CSU FW 662 - CLIMATE FORCING AND DENSITY DEPENDENCE IN A MOUNTAIN UNGULATE POPULATION

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1598Ecology,85(6), 2004, pp. 1598–1610q2004 by the Ecological Society of AmericaCLIMATE FORCING AND DENSITY DEPENDENCE IN A MOUNTAINUNGULATE POPULATIONANDREWR. JACOBSON,1,5ANTONELLOPROVENZALE,2ACHAZ VONHARDENBERG,3BRUNOBASSANO,4ANDMARCOFESTA-BIANCHET31AOS Program, Princeton University, P.O. Box CN710, Princeton, New Jersey 08544-0710 USA2Institute of Atmospheric Sciences and Climate, CNR, C.so Fiume 4, I-10133 Torino, Italy3De´partement de biologie, Universite´ de Sherbrooke, Sherbrooke, PQ J1K 2R1, Canada4Alpine Wildlife Research Centre, Gran Paradiso National Park, via della Rocca 47, I-10123 Torino, ItalyAbstract.Population models in ecology are rarely validated by comparing their pre-dictions to long-term observations of changes in population size. We have used a varietyof analytical tools to examine a 45-year time series of annual censuses of Alpine ibex(Capra ibex) in the Gran Paradiso National Park in northwestern Italy. This ibex populationgrew from about 3300 to almost 5000 individuals in the 1980s during a decade of anom-alously mild winters, and then began to decline in the 1990s. By 1997, the population sizehad returned to previous levels. Adult survival apparently increased and adult sex ratiomay have changed to slightly favor males during the increase in population density. Yearlychanges in total population were correlated with seasonal average snow depth and populationdensity over the 39 years for which climate data were available. Our results show that theibex population size was limited by both density dependence and deep snow. A modelbased on these factors fit to the first 19 years of data was used to forecast subsequentchanges in total population based on initial population size and yearly snow depth. Themodel was able to predict the increase and subsequent decline in total population size overthe final 20 years of the study but failed to reproduce population levels after the eruption.Our results suggest that the 1980s episode of population growth was primarily driven byincreased adult survival, rather than increased recruitment.Key words: climate changes and population fluctuations; density dependence; ibex, Alpine; Italy,Gran Paradiso National Park; model validation; population dynamics; time series, population data.INTRODUCTIONSpecies that live in arctic or alpine environments canbe particularly sensitive to changes in climate (Post etal. 1999) and may thus face an increased extinctionrisk (Derocher and Stirling 1995, 1998). It is thereforeessential to test the ability of population models notonly to explain historic patterns of population fluctu-ations, but also to predict future changes (Forchhammeret al. 1998a), based on climate projections. Reviewsof studies based on long-term monitoring of markedindividuals in ungulate populations suggest that juve-niles are the age class most likely to be affected byboth extrinsic and density-dependent processes, whilethe survival of adult females is generally unaffectedby changes in density or in environmental conditions(Gaillard et al. 1998, 2000). Severe winter weather isgenerally thought to affect juvenile survival by in-creasing the probability of starvation, through a com-bination of greater thermoregulatory costs and de-creased forage availability because of deep snow(Goodson et al. 1991, Forchhammer et al. 1998b, Loi-son and Langvatn 1998, Portier et al. 1998). A ratherManuscript received 6 December 2002; revised 23 August2003; accepted 25 October 2003; final version received 17 No-vember 2003. Corresponding Editor: J. M. Fryxell.5E-mail: [email protected] result of ungulate studies is that high pop-ulation density increases the age of primiparity, so thatfewer age classes of females contribute to recruitment(Fowler 1987, Jorgenson et al. 1993, Langvatn et al.1996, Gaillard et al. 1998, 2000, Swihart et al. 1998).One goal of our research is to determine whether chang-es in ibex population size were due to variations inrecruitment, as might be expected from these previousworks, or to changes in adult survival.The processes that control animal abundance are offundamental interest to ecologists. Recently, severalstudies of ungulates have suggested that an interactionof year-to-year changes in weather and density-depen-dent changes in survival and reproduction may explainchanges in population density in environments wherelarge predators are very rare or absent (Langvatn et al.1996, Sæther 1997, Forchhammer et al. 1998b, Postand Stenseth 1999). Other studies suggest that even inthe presence of healthy predator populations or humanharvests, climate and density may play an importantrole in ungulate population dynamics (Post and Sten-seth 1998, Coulson et al. 2000). Studies of feral sheeppopulations that show strong density-dependent fluc-tuations have suggested that changes in climate maysynchronize the dynamics of populations on separateislands in the same geographical area (Grenfell et al.1998). Despite the wealth of information that has ac-June 2004 1599CLIMATE EFFECTS IN MOUNTAIN UNGULATEScumulated, however, tests of the predictive power ofspecific models using time-series data on populationsize are rare: researchers have mostly used climaticvariables to explain historic variation in population sizeor in specific vital rates (Solberg et al. 1999, Coulsonet al. 2000). The scarcity of attempts to predict pop-ulation changes stems partly from the rarity of studiesthat have accumulated the many years of demographicdata required by this exercise. For example, the abilityof population-viability analysis models to predict pop-ulation dynamics over several years is subject to con-siderable debate (Brook et al. 2000, Coulson et al.2001b). Here we take advantage of the unusual op-portunity of having a 45-year time series of Alpine ibex(Capra ibex) counts to test the ability of a simple modelbased on snow depth and population density to predictchanges in total ibex abundance. We first estimate theindependent effects of snowfall and density on year-to-year changes in population size for Alpine ibex inthe Gran Paradiso National Park, Italy. Having dem-onstrated that both of these factors affect the populationdynamics, we then develop a simple model incorpo-rating both factors. We assess the long-term predictivepower of this model using an out-of-sample predictiontest: the model parameters are determined using onlythe first 19 years of data, then the model predictionsfor the following 20 years of


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