BioEd Online

When the Panama Canal opened in 1914, it was a tremendous boon to the shipping industry. But research shows that it is not only boats that have taken advantage of the shortcut: several of Central America's fish species have also made the journey.

The canal links the Rio Chagres and Rio Grande rivers, which are on opposite slopes of the Isthmus of Panama. When the waterway opened, the once-isolated fish communities of the two rivers were given the chance to intermingle.

Although both rivers are now home to a handful of new fish, no species has gone extinct as a result, report Scott Smith of McGill University in Montreal, Canada, and his colleagues. This is at odds with many experts' belief that intruders upset the delicate balance of ecosystems.

Find your niche

According to the 'niche' theory of ecosystems, every species within a community, be it river or rainforest, has a well-defined role in life. Every species interacts with other species in the group, for example by forming a predator-prey relationship, or forging a mutual bond such as that of corals with algae.

Some theorists argue that ecosystems do not have the capacity to accommodate new arrivals. In other words, all possible niches tend to be filled, so any intruder will be stepping one someone's toes. But the fact that Panama's two fish communities were able to mingle without any extinction argues against this idea.

Scott and his colleagues sampled fish from the two rivers and compared the species living there today with those found in a census taken before the canal opened. The Rio Grande has gained five freshwater species that previously lived only in the Rio Chagres, and three species have spread in the opposite direction. This indicates that the number of species in the Rio Grande has increased by 28%, and in the Rio Chagres by 11%, the researchers report in the Proceedings of the Royal Society of London B¹.

If any species were to go extinct as a result of increased competition, they should have done so by now, Scott says. "Our results suggest that the process has not occurred, within the timescale of 10 to 100 generations that is envisioned by ecologists," he says. And although Scott concedes that species may still go extinct over a much larger timescale, he thinks this is unlikely.

Space invaders

This does not mean that ecosystem invaders are always benign, Scott says. "A small number of case studies have taught the scientific community that invasions may have disastrous effects," he says. This is particularly true when the invader is a top predator such as the Nile perch (Lates niloticus), which was introduced to Lake Victoria in Africa in the 1950s. The perch wiped out an estimated 200 native fish species.

Although ships continue to chug up and down the Panama Canal, there is unlikely to be any more traffic in fish species, the researchers predict. The reason is the introduction of the predatory peacock cichlid (Cichla ocellaris) to Lake Gatun, an artificial lake halfway along the canal's route, in 1967. With such a dangerous gatekeeper blocking the way, the two rivers are effectively once again separated.