Discussion
A researcher sets up ten large outdoor tanks, each containing the same amount of algae and the same number of guppies (herbivorous fish). In five of these tanks, she also adds gourami fish, which are predators of guppies. Tanks were covered in mesh so no new species could enter. After 2 months, the researcher counted the number of guppies and gouramies, and measured the amount of algae. She found that the amount of algae was much higher in the tanks with gourami fish compared to those without gourami fish. Which one of the following processes best explains the differences in amounts of algae between these tanks?
Answer: D
The best explanation is a trophic cascade.
Gouramis eat guppies, and guppies eat algae. In tanks with gouramis the predators cut the guppy numbers, so grazing pressure on the algae falls and the algae build up. An effect at the top of the food chain has passed down two levels, raising the producers by suppressing the herbivores. That top-down, indirect effect is a trophic cascade.
A competition-colonisation trade-off explains how competitors coexist through differing dispersal abilities. The competitive exclusion principle says two species sharing one limiting resource cannot coexist indefinitely. Interference competition is direct harm between competitors. None involves a predator changing the abundance of a plant through its prey.
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