Strange Days

Alien invaders. Mysterious connections. "Unsettling transformations are sweeping across the planet." A new series on the SciFi channel? Another end-of-the-world series like Revelations? No, it's a new nature documentary. Strange Days (PBS, April 20 and 27) is a four-part series from National Geographic about the interconnectedness of nature and the unintended consequences when human beings throw one element out of whack.

The first episode shows what happens when alien species are introduced into an environment where they do not belong. For example, a South American engineer gave some water hyacinths from home to his African bride. But some of these beautiful water flowers floated downstream and found their way to Lake Victoria. Now they clog 80 percent of the shoreline, throwing off the fishing industry and causing major health problems, as rotting hyacinths infect the groundwater and stagnant water breeds parasite-carrying snails and malarial mosquitoes.

The episode on predators shows that mean, scary carnivores play a part in nature's design. (We would say God's design, but the series does not go there.) After wolves were eradicated from Yellowstone Park, nothing hindered the elk from gathering at the streams and eating all of the willow shoots. The result was fewer trees, more erosion, no more beavers (which build their dams from willow), and a loss of the benefits of beaver dams to fish and streams. Now, wolves have been reintroduced-to the understandable consternation of both the elk and local ranchers-and things are going back to normal.

The other episodes-on global warming and chemical pollution-are not so interesting, seeming more like boilerplate alarmist environmentalism. But the way the overall documentary uses the structure of mystery stories and the imagery of science fiction works well.