Ecologists have long been fascinated with body size as the “one functional trait that rules them all”. An organism’s body size is just so good at helping us understand how it fits into the rest of the community. Thus an exhaustive dataset on how communities of organisms (e.g., all the birds that occupy a woodlot, all the spiders collected from a single tropical tree) vary as you move from place to place—we’re talking real Geographic Ecology here—has always been a grail for ecologists. Or certainly, at least for me and my colleagues.

Enter NEON, the National Ecological Observatory Network. Constructed to monitor North American ecosystems for 30 years, NEON generates some of the highest quality community data out there—all in pretty much the same way—and has allowed our team (including coPIs Michael Weiser, Katie Marshall, and Cam Siler, and a host of students) to begin putting together a picture of how insect communities are built and how they vary from the subtropics of Puerto Rico to the Arctic Circle. Already, we’ve generated the first such studies of insect activity, diversity, and invasive species, with many more to come. Today, let’s look at the insect size results, shall we?

So this is what a NEON sample—3-4 pitfall traps sampling a given ecosystem for 2 weeks—looks like. Using computer imaging, we can turn these images into a metric of body size (in this case, the area (mm2). From there we generate community distributions like the 10 below, with the five communities on top representing warm ecosystems with little or no winter, and those at the bottom with some of the longest winters (and shortest growing seasons).

A close inspection reveals that the sites with the longer, warmer growing seasons supported insect communities where the modal bug size was smaller. But at the same time, those same communities had more individuals in the largest size classes than the colder sites with longer winters.

When we do the statistics, we find little evidence for a geography of mean body size, in part because the bigger taxa (like the Orthoptera get smaller as you move North, while the smallest, like the mites, get bigger). But the range of sizes, the size diversity of this functional trait becomes more and more constrained as you approach the arctic. This pattern is consistent across the taxa.

Why is this so? The top two graphs above capture what little variation (11%!) we can account for body size, but this number is a little misleading. This is because the plot of size vs an ecosystem’s plant productivity (GPP) suggests that communities of tiny bugs trend toward depauperate ones, but that large average size is promoted, but not guaranteed, by productive ones.

But we appear to have a much stronger handle on the diversity of sizes–if you want a community with large and small bugs, you want a long growing season/short winter. We think that is because it takes time to grow, and if you have 12 months to do so, your community will be more likely to produce a behemoth than if you only have two.

The upshot? We now have a much, much better picture of another feature of insects at a continental scale. And as the Earth warms, we can begin to make intelligent predictions as to how insects—described by EO Wilson famously as “the little things that run the world”—will respond. For example, if growing seasons are prolonged, and temperatures increased, we may begin to see larger bugs in our gardens and agricultural fields. If so, the NEON monitoring network may be in a position to reveal this change. Check out the manuscript in Ecosphere.