sanderson e trolle 2005 - monitoring elusive mammals

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148 American Scientist, Volume 93 2005 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact [email protected].

Before the arrival of European settlers,the passenger pigeon (Ectopistes mi- gratorius) constituted more than a quarterof the bird population in North America.Ornithologists estimate that there were

once billions of them. So it is no won-der that 19th-century hunters went afterthese birds with abandon. Who couldhave imagined that such an abundantspecies would really suffer? But suffer itdidand surprisingly fast. By the 1890s,the species was effectively destroyed,despite some belated attempts to savethe few remaining in the wild. The lastpassenger pigeon, a captive bird namedMartha, died at the Cincinnati Zoo onSeptember 1, 1914.

All too many species have met simi-lar fates, prompting biologists to regardmodern times as an episode of massextinction. Indeed, dramatic lossesto our planets heritage of biologicalabundance and diversity have becomecommonplace. Some people are try-ing to reverse this tide, often with theofficial blessing of their governments.More than 175 nations, for example,

have now ratified the Convention onBiological Diversity, an agreement firstdrawn up in 1992 that obligates coun-tries to combat the loss of biodiversityin several ways. The signatories to the

Convention decided, for example, toestablish a system of protected areas,giving special consideration to threat-ened species and ecosystems.

In 1996, participating countries wereencouraged to set measurable targets,stepping-stones to achieve their ulti-mate conservation objectives. As a re-sult, these nations established a coreset of biodiversity indicators, whichcould be followed over time. Monitor-ing the status of natural populationsranks high among the various effortsundertaken in this regard because itcan alert managers when they mustdo more to protect biodiversity in theregions under their care. It was thisneed for tracking long-term changesthat spurred Conservation Internation-al to begin a program called TropicalEcology, Assessment and Monitoring,or TEAM.

One of us (Sanderson) began workingon the TEAM program three years agowithin a wing of Conservation Interna-tional called the Center for BiodiversityScience, which was set up with a grant

from the Gordon and Betty Moore Foun-dation to help support the role of sci-ence in shaping conservation efforts. Theparticular mission of TEAM: to monitorlong-term trends in biodiversity througha network of tropical field stations, thusproviding an early warning system thatcan guide conservation action.

With the help of appropriate experts,we developed protocols to monitor vari-ous biodiversity indicatorssuch as theabundance of leaf-litter ants, birds, fruit-eating butterflies and mammals (includ-

ing primates), as well as tree growth and

landscape changeat these relativelyundisturbed sites. To ease the consider-able logistical difficulties involved withsuch a massive endeavor, we decidedthat the TEAM efforts should be carried

out close to existing research facilities. Sofar, four TEAM stations are operational:three in Brazil and one in Costa Rica. Weplan to set up six more stations in theAmericas later this year and, if we obtainadequate support, intend later to estab-lish similar sites in Africa and Asia.

Ideally, one would like to keep trackof everything living near a monitor-ing station, but we have had to limitthe scope of our measurements so as tokeep the project manageable. In particu-lar, the TEAM program does not involveall mammalian fauna but instead targetsthose of medium to large size, whichare the ones most likely to suffer fromalterations to the local landscape, fromhunting or even from climate change.The populations of larger mammalsthus provide a convenient barometerfor the overall heath of the ecosystem.

Regular viewers of nature documenta-ries might conclude that observing largemammals also has the advantage of be-ing a straightforward exercise. In some

Monitoring Elusive Mammals

Unattended cameras reveal secrets of some of the worlds wildest places

James G. Sanderson and Mogens Trolle

Figure 1. Snapshots of Guatemalan wildlifewere captured by the authors camera trap,a device that can take photos autonomouslywhen it senses the motion of an animal near-by. Such images are useful for determiningwhether a particular species is present in anarea and can even be used to estimate popu-lation density. The animals shown here arethe white-nosed coati (Nasua narica, upper left) , great curassow (Crax rubra, upper right) ,margay (Leopardus wiedii, middle left) , grayfox (Urocyon cinereoargenteus, middle right),tayra (Eira barbara, lower left) and ocellatedturkey (Agriocharis ocellata, lower right).(Except where noted, all photographs cour-tesy of James Sanderson.)

