The Elusive Black Saddlebags
One of my ongoing projects is to photograph and identify all the (non-microscopic) species we have on our farm. Sometimes it is the ID process that is tricky; other times I can easily identify the species, but I can never get a picture of it! That has been the situation with the Black Saddlebags dragonfly (Tramea lacerata). All the other saddlebags in Texas are red. I see the Black every fall but can never sneak up on it.
Finally on a recent trip down to the Texas coast, I saw them everywhere, and I found a cooperative one that perched perfectly.
I got to thinking:
- Why is this dragonfly showing different behavior down here on the coast, than it does a little further north where I live?
- Why are there so many species of dragonfly (about 160 in Texas alone), when they seem pretty similar?
- I have identified at least 19 species of dragonfly on our farm. I see some species in small areas away from the water, but I would say most of them live right around the pond’s edge. How are they all co-existing?
- I know they are all predators, but do they go after different prey?
- I know they live aquatic lives for years in their nymph/naiad forms — do the various species go after different prey underwater?
- In short, do they have different niches?????
This question turned out to be more difficult to research than I would have thought, but I finally found a paper from 1994, aptly titled Niche Specialization in Dragonflies, by K. Sternberg.
So the answer to my niche question is a resounding YES, but of course there is even more to the issue than I could have imagined.
First, here is the definition of a niche:
The position or status of an organism within its community and ecosystem is, by analogy, its ‘profession’ and is termed ‘ecological niche’, whereby time, space, and food are suggested to be the three most important niche dimensions among potentially competing species.
The author points out that if species in an area share the same needs, they can only co-exist if there is an abundance of the things they need. If any of those things become sparse, competition increases, and that competition can occur within a species, between species, or both.
One major category of niche specialization within the dragonfly order, is “habitat splitting.” The nymphs live underwater, and the adults live above it. They rely on different food sources, and are therefore not competing with their own offspring. (This same characteristic of habitat splitting occurs with butterflies, moths, and bees, among others.)
Some of the niche specialization relies on the dragonfly equivalent of working flex-time. Adult dragonflies can be split into “fliers” and “perchers.” The fliers can increase their temperature by the contraction of their wing muscles, enabling them to take off from their resting spots earlier in the morning, and remain active later into the day. But if it gets too hot, they stop flying. The Common Green Darner is one of these. (I wrote about my encounter with this one here.)
The perchers, on the other hand, warm up by basking in the sun, so they usually take off later in the morning, but they can continue flying during very hot days.
Different dragonfly species also choose different locations in which to just hang out. They show preferences for certain heights of perches, proportional to their body size. And, Sternberg says that when dragonflies choose roost sites, “The preferred diameter of plant stems is related to inter-eye distance.” The dragonflies with larger head-width choose stems with greater diameters!
So that is just a little of what I picked up about dragonfly niches. Obviously I have a whole lot of new issues to investigate — to see if I can tell what plants our dragonflies roost on, if I can separate the fliers from the perchers, if I can distinguish how large of territories they defend, and so on.
As far as my original question about the Black Saddlebags, it turns out that they migrate! They go north in the spring, and then back south in late summer. It is presumed that it is a new generation that makes each leg of the migration, but that is not known for sure yet. Maybe when I see them on our place, they are just passing through.
I have specifically not included anything in this post about dragonflies’ amazing flight and vision capabilities, because that is told much better in this video:
I hope you enjoy this glimpse into the dragonfly’s world!