Neurons seem to have at least two self-destruct programs. Like other cell types, they have an intracellular death program for undergoing apoptosis when they are injured, infected, or not needed. In addition, they apparently have a second, molecularly distinct self-destruct program in their axon. This program is activated when the axon is severed and leads to the rapid degeneration of the isolated part of the cut axon. Do neurons also use this second program to prune their axonal tree during development and to conserve resources in response to chronic insults?

Many regulated streams are characterized by high variable and unpredictable flow regimes. Since changes in streamflow directly modify physical habitat, streams with such highly variable flows provide highly unstable aquatic habitats. We evaluated the effect of artificial streamflow fluctuations on stream fish communities by comparing fish densities, in species and habitat groups, between two rivers differing in daily flow regime: on with with a natural flow, and one with highly regulated flows. We developed a simple model describing the relationship between available streams habitat and its use by 15 species or size classes of fish in the natural river. Species and size classes that used a specific set of microhabitat conditions were identified by comparing habitat characteristics for samples with and without each type of fish; for fish that used a particular type of microhabitat, we grouped species and size classes according to similarity in microhabitat use. Next, we categorized stream habitat samples in both the natural and regulated rivers into groups on the basis of fish habitat use criteria. Fish densities for each fish and habitat group were ten individually compared between the two rivers. An abundant (>90% of all fish) and diverse (nine species) group of small—fish species and size classes were restricted to microhabitat that was characterized as shallow in depth, slow in current velocity, and concentrated along stream margins. This group of fish was reduced in abundance in the regulated river and absent at the study site with the greatest flow fluctuation. Another fish group included species and size classes that used either a broad range of habitat or a microhabitat that was deep, fast, or both, and was concentrated in midstream areas. The density of fish in this group was higher in the regulated river and peaked at the sites with the greatest fluctuations in flow. Highly variable and unpredictable flow regimes appear to be a high—frequency disturbance that effects fish differently depending on the way they use stream habitat and acts to reduce community complexity.

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