Cascading Impacts from Prairie Management – Now With Actual Research!

All science is cool, but there are some projects that are cooler than others – especially when they describe the astonishing complexity of interactions in nature.

This deer mouse (I think?) might make decisions about when and where to forage in ways you haven’t considered.

During the first several months of this blog, I told a story about an observation we’d made while seed harvesting. You can go back and read the original post if you like, but the quick highlights are these:

While looking for prairie clover seed to harvest back in the summer of 2005, we noticed that most of the seed in an ungrazed portion of a prairie had been recently clipped off, presumably by mice. However, in a nearby grazed portion, where prairie clover plants were tall but were surrounded by short-cropped grass, the seed was still on the plants. Whoa…

Here’s the photo I took back in 2005 when we found a bunch of prairie clover plants eaten by mice – but mainly in the ungrazed portion of a restored prairie.

My hypothesis at the time was that grazing had removed the protective cover that made mice comfortable foraging. Without that cover, I guessed, mice felt too exposed to potential predation, especially from aerial predators. As a result, grazers were changing the activity of mice, which affected seed production by plants, and that probably had further impacts on the plant community and who knows what else?

For years, I used that story when talking to audiences about the complexity of prairie ecology and management, but it was a story built on one observation and a lot of extrapolation. Well, no more! Now, a scientist named Pete Guiden, along with his collaborators, has published actual research that backs up my speculation. However, they went much further than I did and brought moonlight into the picture! You can read a ‘popular article’ on the research here and the full research paper (if you have access) here.

The study took place at The Nature Conservancy’s Nachusa Grasslands in Illinois, one of my very favorite places in the world. Bison have been introduced to the site and there are numerous efforts underway to understand the impact of bison grazing on all aspects of the ecology of the prairie. This particular project looked at how small mammals were using grazed and ungrazed areas, but focused on that use in relation to moonlight.

The Nature Conservancy’s Nachusa Grasslands has some spectacular prairie habitat, both remnant and restored.

Three small mammals were studied: deer mice (Peromyscus maniculatus), white-footed mice (Peromyscus leucopus), and prairie voles (Microtus ochragaster). Live traps were used to catch the animals and see when and how they were using grazed and ungrazed habitats. The deer mouse results were the first ones I looked at because I think it’s likely my prairie clover seed story was tied to that species of mouse. Sure enough, deer mice showed an aversion to grazed sites when moonlight was bright. That fits with my speculation that those little critters wouldn’t want to be out in the open, for fear of being spotted by aerial predators.

Even more interesting, the numbers of deer mice caught in grazed and ungrazed areas was about equal during times of less moonlight. In other words, on dark nights, deer mice seem to feel just fine about feeding in grazed areas because they feel less exposed to owl predation, for example. On bright nights, though, they want to be under cover. Very sensible. Now I want to know whether the feeding and other habits of deer mice are different in grazed areas, though. Just because they were caught there doesn’t mean they’re foraging in the same way, or for the same kinds of food. I wonder if they avoid climbing up plants to get seeds, for example, when neighboring plants have been grazed down. So many questions…

Now that I was thinking about the moon, I obviously had to go back and look at the date of my previous observation to see what the moon had been doing. A photo I’d taken of prairie clover heads clipped off by mice was dated 8/1/2005. According to the internet, there had been a full moon on July 21st – 11 days earlier. It could be a coincidence, of course, but it seems likely the mice had been feeding on prairie clover when the moon was bright. Fascinating!

This photo of cattle at Konza Prairie in the Kansas flint hills shows the kind of selective grazing that can leave prairie clover (bottom right) standing in the midst of grazed grasses. You can also see a lot of lead plant and other ungrazed forbs in the same scene.

So, deer mice are nervous about being out in the open when the moon is bright, but what about the other two species? Curiously, white-footed mice didn’t seem to respond to either grazing or moonlight. They were just as likely to be caught in grazed areas as ungrazed areas, regardless of moon phase. In their journal article, the researchers very responsibly stayed way from speculation about why that might have been.

I wonder if it’s related to the association between white-footed mice and brushy/woody habitat. If they evolved around trees and shrubs, white-footed mice might be less worried about aerial predators because they’re usually underneath woody cover. That’s a wild guess, everybody – feel free to form your own crazy hypotheses.

The third mammal species studied by the researchers was the prairie vole, which is typically associated with fairly dense vegetation. Sure enough, on dark nights, voles were four times as likely to be caught in ungrazed prairie. That makes sense. However, on moonlit nights, the researchers caught just as many voles in grazed areas as they did ungrazed areas. Why would voles venture out into the open on nights that were the brightest?

When they broke the data down more, researchers were able to see a more defined pattern in that vole activity. Most of the increase in vole captures within grazed areas came on nights when peak moonlight occurred around sunset, as opposed to when the night was brighter closer to sunrise. That pattern made the scientists think about snake predation, since snakes hunt most actively during the warm hours shortly after sunset. Further, many of those snakes use olfactory cues (their sense of smell) rather than sight to locate prey.

When you put that all together, it makes sense that voles might be using moonlight to detect predators and stay away from them. If snakes don’t need light to hunt, bright nights don’t make voles easier for snakes to catch, but they do make snakes easier for voles to see. If they see the danger, they can (hopefully) avoid it.

