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Comments

Intervention and Persistence in Small Populations of Bighorn Sheep

JOEL BERGER*

Program in Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV 89512, U.S.A., email berger@unr.edu

Of the many ways conservation biologists assist land man-agers and decision makers, one is the identification of de-mographic markers, which enables intervention prior tothe point of population loss. For example, when black-footed ferret (

Mustela nigripes

) and California condor(

Gymnogyps californianus

) populations dropped below20, all remaining individuals were captured and broughtinto captivity for propagation. Nevertheless, the use ofsingle demographic parameters to predict extinction maybe risky because real populations often are complex. Be-cause knowledge of noncaptive populations will almostalways be imperfect, it makes sense to identify featuresthat reflect reality and permit generalization. The applica-tion of island biogeographic theory to conservation offersa good example: as suitable habitat is diminished, popula-tions become reduced and often suffer local extinctions(Pimm et al. 1988; Wauters et al. 1994).

In my paper on the persistence of bighorn sheep (

Oviscanadensis

) populations (Berger 1990), I reported a rela-tionship between population size and extinction. We-hausen (1999), using a subset of my data and enhancing itwith additional historical records from California, claimsto have found multiple flaws in my paper. He argues thatmy model “may serve conservation poorly through misdi-rected efforts if it is used as the basis for setting policiesor taking conservation actions” and that it is “an artifact ofthe limited data set available” to me. Wehausen’s reanaly-sis is important. It reinforces some components of my re-sults and it brings others into question. He and I are insubstantial agreement that small populations experiencedextinctions more rapidly than larger populations. Our re-sults differ with respect to trajectories: his are less steepthan those I reported. Although Wehausen’s data set of-fers greater breadth, several of his stated arguments arefragile because he (1) uses data that underestimate the

true nature of sheep extinction trajectories; (2) employsquestionable assumptions; (3) distorts what I have writ-ten or implied; and (4) misses my point about when it isappropriate to attempt rescue of small populations. Be-low I elaborate on each of these.

Extinction Trajectories

Wehausen (1999) suggests that his historical records arecomplete and his analyses robust. Indeed, Wehausen usedpopulation “estimates” that were unavailable to me at thetime of my analyses. Nevertheless, his conclusions remainquestionable because he omits data on population extinc-tions from his own prior publications (Wehausen et al.1987; Wehausen 1988), erects new categories for extinc-tion that differ from those used previously (Wehausen etal. 1987: Table 1), and either ignores or did not access in-formation on extinctions in Berger (1990).

Why Wehausen (1999) “did not plot population per-sistence rates for sample sizes

,

10” is curious and car-ries possible culpability at two levels. First, his decisionto dismiss these samples reduces the efficacy of his(1999) reanalysis by eliminating at least 16 data points (apopulation estimate per decade) from trajectories plot-ted in Berger (1990). Second, by electing not to includeestimates of populations with fewer than 10 and poten-tially ignoring others numbering between 11 and 20 in-dividuals that previously were listed as becoming ex-tinct (in Wehausen et al. 1987), Wehausen (1999)guarantees a priori that his results produce fewer totalextinctions. I base these remarks on omissions and in-consistencies between Wehausen et al. (1987) and We-hausen (1999). The discrepancies include (with We-hausen et al. [1987:66] comments in parentheses) CachePeak (“18 seen in 1933”), the Owlshead Mountains (“12seen in 1938”), Olancha Peak (“3 seen in 1926”), the EagleCrags (“1971 population estimate of 7”), the Quail Moun-tains (“4 seen in 1939”), the Bristol Mountains (“1946 pop-ulation estimate of 4”), the Little Maria Mountains (“1946

*

Current address: Wildlife Conservation Society, P.O. Box 340,Moose, WY 83012, U.S.A.Paper submitted September 14, 1998; revised manuscript acceptedNovember 11, 1998.

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population estimate of 5”), and the McCoy Mountains(“1946 population estimate of 4”). Therefore, Wehausen(1999) chose to ignore six known extinctions that hepreviously acknowledged (Wehausen et al. 1987).

These are not the only problems. Wehausen’s (1999) Ta-ble 1 currently ascribes the loss of sheep in the Argus andthe Slate Ranges as a single combined extinction event,even though in 1987 Wehausen et al. and, subsequently, I(Berger 1990) treated them as separate extinctions. Otherpopulations that Wehausen (1999) combined in his presentpaper under the category of single extinction events, buthad failed to do so previously, are the Newberry Mountains(with the Ords), the Quail Mountains (with the Granites),and the Little Maria Mountains (with the Big Marias). And,in defense of omitting two additional extinctions (Cacheand Olancha Peak) from the Sierras, Wehausen (1999)claims that I (Berger 1990) used minimum counts. It makeslittle sense, however, to drop these from consideration sim-ply because counts are minimums. Such estimates wouldlead to more conservative appraisals of population extinc-tions because if populations had truly been larger then pop-ulations might persist longer. Had these now-extinct popu-lations been included in Wehausen’s (1999) analyses,minimal or not, a more realistic picture of the magnitude oftotal extinctions would have emerged.

