Mystery

Today’s Photography 101 topic is mystery, and here’s my photo:

Stream

I post this photo because I’ve been working on this stream for a while to solve a mystery.  The water quality is good here – surprisingly good for an urban stream – and the insects you find here frequently back that up.  I’ve found riffle beetles and stoneflies here on occasion, insects that are only found in very clean waters.  However, you don’t find them every time.  In fact, you usually find almost no insects at all!

For the past three years, I’ve been working with some high school students to study this stream to try to explain the lack of insects.  What we’ve found so far suggests that flooding is the primary factor influencing the insect population in this stream.  While we still need to collect more data, particularly after a flood event, to be sure that it’s the flow that drives the lack of biodiversity in this stream, but I’m feeling more and more confident that this is the case and that we’re close to finally solving this mystery.

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth.

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The Power of Water Moving Downhill

A while back, I wrote about the microhabitats in the stream where I work and mentioned how the stream is missing some insects I would expect to see in this area.  Most significantly, it’s missing the swimmers.  I haven’t seen a single swimmer in the creek, and I’ve gone looking for insects dozens of times.  The absence of swimmers is curious.  To be missing things like the predaceous diving beetles and the water scavenger beetles…  Well, that just doesn’t happen!  That suggests that there is something happening in the stream that prevents those groups of insects from establishing themselves, and it presents a nice little puzzle to solve scientifically.  With the help of a group of high schoolers and a couple interns, I’ve been monitoring several characteristics of the creek.  The water quality isn’t bad, so it is unlikely to blame for the lack of swimmers.  However, there is another, more likely explanation: flooding.

The Prairie Ridge creek is small, but it drains a large part of the surrounding area.  Let’s take a look at a map for a moment:

Map of Prairie Ridge

A lot of the land to the right of or below Prairie Ridge (outlined in white) on the map drains into our creek.  Everything that is dirt in the center of the map is currently covered in either asphalt or buildings, neither of which absorbs water well.  So, every time it rains, the water runs downhill and ends up in our creek.  The result: we get a big pulse of water entering the creek with every rain event.  Small rain events cause a bump in the stream flow, just enough to make everything look muddy for a few hours, maybe overnight:

muddy creek

Muddy creek

Significant rain events, however, can cause the water to rise so quickly and the flow to blast downstream so hard that it carves new channels, rips banks down, pulls trees out by the roots, and washes it all downstream.  It’s impressive to watch the water in that creek during long and/or heavy rains!  It flows hard and very fast.

Now imagine a little beetle in the stream.  Heck, let’s imagine a big beetle, one of the Cybister predaceous diving beetles that are over an inch long and strong swimmers.  It’s there minding its business, swimming around a deep pool and hunting for food, when it starts to rain.  What do you think happens to that beetle when a huge pulse of water suddenly washes into the stream?  When there’s enough water to rip whole sections of the bank away?:

bank collapse

Bank collapse

The substrate of this stream is mostly sand and small cobbles with few larger rocks.  When it rains hard and you get a big pulse of water flowing downstream, that water picks up the lightweight substrates in the stream and washes them away.  That means that not only is there water flowing downstream very quickly and powerfully, but that it’s churning up sand and rocks as it goes.  Now think about that beetle again.  It’s not built to hold on during floods.  It’s got legs adapted for swimming, not clinging, and probably couldn’t hold on if it tried.  It’s left exposed, so down the stream it goes!  It may be carried miles downstream, assuming that it manages to escape being sandblasted by the churning, roiling mix of water and sand along the way.

We do find some insects in the creek, and they tell an important story.  There are caddisflies, tightly attached to the underside of heavy rocks.  Those rocks will move if the water is flowing hard enough, but for the most part they stay in place.  If you’re a small insect in a stream, living under one of those rocks is smart.  We also have calopterygid damselflies (the jewelwings) lurking in the root mass on the upper left side of this photo:

Prairie Ridge stream

Prairie Ridge stream, showing root mass

More of those roots are exposed every time a flood moves down the creek and the tree will likely fall over one day, but for now it provides a safe haven from floods for clinging insects like damselflies.  I imagine that they hold on for dear life in floods, hoping they won’t be swept downstream. 

We’ve found a few black fly larvae in our samples, insects that use silk to secure themselves to the tops of rocks.  In good black fly habitat, you might see thousands of them coating the rocks in huge mats.  Our stream is not good black fly habitat because we’ve only found a few black flies, but that’s not surprising.  Sitting on top of a rock, even a big, stable rock, isn’t necessarily the safest place to be in a flood.  Imagine standing on top of a house, or clinging to a tall tree, as a huge tornado is headed your way.  You can hold on as tightly as you can, but there’s still a good chance you’ll be knocked from your perch or mortally injured if something big hits you as the tornado engulfs you.

We find insects in the protected areas of the stream, but they are conspicuously absent from other areas.  The pooled areas are wholly devoid of insects, which leads to other interesting questions.  Are the insects colonizing the creek, only to be washed downstream in the first big rain?  Are they colonizing, then starving because there isn’t enough food available?  Or are they actively avoiding colonizing this creek altogether?  I hope someday I’ll be able to test these questions, but for now they’re merely interesting questions to ponder.

With the ease and speed with which the creek rises and falls, it’s little wonder that it’s missing a few groups of insects!  I’ll write about this project again when we have more concrete evidence to explain what’s happening in the stream, but for now I leave you with this thought: water moving downhill is an amazing, terrible, and awe-inspiring force of nature.  It carves canyons and carries with it the power of life and death.  There are few things water can’t move when there’s enough of it, so give it some respect!  You won’t be sorry you did.

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

Friday 5: Fun Facts About Giant Water Bugs

This week’s Friday 5 features a subject near and dear to my heart: giant water bugs!  If you aren’t familiar with these beasts, they have some really amazing characteristics that make them a fascinating group of insects to study.  I’ve already covered giant water bug parental care and feeding in other posts.  Today I’m going to share 5 fun facts about giant water bugs.  I hope that knowing these facts will help you fall in love with these wonderful bugs!

Lethocerus medius

Lethocerus medius, the biggest giant water bug in Arizona, can reach lengths of nearly 2.5 inches!

1.  The largest true bug (i.e. member of the insect order Hemiptera) in the world is a giant water bug.

Lethocerus maximus is truly a giant, reaching nearly 5 inches in length!  However, if you want to add one to your collection, you’ll have to visit northern South America.  The Lethocerus in the US are piddly in comparison, topping out at about 2.5 inches – half the size of the biggest species.

Abedus cannibalizing eggs

A male Abedus herberti cannibalizing his own offspring after he scraped them off his back.

2.  Giant water bugs can be cannibalistic.

A hungry giant water bug will eat almost anything it can get its claws on, including its own young (only when very hungry or something has gone wrong with the eggs a male is caring for), the young of other individuals, and each other.  Female Lethocerus are also known to rip apart the egg clutches deposited by other females when there aren’t enough males with good egg laying sites to go round.  However, I haven’t observed giant water bugs eating each other in the field unless there is very little other food available and they are getting desperate.  It would seem they prefer not to eat each other, but they will when they have no other choice.

Abedus herberti mating

Abedus herberti mating.

3.  Giant water bug mating can take several hours, especially in the back brooding species.

Mating is a long, involved process in the back brooding giant water bugs.  First the male does little push ups in the water.  These are thought to send vibrations through the water that the females respond to.  After a male and a female find one another, they mate.  Then the female climbs on the back of the male and lays a few eggs, maybe 4.  Then the male shakes her off and they mate again.  Then she lays a few more eggs before being shaken off again.  This goes on and on until most of the back of the male is covered with eggs, sometimes 150 altogether!  You can see how this might take a long time.  The water bugs in the photo took over 6 hours to lay all of their eggs.

Belostoma micantulum

Belostoma micantulum, a giant water bug from Argentina, is one of the smallest giant water bugs in the world.

4.  Not all giant water bugs are giant.

Belostoma parvum, a giant water bug from northern South America, can be less than a centimeter long.  It’s a not-so-giant water bug!  In fact, several species of giant water bugs in the genus Belostoma are actually quite small and don’t live up to the “giant” in their name at all.  The giant water bug pictured here is Belsotoma micantulum, a tiny little giant water bug that maxes out at a little over a half an inch long.  Pretty cute though, especially when munching on a mealworm that is WAY too big for her!  :)

flood

The only flash flood I've ever personally witnessed, though it's hard to see how big this flood was in this photo! Clicking on the photo will take you to a cruddy, low-res video I shot of it and posted on YouTube.

5.  At least one species has a nifty flood-avoidance behavior.  

Imagine you’re an aquatic insect and a flash flood is headed your way.  You’re going to be ground into a bloody pulp if you stick around.  What do you do?  If you’re the giant water bug Abedus herberti, you climb out of the stream before it floods!  This species crawls out of the water and walks perpendicularly to the bank until it reaches shelter away from the stream.  After the flood passes through, it crawls back into the water and carries on with its regular activities.  Awesome behavior!  And you can see a video of it online by visiting Dr. Dave Lytle’s website.  He filmed Abedus herberti leaving the stream after artificially simulating flood conditions with a fire hose.  The video is hilarious, so I encourage you to take 30 seconds out of your day to watch it.

Aren’t giant water bugs cool?  I love my bugs.  Considering they mostly just sit in one place hoping that food will swim by, it never ceases to amaze me just how many wild characteristics these bugs really have.  Hope you enjoyed this little peek into some of the many fascinating things these bugs have going for them!

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com