparse and specialized. At higher productivity with less effective defences (b), generalist herbivores may be expected in greater abundance as long as they are insufficient to provide a large and reliable food source for predators which would otherwise keep their numbers low. At the highest levels of productivity (c) high rates of population growth by herbivores are possible, but predators effectively suppress herbivore numbers thus protecting the palatable vegetation.
Plant litter produced by competitors tends to be copious but not usually persistent. Litter production in plants with the other two primary strategies is sparse, being persistent in stress-tolerators but not in ruderals.
It is clear from the above that soil and environmental conditions limit plant form and growth - which is why we have different biomes around the world (see Section 1.6.1), ranging from tough evergreen open woodlands in dry areas to dense rain forests in wet areas. The climate and the vegetation both in turn
5.2 The interdependence of producers and consumers (c)
Carnivores and parasitoids Herbivores
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