The role of habitat productivity and plant structure in the determination of herbivore size and the number of trophic levels in terrestrial communities.
Ben Gurion University of the Negev, Israel 84990 1
The number of trophic levels in a community and hence the relative importance of bottom-up and top-down forces that control a given trophic level are often argued to be a function of primary productivity, as well as the size of, and metabolic type (poikilotherm vs. homeotherm) of the consumers. However, I argue that, in terrestrial ecosystems, the interactions among these factors, combined with a fourth factor that is also confounded with productivity, habitat physiognomy, are critical to explaining patterns in the number of functional trophic levels along productivity gradients. At low productivity, small, poikilothermic herbivores such as insects dominate, both because they require less energy per individual so a viable population size is more likely and because they are protected from predators even in the low plant cover. As productivity increases, higher densities of larger herbivores can be supported and be sheltered from predators by the increased plant height. However, when habitat productivity is large enough to support large ungulates, other means of predator avoidance start to be effective, such as running speed and living in herds and plant cover is less critical as shelter. I suggest that this shift in the size of dominant herbivores is a major control over trophic structure along productivity gradients. In the least productive terrestrial habitats (deserts and dry grasslands) insects are the dominant herbivores and arachnids and reptiles are the primary predators. Their small size and poikilothermic metabolism then allow predatory birds to become abundant as a fourth trophic level. In grasslands, shrublands and tundra, small mammals (rodents) become the dominant herbivores and birds and mammals the main primary predators. In bushlands, small to medium size ungulates are the dominant herbivores and they are controlled by large carnivores. However, highly productive grasslands are dominated by large ungulates that live in herds and are less effectively controlled by primary predators. Thus, contrary to current theory, I suggest that the number of functional trophic levels may decrease from four in the least productive habitats to two in the most productive herbaceous ones. This hypothesis suggests profoundly different patterns of trophic regulation along productivity gradients than current ideas.
Keywords: comunity structure, habitat productivity, trophic levels, habitat physiognomy, food chain length
This abstract is being presented at: 9:00 AM in session:
Oral Session #39: Theoretical Ecology.