I felt like this post deserved a bison photo but I don’t have any from Nachusa. Here’s a young bison in the rain at The Nature Conservancy’s Broken Kettle Grasslands in northwest Iowa instead.

All this is really fun to think about. Deer mice may be most afraid of owls and voles may be most afraid of snakes. Each species seems to respond to habitat structure and moonlight in ways that fit those individual fears. Or at least that’s the current hypothesis, based on these latest data.

We’ve long known that animals adjust their activity in response to pressure from predators (including humans). Those changes in behavior can ripple through ecosystems, affecting much more than just prey species. Herbivores choose the timing and location of their foraging based on predator activity, for example, and that changes which plants get eaten, and where. That, in turn, affects resources available for pollinators, seed predators, and many other organisms.

This latest study layers the impacts of moon phases on top of those kinds of predator-prey interactions, further multiplying the complexity. It means that all those cascading impacts of predators aren’t constant, but change over time – even in relation to things like lunar cycles.

If that doesn’t make you love prairies and nature even more, I don’t know how to help you.

Photos of the Week – January 27, 2023

Today, we celebrate Anurans – amphibians with big hind jumping legs and no tails (as adults). Frogs and toads, in other words. And whatever spadefoots are.

Woodhouse’s toad after emerging from its winter hidey-hole. Helzer yard. Nikon 105mm macro lens. ISO 400, f/20, 1/100 sec.

As a photographer, I love toads and frogs (and whatever spadefoots are) as subjects for a few reasons. First, they are often relatively accommodating of me and my camera. Toads and tree frogs, especially, tend to sit pretty still when I approach. Or if they move, they don’t move very far and I can catch back up pretty easily.

There are exceptions to that. Leopard frogs can jump a country mile if they want to, so if they’re within reach of water when I approach, I usually have no chance. The best leopard frog photo subjects are the ones out foraging in or traveling through short grass. They usually know they’re unlikely to escape so they often sit still, either hoping they’re sufficiently camouflaged or hoping I’ll go away faster if they grant me a photo or two.

Plains leopard frog ‘hiding’ in short grass. Helzer family prairie. Nikon 105mm macro lens. ISO 640, f/11, 1/500 sec.

Bullfrogs can be tricky as well. Like leopard frogs, they can jump long distances and quickly disappear into water. Unlike leopard frogs, I almost never find bullfrogs away from the edge of water. As a result, I have to approach very slowly and photograph them before they feel enough pressure to jump or submerge themselves. Alternatively, once they submerge, I can get into position and wait for them to (hopefully) reappear within camera range. I usually don’t have the patience for that alternate strategy and the bullfrogs usually don’t fall for it anyway. That’s one reason I don’t have a lot of bullfrog photos.

A bullfrog that let me very slowly creep up on it. Helzer family prairie. Nikon 105mm macro lens. ISO 400, f/6.3, 1/1000 sec.

Another fun thing about Anurans is their facial expressions. I love photographing them face-to-face when I can because they all share a very similar expression – anthropomorphically speaking. I’m not sure what the equivalent expression would be in humans, but the shape of their mouths is pretty distinctive.

Is it a resigned expression? That would be appropriate since I’m usually imposing (very briefly) on them for a photo before letting them get back to their lives. If you have a better suggestion, let me know, but as you look at these photos, imagine the frogs and toads (and whatever spadefoots are) feeling resigned. I think it fits pretty well?

Cope’s gray tree frog with heath aster. Helzer family prairie. Nikon 105mm macro lens. ISO 500, f/18, 1/250 sec.
A plains spadefoot. Not really a frog or a toad, exactly, and with vertical pupils for extra flair. Niobrara Valley Preserve. Nikon 105mm macro lens. ISO 320, f/14, 1/500 sec.
Northern leopard frog. Niobrara Valley Preserve. Nikon 105mm macro lens. ISO 400, f/14, 1/640 sec.
Woodhouse’s toad (probably) on a river sandbar. Niobrara Valley Preserve. Nikon 105mm macro lens. ISO 800, f/16, 1/640 sec.
Cope’s gray tree frog in my square meter plot back in 2018. Lincoln Creek Prairie, Aurora, Nebraska. Nikon 105mm macro lens. ISO 500, f/16, 1/80 sec.
Plains leopard frog on frozen wetland. Springer Basin Waterfowl Production Area. Nikon 105mm macro lens. ISO 500, f/13, 1/250 sec.
Blanchard’s cricket frog. Niobrara Valley Preserve. Nikon 105mm macro lens. ISO 320, f/18, 1/125 sec.

Whatever the expression is on their face, it’s one I’m always glad to encounter. I’ll never not plop down on my belly to get face-to-face with an Anuran. Often, that means I end up with wet, muddy, or sandy clothes, but I’m not usually in company that cares much about that. Except maybe the frog, toad, or whatever a spadefoot is, and I think they probably get over it pretty quickly once I leave.

Woodhouse’s toad in the Platte River. Platte River Prairies. Nikon 105mm macro lens. ISO 320, f/13, 1/200 sec.