James G. Sanderson earned a doctorate in math-ematics from the University of New Mexico in1976. He has taught in the Department of Com- puting and Information Science at the Universityof New Mexico and in the Department of WildlifeEcology and Conservation at the University of Florida. He is currently a TEAM research scientistat Conservation Internationals Center for Ap- plied Biodiversity Science. Mogens Trolle holds an M.Sc. in wildlife biology from the University of Copenhagen. He specializes in the study of largemammals from South America, where he has spentmore than five years. Trolle was one of the first to publish camera-trap results from South America inscholarly journals. Among other accomplishments,he discovered a new deer species in Peru. Address for Sanderson: Conservation International, 1919 M Street, NW, Suite 600, Washington, DC 20036.

Internet: [email protected]


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places, it is. In East Africa, for example,casual visitors routinely see elephants,zebras, wildebeest, lions, leopards andcheetahs roaming around in the bush.But in most other locations, even withinprotected areas, wildlife is just not thateasy to view. Indeed, both of us have attimes worked for months in the densetropical forests of Brazil, Suriname, Cam-

bodia or Guatemala without spottingany terrestrial wildlife whatsoever. Howthen can biologists monitor the famous-ly elusive mammals of tropical forests?How will investigators know whetherthese species are in imminent danger inplaces that have been exploited for tim-

ber or bush meat?

Without the benefit of firsthand obser-vations, investigators have traditionallyhad to rely on indirect evidence suchas a track or scat to confirm the pres-ence of certain species. Although suchmethods are still employed, a more help-ful technique is now available for thesurveillance of wildlife: phototrapping.This approach makes use of ordinarycameras mounted in rugged enclosuresto automatically snap photos of animalsthat wander into the field of view.Phototraps are usually outfitted with a

flash, so that even the most skittish noc-

turnal animals can be captured on film.Although conservationists have beentaking advantage of phototraps for onlya handful of years, the roots of this tech-nique reach back more than a century,springing from the ingenious tinkeringof a former American statesman.

Shoot, but Not to KillIn the summer of 1888 at WhitefishLake near Marquette, Michigan, GeorgeShiras III, a Yale-educated lawyer andonetime Pennsylvania congressman,perfected a way of photographingwildlife at night with a large-formatcamera and hand-operated flash. Shirassoon gained considerable acclaim for

his stunning night photographs of deerand other animals. At the Paris Exhibi-tion in 1900, he won the gold medalin the forestry division with an exhibitof several wildlife photographs, one ofwhich was of a doe with two spottedfawns. These same photographs helpedwin him the grand prize for wildlifephotography at the St. Louis WorldsFair of 1904.

The technique he used for his naturephotography was indeed highly un-usual: With his large camera mount-

ed on the front of a rowboat, Shiras

would probe the darkness for animalson the shores of Whitefish Lake usinga flashlight. He would then positionhis boat as close as possible before tak-ing a shot, which required him to setoff a powder flash.

Later Shiras set up his camera onland, rigging it so that he could take apicture remotely by pulling on a longtrip wire. Eventually, it occurred to himthat he could arrange the wire so thatan animal would itself trigger the pic-ture-takingan arrangement to whichhe referred variously as an automatic,set or trap camera. In 1913, Shi-ras wrote: I have usually found it awaste of effort to try to get pictures in

the ordinary way; for, even if occasion-ally successful, the loss of time can beavoided by the use of the set camera.Accompanied by exquisitely detailedphotographs of animals, his articles inThe National Geographic Magazine from1906 to 1921 created considerable inter-est in wildlife photography in prefer-ence to the more popular hunting andtrapping of his day.

In the late 1920s, Frank M. Chap-man (a leading ornithologist from theAmerican Museum of Natural His-

tory in New York) used similar cam-

Figure 2. Beginning in the late 19th century, George Shiras III, a onetime Pennsylvania congressman, developed techniques for obtaining photo-graphs of wildlife at night. Initially, he mounted his bulky camera and powder flash in the front of a boat (upper left) , which he used to approachanimals foraging the shores of Whitefish Lake, located on Michigans Upper Peninsula. Shirass night photo of an adult doe and two fawns (right) won him awards in the U.S. and Europe. He also mounted his equipment on land and triggered the shutter remotely, using a long string, a tech-nique that allowed animals to take their own pictures, such as this raccoon tugging on a baited line (lower left) .

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era traps in the tropical rain forest ofCentral America. Chapman had seenthe tracks left by at least two cat spe-cies on Barro Colorado Island, Pana-ma, but he had never observed any ofthese elusive animals. A camera trap,he thought, might satisfy his curiosity.Using the technique Shiras had devel-oped, Chapman stretched trip wiresacross human-made trails because hesuspected that animals used these con-venient paths through the jungle. Hedid not bait his camera traps for thefirst few nights, but subsequently heleft a variety of enticements: meat, fishand fruits, including a banana attacheddirectly to the trip wire. Chapmanused a large-format camera contain-ing a single glass plate. He describedthe sound of the flash charge explod-ing as being similar to the report ofa small cannon and remarked that

it must have been a terrifying experi-ence for the animal to endure a suddenexplosion of blinding light along witha thunderous boom only three metersor so away.

With the help of this noisy device,Chapman quickly demonstrated thatwhite-lipped peccaries, tapirs, ocelotsand pumas all lived on Barro Colora-do Island. Although mice, rats, agoutis(rabbit-sized rodents), coatis (raccoon-like animals), bats, birds and some ofChapmans colleagues were also rou-tinely photographed, several of the rar-er mammals in residenceforest deer,paca (very large rodents) and jaguareluded Chapmans camera phototrapsfor three months.

In his writings, one finds confirma-tion of what we and all other cameratrappers learn firsthand: Results mayvary. Chapman sometimes went weekswithout photographing anything veryinteresting; then hed hit the jackpot.He described, for example, returningfrom one visit to the darkroom withpictures of a puma and two ocelots.

His images of these wild cats weresometimes remarkable. One photo-graph of an ocelot taken in the middleof the night shows the animal unsuc-cessfully trying to step over the tripwire, which was suspended 25 centi-meters or so above the ground, thusrecording behavior on film.

On occasion, Chapman was ableto recognize individual animals. Forinstance, he regarded two puma pho-tographs, obtained three months andone mile apart, as being of the same

animal, because the figures agreed in

size, proportion, markings and thelength of a foreleg. Referring to thesephotographs, Chapman wrote: As-suming, therefore, that this is but oneindividual, the two pictures admi-rably illustrate one of the most dis-tinctive features of camera-hunting,namely, that we may capture the sameanimal an indefinite number of timesand still leave him as free as he was inthe beginning.

Different StripesShirass unique photographs and theirwidespread dissemination in NationalGeographic engendered an interest inwildlife that went far beyond beingabout what one could mount on thewall or serve at the dinner table. But

because the required equipment wascumbersome and expensive, few butChapman emulated his example early

on. It took three separate develop-ments to spark the widespread use ofcamera traps decades later.

First, photography became a lot eas-ier. Nowadays one doesnt need to lugaround a bulky camera, boxes of glassplates, an ungainly tripod and copiousquantities of exploding flash powder.A modern camera phototrap typicallyconsists of nothing more than a toughplastic enclosure containing a 35-mil-limeter point-and-shoot cameraand an electronic controller. In placeof Chapmans trip wires, these unitsuse a passive infrared detector to trig-

ger the shutter when the sensor andits associated circuitry register heat inmotionexactly what goes on in mostautomatic lights and burglar alarms.

The second thing that helped wasthat weekend hunters discovered howautomatic camera traps could aidthem in their search for trophy deerand other game. They have eagerlyadopted this tool over the past decadeor so, creating a mass market for cam-era phototraps of all kinds. Prices arethus quite reasonable, and the varietyof equipment available is enormous.In addition to finding units containingordinary film cameras, one can now

buy phototraps equipped with digitalcameras. Video-camera phototraps arealso being marketed.

