Exercises: Relationships between living things

State whether the following relationships are intraspecific or interspecific: a shoal of fish swimming together, a bee feeding on nectar from a flower, two stags fighting over a female, and a tick feeding on a dog.

In the food chain grass → grasshopper → frog → snake, identify the producer and name the primary, secondary, and tertiary consumers.

Explain what predation is and give an example from a marine ecosystem, stating which is the predator and which is the prey.

The diagram represents a food web; the arrows point from the organism that serves as food to the organism that eats it.

1. a) Which organism is hidden behind the "?" symbol, knowing that it feeds on frogs and mice?
2. b) Name the primary consumers in the food web.
3. c) Predict what would happen to the rabbit and plant populations if foxes disappeared.

Distinguish between mutualism, commensalism, and parasitism, and classify these cases: a lichen (an alga and a fungus benefiting each other), a remora travelling attached to a shark without harming it, and a tapeworm living in a person's intestine.

Explain the difference between a food chain and a food web, and reason why a food web better describes what happens in a real ecosystem.

Along a river stretch, the European mink (native) and the American mink (invasive alien) coexist. A municipal plan proposes: "To stop the decline of the European mink, we will periodically introduce tons of fish and crayfish so there is plenty of food and both species can coexist without problems."

1. a) Relying on the concepts of ecological niche (the set of resources and conditions —food, space, shelter— that a species needs and the role it plays in the ecosystem) and competitive exclusion (two species with the same niche cannot coexist indefinitely in the same place), argue why the municipal proposal will not prevent the disappearance of the European mink.
2. b) Propose a reasoned hypothesis about the direct and indirect consequences that the simultaneous presence of both species may have on native amphibian populations while competition lasts.

An investor states the following: "We can set up a commercial lion farm fed exclusively on beef from calves raised on our pastures, and we will get almost the same amount of lion meat to sell as if we sold the calves directly."

1. a) Numerically demonstrate, applying the 10% ecological efficiency rule and starting from 100 000 kJ of solar energy fixed by the pasture, how much energy reaches the calves and how much would reach the lions.
2. b) Using your calculations, refute the investor's claim and justify why in natural terrestrial ecosystems there are almost never predators in a fifth or sixth trophic level.

Following an epidemic that reduces the fox population in a holm oak forest by 90%, a team of ecologists records these changes over three years:

• Year 1: The rabbit population doubles and the herbaceous plant cover barely changes.

• Year 2: Pasture cover drops by 70% and the soil begins to erode severely.

• Year 3: The rabbit population suffers a sudden collapse and falls to levels well below the initial ones, even though foxes are still absent.

1. a) Explain the trophic cascade that connects the disappearance of the foxes with the degradation of the vegetation in Year 2.
2. b) Formulate an ecological hypothesis that justifies why the rabbit population collapsed in Year 3 if there were no predators hunting it.