dragonflies and vegetation structure

In the December 2010 issue of "Ecological Restoration" that hit my mailbox today, there's an article on dragonfly and damselfly species richness relative to vegetation structure. The study, by Catherine Mabry and Connie Dettman, was done in the prairie pothole region of Iowa.

The study is focused at a fairly fine scale, comparing stands of diverse plant composition to near-monotypic stands of reed canary grass and cattail. Not surprisingly, at least to me, odonate species richness was considerably higher in the diverse stands than in the monotypic ones: 30 percent higher, and 45 percent higher if looking only at species of conservation concern, which tend to have more specialized habitat requirements. The authors also had access to an earlier study conducted in one of the wetlands which had at that time been diverse, but since then had been invaded by reed canary grass. They were able to document a loss of both damselfly and dragonfly species in the intervening years.

Publication of the article is timely, because I've been involved in similar issues in Illinois. There, we've watched a once diverse wetland complex gradually lose sedge meadow and graminoid fen habitat to cattail encroachment. Reed canary grass is common too, although the extent hasn't really changed in the 16 years of observations available to me, which suggests that the encroachment of that species happened much earlier.

I've been looking at a coarser scale than Mabry and Dettman, because I'm working with a large, strong-flying species, the Hine's emerald dragonfly. As early as 1998, we documented a correlation to habitat interspersion. Over 40 percent of our observations were in sedge meadow, which constituted less than 10 percent of the habitat on the site. Observations also tended to be close to an edge between low-growing sedge meadow and much taller marsh. Thus, I'm looking at the physical structure of the entire complex of habitat types. Mabry and Dettman touch briefly on this, but it's beyond the main scope of their study.

It's very valuable data. It's common knowledge that structurally diverse habitat supports more diverse animal assemblages, but it took me longer than it should have to get some otherwise knowledgable people to grasp the importance. In this case, the dragonfly species of interest is at reduced density for other reasons (a severe drought that peaked in 2005 and has since ended), but degradation of habitat structure may be hindering recovery of the species at the site. Having quantitative data available from a nearby region makes it easier to build a case for aggressive habitat management.