Peeps and chirps

One of my favorite parts of summer is sleeping with the windows wide open – and, if there’s a pond or a marshy spot nearby, drifting off to the peeps and chirps of frogs calling to one another.

The global chorus of frogs, however, is getting quieter – amphibian populations are declining around the world due to many factors, including habitat loss, climate change, and disease. The deadliest disease amphibians face is chytridiomycosis, an infection caused by a type of fungus called Batrachochytrium dendrobatidis, or Bd for short.

Bd infections can devastate amphibian populations and even drive them to extinction, though some locations and species seem to be resistant to the fungus. Scientists believe that environmental differences in temperature, altitude, and moisture influence the ability of some amphibians to survive infection, or avoid it altogether. New research reported in the journal Freshwater Biology suggests that the presence of predators might play a role as well.

Scientists exposed a group of wood frog tadpoles to ‘predator cues’ – in other words, they added the excretions of predacious beetle larvae that had been fed a diet of tadpoles to the tanks containing the experimental tadpoles, signaling to the tadpoles the presence of a predator. Another group of tadpoles were kept under similar conditions, but not exposed to predator cues. The researchers also added Bd, the fungus responsible for chytridiomycosis, to some of the tanks.

The two groups of tadpoles exposed to Bd – those that experienced predator cues and those that didn’t – had equal rates of Bd infection; about thirty percent of the tadpoles were infected with Bd. The infected tadpoles in the predator-present group, however, had less than half as many fungal spores in their bodies as the tadpoles that weren’t exposed to predator cues.

The scientists suspect that the stress-inducing predator cues primed the tadpoles’ immune systems, allowing them to lower their Bd infection loads. “This is an important result,” the researchers note, “because virulence is often associated with Bd pathogen load.” In other words, lower infection loads could translate to higher survival rates.

Amphibian decline is a complex, worldwide problem, but studies on chytridiomycosis and the fungus that causes it move us closer to a solution – and a world in which nocturnal frog choirs continue to sing.

An adult wood frog showing off inflated vocal sacs.

(Image by Dave Huth via Flickr/Creative Commons license)

Successful germination

Invasive aquatic plants can cause real harm in lakes and ponds. They can spread between water bodies via many different pathways – they’ve been known to hitch rides on boat trailers, for instance, and can also spread via floods, wind, or even gardening.

Aquatic vegetation can also stow away on more mobile organisms – either when plant fragments cling to fur or feathers (a form of locomotion that crayfish may also take advantage of), or when animals or birds eat their seeds.

As a team of researchers recently reported in the journal Freshwater Biology, aquatic plant seeds eaten by waterfowl fare differently depending on what type of plant they come from. The scientists fed mallard ducks and greylag geese seeds from four different aquatic plants, two of which are invasive in Europe (where the scientists are based).

The researchers found that the seeds of one of the invasive species were particularly successful in making the trip through the gut of the birds intact – they recovered more than a third of the Ludwigia grandiflora (or water primrose) seeds from the birds’ waste, but less than a tenth of each of the other types of seeds they fed to the birds.

The scientists also attempted to grow the seeds they retrieved – after all, if seeds can’t develop once they’ve gone through digestion, it doesn’t matter if birds spread them around. They found that “[f]or mallards, 9.14% of the tested seeds germinated successfully, compared to 24.18% for the greylag geese.” Some of those seeds were retained in the birds’ guts for 72 or even 96 hours before they were excreted, though the seeds of one plant species, the other invasive, Spartina densiflora (or cordgrass), only sprouted if they spent eight hours or less inside the birds.

Ducks and geese can travel much farther than plants can on their own over three or four days. “Ducks and geese evidently have the potential for long-distance transport of alien and native plant seeds,” the authors write, “with maximal dispersal distances of well over 1,000 km,” or 620 miles, about 20 miles farther than the drive from Chicago to Chattanooga, Tenn. That’s a long way for an aquatic plant to hitch a ride.

Seeds from Ludwigia grandiflora, or water primrose, were better able to survive waterfowl digestion than the seeds of other aquatic plants. 

(Image by bathyporeia via Flickr)


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)

Artificial light

Humans interact with streams in lots of ways – we like to swim in them, catch the fish that live there, and fill our water glasses and irrigate our fields with the water flowing through them. We also change rivers and streams to suit our needs – we straighten stream channels in cities, we cut down the riparian vegetation that grows on stream banks when it gets in our way, and, in some places, we put up streetlights with little regard for how that artificial light might affect the animals and plants that live in streams.

These changes influence aquatic organisms and the ecosystem overall, but sometimes it’s difficult to know what effect each change, individually, is having – something conservation officers or resource managers might want to know if they’re trying to decide which restoration project to dedicate limited funds to. In order to know how streams or rivers respond to different anthropogenic changes, it’s necessary to study systems where only one of them is occurring. And in order to experimentally test what the effects of a change are, it’s necessary to conduct the study in a place where researchers can manipulate streams.

“I was trying to find a system to work in that was basically light-naïve and would be simple to manipulate,” says Liz Perkin, currently a post-doctoral researcher at the University of British Columbia and the lead author on a paper recently published in the journal Freshwater Biology.

Perkin found a system that would work for her research – the Malcolm Knapp Research Forest, located in British Columbia. She and her team set up high-pressure sodium streetlights at four streams within the forest, then compared the insect community, leaf decomposition, and fish growth in the experimental reaches with four similar areas that weren’t lit by streetlights at night, to see what effect the streetlights had on the streams.

The scientists found that the number of aquatic macroinvertebrates drifting in the streams – a primary food source for fish – was much lower in the reaches lit by streetlights. This wasn’t surprising; as Perkin and her co-authors write, “[p]revious studies have shown that the activity of aquatic insects is at least partially controlled by light levels.” Insects are more likely to stay on the stream bottom, rather than drift through the water, when there’s enough light for fish to see and eat them.

Other macroinvertebrate activities, as well as leaf decomposition and fish growth rates, were the same at the lit and unlit streams. Perkin has some ideas about why that might be – they were only able to keep their streetlights in the research forest for one month during the summer, and it’s possible that the duration of the experiment simply wasn’t long enough to induce changes in the ecosystem, or that the effects of the streetlights would have been more pronounced during the spring, when days are shorter and nights are longer.

Another possibility is that the type of streetlight the researchers used in the experiment may explain why they didn’t see larger differences between the lit and unlit streams.

“They weren’t as bright as they could be,” Perkin says, of the streetlights they used. “We used high-pressure sodium because they’re very efficient and they’re currently the most commonly used streetlights across the globe.” But LED lights, which are gaining popularity as they become cheaper and more efficient, are brighter and have unique spectral qualities that may make them more influential in aquatic ecosystems. “If you look at the spectrum of a high-pressure sodium light, they tend to be more in the orange and red end of the spectrum, and those wavelengths are very readily absorbed by water, whereas with LEDs you have things more on the blue end of the spectrum, and those are more likely to penetrate water and be brighter in the water.”

Moving forward, Perkin plans to look at fish behavior in more detail. She suspects that even though macroinvertebrates drift less in streams lit by streetlights – meaning less food is available for fish in those streams – the extra light may make it easier for fish to spot the insects that are drifting.

 “What we know from this [study] is that artificial light does have the ability to affect stream communities,” Perkin told me. “The changes [we] saw were pretty small, but because they’re added at an important level in the food web, there’s definitely the potential for there to be bigger changes on a longer timescale.”

Experimental streams with high-pressure sodium streetlights installed above them had fewer drifting insects than streams without streetlights, but didn't seem to differ in other ways.

(Image of experimental streetlight set-up by Nora Schlenker)