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Ecological correlates of regional variation in life history of the moose Alces alces: comment.

Saether et al. (1996) compared some life-history characteristics of four Norwegian moose (Alces alces) populations occupying ranges of varying quality to test the food-limitation hypothesis and concluded (p. 1499) that "a stable high-density equilibrium between moose and their food resources is unlikely to occur." However, their data set may not justify such a conclusion because they neglected to consider the role of a major component in the dynamics of the moose populations studied: human harvests. On the basis of the simple models proposed by Caughley (1976) for plant-herbivore interactions and regulation of large-ungulate populations, Saether et al. (1996) cannot use population dynamics of moose in Norway to reject the hypothesis of regulation by competition for food.

To alleviate any semantic confusion, it is worth defining four concepts that are central to the understanding of Caughley's (1976) approach to population dynamics of large herbivores: K carrying capacity (KCC; Macnab 1985), limiting factors, regulating factors (Messier 1991) and sustained yield (SY). KCC represents "the equilibrium reached between herbivores and their food supply" after dampened oscillations (Macnab 1985:404). Limiting factors refer to "any processes that quantifiably affect population growth and are responsible for year-to-year changes in the rate of population growth" (Messier 1991:378). Regulating factors designate "any density-dependent processes that ultimately keep populations within normal density ranges" (Messier 1991:378). Thus regulating factors represent a subset of limiting factors, characterized by negative-feedback mechanisms that depress population growth as animal abundance increases. Finally, SY equals the annual surplus of births over deaths observed when an ungulate population below KCC is increasing; if these animals are harvested, the population remains stable below KCC.

Sinclair and Arcese (1995) described three alternative hypotheses related to regulation of large herbivores: the predator-regulation hypothesis, the predation-sensitive food hypothesis, and the surplus (or food-limitation) hypothesis. In the first case, density-dependent predation causes herbivores to stabilize at low density (relative to KCC) with access to ample quality forage. Moose populations preyed upon by wolves (Canis lupus) and black or brown bears (Ursus americanus, U. arctos) support this hypothesis (Messier and Crete 1985, Crete 1987, Gasaway et al. 1992). According to the second hypothesis, herbivore numbers stabilize at a density lower than KCC because predators remove some vulnerable herbivores that would otherwise survive. This model might apply to Isle Royale moose where only wolves prey on moose (McLaren and Peterson 1994). In the third case, the food-limitation hypothesis supposes that competition for forage causes density to stabilize at KCC, predators having no influence on herbivore density. This hypothesis should apply to Fennoscandian moose because wolves and brown bears have been reduced to insignificant numbers during the current century (Cederlund and Markgren 1987). Given the food-limitation hypothesis, one should still expect density to fluctuate around KCC, as observed for moose on Isle Royale (McLaren and Peterson 1994) or wildebeest (Connochaetes taurinus) in the Serengeti (Sinclair and Arcese 1995), due to the combined effects of limiting factors (e.g., winter harshness) and forest dynamics (e.g., forest fires).

In order to test the food-limitation hypothesis of population regulation, Saether et al. (1996) had to study moose populations in the proximity of KCC because, in large mammals, regulation generally operates at densities approaching KCC (Fowler 1981). However they provided no figures on either absolute moose density, or density with respect to KCC. Crete (1989) estimated KCC in an area of deep snow of eastern Quebec to exceed 2 moose/[km.sup.2]; the estimate varied between 3.6 and 6 animals/[km.sup.2] in southwestern Quebec due to greater forage production. Very few data have been published on Fennoscandian moose density, and none to my knowledge for unharvested populations. Bergstrom and Vikberg (1992) reported that the density increased from 1.3 to 5.7 moose/[km.sup.2] in a forested enclosure of central Sweden before being reduced by hunting. Most likely, KCC must approach 10 animals/[km.sup.2] in very productive areas of Fennoscandia, particularly because of the limited snow cover (Saether et al. 1996).

The four moose populations studied were harvested annually at a rate of 0.33-0.50 animal/[km.sup.2] (Hjeljord et al. 1994). With the information provided, it is impossible to compute which proportion of the population this SY represented, but such yields are comparatively high (Crete 1987). Most likely, annual harvests have kept densities much below KCC, particularly for the Alpine population that occupied a poor range (Saether et al. 1996). Not surprisingly, Saether et al. (1996) found no evidence of regulation driven by competition for forage.

However, Saether et al. (1996) found that winter climate was a limiting factor for their moose populations, as calf mass increased by [congruent]40% during two snow-free winters (a very unusual situation for moose [Bedard et al. 1974]), and larger body size after mild winters resulted in higher fecundity of yearling females. From this observation, they concluded that regulating factors due to competition for forage likely contributed less to variation in recruitment rate than variation due to climate. Saether (1985) had already identified summer climate as a limiting factor for moose in Norway, its influence on somatic growth differing however between the south and the north of the country. Sand et al. (1995) observed that climatic harshness influenced body mass of adult Swedish moose more than density or browse availability, whereas Crete and Courtois (1997) found a relationship between winter and summer climate and calf production in an unproductive boreal range of northeastern Quebec. As Saether et al. (1996) observed in Norway, Sand (1996) also noted that the relationship between body mass and sexual maturity varied with latitude for Swedish moose. Three conclusions can be drawn from the preceding observations: (1) Saether et al. (1996) could not compare the relative importance of climate and competition for forage for moose fecundity because they had no data for populations regulated by food. (2) Climate might play a minor role in moose demography if influencing only somatic growth and sexual maturity; adult fecundity and survival have greater effects on population growth of cervids than sub-adult fecundity (Nelson and Peak 1982, Crete et al. 1996). (3) Climatic factors exert a variable influence on population dynamics throughout the range of moose. In this respect, the idea first proposed by Haldane (1956) could be true for moose - that regulation might be easier to detect in the core of the species range whereas limiting factors might gain in importance at the periphery (Crete and Courtois 1997).

Sinclair (1991) advocated the utilization of control and manipulated animal populations through wildlife management for scientific experimentation. In order to test the existence of density-dependent regulation by competition for forage in Fennoscandian moose and to test the stability of the equilibrium, one should exclude moose from hunting over an area large enough ([approximately]1000 [km.sup.2]) to reduce the importance of emigration (Crete 1989) and should monitor major demographic variables for comparison with contiguous, harvested populations. Doing the study in the core of the range of moose might yield more conclusive results than at the periphery.


B. E. McLaren, D. L. Murray, J. D. Wehausen, and an anonymous reviewer helped to improve this note by kindly commenting on a previous draft.

Literature cited

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Sinclair, A. R. E., and P. Arcese. 1995. Population consequences of predation-sensitive foraging: the Serengeti wildebeest. Ecology 76:882-891.
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Title Annotation:response to B.-E. Saether, Journal of Wildlife Management, vol. 49, p. 977, 1985
Author:Crete, Michel
Date:Jul 1, 1998
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