Sometime in the next five or six weeks, the ice on the Tanana River in Nenana, Alaska, 55 miles southwest of Fairbanks, will break up. Every year since 1917, local residents have hosted a contest, called the Nenana Ice Classic: anyone who purchases a ticket can guess the exact date and time the ice will go out, and the closest guess wins the pot – in recent years, so many people have entered that multiple people have picked the right minute, and they’ve had to divide the prize money.

Last year’s winners split $363,627.

With that kind of cash on the line, it’s no surprise that the organizers of the Nenana Ice Classic have kept meticulous records. We know when, exactly, the ice on the Tanana broke up outside Nenana each year for almost the last hundred years – that’s the kind of archive that ecologists dream about, because it’s a long enough record to allow us to see changes over time.


The ice on the Tanana River in Nenana, Alaska, breaks up between mid-April and mid-May each spring. Since 1917, when the Nenana Ice Classic began, the average trend has been toward earlier ice-out dates. 

Sources: Data from the National Snow and Ice Data Center and the Nenana Ice Classic

(Figure by Emily Benson)

Environmental cues that organisms use to time their migrations or developmental milestones are changing as the world’s climate changes: plants are blooming earlier than ever before, frozen rivers are thawing sooner and sooner, and in some places, salmon are returning to freshwater to spawn weeks earlier or later than they have in the past.

That can be a problem for organisms that rely on the salmon, and their eggs, for food – if those animals don’t know when the salmon will be arriving, they might miss their chance to chow down. Enough mismatches in timing, and some species might face a serious threat to their survival.

A group of scientists working in a coastal Alaskan stream recently investigated the migration timing of Dolly Varden, a type of fish that often lives in the same streams as salmon, and which sometimes depends on salmon for food. As the authors write, “[w]here salmon remain at historical levels of abundance, Dolly Varden can acquire the majority of their annual energy intake by gorging on salmon eggs.”

The researchers recently reported their results in the journal Freshwater Biology. They compared the timing of Dolly Varden migrations to salmon migrations over ten years, and they also analyzed environmental conditions, like water temperature and precipitation, to see if Dolly Varden were responding to environmental cues (in which case they might be at risk of missing the salmon migration), or to the movement of salmon themselves.

They found that Dolly Varden seem to synchronize the timing of their migration with that of salmon. Dolly Varden migrations “appear to be cued directly by salmon migration rather than environmental conditions,” suggesting that Dolly Varden are less vulnerable to a timing mismatch than they might be otherwise.

Still, not all animals will be as lucky as Dolly Varden. The authors point out that Dolly Varden can likely see or smell salmon as they return to freshwater to spawn, alerting them to their presence; other migrating animals can’t be assured that the resources they depend on will await them at the end of their journey, and must rely on environmental cues as a proxy. Those organisms are the ones most vulnerable to a timing mismatch, and the ones most likely to suffer negative consequences as environmental indicators – like the date the ice goes out on the Tanana River – continue to shift in time.

Dolly Varden appear to base their migration timing on when salmon are migrating rather than on environmental cues. 

(Image by cinaflox via Flickr)

Razor edges

I stepped into the water at the lake’s edge, and only the thin plastic soles of my water shoes protected my feet from the razor edges of the mussels clustered on the rocky bottom of the lake – I could feel hundreds of shells crunching beneath my heels with each step. It was 1993, the year zebra mussels invaded Lake Champlain, Vt., and the striped shells of the mussels were everywhere, coating dock pilings, moorings, and rocks, sharp enough to leave nasty cuts on bare feet. Zebra mussels are small, less than two inches across, and native to the Black and Caspian Seas and the Sea of Azov in Eastern Europe. They’ve now spread throughout much of the Great Lakes and Mississippi River drainages, and where they’ve gone, they’ve taken over.

Zebra mussels were first detected in Oneida Lake, N.Y., about 200 miles southwest of Lake Champlain, in 1991. As reported recently in the journal PLoS ONE, a team of scientists from Cornell University and SUNY Buffalo State conducted a survey of the molluscs – mostly gastropods and bivalves – in Oneida Lake, which they compared to historical surveys conducted every few decades beginning in 1915.

The lake experienced more than one drastic change due to human activity during that time – in particular, the researchers note a period of high nutrient levels and low water clarity, the height of which was in 1967, and the zebra mussel invasion in 1991.

