Researchers are studying a fish population in a small pond that has algae as the only producer in the ecosystem. The scientists are concerned about what might happen to the fish, if the algae suddenly disappear. Why is this a concern? What might be done to prevent this from occurring?

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.