Unit 4: Ecosystems // Section 6: Regulation of Ecosystem Functions
A key question for ecologists studying growth and productivity in ecosystems is which factors limit ecosystem activity. Availability of resources, such as light, water, and nutrients, is a key control on growth and reproduction. Some nutrients are used in specific ratios. For example, the ratio of nitrogen to phosphorus in the organic tissues of algae is about 16 to 1, so if the available nitrogen concentration is greater than 16 times the phosphorus concentration, then phosphorus will be the factor that limits growth; if it is less, then nitrogen will be limiting. To understand how a specific ecosystem functions, it thus is important to identify what factors limit ecosystem activity.
Resources influence ecosystem activity differently depending on whether they are essential, substitutable, or complementary. Essential resources limit growth independently of other levels: if the minimum quantity needed for growth is not available, then growth does not occur. In contrast, if two resources are substitutable, then population growth is limited by an appropriately weighted sum of the two resources in the environment. For example, glucose and fructose are substitutable food sources for many types of bacteria. Resources may also be complementary, which means that a small amount of one resource can substitute for a relatively large amount of another, or can be complementary over a specific range of conditions.
Resource availability serves as a so-called "bottom-up" control on an ecosystem: the supply of energy and nutrients influences ecosystem activities at higher trophic levels by affecting the amount of energy that moves up the food chain. In some cases, ecosystems may be more strongly influenced by so-called "top-down" controls—namely, the abundance of organisms at high trophic levels in the ecosystem (Fig. 12). Both types of effects can be at work in an ecosystem at the same time, but how far bottom-up effects extend in the food web, and the extent to which the effects of trophic interactions at the top of the food web are felt through lower levels, vary over space and time and with the structure of the ecosystem.
Figure 12. Predators impose top-down control on ecosystems
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Many ecological studies seek to measure whether bottom-up or top-down controls are more important in specific ecosystems because the answers can influence conservation and environmental protection strategies. For example, a study by Benjamin S. Halpern and others of food web controls in kelp forest ecosystems off the coast of Southern California found that variations in predator abundance explained a significant proportion of variations in the abundance of algae and the organisms at higher trophic levels that fed on algae and plankton. In contrast, they found no significant relationship between primary production by algae and species abundance at higher trophic levels. The most influential predators included spiny lobster, Kellet's whelk, rockfish, and sea perch. Based on these findings, the authors concluded that "[e]fforts to control activities that affect higher trophic levels (such as fishing) will have far larger impacts on community dynamics than efforts to control, for example, nutrient input, except when these inputs are so great as to create anoxic (dead) zones" (footnote 4).
Drastic changes at the top of the food web can trigger trophic cascades, or domino effects that are felt through many lower trophic levels. The likelihood of a trophic cascade depends on the number of trophic levels in the ecosystem and the extent to which predators reduce the abundance of a trophic level to below their resource-limited carrying capacity. Some species are so important to an entire ecosystem that they are referred to as keystone species, connoting that they occupy an ecological niche that influences many other species. Removing or seriously impacting a keystone species produces major impacts throughout the ecosystem.
Many scientists believe that the reintroduction of wolves into Yellowstone National Park in 1995, after they had been eradicated from the park for decades through hunting, has caused a trophic cascade with results that are generally positive for the ecosystem. Wolves have sharply reduced the population of elk, allowing willows to grow back in many riparian areas where the elk had grazed the willows heavily. Healthier willows are attracting birds and small mammals in large numbers.
"Species, like riparian songbirds, insects, and in particular, rodents, have come back into these preferred habitat types, and other species are starting to respond," says biologist Robert Crabtree of the Yellowstone Ecological Research Center. "For example, fox and coyotes are moving into these areas because there's more prey for them. There's been an erupting trophic cascade in some of these lush riparian habitat sites."