The Northern Yellowstone Elk: Density Dependence and Climatic ...

More documents

Recommendations

Info

THE NORTHERN YELLOWSTONELK: DENSITY DEPENDENCEAND CLIMATICONDITIONSMARK L. TAPER,1 Department of Ecology, Montana State University, Bozeman, MT 59717, USAPETER J. P. GOGAN, U.S. Geological Survey, Northern Rocky Mountain Science Center, Department of Ecology, Montana StateUniversity, Bozeman, MT 59717, USAAbstract: We analyzed a time series of estimates of elk (Cervus elaphus) numbers on the northern Yellowstone winterrange from 1964 to 1979 and 1986 to 1995 using a variety of discrete time stochastic population dynamic models.These models included adjustments for density, an increase in the area of winter range used by elk, lagged effectsof the weather covariates of spring precipitation, snow depth and winter temperature, and the impacts of the 1988drought and fires. An information-criteria-based model-selection process strongly supported evidence of densitydependence. The best model, a Ricker model, distinguished between the 2 time periods. The bulk of the differencebetween the 2 periods is attributed to an increase in the amount of winter range used by elk. Inclusion of thecovariates spring precipitation and spring precipitation squared greatly improved the model fit. We detected ashort-lived increase in elk population growth rate following the 1988 drought and fires. Fertility and survivorshipof adults appeared to have different density-dependent forms that together result in a biphasic relationshipbetween population growth rate and density. This study confirms the presence of density-dependent regulation inthe northern Yellowstone elk herd, and enhances our understanding of population dynamics of these ungulates.JOURNAL OF WILDLIFE MANAGEMENT 66(1):106-122Key words: Cervus elaphus, density dependence, elk, Greater Yellowstone Area, information criteria, Montana, nonlinearpopulation dynamics, population dynamics, Wyoming.Yellowstone National Park (YNP) supports 8 elkherds that intermingle on the summer ranges(Mack and Singer 1993) within YNP but are identifiedby use of distinct wintering areas within andbeyond the borders of YNP. With minimum populationestimates exceeding 17,000 during winters1992-1993, 1993-1994, and 1994-1995 (Lemke etal. 1998), the northern Yellowstone elk herd is thelargest of the 8 herds. The herd is supported by ex-tensive areas of grassland and shrub steppes withinand beyond the borders of YNP (Houston 1982).Many researchers have noted changes in the lifehistory parameters of the northern Yellowstoneelk herd that are consistent with a density-dependentresponse (Houston 1982, Merrill and Boyce1991, Cheville et al. 1998, Singer et al. 1998). However,such changes are considered insufficient proofof density dependence (McCullough 1990). Truedensity-dependent population regulation has beenindicated by regressions of growth rate on populationsize (Houston 1982, Merrill and Boyce 1991,Dennis and Taper 1994). Nonetheless, contentionstill exists on the extent to which the herd exhibitsdensity dependence because in recent years populationsizes have increased beyond the ecologicalcarrying capacity predicted by Houston (1982). Followingrelease from artificial controls within the1 E-mail: taper@rapid.msu.montana.edupark, the herd size increased to 14,000-15,000during the 1970s through mid-1980s (Merrill andBoyce 1991), 17,800 during 1985-1988 (Merrilland Boyce 1991), and >19,000 during 1993-1994(Lemke et al. 1998). However, rigorous reevaluationof the data analyzed by Houston (1982) confirmshis calculations (Dennis and Taper 1994).An interplay of dynamic natural ecologicalprocesses and human management activitiesthroughout the range of the northern Yellowstoneelk herd have continued to impact theherd's population dynamics. For example, expansionof the extent of the winter range by con-tinued pioneering of herd segments northwardfrom the park boundary and extensive use ofthese more northerly areas by greater numbers ofelk have been coincident with acquisition andconversion of rangelands from livestock productionto elk winter range (Lemke et al. 1998).Also, the adult sex ratio shifted from one highlyskewed by intense culling activities within YNP ofapproximately 50 males:100 females during the1960s to approximately 25 males:100 females bythe late 1980s (Mack and Singer 1993). The im-pact of the 1988 wildland fires on northern rangeelk numbers have been predicted to be relativelyshort-lived (Turner et al. 1994) or cause a declineof 5-10% during the first 5 years but increasednumbers 15-25 years post-fire (Coughenour andSinger 1996). Similarly, the reintroduction of106
