Top-Down and Bottom-Up
Regulation of Communities
|1.||What are “top-down” and “bottom-up” processes?|
|2.||A history of converging views|
|3.||A few system vignettes|
|4.||Theory: Seeking generality|
|5.||Moving beyond vignettes: Empirical generality and tests of theory|
|6.||Where do we go from here?|
In this chapter we briefly trace the historical debate and outline the theoretical and empirical evidence for factors controlling the biomass of predators, herbivores, and plants within and among ecosystems.
autotroph. Organisms that make their own food by synthesizing organic compounds from inorganic chemicals, usually via photosynthesis (e.g., algae, vascular plants).
biomass. The total mass of living biological material.
consumer. See heterotroph.
food web. Network of feeding relationships among organisms in a local community.
heterotroph. Organisms that must consume organic compounds as food for growth (e.g., animals, most bacteria, and fungi).
primary producer. See autotroph.
trophic. From Greek, “food,” this term refers to feeding of one species on another, as in “trophic interactions” or “trophic links.”
trophic level. Feeding position in a food chain: autotrophs form the basal trophic level, herbivores represent the second trophic level, and so on.
Humans are dramatically altering the global budgets of elemental nutrients that limit the growth and biomass of autotrophs, or primary producers. Through activities such as fossil fuel combustion and application of agricultural fertilizers, global pools of nitrogen and phosphorus have doubled and quintupled, respectively, relative to preindustrial levels. The impacts of these nutrient fertility bonanzas are most obvious in surface waters of lakes and coasts. Nutrient eutrophication often causes rapid and explosive blooms of algae and microorganisms and equally rapid death, decomposition, and ecosystem-wide oxygen starvation, or hypoxia. The Gulf of Mexico hypoxic “dead zone” at the mouth of the Mississippi River annually swells over areas exceeding 18,000 km2, larger than the U.S. state of Connecticut. Nutrient eutrophication is a jarring example of a bottom-up process, resource supply, that can dramatically alter autotrophs and the food webs that rely on them for energy and nutrition.
Concurrently, humans are changing the role and composition of consumers in food webs via species removals and additions. Habitat loss and degradation and selective hunting and fishing deplete consumers disproportionately from food webs; many top predators such as tigers, wild dogs, wolves, and sharks have been hunted to near ecological extinction. At the same time, humans are adding consumers to food webs for endpoints such as conservation, recreation, and agriculture as well as accidentally introducing invasive consumer species. In a dramatic example, the brown tree snake (Boiga irregularis), a nocturnal predator, was accidentally introduced to Guam after World War II. This single species has eaten its way through