The populations of different species in each trophic level of an ecosystem is an important parameter of that ecosystem. These populations determine how long the ecosystem can survive, how stable it can remain during environmental stresses and how many different species it can support within it.
If the population of one species in a trophic level rises too much, then intense competition will result in ecosystem collapse. On the other hand, if the population of a species falls drastically, it leaves a void in the ecosystem’s processes and chain of energy transfers. The void will prevent the transfer of energy, and the entire ecosystem will collapse.
Therefore, the populations in each trophic level need to be kept in check and not be left to drift around. For a stable and diversified ecosystem, the system itself needs to come up with ways to control the populations of all its species and make sure it remains in the most suitable range.
The top-down control
In the top-down control, the populations of the organisms lower trophic levels (bottom of the pyramid) are controlled by the organisms at the top. This approach is also called the predator-controlled food web of an ecosystem.
Imagine a simplified ecosystem where there are plants, deer and tigers. The plants are the producers, the deer are the herbivores and the tigers are the top carnivores. The presence of tigers keep the deer population in check. If there were no tigers in this ecosystem, then the deer population rapidly increase. As a result, all the plants would be eaten. When almost all the plants are eaten by the huge, unsustainable population of deer, there is no food left for the deer. So, that population of deer would eventually starve to death and the ecosystem would collapse.
Tigers eat the deer and make sure that their population is not too high, so that the plants are not overeaten and the ecosystem will continue to function.
The bottom-up control
The bottom-up control is driven by the presence or absence of the producers in the ecosystem. Changes in their population will affect the population of all the species in the food web, and thus, the ecosystem. This approach is also called the resource-controlled (or food-limited) food web of an ecosystem.
Again, for instance…
In the same simplified example, let us consider the reverse scenario. What if the population of plants in the ecosystem dwindled to extremely low numbers? Then, the deer would have less food to feed on. Consequently, their population would shrink to reflect the lack of food. When the population of deer decreases, the tigers will also automatically face a shortage of food. The tiger population would also fall. If plants disappeared altogether, then the ecosystem would collapse.
It isn’t just the absence of food that can cause a collapse in the bottom-up approach, but also the inaccessibility to food through competition.
If, for example, there was plenty of food available for the deer, but a competing population of blackbucks turned up. Now, there are two populations vying for the same food resource, making this a food-limited ecosystem. The competition for plants in the region would eventually result in the removal of both, or one of either the deer or the blackbuck in the area.
Does an ecosystem have to choose to be either top-down or bottom-up?
No. In most ecosystems, studies have shown that these two approaches are not mutually exclusive. For example, in marine ecosystems that were initially thought to be purely bottom-up, there have been periods of top-down control due to extraction of large predators through fishing.
It seems like the decision of which approach to use depends primarily on the limiting factor; ie. which of the two, the predator or producer, is present in lesser numbers (or biomass). This makes sense for an ecosystem; purely depending on one approach will make it highly susceptible to destabilization if the driver of that approach goes missing.
In reality, it’s not that simple…which makes it all the more successful!
In reality, there are many more carnivores, herbivores and producers in an ecosystem. This adds a great degree of complexity into this approach, which is the reason for it’s success. More the number of predators or top consumers in an ecosystem, greater is the control established on the herbivores in case of the top-down approach. The same will be true for the bottom-up approach.
Isn’t nature magical?
The top-down and bottom-up control is a structure that governs the flow of energy within ecosystems. For more on flow of energy, check out: Y-shaped energy flow model: Who eats whom in nature.