KEYSTONE SPECIESHow PredatorsCreate Abundanceand StabilityWolves, bears, otters, starfish — these

ecosystem engineers affect nature in

overt yet surprisingly subtle ways.

by DOUGLAS H. CHADWICK

The day came clouded and wind-tossed, with 5 inchesof fresh snow in the valley and a lot more piling upoverhead on the peaks. It was early December inMontana in Glacier National Park. Although winter

wouldn't officially start for another two weeks, blizzards andbitterly cold temperatures had long since sent the bears intotheir dens.

MOTHER EARTH NEWS lune/July 2011

But not every bear.Very large, very fresh paw prints on the trail in front of me said

at least one grizzly wasn't ready to call it quits for the year.Sleeping in underground dens keeps bears safe and insulated

through the snow-smothered months while they live off reservesof fat. The biggest and most powerful ones—adult male griz-zlies—sometimes leave their hidden chambers to roam about

during midwinter diaws. Before, few naturalists realized theseheavy-bodied bears could stay out through much colder con-ditions as long as they were able to take in more energy fromfood than they burned trying to Hnd it. Then wolves returnedto the American West.

The Food Web Surrounding WolvesAfter an absence of half a œntury, wolves came back to

Glacier during the 1980s, trotting across the border fromneighboring Canadian wildlands. Suddenly, this RockyMountain landscape held more carcasses of deer, elk andmoose, and those of us who frequented the slopes beganto discover a few scavenging grizzlies later and later into thefrozen season. One valley, with prime wintering grounds forhoofed herds, hosts a big male silvertip grizzly that I'm notsure ever holes up to snooze anymore.

Wolverines, with their unsurpassed nose for left:overs, canfind more meals now as well. So can wintering bald eaglesand golden eagles, along with northern ravens, which ofrenfollow wolf packs on the prowl. Wildlife biologists trackingthe wolves discovered them taking over fresh kills made bymountain lions. In many cases, the packs seemed to be hon-ing in on the sight of circling ravens or the birds' excited callsin order to find the stealthy cats and drive them off their prize.Before wolves were reintroduced to Yellowstone NationalPark in the mid-1990s, cougars had expanded their range toinclude broad valley bottoms. Afrer the wolves' return, thecougars retreated to the steeper, more broken upland terrainthey had normally hunted.

Animals ranging from jays and magpies to martens andblack bears glean scraps from wolf-killed carcasses. Coyotesare on that list of opportunists as well, but the wolves arelethally intolerant of their closest kin. In fact, Yellowstone'snew wolves quickly reduced the number of resident coyotesby nearly half, and forced survivors to take up life in smaller,more scattered territories. Coyotes behave much the sameway toward their smaller relatives, suppressing the numbersand territories of foxes. So, where wolves move in, coyotesend up marginalized while foxes thrive. That in turn shifrsthe odds of survival for coyote prey such as hares and youngdeer, as well as for the small rodents and ground-nestingbirds the foxes stalk. These changes affect how ofren certainroots, buds, seeds and insects get eaten, which can alter thebalance of local plant communities, and so on down thefood chain all the way to fungi and microbes. (For a visualrepresentation of how wolves bring balance to an ecosystem, seePages 46 and 47. —MOTHER)

Trophic Cascades:The Trickle-Down Effect

Ecologists commonly depict the structure of natural com-munities as a pyramid. The different layers are known as"trophic" (or feeding) levels. Green, solar-powered organ-isms—plants—form the foundation. On that broad baseof prim;iry producers rest layers of vegetarian species. Meat-

Above: Cougars in Yellowstone National Park, in the absence of wolves, badexpanded tbeir hunting range beyond its natural boundaries. Below: Tbereclusive wolverine climbs trees and towering mountain walls with equal agility.

www.MotiiEKEARTHNEWS.coM June/July 2011 4 3

eaters then stack up in smaller blocks above the her-bivores until only the predators that have few or nopredators themselves remain at the pyramid's peak.

Experts used to assume the overall richness of anecosystem was determined by bottom-up influenc-es—that is, by the fertility of the soil, climatic condi-tions such as annual precipitation, disruptions suchas flooding or wildfire, and other environmental fac-tors that bear upon plant grov*T:h. Only recendy didresearchers begin to notice how the addition or re-moval of a dominant predator—say, a jaguar or killerwhale—reverberates through all the lower layers of thepyramid. This top-down effect is known as a "trophiccascade. " Picture water spilling fi'om a point source:The main fiow fans out as it descends, splitting andsplitting again, with a few streams jetting away to oneside or another, then flowing on in more rivulets andarcs of spray.

Wolves are one such predator. This endangered spe-cies' natural recolonizadon of the Glacier Park area andsubsequent reintroduction to Yellowstone sparked anarray of close scientific studies, and the results are anongoing revelation. At the start, the elk in Yellowstonewere, in the view of many experts, overpopulated anddegrading their range. Herds practically camped inareas such as brushy wetland meadows and riversideflood plains, concentrating on favorite foods such aswillow, aspen and cottonwood. Over the years, thosewoody plants became prtmed into misshapen bonsaiforms by elk teeth. Finding a young one taller thanshoulder height was rare. In quite a few places, theaspen stands were no longer reproducing at all. Somehad already died out.

Predictably, aft:er the wolves arrived, their poptila-tion grew while elk numbers fell. Yet within a decade,the wolf and elk populations started to approachequilibrium. Lately, wolves have actually declined inYellowstone, pardy because food is no longer as readilyavailable, pardy from disease, and pardy because wolveslimit their own density within a given range throughdeadly fights over territories. From the standpoint ofthe ecosystem, the most important impact wolves hadon Yellowstone elk was not simply readjusting the sizeof herds, but also catising them to spread out into dif-ferent areas and keeping them on the move.

Relieved from the pressure of constant browsing,long stems shot up from the clubbed-looking shrubsand trees. New saplings arose from ground level.Groves thickened, attracting more songbirds and smallmammals. The roots of recovering vegetation stabi-lized the shores of waterways. Lured by the flourishingwoody plants, beavers moved in and raised families.

Wolves and other keystone predators bring ecologicalstability to tbe habitats in whicb they live.

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^ ' ' • • • * -

Réintroduction of wolves to Yellowstone thinned the excessive population ot elk (above), enabling much-needed regrowth of willow, aspen and cottonwoodtrees. Beavers, attracted by the regrowth of trees and other woody plants, built dams that created pond and marsh habitats for moose (below).

Tlieir dams further countered stream bank erosion, and at thesame dme created new f>ond and marsh habitats for moose, ot-ters, mink, wading birds, warerfowi, físh, amphibians and more.Lush regrawth aking the edges drew still more son^irds, and theburgeoning supply of insects fed nestling;?. The insects dropped offoverhanging leaves into the waterways, nourishing trout as well.

As these trends continue into the future, entire watersheds willbeoome more productive and able to support a wider diversity ofspedes. They'll also beconie more droi^ht-resistant, with runoffwater stored in beaver ponds and side channels able to help main-tain streamflows through the hottest, driest months.

Glader Park has denser, more varied forests where white-taileddeer abound, so the wolves there havent been as tightly focused on

elk. Just the same, the recovery of some heavily browsed sites hasbeen as dramatic as in V^owstone.

Long reviled as beasts of waste and desolation, wolves are lookingmore and more like creators of abundance and stability. Squint abit, and you may even see generations of wolves in the keen sensesand grace of elk, in the long-l^gçd bulk and power of moose, inthe electric reflexes of deer. Or, as poet Robinson Jeffers put it:"What but the wolFs tooth whitded so fine / The Beet limbs ofthe antdofje?"

Surprising Keystone SpeciesA trophic cascade doesn't always have to involve a top predator

Back in the late 1960s, zoologist Robert Paine removed the starfish

known as "ochre stars" fitom sample areas on thecoast ofWashington state. Active, aggressive hunt-ers, ochre stars gorge on mussels. Lose that singlepredator, and thick beds of rapidly proliferatingmussels take over, squeezing out barnacles, algae,snails, sponges, tube worms, sea sqtiirts, and therest of the remarkable mix of marine life usuallyat home in the intertidal zone. With ochre stars,Paine introduced the idea of a "keystone spedes,"defining it as one that exerts an outsized effectwithin an ecosystem, not necessarily through sheersize, nimiber or biomass, but because of the pivotalrole it plays.

Sea oners aren't quite at the apex of the foodchain along the North Pacific coast, for they canfall victim to sharks and killer whales. However, theotters dine on a smoig^sbord of mollusks, crusta-ceans and, most of all, sea utdhins. With the dens-est fur of any animal—650,000 hairs per squareinch—diey were once hunted to near extinctionfor their pelts. Not long afrerwaid, people noticedkelp beds that had served as the otters' preferredhabitat turning quite Sf>at5e or vanishing altc^edi-

Restoring wolves to Yellowstone after a 70-yearabsence as a top predator—especially ofelk—set off a cascade of changes that isrestoring the park s habitat as well.

Without Wolves« • YELLOWSTONE WITHOUT WOLVES • 1926-1995

ELK overbrowsed the streamside willows, cottonwoodsand shrubs that prevent erosion. Birds lost nestingspace. Habitat for fish and other aquatic speciesdeclined as waters became broader, shallower and,without shade from streamside vegetation, warmer.

^ ASPEN trees in Yellowstone s northern valleys, where§ elk winter, were seldom able to reach full height. Elk" ate nearly all of the new sprouts.

COYOTE numbers climbed. Though they often kill elkcahfes, they prey mainly on small mammals such asground squirrels and voles, thus reducing the foodavailable for foxes, badgers and raptors.

er. With the otters gone, it turned out, an explodingurchin population was gnawing its way throtigh thosesubmarine thickets of giant algae. Producir^ as muchorganic material per acre as tropical rain forests, kelpforests serve as sheltering nurseries for all kinds of ju-venile fish, including corrunercially valuable species.Where kelp was most abundant, humans harvested itto make additives for products from soap and fertilizerto ice cream and jelly. Here, then, was a trophic cas-cade that tumbled down through unexjxaed levelsand then washed onto us.

