How the dance of predation upends ecology models

Not all predators are equal, and neither are prey. When lots of genetic variety allows two species to co-evolve rapidly, reports a new study, both their population numbers and their evolutionary fitness levels fluctuate, defying traditional ecological models.

|
AP Photo/Jack Smith
A female Canada lynx heads for the woods after being released near South Fork, Colo. in 1999, a year before the species would be declared a threatened species.

The primordial two-step of predation and predator-avoidance has driven evolution since long before our hominid ancestors abandoned knuckle-walking for the upright posture that let us scan the savannas for dangerous cats, who were meanwhile learning more effective ways to flatten themselves and stalk prey.

A hundred years ago, when ecologists began to study how predation shaped populations, they found that many relationships followed a logical cycle: rising prey populations sustain rises in predator populations, which cause prey numbers to drop, thereby sustaining fewer predators, whose decline allows prey populations to grow again, and so on.

These undulating predator populations, when graphed, lag about a quarter of a cycle behind the populations they preyed upon, according to the venerable Lotka-Volterra ecological model. Soon after prey populations rise, predator populations will rise to eat them. So, when hares are hopping through a forest at maximum density, lynx levels will be moderate, and rising. And when lynxes are peering tuftily down from the greatest possible number of tree branches, hare numbers will be moderate and falling. 

This model assumes the lynxes and hares in question are all pretty much interchangeable. Which happens to be true enough, in the case of lynxes and hares, to make them the poster animals for this model.

But elsewhere in forests – and oceans and deserts and peat bogs – individuals often survive thanks to specific strengths, be they fleet feet, an astute mind, or a hard beak. Evolution depends on these differences.

Squirrels with extra-brilliant memories for buried nuts, for example, have an advantage in life, all else being equal. But during peak-owl moments, extra-skittish squirrels may survive to bear far more pups than those with strong memories. In other words, species can evolve over the course of their population cycles, changing how those cycles interact.

"Changes in allele frequencies (and associated phenotypes) can occur at the same rate as changes in population densities or spatial distributions, and alter the ecological processes," explains a report published Tuesday in the Proceedings of the National Academy of Sciences.

The mathematical biologists behind the report already knew this – that evolution and ecological cycles can occur over the same time scales. But, exploring how these two kinds of shifts interact, they made a surprising discovery: when specific prey and predator communities are co-evolving, the interplay between their populations can be reversed.

"I hoped to find something new, but I didn't expect to find this," says Michael Cortez, the study's lead author and a National Science Foundation post-doctoral fellow at Georgia Institute of Technology.

Dr. Cortez took data gathered in past studies, of phages eating cholera bacteria, mink eating muskrats, and gyrfalcons eating rock ptarmigans, and plugged them into one of two ecological math models, which include fitness levels as variable.

Out popped the surprise: in all of those relationships, prey populations actually rose in response to predator increases. In fact, they followed them by between a quarter cycle and a half cycle.

Why would this happen?

"The prediction would be that the prey are eating the predators, but that's completely wrong," says Cortez.

The mechanism behind it, which depends not only on populations' fluctuating numbers, but also on their fluctuating levels of evolutionary fitness, is quite a bit more complex than the Lotka-Volterra model. 

Imagine a community in which a small population of "fit," hard-to-catch prey – perhaps lightning-fast mice – interact with a large population of not-very-effective predators, like visually impaired hawks. Because of the imbalance, the hawk population drops, with only enough food to sustain the very sharpest-eyed among them. The disappearance of so many predators allows the swift mice to become numerous, and at this point we see a reversal of the Lotka-Volterra model: a population peak of inept predators actually leads to a population peak of adept prey.

This abundance of mice, of course, gives the few surviving sharp-eyed hawks fodder to multiply, thus creating a large population of adept hunters. And this population drives the mouse population back down. Now, because the mice are outnumbered by terribly keen-eyed hunters, their genius for lickety-split scampering stops being much of a defense.

At this point – and this is the tricky part – it actually behooves the evolving community of mice to trade in their speed for another quality, perhaps higher litter size. The population of these slow mice is quickly driven down by the hawkish hawks, at which point, surrounded by defenseless prey, the hawks no longer need to be quite so keen-eyed. So they revert to their previous myopia, in exchange for for some more advantageous trait, such as strong memory. The abundance of vision-impaired raptors makes it worthwhile for the mice to re-evolve great running skills, and the cycle begins again.

For this reversal to happen, the relationship needs to foster a brisk pace of evolution. Cortez' team identified three main conditions that enable this:

• Predators' skill sets need to be full of trade-offs. Meaning that, for hawks' hunting aptitude to be as elastic as it is in the scenario above, an improvement in vision must lead to a loss in some other quality – which is related to survival but not hunting – and vice versa.

• The relationship between the two species needs to favor "extreme phenotypes," like lightning-fast scampering or enormous litters, rather than a stable balance of decent speed and moderate litter sizes.

• The defenses of fit prey animals, like fabulous speed, must be effective deterrents against weak hunters, like half-blind hawks.

The new model, a so-called "clockwise" pattern of prey and predator populations, is likely to change how ecologists predict population dynamics. "I think the important part to take from it, is that when studying populations and their fluctuations over time, it's important not just to measure the total numbers of things, but also to measure their different phenotypes," says Cortez.

"The most exciting part to me, is that you have this 90- or 100-year old prediction from some of the founders of mathematic ecology, who predicted that predator and prey cycles should have a certain shape," he adds. "This has been consistent until maybe 10 years ago. My work has sort of shown that, when you have evolution or genetic diversity in popultions, you can actually completely reverse that. To people who have spend their careers on this, it is pretty mind-boggling that you can have adaptation reversing an ecological dynamic."

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to How the dance of predation upends ecology models
Read this article in
https://www.csmonitor.com/Science/2014/0507/How-the-dance-of-predation-upends-ecology-models
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe