The Bug Geek

Insects. Doing Science. Other awesome, geeky stuff.

Arctic beetle trophic structure and shiny new research direction!

I am pretty excited by the next step I’m taking with one of my projects.

I’ve spent the past few months looking at a season’s worth of subarctic beetles from my summer in Kug, back in 2010. In my mid-field-season post that year, I mentioned that the community of beetles seemed pretty darned weird, at least to the naked eye: my traps were full of predatory beetles, but I was hard-pressed to find many herbivores, either in my traps or just by looking around on plants.

Now that I’ve actually gone through all of the samples, it’s clear that what I thought I saw was actually pretty much the case. Out of exactly 2638 adult beetles, 88.3% of them are carnivores. Only 11.2% are plant-feeders of some kind, and less than 1% are scavengers. I see almost identical figures if I consider the animals in terms of their mass and not just their numbers: about 87% of the “bulk” of all beetle bodies is carnivorous.

So why is this so weird?

Usually, when we think about how animals feed on each other, we tend to think of something rather pyramid-shaped, like this:

This is the “trophic structure” of a typical community of organisms. Each level in the pyramid is called a trophic level.

Most places on earth have a lot of plants. There are enough plants to feed, and provide energy to, all of the herbivores. Those herbivores are eaten by, and provide energy to, predators, which are fewer in number. Some trophic structures may have an additional level of “top” predators, that feed on just about everything, including other carnivores.

You can see how each trophic level in the pyramid gets smaller; it’s what keeps the community stable. For example, if there were more herbivores than plants, the herbivores would eat all of the plants (obliterating that level) and then they would in turn die off because there was nothing left for them to eat.

What I have found with my beetles from Kug is a trophic structure shaped something like this:

There are still quite a lot of plants, though not as many as you’d normally find in, say, an old open field in rural Ontario (this is the Arctic, after all). But the rest of the pyramid has essentially inverted: there are few herbivores and lots of predators.  The usual upward flow of energy seems to be disrupted.  Where are all these predators getting their energy?

My answer at this point is: I have no idea.

But I have two guesses:

1. Maybe I’m not seeing the whole picture – the predators might be eating other things!

Beetles don’t necessarily feed on other beetles. Maybe, if I added in other groups of animals, the trophic structure might look a little more “normal”. I don’t actually think this will be the case. I have started to look at the other critters I collected in my traps, and MOST of them are large, heavy-bodied, predatory spiders. There are a smattering of plant-eating bugs, grasshoppers, caterpillars and springtails, but I am almost certain there are not enough to provide energy to all the “bulk” of beetles and spiders.

2.  Who needs herbivores – why not just eat other carnivores?

I think these beetles (and the spiders, too) are actually feeding on each other  – this is a type of cannibalism, called intratrophic predation. In this kind of arrangement, predators get their energy by feeding on other high-energy predators. This is not unheard of; it’s been seen in desert communities, for example, but these kinds of trophic structures are not terribly common.

Anyways, I’d like to figure out exactly what’s going on in this system, and particularly if my second guess is correct. Since I wasn’t able to directly observe what all these beetles were eating while I was up north, I have to rely on some fancy-schmancy and new-to-me lab techniques

(*Gasp!*  TGIQ doing lab stuff??!?  I know, right?  This is all in the name of trying out new binoculars, friends).

The technique I’m going to start working on soon is called stable isotope analysis.

I’ll save the inner workings of this method for another post (not just a little bit because I’m still sorting out all the details myself!), but I’m pretty excited about trying it out. My job will be to carefully prepare beetle specimens by drying, crushing, and weighing tiny samples of their bodies into special teensy little tin cups. Then I’ll send them out to a lab that has a couple of specialized bits of equipment (which, last time I checked, I did not have sitting on my lab bench) that will measure the amount of nitrogen and carbon in each sample.

In a nutshell, this technique should let me figure out the trophic levels of all my predators (i.e., where exactly on the pyramid they sit), mainly by the amount of nitrogen in their bodies.  If they’re eating only herbivores, they’ll have less nitrogen, and will be on a lower trophic level. If they’re eating only other predators, they’ll have lots of nitrogen, and will show up at the highest level. Beetles eating a mix of herbivores and other predators will show up somewhere in the middle, with an in-between amount of nitrogen.

If I see mostly herbivore-feeders, and not predator-feeders, then I’ll know that my guess #2 is incorrect, and that I’m missing a piece of this little trophic puzzle.

Stay tuned for updates in the new year on this project!

15 responses to “Arctic beetle trophic structure and shiny new research direction!

