Aerial Plankton

aerial plankton

Aerial plankton. Image taken from npr.org.

Okay okay.  I know I promised a post about a dragonfly swarming paper this time, but I decided I should talk about a related subject first: aerial plankton. Don’t worry – I’ll get to the dragonfly paper next time!   I am nearly wholly engrossed by the dragonfly swarming information that’s been coming my way and I will definitely go right back to it.  But this topic is likely an important component of the migratory swarms seen in dragonflies and I thought I should discuss it before I do the dragonfly paper.

If you’re like most people, you probably don’t know what aerial plankton is and may never have considered the possibility that it exists.  But most people know at least something about the plankton that live in oceans, marine plankton.  If you do, the concept is very similar.  The definition on Wikipedia for plankton is pretty good: Plankton are any drifting organisms (animals, plants, archaea, or bacteria) that inhabit the pelagic zone of oceans, seas, or bodies of fresh water.  Most of what we think of as plankton are little crustaceans such as krill or amphipods.  These are the things that are eaten by whales using baleen and several other large marine mammals.  Though small, they’re very important in marine habitats, both as a food source and for the many other services they provide.

Aerial plankton is similar in that it is made up of small creatures drifting along on currents.  However, instead of drifting in water currents, they drift through the air!  There are many, many species of insects, spiders, and other small organisms that make up the aerial plankton community and these creatures rely on wind currents to carry them from one place to another.  Basically, any small animal that can catch and updraft or find another way to get high enough into the air to get caught up by an air current becomes part of the aerial plankton.  Different things will use different methods to launch themselves into the currents.  Most insects fly.  Many spiders are known to “balloon.”  They extend strands of silk into the air that are caught in the wind, carrying the spider up into the atmosphere and away on the wind currents.  Ever read Charlotte’s Web?  The spiders leaving the egg sack at the end of the story were doing this exact thing.  Charlotte’s children became part of the aerial plankton!

It just so happens that this topic is one I’ve been just itching to cover recently thanks to a story by Robert Krulwich that popped up on NPR a few weeks ago.  Robert Krulwich does amazing reports on scientific topics and the animations that frequently accompany his stories are truly brilliant. They’re simple to understand, fun to watch, and get the main points across in a wholly engaging manner. I highly recommend that you check out his work if you haven’t already!  Rather than telling you why animals might want to drift around on air currents and how many organisms are floating around on our heads at any given moment, I’m going to direct you to Robert Krulwich’s story on NPR.  You can read the full article using the link, or you can just watch the animation, which sums it all up in a very succinct way:

I think this little video is quite brilliant.  So there are billions of animals floating through the air at any given time!  And they’re trying to move from one place to another efficiently.  There are certainly downsides to this sort of travel, the most important of which is this: these animals are really at the mercy of the winds and have very little control over where they end up.  It’s rather like hot air ballooning in that way.  As May Berenbaum said in the video, sometimes an animal ends up in a worse spot than when they started out.  It happens.  It probably happens a lot!  But many of those animals also make it to a better place than they started and make a good life for themselves in a new area.

Wandering glider (Pantala flavescens)

Wandering glider (Pantala flavescens)

So why am I bringing up aerial plankton?  Well, dragonflies likely use these same wind currents when they migrate.  They are, technically, becoming part of the aerial plankton when they do.  Some dragonflies are superb fliers, such as the wandering glider pictured at the right.  This dragonfly is known to fly over oceans and is found on all continents naturally.  While it can fly for many, many hours without resting, even these insects are probably getting a boost from the wind as they fly across oceans in search of new homes.  By using the power of the wind to propel them along, they can let the wind do some of the work for them and rest their wings to some extent.

But there’s another reason that aerial plankton and dragonflies are related as well.  Aerial plankton is a very valuable source of food for many animals, including predatory and scavenging insects.  What comes up, must come down, and aerial plankton is no exception.  Eventually, all of those billions of animals fall out of the sky.  Sometimes those things come down alive and other times they come down dead, but they’re important as a food source either way.  Storms are particularly hard on the aerial plankton community.  They can knock vast numbers of animals out of the sky and push them toward the ground, making them easy targets for things that might want to eat them when they do.  Dragonflies are thought to take advantage of this occasional shower of food from the sky and feast after storms.  This behavior is likely related to some of the behaviors observed in the big migratory swarms that I’ll be talking about next time, which is why I wanted to discuss aerial plankton first.

Aerial plankton is a vital dietary component for several insect species, but two come instantly to mind.  There is one insect order that is absolutely coveted by entomologists for collections because they are so hard to find and so rare to collect: the Grylloblattidae.  These insects live on snow fields in the Arctic and on the tops of very high and very cold mountains that are typically covered in snow.  Very few invertebrates can survive in these conditions (indeed, few animals live in them period!), so the grylloblattids rely almost entirely on aerial plankton as food.  Things fall out of the sky, land on the snow or ice, and the grylloblattids go skitting around on the snow collecting them.  These insects likely couldn’t survive at all without aerial plankton.   One of the only truly marine insects also depends on aerial plankton as a food source.  The water striders belonging to the genus Halobtes live on the open ocean, on top of the water like their freshwater relatives.  They’ll eat things from the water, but they also eat things that fall on the ocean’s surface from the sky and become trapped.  Aerial plankton likely forms a large part of the Halobates diet.

