The Kaspari Lab
Exam Lady No. 1
When it comes to invertebrate body size, it’s hard to go big toward the poles
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.
Appalachian Understory Study No. 2
The challenge of getting the light, the movement, the depth, and shimmer, using a wet brush with ink, Prismacolor pencil, and Pentel markers.
Appalachian Understory Study No. 1
New Paper: Predicting the number of non-native across North America’s insect communities
When walking through a patch of habitat a key question obsesses ecologists: “How many species are there?”. An ornithologist or mammalogist can usually get a number within a reasonable range, in part because those critters are well studied. What happens when you dip into the realm of the hyper-diverse? In a new open access paper in Ecology and Evolution, our team uses a network of pitfall traps and eDNA to explore the balance of native and non-native insects in North American insect communities.
Insect communities (and by that, I’ll mean all invertebrates) are a challenge to study as they are hyper-diverse, with each taxon studied by a handful of taxonomists–the high priests of identification. As a result, a simple answer to the question “How many species are there, and how does that answer vary as you move from place to place?” is rather sticky (but see our recent contribution highlighted here).” And we need that answer before we can get to the trickier question, namely, “Why?”.
For example, individuals of a novel species are thought to arrive in a patch of habitat with some regularity but fewer “stick” and leave offspring, reflecting their ability to grow and thrive. The schema below captures some of the key ideas. You can increase the number of new species (shorthand “non-natives”) by increasing the community’s Capacity—its ability to support all species, native and non-native alike—or by increasing the likelihood of Establishment once arrived. But first, of course, you need some data to evaluate.
Enter NEON, the National Ecological Observatory Network, the US’s distributed network of monitoring sites that are running pitfall traps from the Arctic Circle to Puerto Rico. Each of these sites samples all the local available habitats, for a total of 51 communities of ground dwelling insects. Our other ace in the hole (methodologically speaking) was extracting DNA from the ethanol in which the bugs were stored. We identified the bugs from their leaky DNA. A longer explanation of the techniques can be found here.
Our first surprise was just how few non-native species (typically < 5%) were found in most North American communities for most major groups. The big exceptions are rather notorious—earthworms and isopods—as much of eastern North America is awash in European worms and sowbugs.
The second satisfying result was using current theory to account for the geograpny of non-invasives. The first is the Capacity hypothesis that diversity begets diversity: communities that support more native species—for all the myriad possible reasons—are able to support more non-natives. We estimate about one non-native species for every 14 native species.
Once we account for local native diversity, other hypotheses more clearly express themselves, accounting for a total of about 2/3rds of geographic variation in successful species invasions. One novel result was the 3-fold variation in fraction of invasives across North American habitats. Deciduous forests, wetlands, and pastures and hayfields all supported more non-natives than grasslands, shrub steppe and evergreen forests. This corresponds with predictions that high levels of disturbance and resource pulses enhance opportunities for establishment.
Likewise, areas surrounded by faster traffic appear better able to promote non-native establishment (good reason to keep cars out of our parks and preserves!). Finally, even for a given native diversity, more productive ecosystems support proportionally more non-natives. A new non-native species was added for every 250 g/C/m2 of NPP over a 35-fold gradient of productivity
The big picture?
The US’s National Ecological Observatory Network is designed to monitor ecosystems and populations across North America over the next 30 years. Using eDNA—the DNA from critters suspended in storage ethanol—we can non-destructively monitor the presence/absence of populations over time. At the same time, long-term studies of abundance remain rare and such distributed networks are vital. We cannot reliably detect—and reverse—insect declines without them.
Cover illustration by Brittany Benson.
Potassium as a game-changer in prairie food webs
We all know how sports drinks full of sodium (Na) and potassium (K) can rescue your performance on a hot day. But what if you’re a plant? Let me tell you about an experiment with Dr. Ellen Welti that shows the super-powers K gifts to plants. Plants use K as their primary way of keeping cells hydrated, plump, and working. But K is slippery—it’s so soluble that it leaks out of plants and must be constantly resupplied. K today, gone tomorrow. PDF: https://bit.ly/3G9SKwO
Plants often get their K pre-packaged with Na in urine. But plants don’t need the Na, and we’ve shown it attracts herbivores who, like us, eat plants and need both. What gives? So, we set up an experiment at @KonzaLTER to find out. #NSFFunded
We monthly fertilized 7×7 m plots w the Na and K you’d find from a herd of bison —or just the Na or K—vs controls. We watched what happened to the chemistry of the plants, and numbers of bugs on the plots. @OUBiology
On control plots, we see the natural course from May through September. Plants get bigger and drier, and a mouthful of May grass is much more nutritious than that in Aug/Sept. Nutrients are diluted in time. PDF: https://bit.ly/3yc5Y6s
But look what happens to all those elements on plots fertilized with K, Na, NK! With repeated fertilization, the net effect sizes become strongly positive by July. The grasses seem to be using the K to help pump nutrients into foliage. By September those same treatments have *lower* nutrients than control plots. NaK is a ‘sports drink’ for prairie plants, allowing them to mobilize nutrients up to leaves when they need them, and down to roots when they need to store them.
Now look at the bug densities: almost the opposite pattern. When the nutrients are high on +KNa plots, the bugs are suppressed. As they decline, the bugs rally, until they are sequestered below round, when they decline again.
One more thing. Perhaps the reason one-time applications of K rarely show an effect in big experiments like #NutNet is that they are applied, w N and P, in May. K is so soluble that it doesn’t stick around. But areas with repeated urine applications—like Bison Lawns and Vole latrines— may be the place to find plants that are true masters of their ionomic domain. But areas with repeated urine applications—like Bison Lawns and Vole latrines— may be the place to find plants that are true masters of their ionomic domain.
All that is green is not nutritious or, The importance of peeing earnestly
Grasslands can be zen places: hectares of rolling prairie. But to an herbivore, all that is green is not nutritious. In a recent paper led by NSF REU Katerina Ozment mentored by Dr. Ellen Welti of OU_Biology we reveal how drought & Bison combine to create a nutritional patchwork.
The paper (PDF:https://bit.ly/39c4cHB) began as a 2018 study of Bison Lawns at
@KonzaLTER, patches revisited by the big herbivores, where they eat, poop, and pee. The bison get a dependable, high nutrient food supply; the local grasshoppers do too. Win:Win!
But 2018 was also a *big* drought year. Enter Nutrient Dilution: where lotsa plant biomass dilutes with carbs the other essential nutrients in a bite of grass *and visa versa*. 2018 nitrogen supplies *off* bison lawns were like those of bison lawns in *normal* years.
So Ellen and I revisited the same 10 bison lawns in 2019, a normal rainfall year that would 1) reduce nutrient densities with the increased biomass, and 2) exacerbate (we predicted) the consequences for grasshoppers of *not* taking advantage of Bison pee-supercharged grass.
Combined, the 2 years tell a story of grasshoppers minding their nutrients. In the wet year (triangles) bison lawns were ‘hopper hotspots in a green desert; in the drought year, where nutritious grass was everywhere (just less of it), grasshoppers were less discriminating.
Katerina’s honors project revealed the interaction of subtle (rainfall) and not so subtle (bison) factors in generating a prairie patchwork for grasshoppers, and the counter-intuitive result that a lush green prairie may not necessarily be a herbivores cup of tea.
