By Kim MacQueen

Nature’s frenzied—even frightening—choreography of predators and prey play grand themes on a small stage.

It’s a puzzle that has bugged biologists since long before “ecology” became a household word in the early 1960s.

How do living things share common ground in a vicious, eat-and-be-eaten world and survive? Or don’t? Which species can live together in relative harmony, and which can’t?What natural forces dictate who lives and who dies? Do Darwin’s ideas about natural selection—about survival of the fittest—explain the whole picture of how diverse populations of plants and animals wax and wane—and physically change—over time?

Natural selection tends to make predators increasingly more cunning and prey increasingly more elusive—there’s evidence aplenty of that. But clearly, this “arms race” among animals can’t go on forever without some breakdown in the system. A planet with too many prey animals running around can become just as life-threatening as a world dominated by predators.

So, what’s the key to the boom-and-bust cycles we see in the wild world around us? How does nature find any sort of balance or stability that will allow predators and prey to do what they naturally do without destroying each other or their environments?

Over the years, biologists who study such questions—community and population ecologists— have developed elaborate mathematical and computer models to explain predator-prey relationships and how, in general, they work. These models use data often laboriously collected from intensive fieldwork studying all sorts of creatures—from butterflies to wildebeest.

Not surprisingly, such studies are often hard to test and typically are expensive and difficult—if not impossible—to do. For example, to properly study the predator-prey dynamics of animals living in an African savannah, scientists would be obliged to remove entire populations of lions to see what effect that would have on gazelles and other lion food.

To look at the same question in a lake, researchers would have to study several different lakes at once—and could rarely control for the subtle variances between different lakes, much less get permission to kill off big schools of predator fish to study the effect of their removal on the rest of the lakes’ populations.

Thankfully, nature provides scientists with a few practical methods for studying big ecological questions in small ways.

FSU’s Tom Miller has found what he says may be the best, most feasible way to find answers posed by some of nature’s sturdiest riddles about how predator and prey coexist.

A casual observer of Miller in the field could assume that his primary research interest is a wonderful bug-eating plant found in the wilds of the Florida Panhandle. After all, Miller, an associate professor of biological science, spends a lot of his time plodding through boggy parts of the Apalachicola National Forest. This is home to some of largest concentrations of carnivorous plants found anywhere in the world, including the focus of Miller’s work, the purple pitcher, Sarracenia purpurea.

A healthy community of these carnivorous, wetland-loving beauties has also taken up residence in Miller’s campus lab and adjacent greenhouse. The plants are left alone to do what pitcher plants do naturally—fill their cup-like leaves with water, soak up sun and suck down any worms, ants, beetles and other bugs unlucky enough to fall into their lovely green and pink-veined leaves.

But Miller doesn’t study the plants themselves—he studies the curious world inside them. Miller studies the plants’ small, deadly pools of rainwater, a potent brew of plant and bacteria-produced enzymes and a swimming menagerie of hungry organisms.

These “killing pools” are alive with swarms of microbes that quickly begin tearing insects’ bodies apart, and a host of other organisms—from single-celled protozoans to insect larvae—that join the feast. This “bug soup” even attracts a top predator—mosquito larvae.

“It turns out that there’s a whole bunch of specialized insects that only live in pitcher plants,” Miller says, ticking off a list of species of critters that aren’t seen anywhere else outside the plant.

Miller says this teeming and diverse group of organisms that create a life for themselves inside pitcher plant leaves make a community of organisms like no other—microbial or otherwise—on Earth.

It also turns out that these communities vary significantly in structure and function from leaf to leaf. It’s these communities that Miller and his students have found to be ideal, natural models for studying some of the classical questions in ecology.

In a typical pitcher plant community, mosquitoes indeed fill the role of top predator in this fascinating food web, says Miller. Mosquito larvae eat the microscopic protozoa and rotifers that live in the water. The protozoa and rotifers in turn eat the bacteria that decompose the dead insects, breaking them into molecule-sized bits that get absorbed by the host as plant food. Meanwhile, the larvae of two other kinds of flies help out the bacteria by feeding directly on the dead insects captured by the plant.

Miller’s lab has already looked into several major ecology puzzles, such as whether the pitcher plant communities follow either the “top-down” model, where predation controls populations, or the “bottom-up” model, which holds that populations are chiefly controlled by the availability of nutrients.

