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

Bottom-up control in ecosystems refers to ecosystems in which nutrient supply, productivity, and the type of primary producers (plants and phytoplankton) control the structure of the ecosystem. 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 due to biomagnification and ecological efficiency. An example would be how plankton populations are controlled by nutrient availability. Plankton populations tend to be higher and more complex in areas where coastal 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 about which type of control affects food webs in certain ecosystems.

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

Bottom-up control in ecosystems refers to ecosystems in which nutrient supply, productivity, and the type of primary producers (plants and phytoplankton) control the structure of the ecosystem. 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 due to biomagnification and ecological efficiency. An example would be how plankton populations are controlled by nutrient availability. Plankton populations tend to be higher and more complex in areas where coastal 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 about which type of control affects food webs in certain ecosystems.