The third advance came when scien-tists interested in wildlife populationsrealized that they could apply analyti-

cal methods that they already had onhand to data collected with cameratraps. Those statistical tools, known asmark-recapture or capture-recap-ture methods, have served for decadesto estimate populations of rodents, rab-

bits and other small animals that can beeasily caught, marked in some way andreleased. But capture-recapture analy-ses have been applied in conjunctionwith camera traps only recently. Indeed,widespread use of this combination didnot come until after 1998, when K. Ul-las Karanth, who works for the WildlifeConservation Society in India and is one

Figure 3. Modern camera traps, which are mostly sold to weekend hunters in search of prizegame, are increasingly being used by conservationists to monitor elusive animals. Here Francisco

Braga Ribeiro Filho (left) and Simone Martins deploy a camera trap in Cxiuana, Brazil.


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of the worlds leading experts on tigers,and James D. Nichols, a statistician atthe U.S. Geological Survey PatuxentWildlife Research Center, showed thatcamera traps and the appropriate ana-

lytical software could together be usedto estimate the population density oftigers in India.

Karanth and Nichols realized that because every tiger is marked with aunique pattern of stripes, it is usuallypossible to identify individuals fromphotographs. Positive identification nor-mally requires images of both sides ofthe tiger, because these animals are lat-erally asymmetric. The same is true formany other wild cats, which can thus

be monitored effectively using pairs ofcamera traps set up to take photos fromeither side of the subject. Species whoseindividuals are distinguishable in otherways are, of course, also candidates forcamera trapbased population studies.For example, it is not uncommon thatwe identify individual cats by notingunusual scars or cuts to the ears.

The Art of PhototrappingThe main challenge to the phototrapperis to position the subjectan animal ofuncertain type and sizein front of,and at a reasonable distance from, the

camera so that useful pictures will betaken. The best strategy is not alwaysapparent. Imagine for a moment stand-ing somewhere deep within a densetropical forest, a place so thick withvegetation that only a smattering ofsunlight reaches the ground and thefield of view is severely limited in everydirection. A rustling alerts you that ananimal is moving in the forest nearby.How would you take its picture? Withdifficulty. Its no wonder that my lo-cal field assistants, whose knowledge

of these forests always exceeds my

own, invariably ask the same question:Where should we put the cameras?Ought they to be set deep in the forest,among the thickets of lianas and bam-

boo, or placed on trails, where there is

slightly more breathing room? Shouldcamera phototraps be positioned near a burrow or nest?

Putting cameras near burrows seemslogical: When the resident emerges toforage, a picture will be snapped. Theproblem is that when this creature re-turns, another picture will also be taken.Indeed, the comings and goings of oneanimal might be all the camera recordsfor the next 30 days, which is not partic-ularly helpful if ones aim is to surveythe general population.

With experience, one can locate trapsin less problematic places frequented

by animals. We both have now devel-oped the knack, having worked withcamera traps for a variety of projects.One of us (Sanderson) has, in addi-tion to his recent work on the TEAMprogram, been using camera trapsto search for small cats in the wild atvarious places around the world. AtBarito Ulu research station in the heartof Kalimantan (the Indonesian part ofthe island of Borneo), Rupert Ridge-way, a consultant to the University of

Cambridge, and Sanderson mounteda camera trap facing a log that hadfallen across a small stream, figuringthat wild cats might well take advan-tage of this natural bridge to cross overthe water. The result, ironically, was afine photograph of a pangolin (a scalyanteater).

Deciding where to place cameraswhen there is nothing so obvious as a log

bridge is difficult; even the general strat-egy to follow isnt obvious. At La Selva,a world-famous ecological research sta-

tion in Costa Rica, David B. Clark, a bi-

ologist at the University of Missouri-St.Louis (the former director of the La Selvafacility and now an adviser for TEAM),argued against placing camera trapson trails. He knew that the biologists

working there often walked these junglepaths, perhaps more frequently than didanimals, and he feared that these peoplewould inadvertently trip the cameras,using up the film. Random placementin the forest, Clark reasoned, wouldserve better. Sanderson disagreed. SoTEAM members tried both strategies.After two months they compared resultsfrom cameras placed on trails againstthose obtained from traps positioned atrandom spots in the forest. The largenumber of photos of people walking thetrails and the complete absence of hu-man images from the randomly placedsites showed that Clarks hunch was cor-rect. There was just one problem: Therandomly placed camera traps returnedonly a handful of pictures of animals.Fortunately, the traps mounted by trailsrecorded many such images, despiteClarks initial concerns.

