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!