The largemouth bass is a top predator in the lakes of the midwestern United States. These bass often eat smaller fish, such as minnows. Minnows eat zooplankton, which eat phytoplankton (microscopic algae).
Small lakes, like the ones where these organisms live, are ideal systems for testing the role of a species in an ecosystem. Lakes have clear boundaries, and their food webs are often relatively simple. Scientists can also use neighboring lakes in comparative experiments, which test and compare the effects of different conditions.
To test how the presence or absence of a top predator affects the lake ecosystem, scientists removed largemouth bass from some lakes and not others. The absence of the bass caused minnow populations to increase. More minnows ate more zooplankton. Fewer zooplankton led to a rapid phytoplankton increase called an algal bloom. Algal blooms use up a lot of the oxygen in a lake, which may kill many of the other species. Blooms of certain phytoplankton species can even turn the water toxic.