Editor's Note: We are republishing this new story from our February 1995 issue as part of our in-depth report on Science at the Movies.

The world was safe all along. Back in the 1950s, moviemakers regularly served up the spectacle of creatures from other planets attempting to take over our bucolic little orb. Heroic earthlings fought the aliens with dynamite, napalm, atomic torpedoes and bad acting. But had the heroes been better acquainted with life-history strategies -- the reproductive behaviors that determine patterns of population growth -- they might not have bothered.

“In general,” says May R. Berenbaum, professor of entomology at the University of Illinois, “none of [the aliens] exhibit the opportunistic sorts of reproductive traits or characteristics of organisms that successfully colonize.” Her findings help to explain why earthlings should be afraid of at least some contemporary invaders “such as zebra mussels, bark beetles, medflies and, perhaps, the sluglike aliens featured in one of last year’s movies, The Puppet Masters.

Berenbaum’s first try at sharing her interest in cinematic biology was an abortive attempt to organize an insect film festival while she was a graduate student in entomology at Cornell University. “I thought it could be a way to attract a large audience to insect issues,” she recalls. Shortly after joining the faculty at Illinois, however, she teamed up with Richard J. Leskosky, assistant director of the university’s Unit for Cinema Studies, to get the bug film festival finally flying. The couple went on to produce several papers on insects in movies and cartoons, as well as a daughter -- the aficionado of entomology and the film buff were married in 1988.

In 1991 Berenbaum was invited to lecture at the Midwest Population Biology Conference. “I thought it might be entertaining to look at population biology in the movies,” she says. “And a recurrent biological theme in films is the idea of invading organisms.” It seemed a testable hypothesis to see whether fictional invaders share the attributes that invading organisms in real biotic communities display.

So Berenbaum and Leskosky looked at the life histories of aliens in science-fiction movies released in the 1950s, a time when movies were lousy with invading organisms. (Film historians attribute the obsession to the recent memory of Nazi aggression and to cold war paranoia.) The two used Keep Watching the Skies, an exhaustive compilation of science-fiction flicks, as their database. Their lecture -- a version of which was published in 1992 in the Bulletin of the Ecological Society of America -- ended with films from 1957, the last year the book covered.

Of the 133 movies described in the text, 67 fulfilled Berenbaum’s requirement for inclusion in the study: they depicted an extraterrestrial species. Analysis showed that invading is a dicey lifestyle choice. “We determined that, collectively, alien beings in science-fiction films do suffer from high mortality,” Berenbaum and Leskosky wrote. Indeed, in only three of the movies do aliens survive to see the credits. They die at the hands of humans and through acts of God or the director -- earthquakes, volcanoes and avalanches all come to the rescue of humans. But heroic deeds or natural disasters were probably less threatening to the long-term survival of the invading species than their own poor fecundity.

Opportunistic species, those good at colonizing new environments, exhibit so-called r-selection. “These species have a set of traits -- small body size, rapid growth, huge brood sizes,” Berenbaum explains. Those qualities lead to a high r, the intrinsic rate of increase, which can cause big problems in real life as well as in real bad movies. “European bark beetles, just reported in Illinois a year or two ago, almost shut down the entire Christmas tree industry,” Berenbaum notes. “A National Academy of Sciences study showed that introduced species have caused about $90 billion worth of economic damage.

“On the other hand, those species marked by slow development, reproduction later in life and large body size -- traits of so-called K-selection -- are good at competing in a stable environment but poorer at colonizing a new one. Thus, California farmers find themselves fearing r medflies far more than K elephants. The typical 1950s alien invader, however, is far closer biologically to an elephant than to an insect.

The aliens also suffer from overconfidence. Berenbaum and Leskosky found that 42 of the movies showcased either a lone invader or a pair. Only 21 films have the earth threatened by more than six intruders. The small initial invading force, combined with failure to go forth and multiply once they reach the planet, renders most movie aliens nothing more than short-term threats. The few invaders who do try to reproduce once they land make efforts that are biologically questionable. For example, the attempts of the title character in Devil Girl from Mars (1955) to mate with humans is “an undertaking fraught with hazards associated with postzygotic reproductive isolating mechanisms,” Berenbaum and Leskosky point out. (Strictly speaking, the humanoid Devil Girl was less interested in colonization than in the abduction of human males that she could import back to her home planet for breeding stock.)

Students of Stanislavsky would therefore do well to contemplate population biology in addition to “The Method” before accepting roles in science-fiction films starring K-type invaders. They would not act so scared.