Relationships Among Living Things
A jaguar hunts deer in the jungle, competes with the ocelot for the same prey, and carries ticks that feed on its blood. No living thing lives in isolation: this whole web of relationships keeps an ecosystem in balance. What would happen if one of these organisms disappeared?
Intraspecific and Interspecific Relationships
In a forest, two male deer clash antlers over territory while, at the same time, dozens of caterpillars devour the leaves of an oak tree. Do living things interact in the same way with members of their own species as with members of other species?
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…
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.
Predation, Competition, and Symbiosis
| Relationship | Species A | Species B | Example |
|---|---|---|---|
| Predation | + | − | Orca and sea lion |
| Competition | − | − | Two birds seeking the same fruit |
| Mutualism | + | + | Lichen (alga and fungus) |
| Commensalism | + | 0 | Remora and shark |
| Parasitism | + | − | Tapeworm in the intestine |
(+ benefits, − is harmed, 0 neither gains nor loses)
Explain what predation is and give an example from a marine ecosystem, stating which is the predator and which is the prey.
Explain what predation is and give an example from a marine ecosystem, stating which is the predator and which is the prey.
Distinguish between mutualism, commensalism, and parasitism, and classify these cases: a lichen (an alga and a fungus benefiting each other), a remora travelling…
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.
Food Chains and Food Webs
In a grassland we see grass nibbled by grasshoppers, frogs that hunt grasshoppers, and snakes that catch frogs. How do we show the path that food follows?
Food webs: what happens if … disappears?
A food web is useful for making predictions. Look at this web from a scrubland:
If the coyotes disappeared, the rabbits would increase (they no longer have their predator), they would eat more plants, and the plants would decrease. When there are almost no plants left, the rabbits would also start to starve. This is how balance is maintained: each organism depends on others (interdependence).
In the food chain grass → grasshopper → frog → snake, identify the producer and name the primary, secondary, and tertiary consumers.
In the food chain grass → grasshopper → frog → snake, identify the producer and name the primary, secondary, and tertiary consumers.
The diagram represents a food web; the arrows point from the organism that serves as food to the organism that eats it. a) Which organism is hidden behind the "?"…
The diagram represents a food web; the arrows point from the organism that serves as food to the organism that eats it.
- a) Which organism is hidden behind the "?" symbol, knowing that it feeds on frogs and mice?
- b) Name the primary consumers in the food web.
- 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.
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.
Competition and the Ecological Niche
Two species that use the same resource compete. What happens when they share everything?
This is what happens with invasive species: a non-native species with a niche similar to that of a native one can displace it, even if there is enough food, because they compete for shelter, territory, or prey. In addition, the invasive species may prey on or spread disease to other species, with direct and indirect effects on the web.
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,…
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."
- 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.
- 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.
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…
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.
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Year 2: Pasture cover drops by 70% and the soil begins to erode severely.
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Year 3: The rabbit population suffers a sudden collapse and falls to levels well below the initial ones, even though foxes are still absent.
- a) Explain the trophic cascade that connects the disappearance of the foxes with the degradation of the vegetation in Year 2.
- b) Formulate an ecological hypothesis that justifies why the rabbit population collapsed in Year 3 if there were no predators hunting it.
Energy Is Lost at Each Trophic Level
In the savanna or the jungle there are millions of plants, thousands of herbivores, and very few large predators. Why are there always fewer predators than prey?
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…
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."
- 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.
- 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.
Back to the beginning: the jaguar hunts deer (predation), competes with the ocelot (interspecific competition), and carries ticks (parasitism). If it disappeared, its prey would increase and the vegetation would suffer: everything is connected.