What exactly do conservation bi-ologists do with trap photos? Mostcommonly, we use the images just todocument the presence of a particularspecies. An example comes from the

Udzungwa Mountains in Tanzania,where one of us (Sanderson) and Fran-cesco Rovero of the Museum of NaturalSciences in Trento, Italy, obtained thefirst photograph ever of a live Abbottsduiker, a medium-sized forest ante-lope. (Remarkably, the duiker is eatinga frog.) Another instance comes fromthe efforts of Sutrisno Mitro of the STI-NASU Foundation for Nature Conser-vation in Suriname, who used cameraphototraps in the Brownsberg NatureReserve, a protected area in that coun-

try. There he obtained photos of a small

Figure 4. Indian tigers, like many other cats, display coat patterns that vary from animal to animal. Paired camera-trap images like these canthus be used to identify individuals. (Photographs courtesy of K. Ullas Karanth, Wildlife Conservation Society.)


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spotted cat called the margay (Leopar-dus wiedii) and of the elusive tayra (Eirabarbara, a kind of weasel), adding to thelist of mammals known to live there. Insimilar fashion, Anthony Simms, work-ing for Conservation International inthe Cardamom Mountains of Cambo-dia, documented the presence of theSiamese crocodile (Crocodylus siamensis),which has been extirpated throughoutmuch of its former range.

The failure to photograph a particu-lar species cannot be used to establishabsencejust because you dont see ananimal doesnt mean it is not present

but the lack of evidence might suggestrelative rarity. Relative abundance, too,can be gauged from the number of timesa species is photographed comparedwith the total number of frames taken.And for certain animals, a full-blowncapture-recapture analysis provides an

estimate of population density.One assumption required of any den-sity study is that all individuals of thetarget species have a nonzero probabilityof being detected. For camera trappingthis means that no individual can live

between phototrap sites without there being at least a possibility that it will becaptured on film. With enough photo-traps and enough time, all individuals ina fixed area can thus be photographedin principle. In practice, the number ofcameras we can afford to deploy is fartoo small to saturate an area with cover-age. For this reason, and because humanscent sometimes discourages shy ani-mals from visiting a site, some individu-als escape detection. Hence statisticalmethods are essential for estimating thesize of the population.

Measuring the InvisibleIt might seem far-fetched that field

biologists can determine the size of apopulation even when they (or theircameras) have captured only a frac-tion of the animals living in the area

under study. In fact, the task is rela-tively straightforward using a capture-recapture analysis. The procedure isperhaps best illustrated with a hypo-thetical example, say, for the mice livingin your cellar. Want to know how manythere are? Simply put out some trapsthat can capture them live.

Suppose that on the first night ofan informal two-night study youcatch n1 of these rodents. Mark eachwith a dab of paint and then releaseit. Then, on the second night, say that

you catch n2 mice. Of this set, sup-

pose you found that m2 were markedwith paint. You could reasonably ex-pect that the ratio of marked animalscaptured on the second day ( m2) tothe total number animals capturedon the second day ( n2) equals the ra-tio of marked mice available for cap-ture (which equals the number youmarked on the first day, n1) to thetotal population of your basement,

which then works out to ( n1 n2)/ m2.

Limiting the sampling to two con-secutive nights helps ensure that thepopulation size being estimated isfixedthat is, one can reasonably as-sume that there are no significant ad-ditions of mice (through birth or im-migration) or losses (though death oremigration). But sampling for a fewmore nights would help reduce thelevel of uncertainly. And the analysis

only gets a little more complicated.

