Bythotrephes longimanus
a.k.a. - spiny water flea
Cascading impact of spiny water flea on ecosystem services in Lake Mendota
Lake Mendota is located at the southwestern edge of the spiny water flea's invaded range in North America. As an invader expands its range it's likely to come across new ecological interactions that may lead to new impacts. Until Lake Mendota, the spiny water flea's impact was restricted to its predatory impact on native zooplankton communities. Lake Mendota may be the first documented case where the spiny water flea's range trips over the "land mine" of agricultural phosphorus loading and cultural eutrophication. Further, we've managed the lake's food web in such a way to promote an abundant and healthy population of Daphnia that controls algal abundance. Daphnia have been particularly vulnerable to spiny water flea predation.
Plot made using package "xkcd" in R (Torres-Manzanera 2014). xkcd is my most very favorite online comic.
This interaction between an invader expanding its range, existing ecosystem stressors, and our management of those stressors may have made Lake Mendota uniquely suited to be impacted by a spiny water flea invasion.
We've observed a 60% decline in our biomass of Daphnia pulicaria (the big algae grazer) and a resulting decline in water clarity of 0.9 m. We updated a "willingness-to-pay" study and found that 1 m of water clarity in Lake Mendota is worth ~$140 million to Dane County.
We've used multivariate autoregressive models to model water clarity time series in Lake Mendota. We've found that we would have to reduce phosphorus loading into the lake by 71% (!!!) to counteract the negative effects of spiny water flea on our algae-grazing friend, D. pulicaria.
This would cost us anywhere from $86 million to $163 million... Assuming we could actually reduce loading by this much. In our 50+ years managing water quality in Lake Mendota, we haven't made any significant reduction in phosphorus loading into the lake.
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Since 1 m of water clarity is worth something on the order of $140 million, these results suggest that it is worth it to redouble efforts to reduce the P load into Lake Mendota. It also suggests that researching control and eradication methods for invasive species - which are often thought to be too expensive or too challenging - may be worth it.
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In Lake Mendota, spiny water flea revealed the value of preventing species invasions and supporting resilient ecosystems (e.g., reducing agricultural runoff into lakes) to ensure that valuable ecosystems services are protected in the future.
Invasive "Sleeper Cells": The problem of detection limits, high propagule pressure, and long-invaded landscapes in range expansion
In spy dramas, a sleeper cell is a group of people who inconspicuously remain dormant in a community until activated for some nefarious plot. Invaded landscapes, particularly those that have been invaded by relatively cryptic species for long periods of time (as is often the case), experience a similar problem. Invasive species can persist at low densities until favorable changes in ecosystems or the invader itself cause it to "activate", become abundant, and have impacts.
We believe that this may be a common mechanism of invasive species' impact in ecosystems and landscapes.
Environmental conditions trigger rapid transition from undetected invasion to high density of spiny water flea in Lake Mendota (WI, USA).
The spiny water flea was first detected in Lake Mendota by Jake Vander Zanden and his limnology class in the fall of 2009. Jake recalls pulling a zooplankton tow so dense with spiny water flea that the sample jar looked more like jar of apple sauce than a jar of water. We later confirmed that the densities regularly reached in Lake Mendota are higher than any other lake on record.
The kicker here is that we've been monitoring Lake Mendota's zooplankton community on a monthly to fortnightly (every two weeks) basis for almost 40 years. How does a species go from absent to record densities in a single summer?
As it turns out... it didn't.
To explain the story, I'll have to back up to a conversation Jake VZ and I had repeated several times a year over the past four years. Could we build a model to "observe" (remember, the spiny water flea went undetected through the majority of 2009) the unobservable population dynamics that lead to the record 2009 densities? Would this help us piece together what happened in 2009?
Early on, the answer was fairly simple. The spiny water flea can reproduce both sexually and asexually, the latter leading to incredibly high population growth rates. Our model predicted that, while it would have taken a fairly large number of propagules (a propagule is a group of invasive individuals introduced to a new system), it was possible that the spiny water flea grew so quickly in 2009 that it surpassed our detection limit and shattered density records.
But we were still unconvinced, or at least, unsatisfied.
The spiny water flea had been in Lake Michigan since the early 1980s and boater traffic between the two lakes is fairly high, so why hadn't it invaded in the 30+ years prior to 2009?
In the years since it had invaded we'd seen summers where the lake heated past the spiny water flea's thermal limit and, as a result, the population crashed in July and August before returning later in the fall. We knew that 2009 was the mildest mid-summer on record in Madison, was it possible that mid-summer hot temperatures had limited the population before?
If so, why did the population persist through 2011 and 2012, two of the hottest summers on record?
