Functional Feeding Groups and the River Continuum

In discussing families of insects in previous entries I've mentioned the phrase "Functional Feeding Group" (FFG) on more than one occasion.   Since this is an important ecological concept in the work that we do, I thought I might say a bit more.   Combined with the idea of the "River Continuum," it helps us to understand which insects we're likely to find in different parts of rivers and streams.

Let me begin by noting that my information for writing this entry comes from two sources.  They are:

1) an article by Richard W. Merritt and Kenneth W. Cummins entitled "Trophic Relationships of Macroinvertebrates," which can be found in F. Richard Hauer and Gary A. Lamberti, eds., Methods in Stream Ecology, Second Edition (Academic Press [Elsevier]: Burlington, MA and elsewhere, 2007), pp. 585-601.

and

2) Glenn B. Wiggins, Caddisflies: The Underwater Architects (Toronto: University of Toronto Press, 2004), pp. 45-47 ("Communities in Streams and Rivers," "the River Continuum Concept," and "Drifting Invertebrates.")

Benthic macroinvertebrates are divided by eating behavior into four different groups.  They are:

I. Shredders (sh): (e.g. crane fly larvae, all stoneflies with the exception of the three that are predaceous, and the Lepidostomatid case-making caddis)

II. Collectors -- a group that is subdivided into

       A. Filtering Collectors (cf): (e.g. black flies, common netspinners, and brushlegged mayflies)

       note: black flies and brushlegged mayflies "filter" particles out of the water using their           "whiskers"and hairy legs, respectively; common netspinners pick the food out of their nets.

       B. Gathering Collectors(cg): (e.g. midges, all mayflies with the exception of the brushlegged
       mayflies, and flatheaded mayflies)

III. Scrapers (sc): (e.g. flatheads, water pennies, riffle beetles, and snails)

and

IV. Predators (pr): (e.g. Perlid [common], Perlodid, and Chloroperlid [Green] stoneflies, the freeliving 

      caddisfly, and all Odonata (all dragonflies and damselflies).

(To be thorough, I should note that Merritt and Cummins, p. 586,  add a fifth group -- the "Piercers-Herbivores" -- but it includes only one taxon, the Micro caddis.  The Micro caddis apparently "pierces" algal cells and sucks out the contents.  Pretty specialized!)

For a full account of the FFG's of all benthic macroinvertebrates by order and family, I urge the reader to go to the StreamWatch website (http://streamwatch.org/volunteers/forms-and-documents) and open the pdf. on "Tolerance Values."

The concept of the "River Continuum" simply asserts that the numbers of insects in each of these groups will vary with the point of the stream being sampled and the food sources available at that point in the stream.  Thus, "Shredders" need leaves and twigs to shred for their food, and they are the dominant group found in the forested headwaters of any watershed (like the site pictured at the top of the page).  What have I been finding in leaf packs all fall and winter?  Crane fly larvae, Lepidostomatid caddisflies, and large and small winter stoneflies.   However, in those same leaf packs, I've found a fair number of common and Perlodid stoneflies -- Predators -- a case of the "eaters" eating the "eaters" (or the eaters becoming the eaten)!   Also present in this headwater habitat are the Collectors -- both types of Collectors (e.g. mayflies and black flies).  Why?  They are there to "pick up the crumbs," so to speak, the fine bits of organic matter left over by the Shredders (and Predators, for that matter).  The food consumed by Shredders and Predators is technically known as "coarse particulate organic matter" (CPOM), while that eaten by the Collectors is referred to as "fine particulate organic matter" (FPOM).

As we move further downstream, the stream widens, trees no longer cover the banks dropping their leaves into the water each fall, and the increased sunlight hitting the water leads to an increased growth of algae on the boulders and rocks.   All of these factors lead to a decrease in Shredders and an increase in Scrapers -- those insects that eat by scraping their food from rocks (e.g. beetles -- water pennies and riffle beetles -- and flatheaded mayflies).  The number of Collectors remains fairly constant as does the number of Predators (though we might start to see more "warm water" predators -- damsels and dragons -- and fewer "cold water" stoneflies).

Finally, as the stream widens into a genuine river,  Scrapers too start to diminish, and the Collectors become the dominant group in the stream.   But the Predators still remain -- Odonata families by now taking over.   StreamWatch samplers all know that we find the greatest number of damsels and dragons in the mainstem of the Rivanna, especially the site near Crofton.