So in the open maws of pitcher plants, what’s driving the system? Is it the availability of nutrients or the number of predators?

Well, neither—and both—Miller says. In pitcher plants, it turns out that removing predatory mosquitoes helps rotifers thrive, but adding dead bugs causes a proliferation of protozoa. So, though protozoa and rotifers compete with one another for food, one is affected by top-down forces, the other by bottom-up.

Miller has yet to determine when one aspect of this dynamic is more important than the other, and how the top-down and bottom-up forces interact. But the same conundrum exists in many other predator-prey relationships that are far less easy to study.

That’s why Miller enjoys his pitcher plants so much. The abundant plants offer tidily self-contained, easily manipulated, endlessly reproducible communities that are essentially microcosms of large-scale communities found everywhere in nature. Just one field of pitcher plants in Miller’s Apalachicola Forest “lab” might provide as many as 10,000 such communities. Removing predators for experiments is as simple as sucking out a little water from the pitcher with a pipette. Adding nutrients is a snap, too—just toss in a couple of extra dead bugs.

A great advantage these pitcher communities have is the fact that the living things in them also work a whole lot faster than fish or lions. Some of the organisms in these communities can go through up to three generations in a single day, Miller says. That’s like being able to observe 100 years of interaction among human inhabitants in a suburban neighborhood in a couple of days, he says. The ability to manipulate and control for evolution is key to the next step in Miller’s research. The lab’s current focus is to study protozoa extracted from young pitcher plants, contrast that with protozoa pulled from older plants and evaluate to see which organisms are better competitors.

“We’ve already tried this experiment with pitcher plant bacteria. The bacteria we pull out from the young pitchers tend to be better competitors than the bacteria from the old pitchers.” Which actually doesn’t make any sense to Miller.

“It could be they evolve to better avoid predation. Maybe there’s a trade-off; maybe you can’t be a good competitor and also be good at avoiding predators. Or there could even be selection to withstand the high heat of the pitchers in the summer. We’re really not sure what’s going on, which is part of the fun.”

Miller and his colleagues also have questioned the role of migration between pitcher plant communities. Since Darwin, biologists have become increasingly aware of the powerful influence that migration has in the evolutionary process, both between and among species. Migration plays a fundamental role in both the relative abundance of plants and animals and their myriad forms.

Miller says that pitcher plants present a superb model for studying what’s known as the metacommunity view of community ecology. This is a relatively young concept of looking past isolated communities and seeing the natural world for what it really is—a vast, interconnected complex of different communities capable of benefiting from—and exploiting—each others’ strengths and weaknesses.

Miller uses this expanded concept and his pitcher plant data to study the impact of migration on biodiversity.

“Ecologists spend a lot of time thinking about what goes on in one single area, but now they recognize that you really can’t understand that unless you also understand what’s going on in the other areas around it and how much movement there is among communities,” Miller says.

Put another way, what exactly does happen to the communities inside a pitcher plant when you take some of the water from one and transfer it to another? Do the “invaders” in the new community die off or become stronger? How does biodiversity help that community to thrive, and which community members benefit most—the predators or the prey? The answers can be extrapolated into an examination of the role of migration in all types of larger communities, Miller says.

“You often don’t end up with a simple answer here,” Miller notes. “If the two communities are really different—are heterogeneous—and you move a species from one to the other, you’re moving it to a place where it can’t live because the conditions are too different.

“On the other hand if these two communities are similar (homogeneous), migration doesn’t have much of an effect because the species you’re moving are already there. So where migration has an effect is when there’s some kind of halfway point.”Ultimately, Miller says his work on migration may lead to a better understanding of how animals develop and use specific paths, or corridors, to move from place to place in the wild. These days, such information is of paramount importance in designing wildlife areas and nature reserves.

William Blake, the 18th-century English poet, wrote about seeing “the world in a grain of sand, and Heaven in wild flower,” metaphors for the immense complexities of nature condensed into a tiny, marvelous thing. And so it is with Miller’s pitcher plants, where some of nature’s cleverest tricks—played out on an enormous scale in the biosphere—get distilled into a space so tiny, yet so useful for tackling some awfully big questions about nature, and more than likely, humans’ role in it.