Relationships Among 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.

Classify the following relationships as intraspecific or interspecific: a pack of wolves hunting together, a tapeworm living in a wolf's intestine, two wolves fighting over prey, and a raven following the pack to take advantage of the leftovers.

Two species of mice in the same forest feed on the same seeds. State what type of interspecific relationship they have and reason what may happen to their populations if food becomes scarce one year.

Define what mutualism is and give an example, stating what each species gains from the relationship.

Explain what commensalism is and give an example from a marine ecosystem, stating what each organism gains.

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.

Some ants protect aphids from their enemies and, in return, feed on the sugary liquid the aphids excrete. Classify the relationship and explain what each species gains.

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.

The Asian hornet (Vespa velutina), an invasive species, captures honey bees at the entrance of their hives. Classify the relationship between the two species and explain two effects its spread can have on the ecosystem, taking into account the role of bees.

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.

In the food chain holm oak → caterpillar → great tit → sparrowhawk, state which trophic level each organism occupies (producer, primary consumer, secondary consumer, or tertiary consumer).

State whether the following organisms are producers, consumers, or decomposers: a mushroom, a lettuce, a cow, a bacterium, an earthworm, and an eagle.

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.

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.

Draw with arrows the food web formed by these organisms: phytoplankton, sardine, tuna, and seagull. The sardine feeds on phytoplankton, and the tuna and the seagull feed on sardines. Then state which trophic level each one occupies.

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.

In a nature reserve, the following annual censuses of rabbits (prey) and foxes (predators) are recorded:

• Year 1: 500 rabbits and 20 foxes.

• Year 2: 800 rabbits and 35 foxes.

• Year 3: 400 rabbits and 50 foxes.

• Year 4: 250 rabbits and 30 foxes.

• Year 5: 550 rabbits and 22 foxes.

1. a) A park ranger states: "In Year 3 the fox population reached its peak because that same year there was a great abundance of rabbits." Refute this statement by interpreting the data and explaining the phenomenon of time lag in predator-prey dynamics.
2. b) Predict with justification what trend (increase or decrease) both populations will experience in Year 6 if no external factors intervene.

On an oceanic island, three key species coexist: ground-nesting seabirds, invasive rats (omnivores that consume seeds and prey on eggs and chicks), and feral cats (predators that hunt both rats and adult birds). In order to protect the birds, the authorities decide to eradicate exclusively the cats.

1. a) Draw the food web relating these species (including the island's producers) and indicate the type of interspecific relationship between each of them.
2. b) Predict the sequence of changes that the ecosystem will experience after completely eliminating the cats and deduce what will happen to the seabird population (mesopredator release phenomenon).
3. c) Design a more appropriate management alternative that the technicians should have considered to ensure the actual conservation of the birds.

Cattle egrets accompany livestock and eat the insects the animals stir up as they walk, without harming them. State what type of relationship they have with the livestock and compare it with that of a tick attached to the same cow.

Explain the difference between a parasite and a predator, and reason why it is not in the parasite's interest to cause the death of its host.

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.

In a restoration project on a hillside scorched by a wildfire, holm oak saplings are planted in two plots with poor, degraded soil:

• Plot 1: Heat-sterilized soil supplemented with abundant soluble chemical fertilizer (nitrogen and phosphorus).

• Plot 2: Non-sterilized soil inoculated with mycorrhiza-forming fungi, without added chemical fertilizer.

During the first summer, marked by an extreme drought, 85% of the saplings in Plot 1 die from dehydration, while 90% of those in Plot 2 survive and grow vigorously.

1. a) Identify and describe the interspecific relationship responsible for the success of the saplings in Plot 2.
2. b) Explain why the chemical fertilizer in Plot 1 was not enough to guarantee the survival of the holm oaks under arid conditions.

Analyze the following biomass data from two different ecosystems:

• Terrestrial ecosystem (meadow): Producers (grass) = 10 000 kg; Herbivores (grasshoppers) = 1 000 kg; Primary carnivores (frogs) = 100 kg; Secondary carnivores (snakes) = 10 kg.

• Marine ecosystem (open sea): Phytoplankton = 20 kg; Herbivorous zooplankton = 120 kg; Small fish = 15 kg.

1. a) Knowing that an adult snake in that meadow weighs about 0.5 kg, calculate the maximum number of snakes that ecosystem can support and justify why it would be unfeasible to maintain a stable population of 60 adult snakes.
2. b) A student claims that the marine ecosystem data are incorrect because "a biomass pyramid can never have less biomass at the base than at the level above it." Refute their statement by explaining how it is possible for that mass of phytoplankton to sustain a biomass six times larger of zooplankton.