I could have sworn that I posted this last weekend, but when I went to post the next part of the report there it was in the drafts folder. Whoops! Well, here’s hoping it will go through this time…
In part III of my report, I addressed the predictions that I made at the end of the season last year and made some new predictions. Today’s part IV will focus on the big picture, i.e. why I think dragonfly swarms form and what their purpose is in the environment based on the information that I’ve gathered from you all thus far. Remember as you read that there are two types of dragonfly swarms, static (dragonflies form tight-knit groups over a well-defined area) and migratory (dragonflies fly together in a single direction as they move from one place to another) and they are very different types of swarms. Please also note that there needs to be some additional experimentation to confirm nearly anything I say in this post – this is all observational data and you can’t show cause and effect with observational data like this. With that disclaimer, however, I think there are several interesting patterns in the data that are suggestive of what’s going on.
First let’s consider how dragonfly swarms form, and let’s focus on the static swarms first. Static swarms are all about feeding. Where there is abundant food available to dragonflies, such as swarms of small flies (i.e. gnats, midges, mosquitoes), ants, or termites, you will often find dragonfly swarms. The majority of people who have made reports of static swarms have noted that the dragonflies were actively eating some form of small insect and the scientific literature supports these observations. It would be interesting to see if you could induce a dragonfly swarm by artificially creating a swarm of small insects as I think that would provide great experimental confirmation of the observational data.
If static dragonfly swarms form only in the presence of abundant prey insects, then what exactly is drawing the dragonflies in? Having witnessed the start of one swarm, I think there are two potential explanations. The first is that each individual dragonfly sees the prey insects and swarms form as each individual chooses to feed in the same place based on the availability of food, i.e. they’re using food as a cue to swarm. An alternative explanation is that one dragonfly finds the food source and then other dragonflies recognize the flight pattern of the “founder” dragonfly, realize there’s food available, and join the dragonflies that have already begun to swarm. In this scenario, the swarm is formed based on the behavioral cues of the dragonflies rather than the food. If you’ve seen a static dragonfly swarm, you know how regular the flight pattern is. Nearly every individual follows the same rectangular or figure 8 path back and forth. Another dragonfly could potentially recognize this flight pattern as a signal that there is food available. Either explanation could be correct (or neither!), so an experiment would be necessary to tease them apart.
What about migratory swarm formation? We need to consider that dragonflies migrate for a variety of reasons. First, there’s the annual fall migration where millions (billions?) of dragonflies fly from north to south in search of warmer overwintering sites. I suspect this behavior is largely weather driven, that dragonflies recognize cues in the environment that tell them they need to move and they do it. Swarms likely form because lots of dragonflies happen to be moving in the same direction along the same paths, but it’s always possible that the dragonflies toward the back are following those in the front. (It would be really hard to test that experimentally though.) Second, many people have reported migratory swarms during some of the static swarming mega-events where there are several million dragonflies in an area at one time. In this case, I suspect that weather is partly responsible as those mega-events are typically associated with storms, but I think that these migratory events are partly a lemming-like response where there are so many dragonflies in an area that the environment can no longer support them all and some of them move to other areas. Then there’s a third situation where conditions in an area deteriorate sufficiently that the dragonflies NEED to move to survive, so they get up and go. I have no idea how the dragonflies in the lemming-like scenario or the deteriorating conditions scenario form swarms, however. Somehow it seems unlikely that the dragonflies would follow the leader to a new area, but I don’t have a better alternative suggestion at this time either.
So that’s how I think dragonfly swarms form based on the information I’ve collected to date. Now let’s consider what role dragonfly swarms play in the environment. Based on the 2500 reports received so far, I can say that the majority of static swarms develop after a disturbance of one form or another – floods, droughts, fires, too many dragonflies in one area, disturbances of vegetation, etc. I’ve discussed this idea before, but I think that flooding leads to localized population booms in a variety of fly species and that dragonflies move in to take advantage of the abundant food. Similar things happen when it rains after a long period of drought in an area. I’ve thought that rain, storms, and flooding play a role in swarm formation since I started the Dragonfly Swarm Project and I still think that’s true. However, this was the first year I started to understand the role that other disturbances play in swarm formation. Droughts were reported many times over the past season, and both static and migratory swarms were observed in drought stricken areas all over the country. I suspect that drought conditions induce movement of a lot of insects and the dragonflies form swarms to swoop up any prey insects as they move around. However, droughts can also lead to migratory movements of the dragonflies themselves. I suspect these migratory swarms consist of dragonflies that are leaving an area that has deteriorated sufficiently to force them to find better conditions elsewhere. Like I said before, I’m not sure why they might do this en masse (perhaps they’re avoiding predation? Maybe they go the direction they do for a reason and every individual happens to be going in the same direction regardless of the other dragonflies?), but it sounds like a pretty amazing thing to see.
I’ve received many reports of dragonfly swarms immediately after or during disturbances to vegetation. Several people have reported swarms forming around them as they mow their lawns or do extensive work in their gardens. I’ve also received reports of smaller swarms forming around people walking through brushy areas. I think these are related phenomena, that by disturbing vegetation people are disturbing the small insects resting in grasses, garden areas, trees, etc, causing a temporary cloud of small displaced insects. The then dragonflies move in to eat the abundant prey.
This year I also had dozens of reports that mentioned fires in areas where swarms occurred, so I began to suspect that fires played a role in swarm formation. When you think about it, fires are massive disturbances to vegetation, a much more severe form of mowing. Just as I described above, fires potentially displace insects that are living in the burning grassland/forest/other area and the dragonflies are drawn to the all you can eat prey buffet that result. This seemed like a crazy idea when I first had it, but let’s consider Colorado. Colorado is an area where there are normally very few reports but had a huge boom in swarm reports in 2012. If you compare the wildfire areas to locations where swarms were reported, all but one of the swarms were observed in areas impacted by fires. I started looking through other reports and found that there were additional reports of swarms associated with fires in several other states as well. Then I got to witness it myself! One of my coworkers did a demonstration of a controlled burn on a small patch of the prairie at work for an event in September. Over the next week there were swarms forming over the burn area every afternoon, and you could clearly see the dragonflies eating insects flying up from the burned patch. All of this leads me to believe that fires can play a role in dragonfly swarm formation.
I estimate that at least 75% of all swarm reports submitted mention some form of disturbance. As a result, I suggest that static dragonfly swarms can form anywhere there is abundant prey available due to a disturbance of some sort. And, if I’m right about that, dragonflies are performing an incredibly important service! They’re restoring balance to a disturbed area by eliminating displaced prey insects that may be a nuisance to humans. A lot of those insects are things you probably don’t want around – gnats, midges, flying ants, etc. Some of those insects are things that can spread disease, such as mosquitoes and black flies, or cause damage to your property like termites. People ask me all the time why we should care about dragonfly swarms and I finally have an answer that a lot of people can get behind (something other than “because it’s an amazing, beautiful, crazy spectacle of nature that only a handful of people will ever see”): dragonfly swarms help restore balance in areas where prey insect population booms have occurred. If that doesn’t make you appreciate dragonflies, I’m not sure what will!
That about wraps up my current thoughts about what’s going on in this system, and I want to stop here as this is getting very long. There will be one final report next week that highlights some of the stories of participants and summarizes attitudes toward dragonflies that the people who make reports have expressed. This is one of my very favorite parts of the project, all those great stories, so look for those next week!
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