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The Limiting Factors in a Food Web

The food web describes the transfer of energy among organisms in an ecosystem. Primary producers such as plants produce biomass from inorganic sources. Primary consumers eat the primary producers and are in turn preyed upon by secondary consumers. Tertiary consumers sit atop the food web, top predators that prey upon secondary consumers. Limiting factors influence each level of the food web separately, which can exert effects on the web as a whole.

Nutrients
- Nutrient availability is an important abiotic, or nonliving, limiting factor for primary producers such as plants and phytoplankton. The amounts of nutrients such as nitrogen and phosphorus that are available limit the amount of biomass created by these primary producers. Reduction in primary producer biomass exerts significant "bottom up" effects on the entire food web.
Climate
- Climate is another important abiotic limiting factor that impacts the food web at all levels. Sudden cold snaps or droughts cause die-offs of both producers and consumers. The level of these effects depends on the characteristics of the individual ecosystem and its organisms. For example, drought might cause die-offs of predatory mammals in the desert, while drought-tolerant plants survive and lower-level consumers thrive in the absence of top predators. Scientists expect climate change to impact ecosystems and alter food webs dramatically as plants and other species that cannot tolerate change or migrate rapidly enough to avoid it are eliminated from the web.
Competition
- Competition is an important biotic limiting factor that also impacts the food web at all levels and is especially important in modern, altered food webs. Introduced species often compete with native species and limit the amount of biomass produced by the native species. When the introduced species does not fulfill the same role in the food web, the entire system can be altered. For example, primary consumers may find introduced plants to be inedible--if the introduced plants outcompete the native species, primary consumers may be limited by the lack of nutrients, and the impacts will continue to trickle up through the food chain.
Predation
- Predation is an extremely important biotic limiting factor that exerts top-down influence on the food web, limiting the amount of biomass produced by the primary consumers. Scientists have identified "keystone predators" in many environments that are crucial to the survival of other species. When an ecosystem lacks a top predator or the number of predators is reduced, the balance between consumers and producers is affected and the consumers may over-graze. As a result, some species of producers could be eliminated from the ecosystem altogether.

Nature