Subsections:
Body Condition and Reproductive Performance
Overview
References

Reproductive success of caribou is highly correlated with nutritional status. The probability of producing a calf varies directly with body weight and/or fat content of sexually-mature females during the previous autumn (Cameron et al. 1993, 2000; Cameron and Ver Hoef 1994; Gerhart et al. 1997). In contrast, calving date and perinatal survival are more closely related to maternal weight shortly after parturition (Cameron et al. 1993) (Fig. 4.8). The likelihood of conceiving is probably determined by body condition at breeding, whereas parturition date and calf survival reflect maternal condition during late gestation.

Figure 4.8. Logistic regressions (solid lines are significant at P < 0.05) of parturition rate, incidence of early calving (i.e., on or before 7 June), and perinatal (>2 days post partum) calf survival on autumn and summer body weights of female caribou, Central Arctic caribou herd, Alaska, 1987-1991. The empirical percentages are shown at arbitrary 10-kg intervals of body weight. Numbers in parentheses are sample sizes. The asterisk indicates inclusion of one female weighing 57 kg. (from Cameron et al. 1993)

These relationships link the nutritional consequences of changes in distribution to the reproductive success of caribou of the Central Arctic herd. West of the Sagavanirktok River, in the petroleum development zone, caribou had reduced access to preferred foraging habitats near roads (Nellemann and Cameron 1996) and shifted their concentrated calving area into habitats with lower plant biomass (P < 0.001) (Wolfe 2000). In contrast, forage biomass remained constant (P = 0.23) within concentrated calving areas east of the Sagavanirktok River where no development was present (Wolfe 2000) (Fig. 4.9).

Figure 4.9. Changes in median Normalized Difference Vegetation Index (NDVI) on 21 June for concentrated calving areas of the Central Arctic caribou herd in the study reference zone (relatively undeveloped) and treatment zone (developed) east and west of the Sagavanirktok River, Alaska, respectively, 1985-1995. (from Wolfe 2000)

Repeated use of lower-quality calving habitats may reduce forage intake by females calving west of the Sagavanirktok River. Likewise, impaired summer movements between insect relief habitat and inland feeding areas could depress energy balance (Smith 1996) and, hence, rates of weight-gain.

Indeed, several data sets suggest reduced nutritional status and fecundity of radio-collared females exposed to oil development west of the Sagavanirktok River. Estimates of July and October body weights, over-summer weight-gain, the incidence of 2 successive-year pregnancies, and perinatal calf survival all tended to be lower for females to the west than for those under disturbance-free conditions to the east, although individual differences were not significant at the 95% confidence level (Cameron et al. 1992a).

In a more recent analysis of data for 1988-1994, however, mean parturition rate of females calving west of the Sagavanirktok River was less than that of females calving east of the Sagavanirktok River, 64% vs. 83%, respectively (P = 0.003, Table 4.1) (Cameron 1995). Corresponding frequencies of reproductive pauses (Cameron 1994, Cameron and Ver Hoef 1994) were significantly higher (P < 0.02, t-test, ratio method) in the west (36%, 26 of 73 observations) compared with the east (19%, 12 of 64 observations), or approximately one pause every 3 and 5 years, respectively (Cameron 1995).

Table 4.1. Parturition status of 43 radio-collared female caribou^a, Central Arctic herd, west and east of the Sagavanirktok River^b, Alaska, 1988–1994. West includes the Prudhoe Bay and Kuparuk oil fields; east was generally free of disturbance during that time. (data from Cameron 1995)

The key constraint on reproduction is lactation, which exacts a substantial cost on summer weight-gain, in turn influencing the probability of conceiving that autumn. During 1988-1991, weights of all lactating Central Arctic herd females sampled averaged 9 kg less than nonlactating females (Fig. 4.10). This resulted in a projected 28% lower parturition rate for the lactating females (Fig. 4.11) (Cameron and White 1992).

Figure 4.10. Mean (SE) body weights of lactating and nonlactating female caribou from the Central Arctic herd, Alaska, in summer (July) and autumn (October). (from Cameron and White 1992) *Significant at P < 0.001.

Figure 4.11. Distributions of observed autumn (October) body weights for lactating and nonlactating female caribou from the Central Arctic herd. The associated parturition rates are integrated estimates derived from the logistic model (Fig. 4.8). (from Cameron and White 1992)

Lower parturition rates of females west of the Sagavanirktok River during 1988-94 (Table 4.1) may reflect a failure to compensate for the metabolic burden of milk production (i.e., through increased forage intake or reduced energy expenditure). Hence, those females of the Central Arctic herd that used the development zone were in consistently poorer condition in autumn, experienced more frequent reproductive pauses, and produced fewer calves (Fig. 4.2).

Yet the degree to which lactation constrains weight-gain does vary. An increase in net calf production during 1996-2000 (Fig. 4.2) suggests the prevalence of forage and insect conditions that enhanced growth and fattening despite the demands of milk production and presence of industrial activity. With the opening of the Badami petroleum development area east of the Sagananirktok River in 1996, however, the undisturbed status of that area was compromised, rendering further comparisons questionable.

Clearly, anthropogenic impacts on caribou must be identified and assessed within the framework of a variable natural environment. Favorable foraging and insect conditions would attenuate the consequences of disturbance-induced changes in quality of occupied habitats. Conversely, adverse conditions would exacerbate those same types of consequences. Unless analyses are based on multi-year observations of marked individuals and incorporate comparative data on an undisturbed control or reference group, conclusions will be equivocal at best. For example, absent a valid baseline, net growth of the Central Arctic herd (Fig. 4.2) is no better evidence of compatibility with development than a net decline would be evidence of a conflict.

The crucial consideration for the future of the Central Arctic herd and other arctic caribou herds is whether changes in distribution associated with surface development, by depressing reproduction or survival, will either retard an increase in herd size or accelerate a decrease.

Our data, in fact, indicate that productivity can and will decline if the cumulative loss of preferred habitat, when superimposed on natural forces, is sufficient to compromise nutrition.

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| Home | Section 1 - Introduction | Section 2 - Land Cover | Section 3 - Porcupine Caribou Herd |
| Section 4 - Central Arctic Caribou Herd | Section 5 - Forage Quantity and Quality | Section 6 - Predators |
| Section 7 - Muskoxen | Section 8 - Polar Bears | Section 9 - Snow Geese | Acknowledgements |