In one bay, the scientists found that by 2012, the number of bivalves – mussels and clams – was ten times as great as it had been in 1915-17. Not surprisingly, almost all of those bivalves were exotic, or non-native, and at least one family of native freshwater mussels appeared to be gone completely – no individuals from that family were found in the 2012 surveys. 

Bivalve density - the number of individuals found in one square meter - increased dramatically between 1915 and 2012; most of that increase was due to two exotic species, the zebra mussel, which invaded Lake Oneida in 1991, and the quagga mussel, which first appeared around 2005.

Most of the change in exotic gastropod density was driven by one species, Bithynia tentaculata, a small snail.

Source: Data from Karatayev et al., 2014.

(Figure by Emily Benson)

Zebra mussels are filter feeders: they eat algae from the lake water around them, and, in the process, they increase water clarity. The researchers suggest that this is the mechanism behind the recovery of underwater gastropods – snails and slugs – that they found; after the number of gastropods in Oneida Lake declined in the 1960s, when water clarity was at its worst, gastropod numbers and diversity rebounded to close to 1917 levels by 2012. Poor water quality during the 1960s, the authors argue, limited the growth of algae on the bottom of the lake, the main source of food for gastropods. When zebra mussels invaded, they ate the algae suspended in the lake, which allowed more sunlight to reach the lake bottom. The scientists suspect the increase in light led to an algae boom on the lake bed, and the subsequent revitalization of the gastropod population of Oneida Lake.

Zebra mussels don't just make life more difficult for bare-foot swimmers – they also clog water pipes, corrode underwater pilings, and choke boat engines. By altering the ecosystems they invade, they may also, at least in the case of Oneida Lake, contribute to the recovery of other organisms. 

Zebra mussels grow on any hard surface they can find, even other mussels.

(Image by U.S. Fish and Wildlife Service via Wikimedia Commons)



Every invasive species is a native species somewhere else. Red swamp crayfish, Procambarus clarkii, aka Louisiana crayfish, also known as crawdads, and sometimes called mudbugs, are native to the south-central United States and northeastern Mexico. Everywhere else they’ve spread to – and that’s a lot of places, including Europe, Asia, and many states outside of their native range in the United States – they’re invasive.

Not all non-native species are invasive – non-native species are organisms that don’t naturally occur in a specific place, while those that are non-native as well as destructive in some way are considered invasive. Red swamp crayfish usually fall into the later category – when introduced to a new location, they typically dominate the local habitat, to the detriment of local crayfish populations. 

Red swamp crayfish were first officially detected in Washington State in 2000, in Pine Lake, a tiny lake (just eight tenths of a mile by four tenths of a mile at its widest point) 20 miles east of Seattle. Five years after they were first recorded in Pine Lake, the invasive red swamp crayfish population was much larger than the native crayfish population. (Native crayfish population was not reported in 2008.)

Note: 2008 values were calculated from 24-hour sampling periods, with the assumption that capture rate was equal throughout the 24-hour period - because crayfish are more active at night, the calculated values may be underestimates. 

Sources: First detection in Pine Lake from Mueller 2001; 2005 values from Mueller 2007; 2008 values calculated from Larson & Olden 2008.

(Figure by Emily Benson)

The problems that invasive red swamp crayfish can cause when they spread to a new location are well known, as are the most common mechanisms of introduction. Most red swamp crayfish dispersal is due to human activity – mudbugs are considered a culinary delicacy, and live crayfish have been stocked, farmed, and traded widely throughout the world. There are, however, other ways that crayfish can spread – a study published in Aquatic Ecology earlier this year suggests that ducks and other waterfowl may be a previously unappreciated vector for transporting crayfish between lakes and ponds.

The researchers who conducted the study were interested in whether or not juvenile crayfish could cling to the feathers of a duck as it flew between bodies of water, and, if they could, how long they could hold on for. They found that crayfish are capable of hitching a ride on waterfowl, particularly in shallower water depths. With trained homing pigeons standing in as proxies for ducks, the scientists found that crayfish could survive flights as long as 37 miles in mesh bags secured to the birds.

As the researchers write, “these findings indicate that waterbird-mediated passive dispersal should be taken into account to explain P. clarkii’s rapid spread and should be considered when managing its invasions.” Humans may be responsible for the majority of the spread of this invasive species, but we’re not the only culprits.

Red swamp crayfish are freshwater crustaceans, but they can survive out of water for up to sixteen and a half hours, depending on the temperature and humidity;  Anastácio and colleagues estimate that a crayfish could walk over 700 yards in that time. 

(Image by Entomolo via Wikimedia Commons)