BOULDER, Colo., Oct. 7 (UPI) -- It should not surprise anyone to learn that large wild animals require large spaces, but new research indicates those animals need not occupy as much range as predicted simply by looking at their food and energy requirements.
The discrepancy between the need for resources and the much greater amount of range occupied by big animals has been the subject of ecological research for 30 years. Now, Princeton University biologist Walter Jetz and colleagues have proposed a clever solution to the conundrum that may make the conservation of large animals a little more politically and economically palatable.
"For large-bodied species, there is a big discrepancy between area-use measures and population density," Jetz told UPI's Blue Planet. Energy requirements can explain how population density decreases, but not the steep rate at which a species increases its home range.
For instance, he explained, consider a conservation area or national park designed to protect bison. Suppose that, based on known, home-range size, each bison requires 5 square kilometers of grassland to sustain itself -- although this figure is hypothetical. With 100 bison in the herd, the necessary conservation preserve would have to be 500 square kilometers.
"But because, in general, large-bodied species have a high degrees of home-range overlap, you can actually fit the bison into 100 square kilometers," Jetz said.
This is because the bison, and other large animals, generally behave like molecules of gas -- but in two dimensions.
"The problem of how mammals encounter each other across the landscape is akin to the problem of how gas molecules bump into each other in two dimensions, explained Steve Buskirk, a professor of zoology at the University of Wyoming in Laramie. "It's been productive before and it is very clever," he told Blue Planet.
"This is an incredibly simplified way of trying to understand the potential constraints of how animals in two-dimensional space encounter each other," Jetz said. "The encounter rates may change with the size of different parameters."
The formula for collisions among gas particles is well known. The relevant parameters that apply to animal populations are the density (the number of animals in a given space), the speed (how far they travel in a day), and the "average neighbor interaction distance" (how far apart two individuals have to be before they react to each other).
What the formula predicts is, because larger animals cannot cover their home range as rapidly as smaller animals, they tend not to run into their fellow bears or mountain lions or vultures as often as, say, mice or squirrels. They cannot defend their space as well against intruders, so considerable home-range overlap results.
Using the mouse example, an individual can cover the area it needs to feed itself three times a day, easily. So, as it runs back and forth to its borders, it is more likely to encounter neighboring mice who are doing the same thing. When they meet, they exchange a few words about the weather, warn each other not to intrude on the home turf, and head off again in search of food or love or nesting materials -- whatever it is mice want out of life.
Now consider an elephant, which pound for pound is much slower than a mouse. An elephant covers the area it needs to feed itself only once every three or four days.
"We can plug these different body-size dependencies into that gas model," Jetz said. "Because smaller-bodied animals cover relatively greater distances than large-bodied ones, they encounter each other more often than large bodied animals. The encounter rate for an elephant will be much lower ... than (for) a mouse."
So, while hypothetical elephant A is lunching in the eastern part of his range, a competing pachyderm, unbeknownst, might be dining on the western part. Elephant A then must wander into some competing elephant's domain to replace its own lost food supply.
The same parameters apply to a chickadee, which probably can sustain itself in the space of a suburban back yard, vs. a vulture, which has to soar over hundreds of square kilometers to find a hearty meal. Even a casual observer of backyard ecology has seen birds defending their territory against frequent intrusions. A vulture, on the other hand, while enjoying its roadkill, might not even be aware of a competitor in another part of its home range.
There is additional, observational support for the gas model applied to animal populations. Peter Waser, a Purdue University biologist, found such evidence in his study of mangabeys, forest primates in Uganda related to baboons.
As a practical matter, this means conservation areas intended to protect certain species can be designed somewhat smaller than previously thought.
"It would lead to nature reserves that are much smaller and more plausible economically and politically," Buskirk said. "We don't need as much space as we thought, based on home ranges. It removes some major constraints on planning for large animal conservation."
There are caveats, of course. These fundamentals "are highly confounded by other factors," he said.
Predators, for instance, need vast home ranges. Lions live and hunt in fairly large family groups requiring enormous amounts of land for their energy requirements. Also, the gas-molecule analogy does not apply to migratory animals, which may require different kinds of seasonal habitat.
Like most scientific results, the information can cut both ways. Conservation planners like large protected areas for a number of reasons, many unrelated to energy requirements or home range necessities -- protection from poachers, reduced impact from roads, enhanced human recreation opportunities, to name but a few. Opponents often call these areas "land grabs."
The work by Jetz and colleagues, along with a commentary by Buskirk, will be published in the Oct. 8 issue of the journal Science.
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