One of the questions that drives a lot of our work is “Of the 25 elements required to build organisms, how many of them help regulate the abundance, activity, and diversity of organisms in ecosystems?”. In a freely available paper just out as a Report in Ecology, we provide a framework for the many ways that nutrients can interact, then test how Sodium (an AntLab favorite) and Nitrogen+Phosphorus (the preferred pair of the “Stoichiometric-Set”) interact to shape the abundance of the invertebrates above- and below-ground.
We were particularly keen on seeing if Na acts as a catalyst, increasing an insect’s ability to efficiently convert Nitrogen and Phosphorus into more insects, or if the two sets of nutrients acted more or less independently.
Four possible ways that two sets of nutrients can interact. We were betting on Serial C0-limitation, where Sodium catalyzes access to N and P. Didn’t always turn out that way.
The experiment was pretty classic field ecology: we furnished m2 plots with either water, N+P in the quantities used by the NutNet experiment, 1% NaCl solution, or both. We measured the soil and plant responses, and bugvac’ed the above-ground invertebrates with a modified leafblower (see picture above). For below-ground invertebrates, we used a very messy, but unfortunately best method available, flotation extraction of soil cores from the center of each plot.
A soil core from one of the experimental plots. Somewhere in there lurk oribatids, collembola, and other tiny inverts.
As an aside, as someone who cut his ecological teeth in the Nebraska Sandhills for his undergrad and master’s work at the University of Nebraska (see below), and has spent a good fraction of my days since then in the tropics, I am very much enjoying spending time in grasslands again. An NSF project with Nate Sanders on the role biogeochemistry plays in grassland food webs will keep me in the grass for the next few years.
A 21-year version of Mike Kaspari above WhiteTail Creek in the Sandhills of Nebraska. Note the Nikon K-100 camera, and the spiffy digital watch. And yes, those are cutoff jeans. It was a different time then.
After the sampling came the microscope work. Leafhoppers are by far the most common insects in the aboveground samples, and their mobility and subsequent ability to respond quickly to manipulations is something we want to investigate further. As predators go, spiders were common and diverse, and clerids were the most common beetle group.
Looking forward to getting to know the leafhoppers, a group whose English name is simple, and whose latinate name defies easy memorization.
One of the big questions going forward is how quickly predators accumulate on plots that first attracted their prey. The spiders were dauntingly diverse on this patch of Oklahoma prairie.
One of the greatest pleasures of invertebrate microscope work is getting acquainted with taxa like these clerid beetles that are common, but that I’ve never had the occasion to look at before.
Results: Both sodium and N+P shape both communities, but in different ways
In the grass and forbs above-ground, all three treatment plots differed from controls. Adding salt increased the abundance of insects, and adding N+P increased it even more so. However, these effects seemed to be independent of each other (strike one for the Na as a catalyst hypothesis). Since plant height and mass increased on N+P plots and not on Na plots (suggesting that while NP was a nutrient for the plants, adding Na had no negative effect on the first trophic level), we think that NP provides a double bonus of more food, and more habitat, while Na just provides the food. We will test this idea more this coming summer.
Below-ground, however, was a different story. Here Na seemed to act like a catalyst. Alone it had no effect on the oribatids and collembola that dominated belowground. But combined with NP, it boosted the modest increase of abundance where NP was added by itself. Here, our working hypothesis is that NP fertilization increases the food supply, and thus increases the demand for Na. We are keen to follow this up this summer as well.
So there it is, so far. The plans are to expand both the kinds of experiments and their distribution, to add experiments farther north in more Na-deprived grasslands. One project has a particular appeal: as NP + NaCl is roughly the recipe for urine, and urine is the single most effective way to fertilize a patch of prairie, how do community dynamics respond to a splash of Bison Piss? Is there a predictable succession? And how do the hundreds of species find, exploit, and deplete this resource? What are the patterns of succession? Lots to learn.