III.6 Top-Down and Bottom-Up Regulation of Communities
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III.6 Top-Down and Bottom-Up Regulation of Communities E. T. Borer and D. S. Gruner OUTLINE 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. GLOSSARY 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. 1. WHAT ARE ‘‘TOP-DOWN’’ AND ‘‘BOTTOM-UP’’ PROCESSES? 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 Guam’s native food web, causing direct reductions or complete extinctions of dozens of native birds, bats, and reptiles, and indirect negative impacts to native arthropods, forest tree seed dispersal, and recruitment. This example highlights an extreme change in topdown processes, or consumption of organic biomass, that can have dramatic effects throughout food webs. Management of algal blooms, crop fertilization, agricultural insecticide use, and wildlife conservation are prime examples in which complex interactions between bottom-up processes (i.e., fertility) and topdown processes (i.e., consumption) challenge us to better understand the critical processes that bridge communities and ecosystems. Thus, understanding the ways in which altered resources and consumer community structure interact to control the biomass of predators , herbivores, and plants is not simply a problem for basic science but one that has an immediate impact on humans. Biological control of crop pests and control of lake clarity are two management realms that draw on knowledge about these interacting processes to bring about planned changes in whole ecosystems. Thousands of scholarly studies report on the implications of fertility manipulations and biological weed or pest control introductions for applied endpoints such as agricultural yield. We focus here on the basic science underlying such bottom-up and top-down applications. Although such factors as genetics, disease , nutrition, dispersal, and spatial structure can be critically important in structuring communities, we focus primarily on fertility and consumer controls of communities, as even this more restricted literature is quite vast. We refer to an extremely simplified theoretical community, or ‘‘module,’’ describing one predator , one herbivore, one plant, and one nonbiological resource (e.g., nitrogen; figure 1). Most common mathematical descriptions of this module treat each level...