Editing Bottom-up and top-down design (section)

=== Ecology ===
[[File:Limiting factors in ecology figure.svg|thumb|right|The [[energy pyramid]] represents the ecosystem and its layers, the symbols represent the various limiting factors.]]

In [[ecology]] top-down control refers to when a top predator controls the structure or population dynamics of the [[ecosystem]]. The interactions between these top predators and their prey are what influences lower [[trophic level]]s. Changes in the top level of trophic levels have an inverse effect on the lower trophic levels. Top-down control can have negative effects on the surrounding ecosystem if there is a drastic change in the number of predators. The classic example is of [[kelp forest]] ecosystems. In such ecosystems, [[sea otter]]s are a [[keystone species|keystone]] predator. They prey on [[Sea urchin|urchins]], which in turn eat kelp. When otters are removed, urchin populations grow and reduce the kelp forest creating [[urchin barren]]s. This reduces the diversity of the ecosystem as a whole and can have detrimental effects on all of the other organisms. In other words, such ecosystems are not controlled by productivity of the kelp, but rather, a top predator. One can see the inverse effect that top-down control has in this example; when the population of otters decreased, the population of the urchins increased.

Bottom-up control in ecosystems refers to ecosystems in which the nutrient supply, productivity, and type of [[primary producer]]s (plants and phytoplankton) control the ecosystem structure. If there are not enough resources or producers in the ecosystem, there is not enough energy left for the rest of the animals in the food chain because of [[biomagnification]] and [[ecological efficiency]]. An example would be how plankton populations are controlled by the availability of nutrients. Plankton populations tend to be higher and more complex in areas where [[upwelling]] brings nutrients to the surface.

There are many different examples of these concepts. It is common for populations to be influenced by both types of control, and there are still debates going on as to which type of control affects food webs in certain ecosystems.