Hence, by deleting population samples in his present(1999) analysis with fewer than 10 or those with fewerthan 10 individuals that went to extinction, and bylumping other populations that previously were listed asextinct (Wehausen et al. 1987), Wehausen (1999) biasedhis findings by assuring that they reflected fewer extinc-tions. Also Wehausen (1999) admonished me for not cit-ing all reports, yet he fails to include the Piute Mountainextinction noted by Weaver and Hall (1971).

Wehausen (1999) perceives other problems, suggestingthat the “transient classification was used in the historicalrecords for mountain ranges visited occasionally by sheep(probably rams)” and also that “three of these sites neverreceived any other classification.” There are two issueshere. First, unless noted specifically, there is no basis forknowing whether rams or ewes used some sites, and We-hausen’s argument for “probably rams” is not substanti-ated. Recent work by Bleich et al. (1997), of which We-hausen is a coauthor, does not show large populationsegments in the Mohave Desert that consist solely of rams.Even more substantive, Wehausen’s second comment iswrong because estimates were listed for at least fourranges: the Pintos (“40 were seen in 1950”), the EagleCrags (“population estimate of 7”), the Bullion (“1940population estimate of 54”)—all quoted from Wehausenet al. (1987:66)—and the Stepladder Mountains, whichWeaver (1987) recorded as an extinction, although thispopulation is omitted from all of Wehausen’s analyses.

These quibbles aside, the overall magnitude of extinc-tions in California can be viewed in a different context. Ofthe 74 California populations that I (Berger 1990) exam-

ined, 31% went extinct. Wehausen et al. (1987) reporteda total of 45 extinctions in California after 1850. Of these,22 were included in Berger (1990). Therefore, of 107 pos-sible populations, 45 (42%) went extinct. But Wehausen(1988) also reported that 16 of 20 became extinct in theOwens Valley region. When the four additional extinc-tions (Lee Vining-Lundy, Keough Hot Springs, WalkerPass, Tehachipi Pass), which were not included in We-hausen et al. (1987), are combined with my data (Berger1990), 44% of the 111 populations that presumably ex-isted in California around 1850 became extinct. My(1990) data indicate that 31% clearly is conservative.

Questionable Assumptions

Wehausen (1999) writes, “For all populations

,

50, itwill be necessary to . . . write the populations off as lostcauses over the next half century.” His statement restson two tacit assumptions: (1) that a metapopulationstructure does not exist and (2) that land managementpractices have not changed. With respect to land man-agement, if practices were to remain the same duringthe next century and reflect conditions indicative ofthose that existed prior to 1938 (the cut-off point forwhich extinctions in my paper had to occur to havebeen considered within a 50-year time frame when myanalyses ended in 1988 [Berger 1990]), then Wehausenmay be correct that small isolated populations aredoomed. Neither assumption, however, is correct. First,Wehausen himself has been a coauthor on multiple pa-pers indicating that sheep distributions fit predictions ofmetapopulation models (Bleich et al. 1990, 1996, 1997).Second, federal practices on many of the public landsoccupied by bighorn sheep have changed, also as indi-cated by Goodson (1994). In fact, the Bureau of LandManagement was not even formed until 1946, althoughsome rudimentary federal services were offered prior toits inception (Clawson 1984). Nevertheless, the currentmanagement of federal lands has resulted in fewerabuses than in the past (Cooperrider et al. 1986), and ex-isting data (e.g., Krausman et al. 1996) cited by We-hausen make it clear that land management may be oneof the reasons sheep populations today persist longerthan they did prior to 1938. Thus, Wehausen has createdan artificial division between reality and populations op-erating in a vacuum when he neglects to mention thatconditions today differ dramatically from those in thepast and suggests that in the next century small popula-tions may have to be written off.

