Morphological change in Newfoundland caribou: Effects of abundance and climate

Shane P Mahoney,Shawn F Morrison,J Glenn Luther,Jackie N Weir,James A Schaefer

doi:10.7557/2.31.1.1917

Shane P Mahoney, Shawn F Morrison + Show 3 more

Open Access

https://doi.org/10.7557/2.31.1.1917

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Abstract

The demographic and environmental influences on large mammal morphology are central questions in ecology. We investigated the effects of population abundance and climate on body size and number of male antler points for the La Poile and Middle Ridge caribou (Rangifer tarandus, L. 1758) herds, Newfoundland, Canada. Across 40 years and 20-fold changes in abundance, adult males and females exhibited diminished stature as indicated by jawbone size (diastema and total mandible length) and the number of antler points at the time of harvest. Associations between jawbone size and population abundance at birth were consistently negative for both herds, both sexes, and all age classes. Large-scale climate patterns, as measured by the North Atlantic Oscillation in the winter prior to birth, were also negatively associated with jawbone size. Declines in male antler size, as measured by the number of antler points, were not well predicted by either abundance or climate, suggesting other factors (e.g., current, rather than latent, foraging conditions) may be involved. We conclude that these morphological changes indicate competition for food resources.

Highlights

Understanding variations in species abundance and life histories remains one of the primary goals in ecology
A growing body of evidence suggests that some Rangifer populations, like other mobile ungulates (Dublin et al, 1990; Mduma et al, 1999), are regulated by food resources (Skogland, 1983, 1985; Messier et al, 1988; Couturier et al, 1990; Skogland, 1990; Mahoney & Schaefer, 2002)
The classic density-dependent responses exhibited by both the La Poile and Middle Ridge herds – a rapid increase in abundance followed by a decline, mirrored by reduced body size – suggest that a forageabundance relationship has influenced the dynamics of Newfoundland caribou

Summary

Introduction

Understanding variations in species abundance and life histories remains one of the primary goals in ecology. Many migratory herds experienced sustained numerical increases (Skogland, 1985; Fancy et al, 1994), followed in many cases by the cessation of growth (Messier et al, 1988; Mahoney & Schaefer, 2002) and decline (Skogland, 1990) These numerical changes provide valuable opportunities to broaden our understanding of the influence of density-dependent factors, such as resource competition, on population dynamics. We examined changes in these morphological indices for two Newfoundland caribou herds (La Poile and Middle Ridge herds) across several decades and dramatic changes in population abundance Such datasets are rare (Gaillard et al, 2000; Wittmer et al, 2005) and allowed us to identify putative limiting and regulating factors which may have contributed significantly to temporal trends in morphology. We anticipated a trend toward diminished size of these caribou – a sign of density-dependence resulting from food competition, consistent with other populations of large, mobile, herbivorous mammals

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