Homogenous rivers, homogenous faunas.

H abitat change and invasions of alien species are usually listed as the two biggest causes of biodiversity loss. When the two coincide, the results can be devastating, because altered habitat often favors alien species adapted to the changed conditions. This is particularly true in streams and rivers of North America, where homogenization of fish faunas through extinction of native species and invasions of alien species have been widely recorded (1). Although dams that alter flow regimes have been implicated as a causal factor behind this shift in North America, the degree to which flow regimes have been altered is not widely appreciated. The study by Poff et al. (2) in this issue of PNAS should help to change this problem. They document that flow alteration by dams in the U.S. is pervasive and that the resulting flow regimes tend to converge on one another, regardless of the original f low regimes of the dammed rivers. Dams homogenize flows mainly by reducing peak flows and increasing minimum flows, creating more uniform flow conditions. They also change the timing and duration of high and low flows. The exact changes to the flow regime vary from region to region but result in rivers with similar flow regimes and reduced variability in flows. From an aquatic organism’s perspective, reduced variability in flows is particularly important, because this results in reduced channel complexity, often with stable channels that have reduced capacity to carry high flows and that are incised at low flows. Less complex channels translate into a lowered diversity of structural features preferred by fish and other aquatic organisms (3). This loss of structure, in turn, results in fewer fish of fewer species. Ironically, these relationships do not change with large hydropower dams that have frequent peaking flows, even though they increase flow variability, because the time scales (often daily) are much shorter than found naturally, whereas other aspects of the flow regime remain similar. Flow regulation is typically coupled with land-use changes that take advantage of the reductions in flow variability, further homogenizing aquatic habitat. The most prominent of these changes is the construction of close levees, which disconnect the river from its f loodplains and reduce or eliminate riparian succession and channel migration. Such restrictions on rivers, of course, are generally perceived as desirable, which is why they are nearly universal. Likewise, the fishes that invade such regulated rivers are often desirable game and food fishes, such as centrarchid basses (Micropterus), catfishes (Ictaluridae), and common carp (Cyprinus carpio). In the cold ‘‘tail waters’’ below dams, productive fisheries for various trout (Salmonidae) species develop, usually sustained by fish hatcheries, because flow regulation degrades natural spawning habitat. Thus, homogenized rivers and fish faunas are often regarded as beneficial not only in the U.S. but also in other countries. For example, rivers