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The flight of the backswimmer: dispersal behaviour in a freshwater insect

By Celina Baines

Have you ever thought about what a pond-dwelling insect might do if it doesn’t like the pond it lives in? People generally assume that these insects are stuck where they are, but actually, many freshwater insects have wings and can fly. This movement between ponds is an example of a process known as dispersal.

Backswimmers, for example, are insects that live in ponds and streams (and sometimes even swimming pools!). Backswimmers have a characteristic way of swimming – on their backs, just under the surface of the water, using their hind legs to propel themselves. It makes them look a little like they are doing the backstroke (hence their common name!). But they also have wings, and can fly between ponds.

A top view of a backswimmer swimming. Backswimmers can often be seen swimming just under the surface of the water, ventral side up. Photo credit: Shannon McCauley.

A top view of a backswimmer swimming. Backswimmers can often be seen swimming just under the surface of the water, ventral side up. Photo credit: Shannon McCauley.

We know from observing these insects that not all backswimmers make the same decisions about whether to disperse. Some individuals spend their whole lives in the ponds they are born in, and some individuals move to new ponds. So why do some individuals stay and some leave? One factor that could influence dispersal decisions is the quality of the pond. Pond “quality” could depend on many things, including the risk of being eaten by predators like fish. Dispersing can be a great way for organisms to avoid habitats that will be bad for them or their offspring.

Once a backswimmer has decided that it wants to disperse, it then has to decide whether it is strong and healthy enough to fly. This could be another factor that determines whether an individual decides to stay or go.

In the summer of 2013, I conducted a field experiment to learn more about how backswimmers make dispersal decisions. I wanted to test whether dispersal was induced by fish. I also wanted to test whether body condition (basically, the general strength and health of an organism) influences dispersal decisions.

I started by collecting backswimmers from a pond at the Koffler Scientific Reserve. That’s a research site owned by the University of Toronto, where I’m a graduate student.

This is me collecting backswimmers from a pond at the Koffler Scientific Reserve. Photo credit: Chris Thomaidis.

This is me collecting backswimmers from a pond at the Koffler Scientific Reserve. Photo credit: Chris Thomaidis.

I brought the backswimmers back to a lab at the University of Toronto. Because I wanted to test the effects of body condition on dispersal, I first had to manipulate the backswimmers so that they had different levels of body condition. I did this by carefully controlling how much food each backswimmer got to eat.

Backswimmers are carnivores, and they aren’t very picky. For this experiment, I fed them fruit flies, because it’s really easy to get lots and lots of fruit flies. So, in what turned out to be one of the most back-breakingly tedious jobs I’ve ever performed for science, I (and many uncomplaining assistants) counted out thousands of individual fruit flies to feed to the backswimmers. Each backswimmer was housed in its own little cup, and received a specific (and carefully counted) number of fruit flies to eat every day. Here’s what the hundreds of drink cups looked like, colour coded and full of bugs.

Left: Cups housing backswimmers at the University of Toronto. Right: A backswimmer in its cup.

Left: Cups housing backswimmers at the University of Toronto. Right: A backswimmer in its cup.

After a few weeks of controlling the backswimmers’ diets, it was time to bring them outside to see if they would fly. I set up some artificial ponds in a big field. These “ponds” are actually just watering tanks that farmers use for cows and horses, but I added algae and artificial plants to make them more like natural ponds. Since I also wanted to test whether backswimmers are scared away by fish, I added a fish to half of the tanks. I put the fish in cages, and that way, the backswimmers could tell there was a fish in the tank (they could see and smell the fish), but the fish couldn’t actually eat the backswimmers.

This is me, checking the artificial ponds for backswimmers. Photo credit: Betty Dondertman.

This is me, checking the artificial ponds for backswimmers. Photo credit: Betty Dondertman.

Then I put the bugs in the tanks, and waited. After a couple days, I went back to the tanks and checked to see which backswimmers were still in the tanks, and which ones had flown away.

Firstly, I found that backswimmers are scared away by fish; they are more likely to disperse when a fish is in their pond.

I also found that the backswimmers with high body condition are more likely to fly, probably because they are strong fliers and have the best chance of successfully finding a new pond.

