Question

In the ecologist Mary Power's work, presented in Section \(18.4,\) the top predators appear to control plant productivity by controlling the abundance of herbivores (their prey). Now suppose we were to conduct a second experiment and reduce plant productivity by using some chemical that had no direct effect on the consumer organisms. If the results show that reduced plant productivity reduces herbivore populations, in turn leading to the decline of the top predator, what type of control would this imply? How might you reconcile the findings of these two experiments?

Short answer

Answer: In the two experiments, top-down control is demonstrated by top predators regulating herbivore populations and impacting plant productivity, while bottom-up control involves reduced plant productivity affecting herbivore and top predator populations. To reconcile the findings, both top-down and bottom-up controls can coexist in an ecosystem, and their influence might vary depending on ecological factors and circumstances. Thus, the complexity and interdependence of the trophic levels in ecological systems show that both top-down and bottom-up controls are at play.

Step-by-step solution

Understanding the Two Experiments

In the first experiment by Mary Power, top predators control plant productivity through their control of herbivores. This implies that a strong connection exists between predators, herbivores, and plant productivity. In the second experiment, plant productivity is artificially reduced, and we need to observe if this reduction affects the herbivore populations and top predator population.

Identifying the Control Type

Since top predators' abundance in the first experiment controlled the plant productivity through herbivore population, it is considered top-down control. If the artificially reduced plant productivity in the second experiment leads to a decline in herbivore populations, which results in the decline in top predator populations, this control would be considered bottom-up control. This is because it must have started with a reduction in plant productivity (bottom) leading to less food available for herbivores, which in turn causes a decline in the herbivore population and, subsequently, have a ripple effect up the food chain (top predator population).

Reconciling the Findings of Both Experiments

To reconcile the findings of these two experiments, we can consider that both top-down and bottom-up controls coexist in this ecosystem. In the first experiment, top predators place top-down control on the system by regulating herbivore populations, which has an impact on plant productivity. However, the second experiment shows that bottom-up control can also exist and affect predator populations through the resource availability for herbivores. Thus, both controls are not mutually exclusive, and their effect might vary according to specific circumstances, disturbance events, or other ecological factors. This highlights the complexity and interdependence of the different trophic levels in ecological systems – and that both top-down and bottom-up controls can be at play.

Key concepts

Trophic Level Interactions

An ecosystem is made up of different levels of organisms, from plants to apex predators, each level interacting within a complex web of life. Understanding trophic level interactions is crucial to grasping ecosystem dynamics. Most ecosystems are characterized by a number of trophic levels: producers (typically plants), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers (carnivores that eat other carnivores).

When primary consumers feed on producers, they transfer energy up the food chain. If their population diminishes, secondary and tertiary consumers might also be affected due to the shortage of food. In Mary Power's study, when top predators, which are higher trophic level organisms, control the herbivores, the plants flourish due to decreased grazing pressure. This direct influence by organisms on different levels can lead to a cascading effect throughout the ecosystem.

Ecological Experiments

Conducting ecological experiments helps scientists gain insights into the functioning of ecosystems. They can manipulate one or more factors in a controlled setting to examine the outcomes. When an experiment is carried out, such as reducing plant productivity through a chemical that does not affect consumers, it is a method of testing bottom-up control. These controlled experiments, as described in the solution's second step, provide evidence of causality—not just correlation—by showing how changes in the lower trophic levels (like plants) can directly affect higher trophic levels (herbivores and then predators).

These experiments must be designed carefully to isolate variables and factors, ensuring that the observed changes are due to the manipulated element. Reconciling the results from different experiments helps clarify the relationships and interactions within the ecosystem, highlighting how complex and multifaceted these interactions can be.

Predator-Prey Relationships

The dynamic between hunters and the hunted is what defines predator-prey relationships. It's a fundamental aspect of ecosystem balance and a key driver of evolutionary processes. Predators affect prey populations by consuming them, which in turn can regulate the prey's consumption of plants or other organisms. This interplay can stabilize the ecosystem, but it can also lead to oscillations in the populations of both predator and prey, as each influences the other's survival and reproductive success.

In our example, the reduced herbivores resulting from fewer predators can lead to increased plant growth. Conversely, if plants are scarce, herbivores may decline, and this reduction impacts predator populations. Such studies shed light on the complex and vital roles predators and prey play in maintaining ecological balance. It's the understanding of these relationships that allows ecologists to appreciate the full scope of interactions within trophic levels and the equilibrium that exists between top-down and bottom-up controls in ecosystems.