A. The short answer to your first question can be summed up by saying a stream with an adequate food base, the proper substratum conditions, and a suitable environmental regime will usually produce many insects. The long answer would take more space than we have, but I'll try to add a bit about each of these. Obviously, if the insects are to survive and reproduce they must have an adequate food base; some utilize algae growing on stones or sticks, others feed on the fine detrital matter in the water column or where it settles onto the stream bottom, others shred coarse organic material such as leaves, and others feed on other insects. Now to have a proper algae food base, the right environmental conditions in terms of sunlight for photosynthesis, adequate dissolved nutrients (mainly nitrogen and phosphorus), and a solid substratum for attachment must be present. Thus, the proper environmental regime includes such things as the proper nutrient content, temperature conditions that do not reach lethal levels, and varying amounts of dissolved oxygen. In terms of temperature conditions, each insect requires a certain amount of thermal history to develop; thus, the colder the stream, the slower the development will be.

Your second question about a little pollution being good is a bit tricky. In certain conditions, and depending on the "pollution," the answer is "yes." For instance, in an arctic stream where the water temperature rarely gets much above freezing, a little thermal "pollution" could enhance development. A stream draining a basin where the rocks were highly insoluble and thus having low amounts of dissolved nutrients (N and P) could benefit from some pollutant containing these nutrients. Now having said this, I don't want to intimate that I condone pollution; I'm just saying that in some restricted examples, your term "a little pollution" might be good.