Tuesday, 12 July 2011

Dragonflies and ways to interpret wildlife sightings

I saw a beautiful dragonfly at the weekend. I have to say I can't identify it to species - I think it's probably Trithemis stictica, a fairly widespread and common species with a nice English name: the Jaunty Dropwing. If you know better than me (and I am only an ornithologist!), please tell me!

Jaunty Dropwing (I think!), Maji Moto, July 2011
It's pretty obvious why this genus of dragonflies are called dropwings from the picture...

Now, I know very little indeed about dragonflies, but as I had a nice picture to share, I thought it would be worth talking through a few of the things I do know, as an example of how wildlife sightings of any kind can lead to interesting conversations. And, of course, other intvertebrates (that's other than termites) are one of my ten things, of course, so any facts I can dredge up might be worth remembering.

So, what's my strategy for interpreting wildlife sightings? I try and think of three basic questions to ask myself: (1) what is it? (2) What's it doing? And (3) What impact does it have / role does it play on the ecosystem and other plants/animals around? Not all of these will be useful to every sighting, but at least it gives me a framework to start thinking about - and I don't think I'll ever be left with just being able to say "oh, err, it's a dragonfly...". So, given my massive ignorace of dragonflies, how would this work if I'd been out with visitors (as it happened, I was with a friend, so it's a real situation).

Me: "Ah, look, isn't that a lovely dragonfly"
Friend: "Beautiful, what is it?"
Me: "No idea, I'll take a photo and we can look it up later..."
Click, click, click.
Me: "I wish I knew more about dragonflies, they do some pretty amazing things, you know?"
Friend: "Oh?"
Me: "yes,you know there's one around here called the global skimmer that migrates here from India."
Friend: "what?!? That's amazing!"

And so we went on - I'd identified it for him as much as I could at the time (a dragonfly, not too precise, but that's all I knew!) - and I've since discovered a fantastic website that will help you identify your African dragonflies here - if you register you can even send your unidentified picture and get help identifying it. Isn't it great what people do for free! So, now I know what it is (I think - I'll send it to them, and if I'm wrong I'll let you know here!), and I could also have carried on the identification part at the time - it was pretty obviously a male, as I already know that most blue dragonflies have females that are yellow. With other species the identification part can go much further - you can identify how old it is, what role it might be playing in a group (alpha male, etc.). For a dragonfly this wasn't going to go too far.

Then the next question was what's it doing? And it was sitting around (with its wings drooping), then buzzing about hunting. A typical sit and pounce type hunting behaviour. So it was feeding and resting - worth pointing out the strategy it uses for this too - there were other dragonflies I could have pointed out using a much more active search and destroy hunting technique. Given any animal, even one you've not seen before and don't know what it is, you can probably have a pretty good guess at what it's doing, as most animal activities fall into a fairly restricted number of classes: feeding/drinking, mating behaviour (including displays, dominance fights, calls, etc., if not actual copulation which probably doesn't take a guide to interpret...), territorial defence (whih can obviously also be related to mating), resting (in fact, I think that's all those boring lions ever do) and other social behaviour (play, groub bonding, etc.). If you think of these five categories as the main ones (and there are others of course), you can probably have a pretty good guess at what's going on. And by watching a bit more, you might learn something interesting!

In fact, in this case I didn't bother, just jumped straight to my most interesting story aout dragonflies - the global skimmer Pantala flavescens. It's rather scarce in Tanzania, but (as well as there probably being a resident population) it does indeed migrate here from India in November, and back again in May, 3500kms across the open oceans (some obviously rest on the Maldives). Amazingly, we think it often migrates at altitudes over 1000m above the sea, and even over 6000m when crossing mountains, literally skimming on the winds associated with the Inter-Tropical Convergence Zone (that's what gives us our rains). (You an read lots about the migration here). Interestingly, it's migration across the ocean occurs at the same time as a particularly good dragonfly predator is doing exactly the sae - the Amur Falcon. The dragonfly is quite a fat beast, yellow in colour and in the same family as my pretty above, so if you see a golden yellow looking dragonfly, it might be this amazing beast.

Finally, the question is what role does it play in the ecosystem. And you might think that a dragonfly doesn't do very much. But actually they do! Not only do they eat rather a lot of nasty bugs (and they're actually recommended as an efficient means of controlling mosquito numbers - a dragonfly can apparently eat it;s own body weight in bugs in just 30mins!), but they're also eaten by lots of things too - those Amur Falcons, and the two species of Hobby we get around here are major dragonfly predators. And the pygmy kingfisher in the riverine forest post was busy eating them too! But their biggest role in the environment is actually probably not as adults, but during the nymph stage. Now remember the dragonfly lifecycle? Eggs are laid in water, hatch into nymphs, nymphs (or naiads) live in pools for anything from a few months to up to five years, depending on species, and then after heaving themselves up some emergent pool-side vegetation a final moult sees the adult emerge direct from the nymph (no pupal stage for dragonflies). And during that long periods in the pond, dragonfly nymphs are major predators - they can move very quickly indeed (by squirting water out of their anal cavity, rather jet propelled. Good story for kids methinks...) and although they mainly eat invertebrates, they'll catch tadpoles and small fish quite frequently too. Not many invertebrates predate vertebrates, so that's pretty neat. They're actually top predators in many pools, and have massive impacts on the numbers and diversity of animals around pools - especially those nasty mosquitos! So, yes, just like top predators on land, they play a pretty important role in the wetland habitat too.

Hopefully something new and interesting will be in this - and if nothing else you might have a new way of interpreting your own wildlife sightings too. We'll try the technique on other vitual sightings in time, but I've had enough for tonight...

1 comment:

  1. Fantastic conference about the global skimmers from Anderson at Ted: