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Backpacker Magazine – September 2008

Predator-Prey Relationships

Learn how carnivores and their quarry interact—and what happens when humans upset the natural balance.

by: Conor Mihell

(Photo by Willard Clay)
(Photo by Willard Clay)
(Photo by Ken Archer)
(Photo by Ken Archer)
(Photo by Ken Archer)
(Photo by Ken Archer)

Who saw more elk at Yellowstone's Mammoth Hot Springs: (a) nineteenth-century explorers, or (b) last summer's motoring tourists? Believe it or not, it's the windshield crowd. Hundreds of years ago, a healthy number of bears and wolves kept the elk population in check. Natural dynamics between predator and prey keep both species in balance, which helps ensure biological diversity across the ecosystem. Here's how it works–and what happens when it doesn't.

Natural Forces
At Michigan's Isle Royale National Park, wolf and moose populations rise and fall in connected cycles. "When wolf populations are high, they eat a lot of young moose," explains Rolf Peterson, a Michigan Tech University biologist who's been studying predator-prey dynamics on Isle Royale for decades. "But 10 years later, there aren't as many old moose." Wolf numbers then nose-dive, and moose thrive again. This in turn affects the island's vegetation: Balsam firs decline when abundant moose eat young trees before they can reproduce, then rebound when lean moose populations allow them to reach maturity.

Effect
Natural negative feedback cycles mean more robust populations of both species. At pristine and remote Isle Royale (where light visitor traffic results in a fairly intact ecosystem), "We're seeing that the wolves tend to remove the weakest moose, leaving the bigger, healthier animals to breed," explains Peterson.

Human Influence
Given plenty of food and scarce predators, any population will grow exponentially. With no wolves left in Colorado's Rocky Mountain National Park, the elk population has swelled to almost double its natural size. At Yellowstone, Yosemite, and Zion, research has linked high numbers of deer and elk to areas with lots of human activity, where wolves and grizzlies just won't go. The result? Skyrocketing populations. "We end up with 'urban ungulate syndrome,'" says Bill Ripple, an Oregon State University ecologist. "It's as though they are using humans as shields to avoid predators, and they increase in numbers in the process."

Effect
Mule deer have over-browsed Zion Canyon's cottonwoods, leading to streambank erosion, reducing vegetation, and making the area less habitable for lizards, amphibians, and butterflies. The elk at Rocky Mountain have also damaged aspen and riparian willow communities–prompting park officials to create a plan to cull the population in 2009.



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Mike Chumbley
Jul 15, 2011

Nice to see Rolf Peteron get some props. Great guy who has dedicated his career to this study. Should also give recognition to John Vucetich.

MN
Jul 14, 2011

This not a "natural end of a species" because of the extreme affect that humans have by their often mismanagement of our natural areas. There are pressures from outside organizations to lobby on which species get controlled based on people's needs. The current position of now allowing hunting of wolves in the Greater Yellowstone Area is driven by Elk hunter guides and ranchers who see a "normal" population of Elk that the wolves were helping control as a loss of income, not as a concern for the overall health of the environment. Bison that leave Yellowstone are killed because ranchers are afraid that they carry Brucellosis even though there has not been a proven case of transmission from bison to cattle. It is ironic that Elk also carry the disease and the rancher are not calling for them to be controlled because hunters bring in money.

Rocky Mt. NP has in the past have given hunters permits to cull the park's Elk herds and probably are still doing that.

John Smith
Jun 12, 2011

Have you taken into account the natural end of a species? Evolution has shown over time that some species die out altogether. Since this is a fact does it not follow that these dramatic peaks and valleys between predator and prey is the natural and proper order of things even if it means the end of a species? Simply because a prolonged death by starvation is harsh does'nt by any means suggest that this is unnatural. What we should be aware of and control is how we ourselves are also subject to these same natural laws. We should intervene to prevent these peaks and valleys from happening to us and if so perhaps we should occasionally intervene to prevent the exinction of our own food sources. To suggest that nature cannot survive without our intervention , however , I find hard to swallow being that nature has done this very thing for millions of years before we were present. You are correct in that we should not waste or lose touch with our survival skills and animals whose population has exploded should be used to feed the people. I'm sure it's true that wolves are opportunist hunting-wise but I hardly agree that this is a make believe pretty picture. I don't think wolves knowingly pick the slow or weakened animals... it is simply a better opportunity and as you yourself stated they are opportunist. They pick the weak and slow because the weak and slow are by their own weaknesses presented more easily as a food source. This is no pretty picture but a theory that seems to me very valid.