Distortions

Wehausen argues that I “focused discussion instead on in-breeding as a probable cause of the rapid extinction of

434

Intervention and Persistence in Bighorn Sheep Berger

Conservation BiologyVolume 13, No. 2, April 1999

small populations” and subsequently that my model(Berger 1990) does not offer “a powerful generality po-tentially supporting inbreeding arguments.” These state-ments offer a false sense of my comments, explicit or im-plicit. Had I focused on inbreeding, the words

genetics

or

population structure

should have appeared in either myabstract or conclusion or both. They do not. In fact, whatI said is that “In captivity, inbred bighorns suffer higher ju-venile mortality . . . (Sausman 1984).

Whether this occursin field populations is unknown

” (my emphasis; Berger1990:96), hardly a focus or a claim for generality in wild,unmanipulated populations. It is surprising that in his zealto dismiss the possible effects of inbreeding on demo-graphic performance, Wehausen (1999) ignores the vastliterature, both theoretical and empirical, on this topic(e.g., Jimenez et al. 1994; Mills & Smouse 1994).

Wehausen states that my “major aim was to dismiss thehypothesis that inadequate forage resulting from dry con-ditions during low precipitation might account for popula-tion declines.” This is not quite accurate. What I attemptedwas a retrospective analysis of multiple correlates of local-ized extinctions. Because precipitation is related to plantbiomass in xeric regions, I analyzed data on past weatherpatterns, a commonly used approach to address questionsabout environmental stochasticity for ungulates and othertaxa (Beissinger & Gibbs 1993; Adams & Dale 1998). IfWehausen (1999) believes a relationship exists or fails to,he should produce and analyze the data.

Wehausen (1999) also points to burros or predators aspossible causes of sheep extinctions. Such speculation isin need of empirical investigation. The mere citation toWehausen’s own recent work in support of a hypothesisof predation by pumas (

Felis concolor

) obscures a criti-cal temporal issue about potential predator-induced ex-tinction. From 1907 to 1963 California offered bountieson pumas and more than 12,000 were killed (Torres etal. 1996). Because pumas today receive full legal protec-tion, populations are likely to be higher now than in thepast. Therefore, Wehausen’s reliance on contemporaryinteractions between pumas and sheep is a poor para-digm to gauge possible

past

predator-induced extinc-tions of sheep, particularly because there is no evidenceof pumas in the Mohave Desert prior to the introductionof mule deer in the 1940s (Johnson et al. 1948; Berger &Wehausen 1991).

Intervention in the Management ofSmall Populations

Wehausen (1999) suggests that if resource managersbase their conservation strategies on my conclusions,“the conservation of this species (and of others . . . ) maybe poorly served.” I disagree. If small sheep populationsdo not plummet to extinction, what has been lost? Noth-

ing. If small sheep populations have gone extinct in thepast, which they obviously have, it still needs to beknown whether this is the case now. Evidence fromboth Goodson (1994) and Krausman et al. (1996) is en-couraging and suggests that conditions for persistenceare improving. Most critically, however, Wehausen’s(1999) own data make clear that agencies are indeedconcerned with the persistence of small populations.

Rather than adopt a laissez faire approach, California’sDepartment of Fish and Game has intervened regularlyon behalf of small populations, not medium-sized orlarge ones. If efforts were randomly distributed across allpopulation size classes, then recipients of demographicintervention would be in proportion to their abundance.This is not the case. Three populations listed in We-hausen’s (1999) Table 1, the Eagle Crags, Chuckwallas,and the Sheepholes, were each augmented when popu-lation sizes were below 15 individuals, respectively. TheBullion and Argus-Slate areas also received additionalsheep when their populations dipped from about 12 orfewer animals to transients only, and the Bristol, Mt. Lan-gley, Pine–Rock Creek, and Whipple, sites of past ex-tinctions, all received reintroduced sheep. Further, I(1990) presented data on the efficacy of reintroduction,detecting a relationship between the size of foundingpopulations and improved chances for success. So theobvious question is why agencies should consider aug-mentations or reintroductions by taking population sizeinto account. The conspicuous answer is that size mat-ters (Griffiths et al. 1989).

The Bottom Line

Reports and commentaries such as these should be con-structive. Who really cares whether population esti-mates are 10 or 5 or 15? The point is that historicallysome populations were small and others significantlylarger. It would have been far more productive had We-hausen and I compared notes, worked together, andproduced a joint paper to address critical concernsabout methodology, analyses, and how best to increasesheep population viability. Instead, attacks are followedby counterattacks. Those between Wehausen and me in-volve minutia about which population to include oromit and what was said and what was not. Fortunately,there is some agreement between us: small populationsare predisposed to greater overall risks of extinction,and management ought to be skewed to prevent them.

Acknowledgments

For comments and discussion I thank C. Cunningham, E.Main, K. Snow, P. Stacey, and an anonymous reviewer.

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