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.
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…
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.
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!
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.
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.
I love reading these as I learn good questions to ponder . . .
It is really fascinating how predator impacts can cascade in unexpected ways and even be dependent upon grazing. I remember seeing a paper (the exact citation alludes me right now) where they experimentally altered spider species within an eastern US grassland (for different hunting strategies), which in turn changed the grasshopper foraging patterns, and then reduced the abundance of otherwise dominant Solidago canadensis. I wonder how bison grazing might impact the herbivory preferences of insects—perhaps shifting generalist insects to less-palatable (for bison) plants?
It’s all fascinating, isn’t it? If you find that citation, I’d be interested in reading that spider/grasshopper/goldenrod story! Bison (and cattle) grazing certainly changes insect communities, which would affect what plants are eaten by insects, but I like your idea too – that insects would also change to eating different plants. Whether they’d compete more or less with those large grazers… I could see that going either way!
(I guess my phone and computer has slightly different wordpress accounts…who knew)
That paper is actually from my PhD advisor Os Schmitz – https://onlinelibrary.wiley.com/doi/full/10.1046/j.1461-0248.2003.00412.x – and he has a ton of other work on the topic of predator exclusion impacting grassland plant diversity.
Seems like a good project would be to look at owl pellets to see what and when they are eating. Is there an easy way to figure out the snake diets? Who else is around and consuming small rodents?
Good questions. There have been a lot of studies of owl pellets, but I’m not sure how easy it is to tie them to specific habitat patches because you’d have to know exactly where the owl was hunting. Not sure how to tell what snakes are eating without catching and dissecting them or forcing them to regurgitate. Small mammals are food sources for a huge variety of animals, including a lot of ‘mesopredators’ like coyotes, foxes, badgers, bobcats, etc. Definitely lots of raptors too, as well as big snakes.
I did an undergraduate small mammal study comparing burned and grazed pastures at K-State. Annually burned and grazed pastures contained almost exclusively deer mice until fall when dead grass is available for meadow voles to line runs. Unburned pastures contained deer mice and meadow voles with a very few cottonrats, but very few white-footed mice until a shrubs, such as buckbrush and roughleaf dogwood, showed up. Longterm unburned and ungrazed sites had the most white-footed mice and a few woodrats appeared.
I would be curious to know if weather ( i.e. cloud cover) was recorded along with the moon phase element of the study.
Hi! My lab led this research and we did account for cloud cover in our moonlight illuminance measurements :) We obtained hourly cloud cover data from a local weather station and used it to adjust illuminance measures.
I’m not a scientist, so this is pretty much over my head. Nevertheless, I admire, respect, and am grateful for the analyses you’ve presented. While I’m unable to do what you’ve been trained to do, I absolutely enjoy having access to learning more each day. I really appreciate your erudite offerings! More, more, more…please!