Vegans as Ecosystem EngineersIn another type of trophic cascade involving a key-

stone species, the lead role is played by a v^an: theAfVican elephant. Gonsuming upward of 400 pwundsof plants f)er day in the case of large males, these dtansrip down tree limbs for meals, girdle tree trunks bystripping offnutddotis bark, and simply push smallertrees over to get at the branches. As for shrubs, ele-phants not only eat the stems but tear whole plants topieces, uproot others and trample sdll more underfootwhile foraging.

With WolvesYELLOWSTONE WITH WOLVES • 1995-PRESENT ^

ELK population has been halved. Severe winters early intbe reintroduction and drought contributed to the decline.

A healthy fear ot wolves also keeps elk from lingering atstreamsides, where it can be harder to escape attack.

ASPENS tbrive as tbe number of new sprouts eaten by elkhas dropped dramatically. New groves in some areas now

reach 10 to 15 feet tall.

COYOTES bave been reduced by wolf prédation. Fewercoyote attacks may be a factor in tbe resurgence of tbe

park s pronghorns.

WILLOWS, cottonwoods and otber riparian vegetation havebegun to stabilize stream banks, helping restore natural

water flow. Overhanging branches again shade tbe waterand welcome birds.

BEAVER colonies In nortb Yellowstone have risen from oneto 12 now that some stream banks are lush with vegetation.especially willows (a key beaver food). Beaver dams createponds and marsbes, supporting fish, amphibians, birds andsmall mammals, plus a rich insect population to feed them.

CARRION left uncovered by the wolves boosts tbe foodsupply for scavengers—notably bald and golden eagles,

coyotes, ravens, magpies and bears.

When fiilly grown, elephants are virtually im-mune to predators, and herds tend to increase un-til checked by drought, food shortages or disease.At high population levels, their quest for food cantransform wooded habitats into open plains. In co-operation with wildfire, elephants also maintain ex-isting savannas by removing the woody plants thatinevitably invade grasslands. Elephants shape thewoodland-savanna balance in an ecosystem by de-termining the proportions of grazers such as gazelles,zebras and wildebeest to woody plant browsers suchas giraffes and kudus, and of lions prowling the plainsto leopards waiting to pounce from an overarchingtree limb. That is, elephants shaped the communi-ties in which our primate ancestors developed, stoodupright and staned walking toward the future.

Ecologists sometimes refer to elephants as "ecosystem ar-chitects" or "ecosystem engineers." Here in North America,grizzlies serve the same function. In addition to their effectsas predators and scavengers of hoofed animals, the big bearsfertilize streamside habitats with their waste and the remainsof the salmon they eat. (Both are rich in nitrogen and otheressential nutrients.) Grizzlies also distribute thousands of seedsfrom shrubs afrer eating the berries, and they rank as the chiefanimal earth-movers of the upper elevations in portions ofthe Rockies. Excavating acre upon acre to get at hibernatingrodents and the bulbs and starchy roots of various herbs, theybring up nitrogen from deeper soil levels just as a farmer doeswhen tilling fields. The seeds that fall into such freshly turnedsoil yield a more robust crop of new alpine and subalpine plantsthan seeds in undisturbed patches. Next time you're on a trailin grizzly range, instead of recalling some cheesy horror moviescene involving monster bruins, try thinking "heavyweightwildflower gardeners." It can't hurt.

Elepbants so profoundly affect their tiaoitat tnai iney re Known as ecosystem engineei s.

Nature Is a CommunityEveryone seems to have an opinion on ecology these days.

Keep in mind, though, how new this field of science actually is,and how much all of us have yet to understand. Henry Ford hadinvented the automobile before biologist Ernst Haeckel evencoined the term "ecology" early in the 20th century. The worddidn't enter popular vocabulary until the 1960s.

Feeding-level pyramids, top predators, trophic cascades, keystonespedes, ecosystem engineers—these are all different expressions ofthe central fea that everything in an ecosystem is connected toeverything else. A natural community is by no means just a collec-tion of individual spedes. Its vitality comes from the relationshipsbetween those organisms—the interactions, the flow of energyand nutrients frwm each life to the next.

Comparatively rare to begin with, most of the world's apex car-nivores are imperiled at present, and many keystone spedes arealso at risk. There may be no more important lesson from theyoung sdence of ecology than this: If we vivant to keep ecosystems

healthy and resilient, the first step is topay closer attention to the way naturedoes this from the bottom up, and backdown again from the top. •

Aspen trees in Yellowstone once again flourish because wolves keep the elk population In check.

4 8 MOTHEREARTHNEWS June/July2011

Douglas H. Chadwick is a

wildlife biologist and the author

of 11 books — most recently.

The Wolverine Way (Patagonia

Books, 2010)—and hundreds

of magazine articles on natural

history and conservation. He

conducts research on wolverines

as a volunteer in Glacier National

Park and serves as a founding

board member of the conservation

land trust Vital Ground.

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