So we spiced up our model. We rigorously and carefully modeled mortality, reproduction, and growth rates. We added temperature dependence to individual growth and mortality rates. We built in an egg bank (spiny water flea sexual reproduction leads to resting eggs which are deposited in lake sediments until they hatch under favorable conditions - this is a strategy for getting through the harsh conditions of winter, "overwintering"). Finally, we retooled everything to fit our data from Lake Mendota, which is the only lake in the literature to exceed spiny water flea's thermal tolerance.
Our model revealed several key pieces of information that lead us down this "sleeper cell" path:
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The production of the egg bank in the summer is essential to the spiny water flea persisting in the warm Lake Mendota. We observe a peak in production before unfavorable summer conditions and those eggs hatch in the fall to replenish the population after it crashes in the summer. The egg bank allows for both overwintering and "oversummering".
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By this mechanism, the spiny water flea could have persisted indefinitely at a low density in Lake Mendota prior to 2009. This is also the same reason why the spiny water flea maintains record densities even though the lake is periodically lethal to the population. For example, 2011 and 2012 were two of the three hottest years on record, but spiny water flea persisted at high densities both in real life and in the model simulation.
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Thermal conditions in 2009 (the coolest July on record) allowed the population to explode. No other year could have produced the same explosion. A cool summer in an otherwise warm lake "activated the sleeper cell". Put another way a single month of strange climatic conditions lead to a permanent, harmful change in the food web.
Oh... and a couple months ago I went through our archived samples from before 2009. The spiny water flea was in Lake Mendota in 2008 at incredibly low densities (2 found in a single sample from October - we typically find hundreds in our October samples).
So the question is no longer, "were they here before?" but instead "how long was the sleeper cell here before?"
Observing the sleeper cell through lake sediments
The sleeper cell story is corroborated by sediment cores where we find evidence (spiny water flea tail spines) in deep core layers that correspond to past decades. Sedimentation in lakes works like rings on a tree, so we can see through time by going deeper into sediments. Most "paleoecology" studies go much deeper into the lake sediments (like 50-100 feet) to ask much larger questions (e.g. what was climate like before, during, and after glaciation?). We're interested in the top foot or so of sediment... Not a very "paleo" study.
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We've found spines in sediments dating back to 1994 (plus or minus 5 years), supporting our "sleeper cell" hypothesis.
We're in the process of using the population model we built to evaluate the potential range of the spiny water flea in North America and to predict where these "sleeper cells" may occur. We've had promising results with both and are looking forward to combining these with correlative niche models to get a more complete picture of the spiny water flea's potential range in North America.
When we run the model across lake temperature data generated primarily from mean July air temperature taken from latitude and longitude cells across North America, we see that Lake Mendota does in fact lie in a band of "sleeper cell" geographical space (Plot D).
In an average year, annual egg bank growth is near zero, so the population doesn't increase or decrease (Plot A). Under a cool year, annual egg bank growth increases substantially, allowing the population to explode (Plot C).
There are other lakes like Mille Lacs Lake in MN and Lake Calumet in IL that are near this band of sleeper cells and seemed to follow a similar dynamic of an explosion in 2009.
Also interesting is the reduction of the range of suitable thermal habitats given an increase in mean July air temperature (Plot B). This would predict that under a warmer climate, lakes like Lake Mendota may no longer be thermally suitable for spiny water flea population growth.
If you simulate our Mendota population model into the future, you see that 2-3 hot years followed by average conditions could "crash" the Mendota population of spiny water flea and that 3-5 hot years followed by a cool year would crash the population. However, it is rare that we see back-to-back (or back-to-back-to-back :) ) hot years.
Modeling the potential range of an invasive species through combining mechanistic and correlative niche models
In reality (not in the model), in Madison, 2011 was the 3rd hottest July of all time and 2012 was the hottest July of all time and we did not see the population crash out. In fact, while the summer was very stressful for the population, later that fall in both 2011 and 2012 the population eclipsed the record densities we saw in 2009. From this we can infer that either 1) it's going to take more hot years to collapse the population or 2) the model doesn't capture the relationship between fall temperatures and population growth well. In almost all of North America (at least north of 35 N latitude), at some point in the year a lake will cool to favorable growth conditions for the spiny water flea and since it can both "oversummer" and "overwinter" the population can endure relatively short periods of hot or cold weather.
It's likely a combination of both hardiness in spiny water flea's life history and model inaccuracies. Either way, this highlights how important it is to ensure that spiny water flea does not establish in a lake in the first place as we likely can't count on climate change to eradicate populations along the spiny water flea's southern edge.