Both of these results were really cool and answered some questions for us about how backswimmers make dispersal decisions. But they might also tell us a little about how other organisms move around in natural ecosystems. Dispersers are the only individuals that can find new ponds and start new populations. If dispersers tend to be the strongest and healthiest individuals, that’s great for native species that we want to encourage to start new populations. But having strong, healthy individuals from exotic species start new populations is probably bad news. Dispersal can therefore have important consequences, which is why we need to understand more about how and why organisms disperse.

For more information about my study, check out the recent publication:

Baines, C. B., McCauley, S. J., & Rowe, L. (2015). Dispersal depends on body condition and predation risk in the semi‐aquatic insect, Notonecta undulata. Ecology and Evolution 5(12): 2307–2316

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Recreational boating affects dragonflies

—-By Aaron Hall—-

A typical adult dragonfly. Note the spiked legs, which are held in a basket shape to help catch prey while flying.

A typical adult damselfly. Note the spiked legs, which are held in a basket shape to help catch prey while flying.

Dragonflies are charismatic insects, and most of us can probably remember chasing them or watching their acrobatic flights when we were children. But what most of us didn’t realize when we were kids, is that dragonflies spend the majority of their lives as toothy, alien-looking predators living underwater before they become adults. Depending on the species, they can live in the water for several weeks up to several years.

A typical larval dragonfly, which feeds on other aquatic animals - and even other dragonflies!

A typical larval dragonfly, which feeds on other aquatic animals – and even other dragonflies!

By living part of their lives in water, and part on land/in the air, dragonflies represent an interesting conservation challenge. Historically, conservation science has focused on single habitats, such as lakes, streams, forests or grasslands. Little attention has focused on incorporating multiple habitat types, such as those required by dragonflies, into conservation, potentially leaving species like dragonflies in danger.

In the Waubaushene area of Georgian Bay (Lake Huron), recreational boating is very common. These boats create waves that can dislodge both adult and larval dragonflies, affecting their ability to find food and avoid predators. The overall number of boats, the speed of these boats, and how close they are to coastal wetlands are the most important factors that determine how impactful boat-generated waves are on dragonflies. My colleagues and I at the University of Toronto investigated how much influence these recreational boats have, relative to more natural processes, on dragonfly communities in Georgian Bay.

A Google Earth image of an area in Georgian Bay. Note the many waves created by boats as they travel through this region.

A Google Earth image of an area in Georgian Bay. Note the many waves created by boats as they travel through this region.

Taking the lead on this project, I counted dragonflies from 17 islands in Waubaushene. The coastal wetlands around these islands are inhabited by dragonflies. The islands studied in this project were selected to represent a range of influence from boats in the area, determined by their distance and orientation to marked boating channels and area marinas.

Aaron Hall counting adult dragonflies at one of the islands in Waubaushene.

Aaron Hall counting adult dragonflies at one of the islands in Waubaushene.

The results show that boats do have an influence on dragonfly communities, providing a link between recreational boating and dragonfly communities. This research provides important insights that can be applied to the protection and conservation of dragonflies, and suggests that some very simple changes in boater behaviour could have big implications. For example, if boats travel slower or further away from dragonfly habitats, they would have less impact. These two factors might be simple to change. In areas where boats mostly stay within marked boating channels, if these channels were moved or adjusted so they are as far away from dragonfly habitats as possible, impacts would be minimized. Additionally, speed limits could be set in these channels to reduce the size of waves created by boats. These simple measures could have a positive impact on dragonflies, which are a critical component in the aquatic and terrestrial foodwebs of this region.

Want to know more? This research is published in the journal Insect Conservation and Diversity. You can also follow me on Twitter @aarohall.

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Physiology Fridays: Trace-metal phantoms

Living in metal-contaminated lakewater is just another day’s work for phantom midge larvae. 

In the lakes surrounding Sudbury, Ontario and Rouyn-Noranda, Quebec, over 75 years of smelter operations have left their mark by contaminating soil and water with the trace metals cadmium, nickel, copper, and zinc.

This contamination led Maikel Rosabal, Landis Hare, and Peter Campbell, all from the Institut national de la Recherche scientifique in Québec, to study how aquatic animals tolerate these contaminants.  To do so, they needed a study organism that was abundant, easy to collect, and could accumulate and tolerate trace metals.  The best option turned out to be larvae of the phantom midge Chaoborus.

