Wolf Predation on the Nelchina Caribou Herd: A Reply

Abstract

We review arguments on the roles of wolf predation and winter starvation in recruitment of the Nelchina caribou (Rangifer tarandus) herd, Alaska. Van Ballenberghe (1985) originally proposed that high calf mortality in the 1960's and 1971-72 was caused by winter severity and that wolf (Canis lupus) predation played a minor role. Van Ballenberghe (1989) now argues that the data are not adequate to decide between hypotheses, and winter starvation is no longer stressed as the valid explanation for low calf survival. His basis for arguing that predation played a minor role in calf survival was that the wolf population was low and stable from 1962 to 1974 (Van Ballenberghe 1985:fig. 2). However, he revised 9 of 10 original estimates and incorporated them into a graph where there were no estimates for 15 years. Original estimates indicated that the wolf population increased at a finite rate of 1.30 from 1953 to 1965, and wolf population estimates based on original census data were negatively correlated with recruitment (Bergerud and Ballard 1988). Additionally, recruitment improved 3 times when wolf populations were reduced through hunting and removal programs. The decline of the Nelchina herd from 1962 to 1972 can be explained by excessive human harvests in the absence of adequate recruitment. J. WILDL. MANAGE. 53(1):251-259 In discussing the decline of the Nelchina caribou herd from 1962 to 1972, Van Ballenberghe (1985) attributed low recruitment in some years to survival of calves in winters with deep snow, and minimized the role of wolf predation in calf survival and dynamics of the herd (for 195084). Bergerud and Ballard (1988) suggested that the roles of starvation versus predation had not been adequately tested by Van Ballenberghe (1985) because his arguments were based on inspection of snow and calf mortality statistics without recourse to statistical analysis, and his argument against wolf predation was biased because he drastically lowered the original estimates of wolf numbers. We concluded the converse of Van Ballenberghe relative to starvation and predation. CAUSE AND EFFECT EVIDENCE Van Ballenberghe (1989) discusses Romesburg's (1981) view that alternative hypotheses should be tested to avoid confusion in cause and effect. In Bergerud and Ballard (1988), we tested 2 major alternative hypotheses of density-dependent starvation versus predation as the agent of control of the Nelchina caribou population (Bergerud 1974). We concluded, in part on the basis of correlation and regression analysis, that predation and reduced calf survival were cause and effect, respectively. However, there was also experimental evidence of cause and effect; wolves were reduced in the early 1950's and again in the mid-1970's and in both cases recruitment of caribou improved (Bergerud and Ballard 1988:table 1). When wolves were reduced in 1967 by hunting and again in 1976 in a reduction program, calf survival again improved (Bergerud and Ballard 1988:figs. 2 and 5). In contrast to our approach, Van Ballenberghe (1985) concluded cause and effect between winter severity and calf mortality, but he did so only by inspecting the data he presented (Van Ballenberghe 1985:fig. 1 compared to fig. 2). In fact, there was no compelling correlation between winter severity and calf mortality (r = 0.41, n = 18, Nov-Apr depths; r = 0.31 for years, >50,000 caribou, n = 9) (Van Ballenberghe 1985: figs. 1 and 2).