I. Shredders (crane fly larva and Lepidostomatid caddis)

IIA. Filtering Collectors (black fly larva, brushlegged mayfly)

IIB. Gathering Collectors (small minnow mayfly, spiny crawler mayfly)

III. Scrapers (flatheaded mayfly, water penny)

IV. Predators (common stonefly, emerald dragonfly)

Study Aids: A Couple of Notes

Just a couple things I thought I would mention.

1.  When I put together my "Amateur Entomologist's Bibliography" (2/11/11), I had in mind those books I've been using in putting together this Blog.   I would assume that a lot of my readers (most?) are volunteers who are still working on "Family" identification.  When I was working on family identification, there were two books I was never without, and I'd really recommend that you get them.

1) J. Reese Voshell, Jr., A Guide to Common Freshwater Invertebrates for North America (Blacksburg, VA: The McDonald & Woodward Publishing Company, Fifth Edition, 2007).

2) R.W. Bouchard, Jr., Guide to Aquatic Invertebrates of the Upper Midwest: Identification Manual for Students, Citizen Monitors, and Aquatic Resource Professionals (Minneapolis, MN: University of Minnesota Press, 2004 -- reprinted in 2008).  This is available on line, or can be purchased by going to:  http://www.entomology.umn.edu/midge/VSMIVP%20Key/English/VSMIVP.htm.  I recommend that you purchase a copy and take it with you to the stream.


__________


2. The second thing I wanted to note is that, for those of you who are primarily interested in looking at photos, I have made a collection of my "Personal Favorites" (of the photos I've taken).  You can find this at my Flickr website: http://www.flickr.com/photos/aquaticinsects_of_central_virginia.  Just click on the "Personal Favorites" icon on the right side of the page.

Omissions: Additional Stonefly and Caddisfly Genera

In describing and commenting on stonefly and caddisfly genera in previous entries -- specifically, in "Common Stoneflies" (12/27/10), "Perlodid Stoneflies" (12/26/10), and "The 'Common Netspinner Caddisfly'" (1/5/11) -- I have omitted genera that I've personally never seen in our rivers and streams -- with one exception.   There is one genus of "common stonefly" I have found in our streams that I simply failed to mention.   But there are two Perlodid stoneflies and one "common netspinner" caddis not mentioned above that may in fact be in our streams waiting to be discovered (at least they're included as "northeastern genera" in Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America.)  So let me comment on these.

1. Common stonefly (family: Perlidae): genus Agnetina.  First, the common stonefly genus I forgot to mention: Agnetina (pictured above).   I found two of these last summer in the main stem of the Rivanna.  They just don't belong there!  I think they took a wrong turn.  This is a stonefly with a tolerance value of "0" -- what is it doing in what is, at best, mediocre water?!   This is why we (I!) have to learn to be less rigid in my expectations.

How do we know this nymph is genus Agnetina?  There are two features that we have look for, the first being an "occipital ridge" (a distinct line across the back of the head) that is made up of closely set spinules.  And, it's certainly there.

Now, this feature is also present on genus Paragnetina (covered in the previous entry on common stoneflies) -- but Paragnetina nymphs lack "subanal gills," Agnetina nymphs have them, and this nymph has them.  Hence, Agnetina.

2. Perlodid stonefly: genus Isogenoides.  This is a genus I hope to run into some day in a high elevation, very clean stream: it has a tolerance value of "0".  (Oh, well, then it might be in the Rivanna!)
The only place I've seen them so far is Montana -- and the pictures below are of one of those nymphs.

Peckarsky, et.al. (Freshwater Macroinvertebrates, p. 71) note two features for identifying this particular genus: 1) "submental gills at least twice as long as greatest width," and 2) "median ridge of mesosternum extends anteriorly beyond fork of Y to transverse ridge."   The microscope pictures tell all (note: the "submentum" is essentially the bottom part of the chin, and the "mesosternum" is the middle part of the chest).

First, the "submental gills":

And then the "median ridge of the mesosternum":

Isogenoides is a Perlodid genus that has been found in Virgina -- some of you may have seen it.

3. Perlodid stonefly: genus Arcynopteryx.  This is a genus that may not be found in Virginia -- I'm not really sure.  It is found in the states of New England.  Again, my sample was brought from Montana.

You'll love the defining feature on this one!  Again I'll quote from Peckarsky, et.al. (p. 69): "Anterior ends of arms of Y ridge of mesosternum meet anterior corners of furcal pits."  Thank God that the microscope can pick all of this up.

If anyone sees this genus in the state of Virginia, please let me know.  Oh, and this genus also has the long, thin, submental gills as we find on Isogenoides.

