Friday, September 23, 2016

Habitat, species variation, and tolerance values: the common stonefly Acroneuria abnormis

I have written before about the issue of habitat and species variation -- in some detail on 8/26/15.  Let me remind you of two examples.

First, we have two Isoperla holochloras, one light and one dark.  Light,

and dark.

The light we find in all sorts of streams that vary in quality from Buck Mt. Creek, a stream that flows through pasture land, to the Rapidan River in the mountains at the entrance to the Shenandoah National Park (Graves Mill).

Buck Mt. Creek

the Rapidan River

But the dark form is one that I've only seen in the mountains.  In the Rapidan River, where it co-occurs with the light, and in a very clean tributary to the Rapidan River, the Staunton.

In fact, in the Staunton I've seen nothing but the dark nymphs.

Another case that I've mentioned before, Isoperla orata.  We have the "classic" form that we've found in the Rapidan River and in the small, pristine streams of Sugar Hollow,

and a variant form

which is also found in the Rapidan River, but I've seen it as well in the Doyles River and Buck Mt.
Creek, again, streams that are a step down in quality terms.

It seems to me that we have the same situation with the common stonefly, Acroneuria abnormis. A reminder of Beaty's species description.  "A. abnormis ... : dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or dorsum of head without M-shaped head pattern and abdomen uniformly brown; anal gills always absent."  He adds "widespread and nymphs occur year round." 

I'd like to suggest that again, habitat is relevant in determining where these two forms are found.  The first, with the well defined M-pattern and the banded terga

is indeed widespread, and I see it in almost all of the streams I visit.   But in the Rivanna, we find the "brown" form (no M-pattern, and terga completely brown), a form that, so far, I've not seen anywhere else.

 (Not sure if the first type co-occurs in the Rivanna.  Something I have to look into.)  All of this makes me wonder about species variation and tolerance values.  It has to be true that the "brown" form of A. abnormis is more tolerant than the typical pattern we see, the same being true for the two types of I. holochlora and I. orata.  The NC list of tolerance values actually does distinguish between the light and dark holochloras: the former has a TV of 0.7, the latter is 0.0.  Makes very good sense.  But no distinction is made for our other two taxa.  There's only one value for I. orata -- 0.0 -- and the same is true for A. abnormis -- 2.1.   I think that there too, a distinction has to be made.

Have to contact Beaty on this one.  Could well be that this is one of those things that needs some revision.

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