Controls in Ecosystems:  Top-down or Bottom-up?

To understand the relative importance of top-down and bottom-up controls in ecosystems, it is important to look at the evolution of an ecosystem over time.

At the INITIAL stage, bottom-up control seems to play a more important role as what plants first grow in a place determines what (both plants and animals) come later, and hence the composition and the structure of the ecosystem. And then, top-down control comes into play.

At any given time, both controls are in play although one may be more important than the other. An ecosystem experiences disturbances (from either biophysical environment or humans) from time to time, and often one trophy level or one species gets affected more than others when a disturbance occurs. When the bottom level is affected the most (such as in a fire disturbance, excessive nutrient input or logging by humans), bottom-up control starts to play a more important role. When the top level is affected the most (for instance, big fish harvest by humans in a lake), top-down control becomes more important. The disturbance can also affect a middle level the most (for instance, people may like and catch more of a certain type of fish that is food of other fish), and in this case both controls are important and began to play more actively. However, all the species in the system will gradually adjust to these changes, and the FEEDBACKS eventually make the other control come into play again, and after a certain time the system as a whole stabilizes if no other disturbances occur.

Over a VERY long time, all the species in an ecosystem adapt to the environment and other species, and adjust themselves to changes in the environment and changes of other species. So they INTERACT with each other and with the environment, and CO-EVOLVE to maintain the function of the ecosystem. Not only the interactions between trophy levels are important, the composition at the same level also matters. The biophysical environment (climate, soil, nutrients etc.) sets up the potential and limit for an ecosystem, and also imposes disturbances to the ecosystem. Humans have huge influences on the ecosystem which may be treated as disturbances. Or if we include humans as a part of the ecosystem, humans impose both top-down and bottom-up controls, and it is even harder to tell which control is more important.

Therefore, it would be overly simplified to say either top-down or bottom-up is more important.

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