4. "Common netspinner" (family: Hydropsychidae): genus Arctopsyche.  Wiggins notes one species of this genus that's present in the "southeast," so, we might keep an eye out for this.  (Until then, the only place that I've found them is Montana.)

To identify this netspinner genus we have to look at the chin where the "gula [is] narrowed posteriorly" (Peckarsky, et.al., Freshwater Macroinvertebrates, p. 101).

The "gula" is the sclerite that separates the two sides of the chin -- called the "genae".  In most common netspinners, the genae touch at some point.  Here the "gula" completely separates the two, and as you can see, does indeed "narrow" as it moves from top to bottom.

Rapidan River off Graves Mill Rd., Pt. II.

Well, this is very interesting.  I made two stops on my trip to the Rapidan yesterday.  In my last entry,  I reported my findings at the first stop -- about 1 mile below the entrance to the National Park.  My second stop was about 5 miles downstream from that location, not far from the village of Wolftown.  There, I didn't take any live photos -- I just collected some insects for preservation, and this morning I took a look at them with my scope.

The insects I found at this second stop were entirely different than those I found at the first.  First of all, I did find large winter stoneflies (only a couple) -- both genera (Strophopteryx and Taeniopteryx).  And I found one tiny Perlodid stonefly -- genus Isoperla.  But I found lots and lots of Nemourids, all genus Nemoura (photo above), and note that their wingpads are already fully developed (we've not yet started to see "baby" Amphinemura Nemourids).  I regret keeping so many: I thought I was picking up something else.   Could it be that the reason we've seen this genus so rarely in our watershed streams is that it's less tolerant of stream impairment than the genus we do find -- Amphinemura (I can't find "tolerance value" information on the genus Nemoura; the general tolerance value for Amphinemura is 3.4)?   It's certainly true that I've only found this genus in pretty good streams (Long Island Creek, the Lynch River, Elk Run, the Moormans River, and now the Rapidan).  Nemoura Nemourids may be "rare" in the streams that I've commonly sampled; I cannot say they're rare in this region.

And secondly, the mayflies I found were not those I had found just 5 miles upstream.  There, you'll recall, I found a whole lot of Pronggilled mayflies, and a lot of Epeorus flatheads.  I also found two spiny crawlers, genus Ephemerella, but these were the beautifully colored species that I've seen nowhere else.  This one.

Here (the second stop), there were no Pronggilled mayflies at all, only a few flatheaded mayflies (still Epeorus), one small minnow mayfly (genus Baetis -- the same that I've found in the Doyles), and -- once I had looked at them with my scope -- four decent sized spiny crawlers, genus Ephemerella, but all a drab brownish green (note the photo below).  This is the spiny crawler I'm used to seeing in our watershed in large numbers throughout the spring.  So, they've arrived!

What accounts for the difference in the nymphs found in the two different sites on the very same stream?
That's something I'd like to study -- but I lack the training and knowledge I need.  Still, a couple of comments and thoughts: the first being that we need to be clear that the mayflies we found yesterday at the upstream location -- Pronggilled mayflies and Epeorus flatheads (lots of them!) --  indicate very good water; those downstream, not necessarily so.  (I wonder about the two different species of spiny crawler mayflies...)  So, it looks like there has been a decline in the quality of the stream even though we've only moved five miles downstream.   Moving from that observation, we have to ask, has the habitat changed?  And the answer is "Yes".  The upstream location is coming out of the national forest, with no human habitation above it; but between it and the downstream location is farm after farm after farm -- less trees by the water, more open land.  And I guess the last point I'd make is that the stream character has radically changed.  The upstream site is riffles and cobble with very few pools -- fast flowing water; downstream, the Rapidan has assumed the more common form we find as streams move out of the woods -- a constant succession of riffles and pools, with the pools being long and the riffles fewer and farther between.

Further comments on these changes are welcomed from readers.   One thing I've always wanted to do is take a good stream and have 5 - 10 teams to sample that stream at different locations, maybe 5 to 10 miles apart on the same day.  What would we find in terms of the types of insects found?  And what would we find in terms of the "scores"?  Ah...  so many fun things to do, so little time (and so few helpers)!

Stream Report: The Rapidan River off Graves Mill Rd.

The site I explored today was about a mile below the entrance to the National Park.  And I made some interesting finds.  (What I did not find was any large or small winter stoneflies!)

1. The insect pictured above is a "predaceous diving beetle" (family: Dytiscidae).   This is an insect we hardly ever see, and I actually found two of them here.  Both were in leaf packs.  They are fittingly named, since they'll go after just about anything they can get into their jaws: today, their game would have been common stoneflies, pronggilled mayflies, and black fly larvae.

