Sunday, April 22, 2012

Thoughts on the Spiny Crawler "Explosion" and Evaluating our Streams

As I've noted in entry after entry for the last month or so, our streams -- all of them -- are loaded at the moment with spiny crawler nymphs.  When I go through the leaf packs or pick up a rock that's covered with moss, there is so much movement and activity that it's almost like looking into a beehive.  Spring samples done by groups that monitor streams are often overwhelmed by spiny crawlers -- I speak from personal experience.  But this can have a negative impact on the score of a stream for two different reasons.

Reason1.  One of the factors that is important in "scoring" a stream is the tolerance values of the insects we find.  At the moment in the state of Virginia, the DEQ sets a value on ALL spiny crawlers, regardless of genus or species, of 4.0.  That's rather high for a mayfly.

Now, how many spiny crawlers, and which spiny crawlers in terms of genus and species, have we seen so far this year in our streams?   We've seen 6 different spinys, and we'll see a 7th come the summer.  They are --

1. Ephemerella subvaria.

2. Ephemerella invaria:

3. Ephemerella dorothea:

4. Drunella walkeri:

5. Drunella tuberculata.

6. Eurylophella

and 7. Serratella serratoides (the spiny we see in the summer)

North Carolina, unlike Virginia, assigns tolerance values by genus and species.  The tolerance values they use for these species are as follows:

1. Ephemerella subvaria:     undetermined (species too uncommon)
2. Ephemerella invaria:       2.6
3. Ephemerella dorothea:    3.3
4. Drunella walkeri:            0.6
5. Drunella tuberculata:      0.0
6. Eurylophella spp.:           4.0
7. Serratella serratoides:     1.7

If we use -- as we do in Virginia -- a TV of 4.0 across the board for all spiny crawlers, are we getting an accurate reading on the health of our streams?  Now, a caveat here: I would bet that 90-99% of the spiny crawlers we find in March -- May in this part of Virginia are E. invaria (2.6) or E. dorothea (3.3): those are the species that really "explode".   Still, with the large number of spiny crawlers we find in the spring, the NC tolerance values will result in a quite different score than the one we'll find using the 4.0 of Virginia.

Reason 2.  Biological diversity is another factor that's taken into account in evaluating the health of a stream, and many groups that monitor streams aim for a sample of around 200 insects.  At this time of year, depending on where you put your net into the stream, you could easily end up with, say, 150 spiny crawlers -- or more -- in your sample of 200.  If you're hoping to have 15-25 "families" represented in a balanced way -- the sign of a "healthy" stream -- you'll be mightily disappointed.  But, this is their season.  I go to some streams that would have to be considerd "pristine," but at the moment any samples done in those streams would be dominated by spiny crawlers.  Is it right to "downgrade" a stream based on what is simply a fact of life?  And trust me, if you go back to those streams in June, you'll be hard pressed to find any spiny crawlers at all.

Random thoughts and concerns of an amateur entomologist, and I'm sure that people who assess the health of our streams must take these things into account.  If not, I sure think they should.

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