Population declines among a crayfish species which has devastated lake ecosystems in the US since being deliberately released in the 1950s are due to its own voracious appetites, researchers have found.
The rusty crayfish (Orconectes rusticus) is an American native species, but its range was artificially expanded mid-century when fishing enthusiasts introduced it into a series of lakes in Wisconsin. The crays, the anglers reasoned, would make an excellent and permanent source of live bait.
The crustaceans certainly adapted to their new surroundings well enough – so much so that their numbers exploded. Hungry for small fish and freshwater invertebrates, and not averse to a spot of landscape modification, they have been so successful that today in many lakes there are scarcely any fish left for anglers to catch.
In some of the bodies of water, a single trap can attract as many as 100 of them.
Recently, however, ecologist Eric Larsen from the University of Illinois noticed something strange. In some lakes the crayfish population was plunging, while in others it remained robust. Teaming up with some colleagues, he decided to find out why. The results are published in the journal Ecology.
The critical factor in the fortunes of the cray, it turns out, is the nature of the lake bed. In those in which the bottom is liberally strewn with rocks and boulders, the crustaceans are thriving. If the dominant surface is mud, however, numbers are declining rapidly – and the reason, the researchers discovered, is the behaviour of the crays themselves.
“The rusty crayfish wants to be in rocky substrate,” says Larson.
“It uses that rock to avoid predation by fish or even wading birds, river otters, or raccoons. If it doesn’t have that rock, it uses aquatic plants as secondary habitat as shelters to avoid predators.”
And therein lies the problem. The crustaceans spend their time burrowing and foraging around in the muck, uprooting aquatic plants in the process. They also clip plants in half while pursuing prey.
The effect has been overwhelming.
“From a condition decades ago where 50 to 60% of our trap locations in some lakes had plants, it’s now about 5%,” says Larson. “It’s a pretty big reduction of coverage.”
And that’s a problem for a species that needs to shelter under things to avoid become a meal for hungry birds and mammals. With nowhere to hide, and nowhere to run, the rusty crayfish, it appears, is now preyed upon in unprecedented numbers.
Larsen acknowledges, however, that this might be too simple a theory to completely explain the situation. He and his colleagues want to conduct further research to determine whether the crays’ impact on sandy-bottom lakes has also affected other factors, such as the presence of parasites or predatory fish.
“We’d also like to study more of the effects of climate on these crayfish from year to year through lake temperatures, drought, and duration of ice cover,” he adds.
Whatever the reasons for the species decline, and however welcome that may be, the end of the rusty crayfish in some of the lakes of Wisconsin is at best a Pyrrhic outcome.