“The lakes in the area, and their watersheds, have been contaminated by the deposition of atmospheric aerosols and particles.  Metal concentrations in lake water tend to be higher in the lakes that are downwind from and close to the smelters, than in lakes that are upwind and distant from the smelter stacks,” says Dr. Peter Campbell. “The presence of Chaoborus in lakes with high metal concentrations implies that they are highly metal tolerant.”

The researchers chose a total of 12 lakes around Sudbury and Rouyn-Noranda with differing concentrations of trace metals, and collected water samples, using diffusion samplers that excluded particles, and midge larvae using a plankton net. After homogenizing the larvae, the researchers used a series of centrifugation, heating, and sodium hydroxide digestion steps to separate the subcellular components of the larvae.  They then measured the amount of metal in each fraction as well as the concentrations of dissolved metals in samples of lake water.  This allowed them to relate the concentrations of each trace metal in lake water to the concentrations in larvae.

They found that the majority of each metal accumulated in the cytosolic heat-stable protein fraction that they isolated from the larvae—a fraction that contains large amounts of metal-binding proteins. And while other fractions also contained small amounts of metals, it was in the heat-stable protein fraction that metal concentrations responded most obviously to the increasing metal concentrations in lake water. This suggests that the Chaoborus larvae were able to bind and detoxify increasingly large amounts of these potentially toxic metals.

“This suggests an important role for these metallothionein-like proteins in the detoxification of metals,” says Dr. Campbell.  “Presumably this contributes to the presence of this insect in highly metal-contaminated lakes.”

While laboratory studies usually focus on the effects of exposure to a single trace metal (usually dissolved in the water), animals in this study were exposed in the field to many trace metals both in the water and in their planktonic food. The researchers suggest that Chaoborus larvae would be effective “sentinels” for estimating trace-metal exposure to lake plankton, which is a key component of ecological risk assessments.

“Rough estimates of trace metal exposure are often obtained by measuring total metal concentrations in the water or the sediment.  Such values usually overestimate metal exposure because much of the metal present is not available for uptake by organisms because they are bound to substances such as organic matter or iron oxides,” explain the researchers. “For these reasons, measurements of trace metals in organisms are increasingly used to estimate exposure in risk assessments.”

Rosabal, M., Hare, L. & Campbell, P.G.C. (2012). Subcellular metal partitioning in larvae of the insect Chaoborus collected along an environmental metal exposure gradient (Cd, Cu, Ni and Zn), Aquatic Toxicology, 120-121 78. DOI: 10.1016/j.aquatox.2012.05.001

Pubmed: http://www.ncbi.nlm.nih.gov/pubmed/22647479

Chaoborus larvae

Photo: Maikel Rosabal

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ESC Joins Effort to Save the Experimental Lakes Area (ELA)

On June 15, 2012, Rebecca Hallett, Chair of the Science Policy & Education Committee, sent a letter on behalf of the ESC to Prime Minister Stephen Harper, Hon. Keith Ashfield (Minister of Fisheries and Oceans) and Hon. Peter Kent (Minister of the Environment) asking the Government to reverse their decision to close the Experimental Lakes Area. You can read the full text of the letter attached here. The letter was cc’d to Save ELA, and MPs Elizabeth May (Green Party Leader), Tom Mulcair (New Democratic Party Leader) and Hon. Bob Rae (Liberal Party Leader). The ESC was also added as a signator to an ad in support of saving the ELA printed in the Globe & Mail and the Winnipeg Free Press on Saturday June 16.

You can read the ESC’s letter to the government here.

Anyone interested in learning more about the ELA and/or adding your individual support to this initiative, should visit the Save ELA site.

Responses:

On Monday June 19, a reply was received from Elizabeth May, leader of the Green Party of Canada, expressing her dismay at the closure of the ELA and her intention to table petitions in the House of Commons supporting the continued operation of the ELA in hopes of reversing the government’s decision.

You can read Elizabeth May’s response here.

On Friday June 29, a reply was received from Hon. Keith Ashfield (Minister of Fisheries and Oceans).

You can read Hon. Keith Ashfield’s response here (PDF)