Cycling through an ecosystem

This exercise is a part of Educator Guide: Whales Eat More Than We Thought / View Guide

Directions for teachers: Ask students to read the online Science News article “Baleen whales eat (and poop) a lot more than we realized” and answer the following questions with a partner. A version of the article, “Whales eat more than we thought,” appears in the December 4, 2021 issue of Science News.

Want to make it a virtual lesson? Post the online Science News article to your virtual classroom. Discuss the article and questions with your class on your virtual platform.

Conserving cycles

1. What cycles through an ecosystem and how does it cycle? Explain and give a specific example from the article.

Matter, including nutrients, cycles through ecosystems via processes that include physical and chemical changes. For example, plants and animals consume and then excrete elements found in other organisms and in the physical environment, making those elements available for yet other plants and animals. Those elements combine in different ways into different molecules as they move through. Iron is an example of a cycling nutrient in the article. Whales eat iron-rich krill and excrete the iron. Phytoplankton blooms rely on that iron and in turn feed krill.

2. How does the law of conservation of mass support cyclical processes? How can a cyclical process in an ecosystem be disrupted? Give an example from the article.

The law of conservation of matter states that in a closed system, the mass of the system must remain constant over time. Instead of matter being created or destroyed, it changes forms. That changing of forms allows the same matter to be used by different organisms, so it cycles through the ecosystem. Such cycles can be disrupted if the size of a plant or animal population changes or if a natural disaster impacts the physical environment, for example. Sometimes a cycle is disrupted by humans. For example, humans hunting whales have decreased the size of whale populations. That decline appears to have altered how iron cycles through ecosystems.

3. What role does energy play in ecosystem cycling?

The cycling of matter is supported by the flow of energy. Energy enters most ecosystems from the sun. Photosynthesizing plants use carbon dioxide, water and energy from the sun to create sugars within the plants, thus transforming the sun’s energy into chemical energy. This process supports primary consumers, who serve as food and energy sources for secondary consumers, and so on. All physical and chemical changes of matter in a food web, for example, involve energy transformations. Some changes rely on energy, while others release it. As energy flows, some energy is lost from the system in the form of heat, which is why an outside energy source, such as the sun, is needed to sustain an ecosystem.

A whale of an ecosystem

1. What does it mean to talk about an organism’s “role” in an ecosystem? What does the term “role” in this context mean? Give an example from the article.

An organism’s role in an ecosystem refers to how it impacts the cycles that exist within the ecosystem. Some examples include producer, consumer, decomposer, parasite, predator, prey and so on. One role that whales play in a marine ecosystem is to keep iron moving through the system. This makes them a nutrient cycler.

2. When ecosystems experience a dramatic change, it can be challenging to determine why. Explain the unexpected impact of whaling on the nutrient cycle as described in the article.

Scientists initially expected that the decrease in the whale population from whaling would lead to an increase in krill because there are fewer whales to eat the krill. Instead, krill populations decreased. Whale poop provides essential nutrients, including iron, for phytoplankton. Krill eat that phytoplankton. By removing iron, phytoplankton numbers dropped, and thus krill numbers dropped too.

3. The article describes how whales’ food consumption was quantified. How can putting specific numbers to animal behavior help scientists understand an animal’s role in the ecosystem?

Quantifying the amount of krill consumed by whales allowed scientists to determine how big a role whales play in the cycling of iron in the ecosystem. This helped scientists gain a better understanding of the degree to which the whales’ disappearance affected various aspects of the ecosystem.

4. Given the complexity of ecosystems, how can scientists determine if one trend or event causes another trend or event?

In a complex ecosystem, there are many possible factors that may contribute to an observed change. In order to prove that one trend or event causes another trend or event, scientists need to identify and explore other contributing factors in order to isolate the variables of interest. Often scientists rely on lab experiments to control for confounding factors — variables that have the potential to distort the relationship between the study’s variables of interest. Confounding factors can mask a causal relationship between a study’s variables of interest or falsely suggest that a causal relationship exists. Controlling for confounding factors helps scientists gain evidence for causal relationships.

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