Robert Buchkowski: Weak interactions between herbivores and detritivores in an old-field ecosystem

In our newest post, Robert Buchkowski from University of Wester Ontario presents his last work ‘Weak interactions between strong interactors in an old-field ecosystem: control of nitrogen cycling by coupled herbivores and detritivores’. He discusses the importance of considering multiple interacting organisms when evaluating ecosystem functioning, the challenge to extract earthworms from the soil and warns about doing ecology under the supervision of an expert.

About the paper

There are many reasons to expect strong interactions between the organisms that eat plants and those that decompose plants. For instance, a grasshopper eats leaves during the summer and so removes tissue that could have been earthworm food in the autumn. Plants facing an army of woolly bears turn on chemical defenses that inadvertently deter woodlice. There are so many reasons to expect these differences to accumulate over time and affect ecosystem productivity, nutrient movement, and species composition.

Earthworm removal device
Earthworm removal device

Yet, there are also many reasons to expect these interactions to be unimportant in real ecosystems. Field and forests are complex places where plant communities assemble stochastically and soil nutrient cycles are built upon a huge bank of available organic matter. Turnover of soil organic matter and plant community composition can be resilient and slow.

The question is what: happens when we mix plant-eating and plant-decomposing animals? Do we see interactions that effect ecosystem productivity and nutrient cycling? Our three-year field experiment manipulated grasshoppers and earthworms using cages in an old field to measure whether their combined effect was different from the sum of their individual effects. We also built a mathematical model to test some of the reasons why we may or may not see interactions.

We found no evidence that the combined effects of grasshoppers and earthworms were any different than the sum of their individual effects. Our field data suggested one reason might be that the plant community assembling in our cages was driven by standard successional changes instead of by the animals. Our theoretical model showed that the mechanisms behind interactions between grasshoppers and earthworms are probably occurring, but that big pools of soil nutrients and the inertia of annual processes obscure them in a real field. Our results agree with two other studies of plant-eating and plant-decomposing animals carried out by Annise Dobson and Chelsea Prather in temperature and tropical forests, respectively.

About the research

Our experiment took place in a small, abandoned field between the foundation of an old barn and a forest. The sun rises slowly above the tall trees to the east of the field and shines over a shallow valley to the west right up until it sets. It’s a wonderful spot.

Our experimental cages replicate the design used by my co-author, Os Schmitz, for his work on grasshoppers and spiders, while the belowground part added landscaping cloth to help keep the earthworms out. We built the cages from plastic fencing and window screen curved into long cylinders with screen lids and bases. We dug the holes in which to install the cages using a Ditch Witch and fitted with an 18” auger to rip out the soil. I often say this was the best money I spent as a doctoral student, because I can’t imagine digging all 60 holes by hand. Some wonderful people helped us fill back in the soil once we set the cages. Then we planted rhizomes and seeds in the cages to add back in some plant functional diversity.

Experimental cage set up

We had two big challenges to overcome during our field work. The first was how to build cages that included the belowground section of the field and the second was how to get earthworms out of the soil.

Figuring out how to remove earthworms from the soil was more challenging. There are three options to get earthworms out of the soil: digging, mustard solution, and electroshocking. Since the first two would wreak our carefully curated experiment, we chose the latter. Electroshocking, as the name suggests, can be dangerous, so it isn’t something you should try unless you know how to do it safely – do not try the technology without clear knowledge and implementation of safety protocols. It also isn’t something we could start quickly because electroshockers are not an off-the-shelf product. Thankfully, I had help from some expert electroshockers and some engineers in my family to build something functional and safe.

A large Lumbricus terrestris removed from one of the cages.
A large Lumbricus terrestris removed from one of the cages.

Our experiment then came down to a lot of field time once we had the cages established. Electroshocking the cages to sample and remove earthworms took 60 hours of rain-free daylight every spring and fall. Electroshocking isn’t effective if it is too cold, too hot, too wet, or too dry. So, every season was a race against the coming summer or winter. I was up before dawn to start as soon as I could see, and I was packing up at dusk before it became too dark. The other parts of the experiment were much more relaxing because Os had perfected the methods. Catching grasshoppers and stocking cages, measuring plant biomass, collecting soil samples were wonderful field jobs after weeks of racing the weather each spring.

In the end, our last earthworm sampling round in the fall of 2018 had the worst conditions, but we managed to get the data and pull out the experiment. It was good to be done.

About the author

I became an ecologist because I enjoyed working outside and doing math. As a biology undergraduate, I started with a general interest in environmental studies but wasn’t keen on the generic approach taken in the introductory classes. I was inspired by the limnologist Maggie Xenopoulos after working with her for two summers and got excited about collecting ecosystem data (i.e., nutrient fluxes, discharge). At the time, I was not excited about ecology because my second-year course skimmed over the mathematical parts of the discipline. I was asked by my thesis advisor, Doug Morris, why I wasn’t planning to pursue ecology. I told him I wanted to go into a discipline with some math in it—oops. I stuck with ecology after he gave me some theoretical ecology papers and I realized I could keep doing math and spend summers outdoors.

Robert enjoying canoeing
Robert enjoying canoeing

These days, I am a postdoctoral fellow at the University of Western Ontario funded by the Natural Science and Engineering Research Council of Canada. I am building some new soil food web models with the soil ecologist Zoë Lindo. After committing my changes to GitHub, I head out into the forests and fields of Ontario with my partner and our terrier. I still haven’t trained our terrier to sniff out earthworms—best to keep work at work, I think.

Read the research in full here.

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