Answer:
Option C: Minnows have a direct negative effect (⇒(-)) on the population size of zooplankton. Basses have a negative effect (⇒(-)) on the population size of minnows. Basses have an indirect positive (---> (+)) effect on zooplankton.
Explanation:
In the trophic web occurs a process of energy transference through different organisms that places at different levels. Each level feeds on the preceding one and becomes food for the next one. The first link is an autotroph organism or producer. The following links are the consumers: herbivores are primary consumers and feed on producers. Carnivores are secondary consumers and feed on herbivores, and so on. The last links are the decomposers, microorganisms that act on dead animals degrading organic matter.
If any of the links change their population size (increasing it or decreasing it), it affects the superior links and the immediately anterior link.
In the exposed example, zooplankton, minnows, and basses are continuously interacting.
Minnows feed on zooplankton. So they affect direct and negatively the invertebrate population. The more minnows there are, the more zooplankton they eat. So, as long as the minnows population keeps growing, the zooplankton population will be more affected because they will not have enough time to reproduce and recover from predation.
Bass feed on minnows, so they also affect direct and negatively the fish population. And the more the bass population grows in size, the more affected the minnows will be because many more basses will be eating them.
- Bass ---->(+) Zooplankton
On the other hand, by negatively affecting the minnows population, basses will be indirectly and positively influencing the zooplankton population. This fact is because the more minnows basses eat, the fewer fishes there will be to prey on zooplankton. So the invertebrate will have enough time to recover its population size. The predation pressure acting on zooplankton will decrease thanks to the action of basses preying on minnows.
Here there is a natural equilibrium between the three species. It is a representation of typical cycle predator-prey.
- Minnows prey on zooplankton. Minnows population grows, and the zooplankton population decreases.
- As the minnow population grows, there is more available food for basses, who eat more minnows. As the bass population increases, the minnows population decreases in size. And as there are fewer minnows, the zooplankton population will increase.
- If minnows decrease in number, basses will decrease in number too. Minnows will not be severely affected by basses, so their population increases again.
- As zooplankton also increased, there will be more available food for minnows that will keep reproducing. And the more minnows there are, the more available food for basses there will be. And basses population will increase again. So you can see that zooplankton is indirectly and positively affecting the basses population too (not shown in images).
