Question

Consider a grassland with five trophic levels: grasses, mice, snakes, raccoons, and bobcats. If you released additional bobcats into the grassland, how would grass biomass change if the bottom-up model applied? If the top-down model applied?

Short answer

The lower trophic level, such as vegetation, would be affected by the release of additional bobcats if the bottom-up model is applied. In contrast, if the top-down model is applied, the increase in bobcats decreases the number of raccoons. In the same way, the number of mice decreases with the rise of snakes.

Step-by-step solution

Bottom-up model

The bottom-up model suggests the unidirectional flow of energy from lower to higher trophic levels. The linkage $\mathrm{V}\to \mathrm{H}$represents the bottom-up model. V indicates vegetation, and H stands for herbivores.

For example, if vegetation increases in a particular ecosystem, the herbivores in the region also increase. However, if herbivores increase in the ecosystem, the vegetation decreases. Hence, the bottom-up model is unidirectional.

Top-down model

The top-down model stands opposite to the bottom-up model. The composition of the community is controlled by predation. The flow of energy takes place from a higher trophic level to a lower trophic level. The linkage $\mathrm{N}\leftarrow \mathrm{V}\leftarrow \mathrm{H}\leftarrow \mathrm{P}$represents the top-down model. N indicates nutrients, and P is predation.

For example, if predators are present in a large number, the number of herbivores tends to remain under check. In the same way, it controls vegetation and nutrient uptake from the plants.

Bottom-up and top-down models in grassland with four trophic levels

With the addition of bobcats in a grassland when the bottom-up model is applied, the vegetation of the grassland shows little effect with an increase in the grass.Bobcats are predators, and capture herbivores.

When the top-down model is applied, it shows a decrease in the number of raccoons and a rise in snakes. When the number of snakes increases, it results in the reduction of mice and also an increase in vegetation.