recap

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55.1 recap

Species interactions may be classified into four basic categories: predation (a trophic interaction in which a predator kills and/or consumes its prey, or part of its prey), competition (a nontrophic interaction in which species use some of the same limiting resources and have negative effects on one another), positive interactions (a trophic or nontrophic interaction in which at least one species benefits from the interaction and none are harmed) or amensalism (a nontrophic interaction in which one species is harmed but the other is not affected). Species interactions are not always clear-cut but instead fall along a continuum in which species vary in how strongly they affect one other. Some species interactions result in evolutionary change.

learning outcomes

You should be able to:

  • Use examples to distinguish among predation, competition, positive interactions (mutualism and commensalism), and amensalism.

  • Give examples of species interactions in which the responses of two species are asymmetrical, and evaluate possible reasons for the asymmetry.

  • Compare biotic and abiotic factors as agents of natural selection and evolution.

Question 1

Classify each of the following species interactions as carnivory, herbivory, parasitism, competition, amensalism, mutualism, or commensalism.

  1. Leafcutter ants harvest and transport leaf fragments to their nests, where the vegetation will support fungus, which the ants consume.

  2. Elephant seals lie on rocks, crushing and detaching algae underneath them.

  3. Head lice inhabit the human scalp and feed on blood.

  4. Wolves and cougars both prey on the same deer population, which is limiting to both predators.

(a) Mutualism
(b) Amensalism
(c) Parasitism
(d) Competition

Question 2

Name some conditions that can modify or change the outcome of a two species interaction.

The conditions that can modify or change a two species interaction include physical conditions, biological processes such as dispersal, and other interacting species.

Question 3

Do you think the interactions between lionfishes and their fish prey in the Atlantic Ocean have resulted in evolutionary change for the prey? Explain.

No, it is unlikely that the lionfish prey in the Atlantic have had enough time to adjust in an evolutionary way to their novel predator. This conclusion is supported by the data, which show that some reefs in the Atlantic have suffered a 65 percent decline in small-bodied coral-reef fish.

Trophic interactions are critical to life on Earth. As a result, the variety of ways that heterotrophic organisms acquire food plays a critical role in the success of every species on Earth. In the next section we will consider how predation influences the ecology and evolution of both predator and prey species.