A. Despite apparent similarities in streams in terms of basic habitat, there are significant differences in terms of microhabitat, food supplies, and basic environmental requirements that govern which species can successfully live in a given stream. If one basic constituent that a species requires (temperature range, suitable food, competition, etc.) is missing, then it doesn't matter if everything else is there - it simply can't exist where its basic requirements are not present. Now, that doesn't explain the absence of some species, such as the Giant Stonefly, in some streams and not others, but it's a pretty good bet that some vital factor is missing. There absence in a particular stream may just mean that they haven't colonized that stream.