Amphipods are, unfortunately, not very photogenic. But here you can see some of my study organisms swimming around in a mesocosm in the laboratory, shredding some leaf litter like it’s their business (because it is).
It can be intimidating to try to turn your research into an academic paper. I think that sometimes we have the idea that a project has to go perfectly, or reveal some really fascinating new information, in order to be worth spending the time and effort to publish.
This is the story of not that kind of project.
One of my dissertation chapters was just published in the journal Aquatic Ecology. You can read it here.
The project originated from a need to show that the results of my lab experiments were relevant to real-world situations. To start out my PhD, I had done several experiments with amphipods – small crustacean invertebrates common to central European streams – in containers, which we call mesocosms. I filled the mesocosms with water and different kinds of leaves, then added different species and combinations of amphipods. After a few weeks, I saw how much leaf litter the amphipods had eaten.
We found that there were some differences between amphipod species in how much they ate, and their preferences for different kinds of leaves based on nutrient content or toughness (that work is here). But the lab setting was quite different than real streams.
So I worked with two students from our limnoecology course (which includes both bachelors and masters students) to develop a field experiment that would test the same types of amphipod-leaf combinations in streams.
We built “cages” out of PVC pipe with 1-mm mesh over the ends. We would put amphipods and leaf litter inside the cages, zip tie them to a cement block, and place the cement block in a stream. We did this in two places in Eastern Switzerland, and with two different species of amphipod.
After two weeks, we pulled half the cement blocks and cages out. After four weeks, we pulled the other half out. Moving all those cement blocks around was pretty tough. I think of myself as strong and the two students were burly Swiss guys, but by the time we pulled the last cement block up a muddy stream bank I was ready to never do this type of experiment again.
Elvira and our two students, Marcel and Denis, with an experimental block in the stream. This was the stream with easy access; the other had a tall, steep bank that was a real haul to get in and out of.
Unfortunately, when I analyzed the data, it was clear that something had gone wrong. The data made no sense.
The control cages, with no amphipods in them, had lost more leaf litter than the ones with amphipods – which shouldn’t be the case since they only had bacteria and fungi decomposing them, whereas the amphipod cages had shredding invertebrates. And the cages we had removed after two weeks had lost more leaf litter than the ones we left in the stream for four weeks.
These are not the “results” you want to see.
We must have somewhere along the way made a mistake in labeling or putting material into cages, though I couldn’t see how. I tried to reconstruct what could have gone wrong, if labels could have gotten swapped or material misplaced. I don’t have an answer, but the data weren’t reliable. I couldn’t be sure that there was some ecological meaning behind the strange pattern. It could have just been human error.
I felt bad for the students I was working with, because it can be discouraging to do your first research project and not find any interesting results. It wasn’t the experience I wanted to have given them.
My supervisor and I agreed, with regret, that we had to redo the experiment. I was NOT HAPPY. I wasn’t mad at him, because I knew he was right, but I really didn’t want to do it. I’ve never been less excited to go do fieldwork.
But back out into the field I went with my cages and concrete blocks (and no students this time). In case we made more mistakes, we designed the experiment a bit differently. We had one really well-replicated timepoint instead of two timepoints with less replicates, and worked in one stream instead of two.
Begrudgingly, we hauled the blocks to the stream and then hauled them back out again.
Cages zip-tied to cement blocks and deployed in the stream. You can see the brown leaf litter inside the enclosure.
And then for 2 ½ years I ignored the data, until my dissertation was due, at which point I frantically analyzed it and turned it into a chapter.
The draft that I initially submitted (to the journal and in my dissertation) was not what I would call my best work. My FasterSkier colleague Gavin generously offered to do some copy-editing, and I was ashamed at how many mistakes he found. I hope he doesn’t think less of me. A fellow PhD student, Moritz, also read it for me, and had a lot of very prescient criticisms.
But through all of that and peer review, the paper improved. Even though it is not going to change the course of history, I’m glad that I put together the analyses and published it, because we found two kind of interesting things.
The first was about species differences. I had used two amphipod species in the experiment (separately, not mixed together). Per capita, one species ate a lot more/faster than the other… but that species was also twice as big as the other! So per biomass, the species had nearly identical consumption rates.
The metabolic theory of ecology is a powerful framework that explains a lot of patterns we see in the world. One of its rules is that metabolism does not scale linearly with body size (here’s a good blog post explainer of the theory and data and here’s the Wikipedia article). That is, an organism twice as big shouldn’t have twice the metabolic needs of a smaller organism. It should need some more energy, but not double.
This relates to my results because the consumption of leaf litter was directly fueling the amphipods’ metabolism. They may have gotten some energy and resources from elsewhere in the cages, but we didn’t put any plant material or other food sources in there. So we could expect to roughly substitute “consumption” for “metabolism” in this body size-metabolism relationship.
Metabolic theory was originally developed looking across all of life, from tiny organisms to elephants, so our twofold size difference among the two amphipod species isn’t that big. That makes it less surprising that the two species have the same per-biomass food consumption rates. But it’s still interesting.
The second interesting result had to do with how the two species fed when they were offered mixtures of different kinds of leaves. Some leaves are “better”, with higher nutrient contents, for example. Both species had consumed these leaves at high rates when they were offered those leaves alone, and had comparatively lower consumption rates when offered only poor-quality leaves.
In the mixtures, one species ate the “better” leaves even faster than would be expected based on the rates in single-species mixtures. That is, when offered better and worse food sources, they preferentially ate the better ones. The other species did not exhibit this preferential feeding behavior.
I thought this was mildly interesting, but I realized it was even cooler based on a comment from one of our peer reviewers. (S)he pointed out that this meant that streams inhabited by one species or the other might have different nutrient cycling patterns, if it was the species that preferentially ate all of the high-nutrient leaves, or not. We could link this to neat research by some other scientists. It was a truly helpful nudge in the peer review process.
So, while I had hated this project at one point, it’s finally published. And I think it was worth pushing through.
It was not a perfect project, but projects don’t have to be perfect for it to be worth telling their stories and sharing their data.
The Little Ecology Group 