A company claims that its fish are grown in large tanks, "each one a complete ecosystem that produces no biological waste." This claim implies that the tanks contain which of the following?
primary consumers
fish predators
at least four trophic levels
a diverse variety of species

The correct answer is "at least four trophic levels".

Explanation:

Ecosystems are biological communities of species that interact with each other at different trophic levels. Species of the four trophic levels include producers that make their own food, as well as primary, secondary and tertiary consumers that obtain their food from other species. In this case, a company that claims that their fish growing in large tanks represent complete ecosystems that produce no biological waster, imply that their tanks contain at least four trophic levels. This ensure that the fish can live and prosper without any external source of food and that they live and die without leaving biological waste.

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