J. Wildl. Manage. 66(1):2002DENSITY DEPENDENCE IN YELLOWSTONELK * Taper and Gogan 107wolves (Canis lupus) to YNP has the potential of more, National Park Service, unpublished report).dramatically affecting the size and population Censuses from fixed-wing aircraft began duringparameters of the northern Yellowstone elk herd winter 1967-1968 and continued to 1994-1995.(Garton et al. 1990, Boyce and Gaillard 1992, Boyce The elk herd was counted several times annually1993, Mack and Singer 1993, Messier et al. 1995). from a single fixed-wing PA-18 Supercub fromWe investigated the northern Yellowstone elk 1968 to 1979 (Houston 1982), with the highestpopulation data from the inception of aerial counts obtained between mid-December and midcountsduring 1965 through 1995 to understand January. Annual counts since 1986 have been conthenature of the population dynamics. Rigorous ducted with 4 Supercubs, each covering a segmentevaluation of density dependence in the north- of the northern range (Lemke et al. 1998), ideallyern Yellowstone elk herd is important for scientif- on a single morning. However, in some instances,ic interest but also for evaluating the appropriate- flying conditions and logistical constraints haveness of human management actions. We explored forced the census to be staggered over several daysthe effects of range expansion, wildfires, and cli- (Lemke et al. 1998). The counts are made duringmate on population changes. We addressed the early winter when elk occupy non-forested porquestionsof whether or not the northern Yellow- tions of the northern range and are more readilystone elk herd exhibited density dependence and detected against a snow-covered landscape. Fromassessed the shape of density-dependent functions 1968 to 1975, we combine information from W. J.influencing the population. Such an assessment Barmore (National Park Service, unpublished re-provides a backdrop for understanding the im- port), Houston (1982), and Lemke et al. (1998) aspacts of natural processes and human activities, our source of data for 1976 to 1995 (Table 1).such as hunting and restoration of wolves on thenorthern Yellowstone elk herd.Age and Sex CompositionHerd ageSTUDY AREAand sex composition were determinedeach January from 1968 through 1979Elk of the northern Yellowstone winter range from ground surveys adjusted for under-repreoccupyan area of 1,410 km2 of foothills and valley sentation of adult males by the percent adultbottoms along the Lamar, Yellowstone, and Gar- males sighted during aerial surveys (Houstondiner rivers, with approximately 1,000 km2 within 1982). During winter 1985-1986 through winterthe boundaries of YNP and 500 km2 on the Gal- 1989-1990, herd composition was assessed from alatin National Forest and privately owned lands helicopter covering one-third of the northernnorth of the YNP boundary in Montana (Houston range (Singer et al. 1997) during early winter1982, Coughenour and Singer 1996, Lemke et al. (an) and late winter (mid-Feb through mid-Apr).1998). Elevations range from 1,500 to 2,500 m. During winter 1990-1991, the early winter com-Mean annual precipitation ranged from 35 cm in position was determined from a similar helitheupper Lamar River valley to 24 cm on the Yel- copter count, but the late count was conductedlowstone River at the YNP boundary (Coughenour from the ground. Thus, the proportion of malesand Singer 1996). Vegetation of the area is pri- in this late count is an underestimate (Houstonmarily grassland or sagebrush steppe (Houston 1982). Early and late winter herd composition1982). Dominant plant species of grassland and estimates were made during winter 1991-1992 folsteppeare Idaho fescue (Festuca idahoensis), blue- lowing the sampling regime used by Singer et al.bunch wheatgrass (Pseudoroegneria spicata), and big (1997). No composition counts were conductedsagebrush (Artemisia tridentata; Houston 1982, during winters 1992-1993 or 1993-1994. Only lateTurner et al. 1994). Coniferous forest of Douglas- winter (Mar) composition estimates were madefir (Pseudotsuga menziesii) and lodgepole pine from a helicopter during winters 1994-1995 and(Pinus contorta) occurs at higher elevations at the 1995-1996. In some instances, we found discrepperipheryof, and as isolated stands within, the ancies between early winter composition estimatesgrassland and sagebrush steppe (Houston 1982). determined from the original data sheets on fileat YNP and those reported by Coughenour andMETHODSSinger (1996:582). We present the YNP file datacorrections used in our analyses (Table 2).Census DataThe elk herd was censused from helicopter dur- Data Selection and Adjustmenting winters 1964-1965 and 1966-1967 (W. J. Bar- Several counts (1976-1977, 1988-1989, and
Page 1: The Northern Yellowstone Elk: Densi
Page 5 and 6: J. Wildl. Manage. 66(1):2002DENSITY

The Northern Yellowstone Elk: Density Dependence and Climatic ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?