Ant Science: litter ants follow sunflecks


When something fresh does come out of my lab, I’ll share it. I also will share the process by which the paper got published, in the context of working in a teaching institution.

Here’s the latest.

Plants living on the forest floor are starving for light.

I was wondering if the same is true for ants, as light brings heat, which regulates ant activity.

In the 1970s, ecologists sorted out some big ideas about how communities assemble (even if they didn’t use the word “assembly” often). It turns out that access to light is a key variable, for plants. This is especially true for plants that live at the bottom of the forest, chained to the ground and not yet reaching the sky. They live and die by the tiny little flecks of sunlight that poke through the forest canopy to provide energy.

Myrmecologists were late to this story, because they were sidetracked by an obsession with interference competition. When you drop some tuna or a cookie on the ground, it turns out that ants go nuts. Now that many of us have accepted the fact, in real life, they don’t get regular tuna and pecan sandies from the sky, we are focusing on what really makes ant communities tick on a day to day basis. Particle physicists might have to smash their subjects in order to understand them, but as ecologists we can be more subtle.

One fundamental way that ant colonies are different from the plants is that they move around on the forest floor. The ants in rainforest litter move their colonies every few weeks from twig to twig. Maybe their nest movements reflect changes in the light coming through the canopy. Some ants might want to avoid the bits of sunlight and stay in shaded areas.

So, I ran an experiment by giving ants free nests, and putting them in a variety of light environments, including experimental shade treatments.

There were some taxonomic differences in who arrived at the supplemental nests, regardless of light environment. That’s not much of a surprise. What was more interesting was that the shade treatment radically affected the size of the colonies that occupied the nests. Generalist foragers like Pheidole and Solenopsis had much smaller colonies in the shade. In fact, the shade treatments had multiple Solenopsis foundress queens, who apparently prefer a shade microhabitat to start out their nests. On the other hand, the slow-moving specialized predators of litter microfauna, the dacetines (mostly Pyramica and Strumigenys) had larger nests in the total shade and much smaller ones in when there was some canopy light. Across the board, among all ants, colonies were smaller as the canopy became more closed. Just a few percentage points of change in the canopy radically affected the ants on the forest floor.

So, light – and its consequent effects on the temperature microclimate – affects how these ants move around their nests in the litter.

In short, sunflecks matter for ants as much as they do for plants, it seems. There is still a lot to learn about this, and we’re working more on it this upcoming field season. With fancier equipment than photosensitive paper to measure light, like we did in this one.

How this paper happened, from start to finish, in my teaching institution:

I went down to my field station in Costa Rica in January 2009, for two and a half weeks, with a group of five students. Travel was mostly funded by our campus AMP program (and some from an NSF grant to bring students to Costa Rica). I left for Costa Rica as soon as my son in first grade went back to school, and stayed down as long as I could before I had to return for classes in late January. My spouse was helpful in parenting 100% while I was gone, counting on ‘the village‘ once in a while.

All but one of us worked together on this single project, which is the foundation for this paper. (One student was prepping part time for the next field season for her MS thesis). The undergraduate in charge of data management was Melinda Weaver (third author). We found that the time span wasn’t enough to get enough of a sample size – it was very cold at the start of that dry season so the ants were not moving their nests much. So, we set up the experiment to cook over the whole season. In the summer of 2009, I took on an REU student from the field station’s REU program, Aura Alonso-Rodríguez (second author), from the University of Puerto Rico. At the end of the summer, a few people pitched in with Aura to finish data sampling (especially those who Aura had already helped out earlier in the summer). That summer, I was on station for a few weeks at the start of the summer, and then returned for a week and a half as the field season was winding down. That was a lot of time to be away from home, but I also had to return to handle a personnel issue, to prevent a different experiment from exploding or melting. This is a rarity.

We worked on the draft of the manuscript, on and off, for a year, and first submitted it in August 2010. (On my website, along with reprints, I provide the reviews of my papers throughout the review sequence). It got rejected after review, and then in the summer of 2011 I got around to submitting it out again, with some big changes to the analyses but mostly ignoring the biggest criticisms in the first rejection. This came back as a reject/resubmit, and I got it back just a couple days before my three month window for revisions expired. The reviews were far more useful than normal, as they caught a couple egregious errors in the analysis. It came back for minor revisions which I knocked out within a month.

Aura is now in grad school at the University of Puerto Rico, with her fieldwork based in Costa Rica. Melinda is now in a PhD program studying animal behavior at Arizona State.

I’m now developing a grant to examine the thermal biology of the community in more detail, in the context of community assembly and biodiversity, as the lowland rainforests get hotter than they already are. Even if the grant doesn’t come in, we’ll still find a way to work on this project for the next couple years.