Dytiscids are aeropneustic insects.  That means they get their oxygen out of the air -- not out of the water (hydropneustic).   To do this they use a "breathing tube" which is located between their tails.  Since it's hard to see in the picture above, let me show you a specimen that's been preserved.

To breathe, they must surface every so often or live close enough to dry land where they can quickly access the air.  Both of the Dytiscids that I found today were in leaf packs, but in leaf packs that were right next to shore and were only partly submerged.

2. Also in the leaf packs were lots of pronggilled mayflies.  And they appear to be fairly mature.  The reader may recall that the only genus I've seen in this region is Paraleptophlebia: to fly fishermen, that's the "Blue Quill," and it hatches in early spring.  A nice photo follows.

3. Also in leaf packs -- lots of Peltoperlids (Roach-like stoneflies).  I had mentioned in an earlier entry that we often find them in spring, and they're often in leaf packs, and that when we find them, we often find lots of them in the same place at the same time.  Exactly what happened today.  I didn't keep any for my reference collection, but I did get a few photos.

And another hovering by a Uenoid caddis and a Glossosomatid (Saddle case-maker) caddis that's slipped out of its case.

4. I also learned a good lesson today which is one that we all should remember:  common netspinning caddisflies -- even those of the same genus -- can often be quite different in pattern and color.  I was curious about the genera of the two larvae below, so I preserved them to look at at home.  The first has a very black head with brownish abdomen; the second was light green in color with a yellow pattern on the head and the thoracic sclerites.  Surely, I thought, these are different genera.

Wrong.  Both were genus Hydropsyche, one of the common genera we see.  What identifies this particular genus?  There are two things to look for.  1) The fore trochantin (shoulder blade?) is forked.
Take a look at the picture below.

The "forked fore trochantin" means a larva is either genus Cheumatopsyche or genus Hydropsyche.  So, we've narrowed it down.  2) For the other defining feature, we have to look under the neck.

Under the neck is a horizontal dark bar known as the "prosternal plate".  If there are two dark marks (sclerites) below it (as in the photo above), our larva is Hydropsyche; if the sclerites are missing, it's Cheumatopsyche.  (Neither of these genera -- you may recall -- has a very good tolerance value; it's between 5 and 6).  Both of our common netspinner larvae turned out to be genus Hydropsyche.

5. I once again found that beautiful spiny crawler, wearing its "coat of many colors" (see previous Rapidan entry).  And as I was pushing it to pose for some pictures, it moved into a stance that we commonly find with spiny crawlers.

It raised its abdomen up and pointed the tips of its tails towards the front of its body.  This is a defensive posture.  It's a way of warding off enemies when they get close.  And, it's something we often see when we collect these nymphs in the spring.  If you see a mayfly curled up like this on your net in the spring, you can be pretty sure that you've got a spiny!

6.  One final photo -- a brushlegged mayfly.  I also found a fair number of these, and they too were snugged up in the leaf packs.   (The bottoms of rocks were the domain of Epeorus flatheaded mayflies, Uenoid case-making caddisflies, and a few saddle case-makers).

The Midges from Heaven!

(As opposed to what we normally see -- the "midges from Hell"!)

Since I've not really "officially" sampled this winter -- with StreamWatch or a VA SOS group -- I don't really know what they've been finding for midges.  The midges I've seen on rocks have been HUGE.
The one in the picture above -- found in the Moormans River this morning -- was about 1/2" long.  Anyone who samples knows that that is not what we normally see.  They can be so tiny in the summer (and other times of the year), that "if" you can see them, and "if" you can pick them up with the tweezers, you may or may not be able to see them in the tray when it's time to total them up.  The biggest compliment any sampler receives is to be told "You have 'midge' eyes" -- i.e. you're able to see the tiniest insects!

A smile for the camera -- though this is from a specimen I had to preserve.  His little "prolegs" show up very well.

Fly fishermen, by the way, would not be surprised at the size of the midges I've found.  The winter is really "midge" season for those of us who go to the streams.  This is one of the main times all year that they're hatching in significant numbers, and if we want success with the trout, we have to carry those size 18-22 midge imitations that nobody likes to tie on!

