A freshwater lake food web consists of 1 species of phytoplankton (producer), 2 species of zooplankton (primary consumers), 2 species of small fish (secondary consumers) and 1 species of larger predatory fish (tertiary consumer). How many trophic levels are present in this food web? Also, name a keystone species that, if removed, could cause a major shift in this lake ecosystem.
Answer
Four trophic levels; the keystone species could be the zooplankton.
The correct answer is Four trophic levels; the keystone species could be the zooplankton. — State the keystone species as zooplankton.
Step-by-step solution
- Define a trophic level: each step of feeding from primary producers upward.
- Count the levels: producers = level 1; primary consumers (zooplankton) = level 2; secondary consumers (small fish) = level 3; tertiary consumer (large predatory fish) = level 4.
- Thus there are four distinct trophic levels.
- Identify a keystone species: a species whose impact on its environment is disproportionately large relative to its abundance. In many lakes, zooplankton regulate phytoplankton blooms and provide food for higher levels, so their removal would drastically alter community structure.
- State the keystone species as zooplankton.
Common mistake: Counting producers as level 0 and giving three levels instead of four
Concept tested: Trophic levels and keystone species
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