  1. Morgan Jackson November 28, 2011 at 2:49 PM

    That’s really cool Geek, I love to see taxonomy creating interesting, testable hypotheses! Good luck breaking new ground!

  2. africagomez November 28, 2011 at 4:28 PM

    Very cool, especially as I am really hooked with the new BBC documentary Frozen Planet, showing some of the coolest footage (no pun intended). This new paper is quite shocking regarding the extent of intraguild predation in ladybirds
    And, I was wondering the extent to which these predators could be actually be scavengers, feeding on all sort of stuff preserved frozen?
    All the best with your research!

    • TGIQ November 29, 2011 at 7:01 PM

      I don’t know how much “preserved” stuff there is…I have a sense that (now this is just my own observation) there isn’t a whole lot of dead stuff lying around – things that die over the winter (large-bodied mammals/birds) are scavenged rather rapidly by other mammals/birds, I think. But this is an interesting idea…cool! (And a colleague out the lab that put out that ladybeetle paper just pointed it out to me 🙂 There’s some good stuff in there!)

  3. Ted C. MacRae November 28, 2011 at 6:48 PM

    What exactly are the main beetle groups you’re finding – carabids? At least among tiger beetles there is interspecific predation, with larger species preying on smaller species. For the most part this has caused shifts in temporal/spatial overlap within a given habitat, and I’m still hard pressed to believe the system is stable with predators all just eating each other. Any possibility of sampling bias that has missed an important (maybe cryptic) herbivorous guild? Not questioning your thinking or methods, just trying to think of other possible scenarious for further consideration. Cool stuff!

    • TGIQ November 28, 2011 at 7:06 PM

      Over 94% of the beetles were Carabids. The next most abundant group was Staphs, with only 58 individuals. And I agree that it’s possible there is another guild somewhere, but it might be pretty darn cryptic. There are, of course, non-beetle gall-makers and some root/stem borers (weevils and non-beetles), but I would think that those would be somewhat inaccessible for the most part. There is quite a bit of size-related spatial partitioning between the two major habitat types I sampled, which might explain things a bit…oh MAN you just gave me a great idea for a project!

  4. dragonflywoman November 29, 2011 at 1:51 AM

    Any chance that your predatory beetles could be relying on aerial plankton the way the grylloblattids do? Seems like the right kind of place for this to be an abundant food source – and maybe something to consider if guess #2 IS incorrect and you’re missing something.

  5. Jon Q November 29, 2011 at 3:15 PM

    Great experiment!! not many insects out where i am right now (northern utah) or i’m just not looking hard enough…. anyway great data i’ll remember that. perhaps the time of year is affecting your data, for instance ground beetles are generally year around but leaf beetles are most abundant in spring. What family did you collect the most of? -beetlebrained

  6. Jon Q November 29, 2011 at 3:18 PM

    don’t need to answere that…. just read through other comments. anyway! fascinating. -beethebrained

    • TGIQ November 29, 2011 at 7:11 PM

      There are ZERO insects out right now where I live too. I’m contenting myself with specimens-in-alcohol for now. It IS a pretty neat data set, and I’m pretty excited about pulling the rest of the story together!

  7. macromite December 1, 2011 at 10:47 AM

    Looks like a fun project. Stable-isoptope analysis can be hard to interpret (e.g. how do scavengers come out, or those that feed on microbes that grow on dead arthropods). Is there a reason you aren’t trying COI or another molecular marker in the gut contents?

    I like Ted’s idea in the EWWW post about the carabids going after aquatic dipteran larvae. We tend to think of food webs as fitting in discrete habitat boxes, but they rarely do (think dragonfly adults and larvae or mozzies for that matter). Also, animals tend to be a lot more omnivorous than we give them credit for (and some of those carabids must be primarily seed predators). Then one also wonders what their larvae are doing (which may or may not be what the adults do, but must represent a significant part of the energy transfer in this system). Don’t foget the ‘Paradox of the Plankton’, it’s possible you have some lower trophic levels that turn over much faster than your beetles. Yeah, this will be a fun project.

    • TGIQ December 1, 2011 at 5:52 PM

      I’m working out the logistics of gut content analyses also – I think it will be a great compliment to the work. I am definitely very intrigued by the possible links here btw aquatic and terrestrial habitats. I collected VERY few Carabid larvae – perhaps a dozen at this one site? And yes, there are certainly seed-feeders. I’ve got a lot of fun stuff to chew on (and yes, this is a super-fun project! :))

  8. Pingback: A few reasons to study Arctic entomology | Arthropod Ecology

  9. Pingback: Pyramids of species richness | Arthropod Ecology

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