Figure 5. A classic mark-recapture analysis is often used to gauge population density (panelsat left) . In such studies, animals are captured (a) , marked in some way (here with splotchesof paint, b) and then released. A second round of trapping (c) then allows the total number of

animals to be inferred by tallying the proportion marked (d) , assuming that the newly capturedones constitute a representative sample of wild population. Exactly the same type of analysis canbe done using camera traps when individuals can be identified from their markings (right) .

a

b

c

d


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In a nutshell, each night (after thefirst one) provides one estimate of thetotal populationderived as before byequating the ratio of marked animalscaught to total animals caught with theratio of marked animals in the popu-

lation to total population. One then

averages over these results to obtainthe best overall estimate of total popu-lation and to determine the size of theerror bars.

Are such procedures really that use-ful? Indeed they are. One of us (Trolle)

once conducted an intensive three-

month mammal survey in the Panta-nal wetlands of Brazil and during thattime made only one direct observa-tion of an ocelot. Had this been theonly datum, little could have been saidabout the size of the local population.

Yet during the same period, camera

Figure 6. Workers at Conservation Internationals Center for Applied Biodiversity Science and their overseas colleagues have so far establishedfour permanent monitoring stations in Central and South America (purple dots on map) . Six other monitoring sites are in the process of beingset up (yellow dots) . Camera-trap photos from these locales and others around the world have confirmed the presence of many rare animalsincluding (clockwise from upper left) Abbotts duiker (Cephalophus spadix) , Siamese crocodile (Crocodylus siamensis), pangolin (Manis javan-ica) , giant anteater (Myrmecophaga tridactyla) , Andean mountain cat (Oreailurus jacobita) and leopard (Panthera pardus). (Images are, respec-tively, courtesy of Francesco Rovero and James Sanderson; Anthony Simms; Rupert Ridgeway and James Sanderson; Kamajna Panashekung,Kupias Tawadi and James Sanderson; Lilian Villalba, Elisea Delgado and Juan C. Esquivel; Francesco Rovero.)


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traps emplaced in these wetlands re-turned 55 pairs of photos of ocelots.Because their spots differ from animalto animal, Trolle and Marc Kry of theU.S. Geological Survey Patuxent Wild-life Research Center were able to iden-tify individuals, of which there werenine. Of course, the number of ocelotsthat eluded the camera traps was un-known. To estimate that, Trolle andKry did a capture-recapture analysis.

Each of nine ocelots had a uniquecapture history, that is to say, a re-cord of when it was photographed.Such a set of capture histories can besuccinctly summarized in the form ofa binary matrix where each row cor-responds to an individual ocelot andeach column represents a cameratrapping occasion: a fixed numberof days (in this case, a week) duringwhich an individual ocelot was either

recorded or not by any or all of thecamera traps. If, for instance, ocelotnumber 7 was photographed at leastonce during the third camera-trappingoccasion, one would record a 1 in theseventh row and third column of thecapture-history matrix. Even if this oce-lot was recorded multiple times or bymultiple cameras during that time, itwould still only rate a 1 in this columnof the matrix. If no camera snapped itspicture, this element would be set to 0.

Feeding this matrix into standardcapture-recapture software produceda population estimate of 10 4. In thiscase, Trolle and Kry knew there wereat least nine ocelots (having identifiedthat many in the photos), so the rangeon the population size was in fact be-tween 9 and 14. To convert this resultinto population density required anestimate of the amount of ocelot terri-tory being sampled. The area spanned

by the cameras (9.3 square kilometers)was easy enough to calculate. Whatwas trickier was accounting for the factthat some ocelots had home ranges that

extended well beyond this core area.Fortunately, Trolle and Kry couldreadily work out how far these oce-lots moved around by determining themaximum distance between photos ofeach of the nine animals and averagingover the whole set. They then added astrip around the core area of the cam-era traps that was one half of this dis-tance wide. This addition brought thetotal range being sampled up to 17.7square kilometers. Dividing the popu-lation size by this area gives a density

of a little over half an ocelot per square

kilometer. This result not only showedthat the technique Karanth and Nich-ols had used for Indian tigers workedfor another cat species, it also revealedthat the ocelot population in these Bra-zilian wetlands is considerable.

