Study: Stories of Invasive Species Often Begin with Undetected “Sleeper Populations”

When an invasive species overruns an ecosystem, it is often assumed that the organism recently arrived at its new home and rapidly took over. But a report published in the journal, BioScience, says that many new arrivals aren’t nearly as impatient as this narrative implies.

In fact, it is not uncommon for “sleeper populations” of nonnative species to spend years or even decades persisting at such low population levels that they go undetected, only making their presence known once their numbers become super abundant.

While a lot of research focuses on impacts and consequences after an ecosystem is already invaded, less time is spent identifying the drivers behind these invasions, says Jake Vander Zanden, director of the University of Wisconsin-Madison’s Center for Limnology and a co-author of the paper, which was published January 27th.

Nonnative species can occur as small, self-sustaining populations “smoldering like embers” in a new habitat, he says. Many will continue on with that modest life history, never having the negative ecological impacts that would get it slapped with the “invasive” label. But, sometimes, an “environmental trigger” gets switched and the smoldering embers of these sleeper populations are fanned into a five-alarm fire.

An Unexpected Find in “The Most Studied Lake in the World”

Spiny water fleas coat the hand of grad student, Vince Butitta. Photo: V. Butitta

Vander Zanden had a front row seat to one such invasion. In 2009, a class of University of Wisconsin undergraduates was taking zooplankton samples in Lake Mendota as part of a lab field trip. When they pulled in their nets, they discovered thousands upon thousands of a species of tiny crustacean that had never before been seen in the lake.

The unexpected find was a little, free-floating organism called the spiny water flea and its discovery was made more surprising by the fact that it was in a place scientists had previously predicted would be unsuitable for this cold-water species originally from European and Asian lakes.

Yet in Lake Mendota the nonnative transplant became invasive, swelling to a population density that had never been recorded in any lake anywhere.

According to the report, this extreme abundance of the spiny water flea was particularly shocking because zooplankton populations in the lake are monitored regularly by Center for Limnology researchers working in the North Temperate Lakes program (NTL-LTER), a research site of the US Long Term Ecological Research Network.  Since the 1980s, NTL-LTER field technicians have conducted routine sampling in Lake Mendota and, in more than two hundred sampling trips spanning those three decades, they hadn’t turned up a single spiny water flea.

But, when researchers dug into Lake Mendota sediment cores and combed through old samples in the UW-Madison Zoological Museum, they discovered evidence of spiny water fleas dating back at least ten years before their sudden population explosion in the lake. The spiny water flea had been an ecological stowaway, existing at such low numbers that it had avoided detection in those two hundred NTL-LTER sampling events and several dozen undergraduate “lake labs.” This discovery led scientists to confront an uncomfortable truth.

“It’s quite possible that the spiny water flea is established in lots of other lakes,” says Vander Zanden. “Currently, invasive species management is aimed at minimizing spread, but what if they’re already there? What if, instead, management practices were aimed at not setting off that trigger that would make populations explode?”

Understanding “Environmental Triggers”

For their study, Vander Zanden and his colleagues highlighted what they call “environmental triggers” that have led to invasive species events in the past.

For example, the study points to Santa Cruz Island off the coast of California, where humans brought cattle to the island to graze. The cattle, in turn, tracked seeds of a nonnative fennel plant in with them. That plant got established on the island but was held in check by the grazing cattle. Once the cattle were removed from this new food web, however, the fennel took over.

In other parts of the world, climate change is triggering invasions, allowing long-simmering populations of rodents and weeds to thrive in ecosystems that had previously been too cold.

And, sometimes, we just get unlucky. In the story of the spiny water flea invasion of Lake Mendota, one abnormally cold summer was all it took for the sleeper population to explode. That summer, water temperatures in the lake hovered within a range similar to what the spiny water flea preferred in its native habitat. Not only did that result in billions of spiny water fleas thriving in the lake, the invaders also produced billions of what are called “resting eggs,” which fell to the bottom of the lake, waiting for the next year to hatch and swarm all over again.

Mike Spear in scuba gear sampling for for invasive zebra mussels in Lake Mendota.

Understanding the environmental triggers that set a species on the path to invasion is crucial, says Mike Spear, lead author of the paper and a current post-doctoral researcher at the University of Illinois and Illinois Natural History Survey.

“We are living in an era defined by environmental change. So, if environmental change is the trigger for them to explode and become super abundant and harmful to the ecosystem, then we’ve got a pretty big problem on our hands,” he says.

Spear says that the paper “asks a lot more questions” than it answers but hopes that it can help people realize that detection of a nonnative species in an ecosystem doesn’t mean that it only recently arrived and, more important, doesn’t mean that an invasion is imminent.

“What we’re trying to focus on are the external environmental factors that could trigger an outbreak and what implications that has for how we study and manage invasive species,” he says. “This paper argues for placing a real emphasis on finding where nonnative populations are and where they are at low levels, because that’s really our only chance of preventing the damage before it happens.”

Getting Better at Detecting Nonnative Species

Of course, finding these “sleeper populations” is easier said than done. If the spiny water flea could set up shop undetected in one of the most studied lakes in the world, how can researchers hope to find potentially invasive species in other bodies of water?

“This paper brings up a lot of questions about how prevalent nonnative species are at low population levels on the landscape,” Spear says. “We don’t know the answer to that. And so it’s sort of impossible to prioritize [management decisions] correctly if you don’t even know how many are out there.”

Field technician Dane McKittrick, helps Spear collect water samples, to be analyzed for eDNA. Photo: Sydney Widell

Fortunately, says Spear, there are promising advancements in scientific methods – like analyzing water samples for the environmental DNA (eDNA) of specific species. As an example, the report points to the detection of invasive Asian carp in a Chicago waterway. Using eDNA methods, a crew was able to collect water samples and detect DNA from the invasive species after only a single day of work. By comparison, traditional electrofishing methods of searching for the fish took 93 days to yield a detection.

The report also highlights how crucial long-term research is to documenting invasion dynamics and placing the story of an invasion in context. Without the NTL-LTER’s decades of samples from Lake Mendota, for example, the narrative that the spiny water flea was new on the scene could have ended up accepted as fact. Instead, researchers were able to comb through a sort of ecological archive to get at the full story.

Spear says that he hopes anyone reading his study will see it as a call to rethink their assumptions about any invasive species they encounter in the future. It’s possible, he says, that that newly detected organism “could have been there a lot longer than you think [indicating] a much more complex ecological process of an invasion over decades with different stages of growth.”

In other words, it’s possible a new arrival is really an established member of the neighborhood – just waiting for the right conditions to make itself known.