The accidental introduction of crayfish into a lake in British Columbia, Canada, by humans has caused the extinction of two species of fish that had lived there for some 15,000 years—well, sort of.
Life, ah…finds a way, and what has replaced these two fish could have dire consequences for the local environment.
The two fish were similar species of endangered threespine stickleback fish, both of which coexisted peacefully in Enos Lake in BC—until they didn’t.
“When two similar species are in one environment, they often perform different ecological roles,” said co-author Seth Rudman, a PhD student in zoology at UBC, in a statement. “When they go extinct, it has strong consequences for the ecosystem.”
In the case of Enos Lake, one species tended to live in the middle of the lake, where it preyed on zooplankton, while the other lived nearer to the shore and ate mostly waterborne insect larvae. They more or less left each other alone, breeding only with their own species, for thousands of years.
But then, in the early 1990s, American signal crayfish were accidentally introduced to the lake, likely through fishermen who were using them as bait on Vancouver Island.
From then on, the crayfish began to clear-cut the vegetation on the bottom of the lake—forcing the middle-of-the-lake sticklebacks to search for food and mates in the open water above, where the shore-preferring species lived.
Forced together, these two species of fish began to interbreed between 1994 and 1997—and then totally disappeared, leaving behind only a hybrid species of stickleback.
How did this happen?
The current study, which can be found in Current Biology, aimed to figure out just how this human-driven evolutionary process—which is known as reverse speciation (or despeciation)—affected the ecosystem. What is different now that there is one species where two used to be?
As it turns out, the two-species hybrid has lost some of the ecological roles that both fish used to carry out; it spends most of its time near the lakeshore, where it eats large insects—which has led to an increase in the number of small insects coming out of the lake. Moreover, after the advent of the hybrid, there has actually been an effect on the terrestrial ecosystem, as leaves that fall into the lake no longer decompose as quickly as they used to.
In other words, by inadvertently introducing crayfish into Enos Lake, humans have triggered an evolutionary process that has consequences for the ecosystem all around the water—and may accidentally cause more in the future. Reverse speciation is especially likely to happen in “young” species—ones that are, say, thousands of years old instead of millions—and is a process that seems to be on the rise, thanks to humans.
Which means that, down the road, Canada could be facing a huge ecological issue as more and more species disappear into hybrids.
“Much of Canada’s biodiversity, particularly fish in lakes and rivers, are considered to be ‘young’ species that formed in the last 12,000 years or so,” said Rudman. “This type of evolution, known as reverse speciation, happens remarkably quickly and can cause alterations to the ecology of the ecosystem. It means we need to consider evolution in our conservation efforts.”
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Image credit: Ernie Cooper
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