Shirass LegacyShiras was interested in more thanphotographing wildlife: He was alsointent on protecting it. In 1904, thisPennsylvania congressman introduceda bill to allow the federal governmentto regulate the conservation of water-fowl. That bill failed to pass, but 14years later a more expansive measurewas enacted to protect all migratory

birds. That law came too late to helpthe migratory passenger pigeon (whichwent extinct the following year), butclearly the national mind-set was be-ginning to turn toward conservation.

Writing in 1927, Shirass fellow photo-trapper Chapman expressed concernabout peoples actions disturbing nat-ural systems:

A satisfactory study of the rela-tion of an animal to its surround-ings, physical and organic, can bemade only when these surround-ings are essentially natural. Theremoval of but a single speciesmay affect the entire fauna. Theintroduction of a species may befollowed by equally far-reaching

results. In other words, the originof structure and habit, the func-tion of form and color, should bestudied where the conditions oflife have been undisturbed.

Such unspoiled areas must be iden-tified now while there is still time tocall attention to them so that they can

be set aside and protected for posteri-ty. Although few people will ever visitthese sites, and the ones that do willnot stay long, carefully placed cameraphototraps can provide a glimpse of

the rich goings-on inside. The viewsof animals these automatic devices re-turn are ones that even seasoned field

biologists, including those of us whowork regularly in some of the mostremote and undisturbed places on theplanet, will likely never experience di-rectly. Such photographs will increasescientific understanding and, despitetheir often haphazard composition,should boost peoples appreciationof nature, just as they did for view-ers of Shirass wildlife photographs a

century ago.

BibliographyBalmford, A., R. E. Green and M. Jenkins. 2003.

Measuring the changing state of nature.Trends in Ecology and Evolution 18:326330.

Chapman, F. M. 1927. Who treads our trails?The National Geographic Magazine52:330345.

Debinski, D. M., and P. S. Humphrey. 1997.An integrated approach to biological di-versity assessment. Natural Areas Journal17:355365.

Hellawell, J. M. 1991. Development of a ra-tionale for monitoring. In Monitoring forConservation and Ecology, ed. B. Goldsmith.London: Chapman and Hall.

Henschel, P., and J. Ray. 2003. Leopards in Af-rican rainforests: Survey and monitoring tech-niques. New York: Wildlife ConservationSociety.

Kremen, C., A. M. Merenlender and D. D.Murphy. 1994. Ecological monitoring: Avital need for integrated conservation anddevelopment programs in the tropics. Con-servation Biology8:388397.

Karanth, K. U., and J. D. Nichols. 1998. Estima-tion of tiger densities in India using pho-tographic captures and recaptures. Ecology 79:28522862.

Karanth, K. U., J. D. Nichols, N. S. Kumar, W.A. Link and J. E. Hines. 2004. Tigers andtheir prey: Predicting carnivore densitiesfrom prey abundance. Proceedings of the Na-tional Academy of Sciences of the USA101:48544858.

Lambeck, R. J. 1997. Focal species: A multi-species umbrella for nature conservation.Conservation Biology11:849856.

Lawler, J. J., D. White, J. C. Sifneos and L. L.Master. 2003. Rare species and the use ofindicator groups for conservation planning.Conservation Biology17:875882.

Shiras, G. III. 1906. Photographing wild game

with flashlight and camera. The NationalGeographic Magazine6:387446.Shiras, G. III. 1913. Wild animals that took

their own pictures by day and by night. TheNational Geographic Magazine 7:763834.

Shiras, G. III. 1915, Natures transformation atPanama. The National Geographic Magazine 8:159194.

Trolle, M. and M. Kry. 2003. Estimation ofocelot density in the Pantanal using cap-ture-recapture analysis of camera-trappingdata. Journal of Mammalogy84:607614.

Yoccoz, N.G., J. D. Nichols and T. Boulinier.2001. Monitoring of biological diversity inspace and time. Trends in Ecology and Evolu-tion 16:446-453.

For relevant Web links, consult thisissue of American Scientist Online:

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