Literature Review: Bluets

by Carl Strang

One of the papers I found most interesting in this year’s literature review had to do with damselflies (Siepielski, Adam M., Ken-Lou Hung, Eben E.B. Bein, and Mark A. McPeek. 2010. Experimental evidence for neutral community dynamics governing an insect assemblage. Ecology 91:847-857).

Familiar bluet. I can believe that this one is ecologically interchangeable with most of the others.

In particular they focused on the 34 species of North American bluets that live in lakes with fish in them (among both dragonflies and damselflies there are a few species that live only in the relatively few bodies of water in which nymphs are free of predation pressure by fishes). Using a variety of experimental, observational and comparative methods, the researchers concluded that the nymphal stages of all these species are completely interchangeable. Competition, separation into different chunks of niche space, differential avoidance of predation, none of these processes apparently separate these bluets ecologically.

Tule bluet. This species, on the other hand, in my experience sticks to the larger, more open lakes.

These bluets as a group are, however, separated from the forktails, a different group of similar-sized damselflies. Bluets are better at avoiding predators, while forktails are more efficient “at converting prey into their own biomass.” I’m not entirely sure what that means; perhaps being more predation prone, they need to be able to get by on less food and so minimize their own exposure.

Eastern forktail. The authors seem to believe that the forktails likewise are ecological equivalents. Someone needs to get the data.

If the larvae are all alike, perhaps the adults are ecologically subdivided? Siepielski et al. didn’t look into this, but they pointed out that the damselflies live in their adult stage for an average of only 4 days, compared to most of a year as nymphs. The diversification of bluet species “seem[s] to have been driven primarily by sexual selection for differentiation in reproductive structures and little else.”

Slender bluet. Another question is, why are some species much more common than others? I run into a lot more familiar bluets than slender bluets.

They suggest that similar neutral community dynamics may operate commonly among insects, given the many sibling species groups. This raised my own metaphoric antennae, as many singing insects (cicadas, crickets and katydids) belong to clusters of sibling species. Along the way, Siepielski et al. mentioned the interesting fact that dragonfly nymphs have been shown elsewhere to feed negligibly on bluet nymphs.

Orange bluet. On the whole I accept these results, but as hinted in the above captions, there may be a little more complexity to this story.

A related theoretical paper published this year (Van Doorn, G. Sander, Pim Edelaar, and Franz J. Weissing. 2009. On the origin of species by natural and sexual selection. Science 326:1704-1707) developed a model that supports the possibility of sympatric speciation where female selection of mates produces divergence. It requires the appearance of identifiers (e.g., color patches) that correlate with the different subpopulations. Females in the different environment patches then are favored to the extent that they identify and mate with the appropriate local males.

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