The next two posts will focus on dragonfly swarms.  First up is the discussion of the migratory swarm paper.  Then, I’m going to give another update on swarming activity in the US, including a map I’m developing of all of the sightings I’m collecting.  And as always, if you happen to see a dragonfly swarm, I’d love to hear about it!  Head over to my Contact page to submit a report.  I’m averaging about 10 reports a day this month so far, so keep them coming!

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Unless otherwise stated, all text, images, and video are copyright © 2010 DragonflyWoman.wordpress.com

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From the Literature: Nice Guys Get the Girls!

I love aquatic insects!  They do some amazing things and are incredibly interesting animals.  That said, I feel like most people know very little about aquatic insects and the role they play in our world.  Heck – some people don’t know that aquatic insects even exist!  So, for my first From the Literature post, I thought I’d discuss a recent paper dealing with aquatic insects.

In biology, it is thought that males benefit from mating with as many females as possible.  Because males usually do not care for their offspring and invest little in producing sperm, it is best for them to mate with as many females as they can, thereby contributing their genes to as many offspring as possible.  There are, of course, exceptions to this general rule (the giant water bugs I study are an excellent example!), but it holds true for many species.  Because females usually make a greater investment in their children – if nothing else, eggs are much more nutritionally expensive to produce than sperm – they often cannot mate as often as males.  As they contribute their genes to fewer offspring, it is to their benefit to choose the best mates, the ones that will likely produce strong and robust children that have a high chance of surviving to adulthood.  In essence, there is a battle of the sexes going on: males want to mate all the time with every female they can find while females want to mate with only the best males.  It pays for a male to be aggressive and secure as many mates as possible while it pays for a female to be choosy.  In essence, there is a trade-off between what males want and what females want: when one sex succeeds, the other suffers.

water strider interactions during mating

In this image by Omar Eldakar, a hyper-aggressive male (on the right) attempts to break up a mating pair of water striders. The colored dots were used by the researchers to keep track of individuals.

This idea sets the stage for a recent paper by Dr. Omar Eldakar (currently a postdoctoral fellow at the University of Arizona) and his colleagues published in the November 6, 2009 issue of Science.  Eldakar and his team studied a type of aquatic insect called water striders (order: Hemiptera, family: Gerridae).  Water striders (also known as Jesus bugs, water spiders, and pond skaters) are long, skinny insects that live and hunt on the surface of water.  Water striders are typically found in groups of several individuals called aggregations in calm areas of streams and ponds.

The species Eldakar studied is Aquarius remigis and it is well-known for its battle of the sexes.  Many male A. remigis individuals are highly aggressive when pursuing females, lunging at and jumping on their potential mates.  While the females often resist  mating with these hyper-aggressive males, the behavior has been known to improve mating success.  Aggressive males are usually more successful at securing mates than non-aggressive males.  This leads to a question: if aggressive males mate with more females than non-aggressive males, why aren’t ALL males aggressive?  Eldakar and his colleagues wondered if the fact that most studies of sexual conflict in water striders do not allow individuals to migrate between aggregations might explain why hyper-aggressive males are reportedly so successful.  If females are forced to remain in an area with hyper-aggressive males, the males might have a higher mating success than if the females could be choosier about who they mated with.

Eldakar and his colleagues set up an ingenious experiment to test this idea.  First, they placed water striders in an artificial pond and observed male aggression, movement of females, and mating attempts/successes.  Water striders were able to form their own aggregations and move freely between them.  The researchers then divided the pond into several sections, placing males of various aggression levels with females in each section.  The same observations were made, though this time individuals were not able to move between groups.  Finally, the group compared their observations of the open treatments to the closed treatment to see how movement contributed to the mating success of aggressive and non-aggressive males.  They discovered some interesting things.

As had been reported in other water strider studies, hyper-aggressive males had more successful mating attempts than the non-aggressive males in the closed system.  If females were not able to move to another aggregation (i.e. their choices in mates were restricted), they mated more frequently with the aggressive males than the non-aggressive males.  However, when the striders were able to move between groups, Eldakar observed that females moved to other aggregations when harassed by aggressive males.  Aggressive males repelled the females they wanted to mate with!  The females would, however, readily mate with the less aggressive males in their new areas.  This meant that less aggressive males were able to secure many more mates in open systems than in closed systems.  In other words, the females avoided the aggressive jerks and the nice guys were getting the girls!

I think this is a great paper.  The experiment was very simple, but it ended up revealing a lot of information.  Many scientists call this sort of experiment “elegant” and I think this research certainly qualifies as an elegant experiment.  Eldakar was able to refute the findings of several previous papers with an experiment that should be very easy for others to duplicate, one of the conditions for good science.  He also clearly reminded biologists of one of the perils of doing behavioral research in the lab: unnatural conditions sometimes lead to unnatural behaviors.  As a behavioral researcher myself, I think this isn’t reinforced enough.  By modifying what had been done in the past, by allowing aggregations of water striders to form naturally rather than forcing them into pre-determined groups, Eldakar learned that aggressive males are not always the most successful in nature.  This corrected what we already knew and gave us some great information that will be useful for many future studies.  Thanks, Dr. Eldakar!

Literature Cited:

Eldakar OT, Dlugos MJ, Pepper JW, & Wilson DS (2009). Population structure mediates sexual conflict in water striders. Science (New York, N.Y.), 326 (5954) PMID: 19892974

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Text copyright © 2009 DragonflyWoman.wordpress.com.

This post was chosen as an Editor's Selection for ResearchBlogging.org This blog post was an Editor’s Choice selection at ResearchBlogging.org!