Print PageIncreasing habitat productivity in terrestrial communities results in several emergent properties: Increase in plant height, and the size of key herbivores and predators. Also, the size of animals is positively correlated with their trophic position. As a result, community properties cannot be inferred from energy alone. Using these emergent properties I present a verbal model that predicts the trophic structure of the major terrestrial biomes
Results/Conclusions
Energy is believed to serve as the main determining factor of community structure. However, energy as such, does not flow along food chains but is transferred as biomass. Above the herbivore level, biomass is consumed as individual prey items and its transfer is therefore discrete, in quanta represented by individual prey items. As each predator consumes a certain size-range of prey size and, in general, predators are larger than their prey, in all communities food chains are size structured.
The size of key organisms in terrestrial communities increases with habitat productivity. Plants increase in size as soil nutrients are less limiting and competition for light becomes important. Consequently, the whole plant becomes less nutritious to herbivores as a higher proportion of its biomass is allocated to supportive tissues. Major primary consumers also increase in size with increasing productivity since increasing plant biomass enables larger herbivores to meet their daily energy requirements, and larger mammalian herbivores are more efficient in digesting plant material of low nutritional quality. Following suit, the size of the top predators also increases with productivity as larger herbivores become more common and their biomass meets the large predators’ food requirements. Yet, the size-ratio between top predators and major primary consumers decreases with habitat productivity, being about 1000 times larger in deserts, but10 time smaller than the key herbivores in grasslands. As a result in a decrease in the number of effective trophic levels with productivity from four trophic levels in deserts to only two in grasslands is expected. This contrasts with energy-based theories that predicts either a fixed number of trophic levels at all productivities or an increase in their number.
The nutritious biomass in forests is concentrated in the tree crowns and only small herbivores and predators are able to forage on the crowns. Therefore, though energy considerations allow five trophic levels in forest, the differences in size between the small top predators and the major herbivores in forested biomes allow the existence of only four trophic levels.
