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)

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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