Density dependence in a fluctuating wild reindeer herd; maternal vs. offspring effects

Summary

The Hardangervidda wild reindeer herd in Norway is the largest in Western Europe. It has fluctuated between 7000 and 32000 animals during the last 35 years. Four density-dependent effects were found: 1. A food limitation effect due to a shift in diet after overgrazing lichen on the winter range. This led to increased tooth wear and lowered body size and fat reserves. 2. A significant correlation between population density and juvenile winter survival rate. No effect on adult female survival rate was found. 3. A cohort effect. After population increase and overgrazing, recruitment was reduced by 30% and remained so after population reduction. Birth weights had increased by 30% 5 years after population reduction and the mean calving time was earlier. As a result, after population reduction weights of newborns were 40% greater at a comparable date. Neonatal survival rate was related to maternal condition during the last part of gestation which coincides with the peak winter snow accumulation. The slow increase in adult dressed body weights (DBW) after population reduction is due to the combined effects of increased tooth wear when winter range was limiting and to the cohort-generation time, so that an improvement in neonatal survival and size was first expressed in subsequent offspring cohorts. 4. An inter-generation effect. During 30 years of resource limitation, DBW decreased by 23%, birth rate was unchanged after the first peak, while fecundity increased by 15%, suggesting increased reproductive effort per unit body weight. Natural selection for increased reproductive effort by smaller females when food was limiting was suggested. Some size-effect due to hunters selecting the largest adult phenotypes was possible but not the main cause. These results do not support some earlier hypotheses about the effects of population density on size at maturity in ungulates.