John Smith
Jun 12, 2011

Have you taken into account the natural end of a species? Evolution has shown over time that some species die out altogether. Since this is a fact does it not follow that these dramatic peaks and valleys between predator and prey is the natural and proper order of things even if it means the end of a species? Simply because a prolonged death by starvation is harsh does'nt by any means suggest that this is unnatural. What we should be aware of and control is how we ourselves are also subject to these same natural laws. We should intervene to prevent these peaks and valleys from happening to us and if so perhaps we should occasionally intervene to prevent the exinction of our own food sources. To suggest that nature cannot survive without our intervention , however , I find hard to swallow being that nature has done this very thing for millions of years before we were present. You are correct in that we should not waste or lose touch with our survival skills and animals whose population has exploded should be used to feed the people. I'm sure it's true that wolves are opportunist hunting-wise but I hardly agree that this is a make believe pretty picture. I don't think wolves knowingly pick the slow or weakened animals... it is simply a better opportunity and as you yourself stated they are opportunist. They pick the weak and slow because the weak and slow are by their own weaknesses presented more easily as a food source. This is no pretty picture but a theory that seems to me very valid.

Anonymous
Mar 15, 2010

great summary

Carm
Nov 14, 2008

Having been to Isle Royale and Very Fortunate to have seen the Both, a Bull and a cow moose and starving timber wolves. On my first visit I'd have to disagree. Wolves when hungry will try to attack but will not always be successfull. Large animals such a moose will not always be caught and most likely not than will. So the balance is a natural. The reintroduction of the Wolf in Mid west is taking off be cause of the large populations of Mule and whitetail deer the herds are the largest ever in part of regulated hunting season and management by humans. Wolves are opputinistic and taking rabbits and other varments that aswell at large numbers overwellming there eviornments aswell. The wolf is only catching up to meet the availability of prey.

isnarewolves
Nov 09, 2008

nice idea's but the real facts are.
1) there is never no "balance" between predator/prey with out human involvement. one is always thriving while the other is in decline. only proper management by humans, can any consistence and long term balance be attained.
2)i always like the pretty picture that wolves kill the weak, the young, and the lame. this idea has also never proven. in fact all predators are opportunist. they kill when they can and are able to. and never thinking to themselves this animal is to healthy to kill and eat. as far as Peterson's observations on the weakest moose being killed. this according to the graph.moose have never regained half of their normal populations.(the last 12yrs.)where as the wolves have done nothing but increase since the very early 90's. so much for that breeding idea.
3)Peterson and other like him or maybe the author of this article don't want you to know or tell you. the HORRIBLE deaths of either the predator/prey. when their populations are crashing or at bottom. they die a death of starvation! and disease. this is not a quick death but a prolong death of weeks and months. of hunger and weakness and towards the end of painful,excursion death their motor skills fail. how healthy is that for the eco-sys.
4)humans have to be involved in the eco-sys. if you want "balance"? i also believe very strongly that if you want a "balance" predator/prey sys. you must control both populations. threw game management plans. that mean game will have to harvested. this is a "natural balance". where all vegetation and wildlife thrive with very moderate fluctuation in their populations. verses the peaks and valleys and most of time take years and years of recovery that may never occur to historical populations.
we are suppose to be the the intelligent ones. and be the stewards of the land and animals.

gun guy
Nov 08, 2008

I somewhat agree open up to a few(charge a hefty fee for a tag) but more importantly they should also reintroduce the wolves, this was very succesful in yellowstone, and it changes the animals behaviors to a more natural one and makes the elk, deer , etc heathier

Carm
Nov 07, 2008

Well the NPS could open up these parks to a few hunters and help the control the population. Few hunters selected closed park days and harvest some animals and donate the meat to food banks, and feed the people of this country. But, its a crazy idea, hunting and thousands of years of human survival skills go to waste. With animal populations exploding and enviromental destruction abound. There is always a cause and effect, one animal grows and pushes another out of the way.

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