I went to two streams this morning -- the upper Moormans River, just downstream from the reservoir, and the Doyles River near Doylesville.  I didn't see anything really unusual: large numbers of Strophopteryx large winter stoneflies, and large numbers of black flies (at the Doyles, the size of the colonies on the rocks is absolutely disgusting!).   At the Doyles, I again found a lot of small minnow mayflies -- all genus Baetis.  It's fair to see this is a "small minnow" stream.  I also found a fair number of flatheaded mayflies at the Doyles: two genera, Epeorus and Maccaffertium (again with the red bands on the femora).  I got two gorgeous shots of an Epeorus flathead that have to be shared.

Probably as well as I'll do as an amateur photographer who likes to play around with these things.

I also found, in the Moormans, three more Nemourid stoneflies, all genus Nemoura (see the previous entry).  I may have to revise my proposal that this is a genus that is rare in our waters.  Perhaps we're just not sampling at the right times and right places to find them.  Here's a live shot of one of those nymphs.

One final observation to share -- or maybe it's a question to ask.  I may, at the moment, be seeing as many "adult" (i.e. hatched) small and large winter stoneflies as I'm seeing nymphs.  They're out in good numbers.  But I'm finding them where I don't expect to see them at all -- in leaf packs!  This defies conventional fly fishing wisdom.  We're taught that stoneflies "crawl" along the stream bottom, then crawl out on the rocks (either by the shore or in mid-stream), where they crack open their shucks, spread their wings, and take to the air.  So, why am I seeing so many -- with wings totally open -- crawling around in the leaves!

(Below,  large winter stonefly adults sunning themselves on a rock in the middle of the North Fork of the Moormans.  Photo taken last spring.)

Elk Run: First Sighting of a Nemourid Stonefly

As I was sorting through the nymphs and larvae I found at Elk Run on Friday, I made a note that I'd better check this one with care.  I suspected that it might be a Nemourid stonefly (see the entry above from 1/30/11 -- "The Super Hatches of Spring"), and with the help of the microscope I now know I was right.

This Nemourid is genus Nemoura -- not the genus we normally find.  The Nemourid that shows up in large numbers in samples done in the spring is genus Amphinemura, and you may recall that it is very distinct because of its "cervical" gills.  Here's a reminder.

No frilly gills sticking out from both sides of the neck on the Nemourid I found on Friday.  Still my first thought when I saw the nymph at the top of the page was "It's a large winter stonefly."  These stonefly families can be confused because the wingpads on mature nymphs are shaped exactly the same.
But, this stonefly lacks the "mottled" green/yellow colors on the wingpads and pronotum that we find on the large winter genus Strophopteryx; it also lacks the distinct light stripe that runs the length of the body on the large winter genus Taeniopteryx -- more importantly, it has no "coxal" gills as we would find on the latter genus.

But could it be some other genus of large winter stone, one that I haven't seen?  The question for which we need an answer is -- is there some way to distinguish Nemourid stoneflies from large winter stoneflies?  The answer is "Yes," and for this we must look at the tarsus.  The tarsus is the third part of the leg out from the body (the order is: femur, tibia, tarsus, tarsal claw), and the tarsus divides into three segments.  Here are two pictures of the tarsus of the stonefly whose identity is in question.

This is a challenge for microscope, camera, and reader...but, I hope you can see that "segment 1" is larger/longer than "segment 2".  Because that's the end of the story: that means that this is a Nemourid.
On a large winter stonefly, tarsal segments 1 and 2 are equal in size.

The next question to ask is how do we know that this particular Nemourid is genus Nemoura?  Well, we've already established that it's not genus Amphinemura since it lacks "cervical gills".  The Nemoura identification is made based on the nature and shape of the pronotum.  Let's have a look.

I'm not sure this is clear in the blog photo, but there are short, spiny hairs on the sides of the pronotum, but none on the bottom.  Also, the pronotum corners are "rounded": this gives us Nemoura.

I have personally seen very few Nemoura Nemourids in our streams, and I think that they're rare: in addition to this one, I found two at Long Island Creek in Fluvanna County last winter and one in the Lynch River.  Also,  these seem to appear and mature earlier than the Amphinemura Nemourids that we see in the spring -- in large numbers.  The two I found in Long Island Creek were found on March 12th, and they were mature.  Our "frilly neck" Amphinemuras tend to mature in late April and May.  One final suggestion: Nemoura Nemourids seem to prefer small, narrow streams (like Elk Run and Long Island Creek).  (If someone knows that I'm wrong about that, please let me know.)

One more look at a Nemoura Nemourid, this one from Long Island Creek.  And then a look at the labium of the Nemourid: note how similar this is to that of the large and small winter stoneflies (see the entry on "Looking under the hood," 2/8/11).