Density‐dependent reproduction in the European rabbit: a consequence of individual response and age‐dependent reproductive performance

Abstract

Density dependence of reproduction has generally been proposed to be caused by habitat heterogeneity and by the individual response of reproductive output. However, a further mechanism might generate density dependence of average reproductive rates. High density situations might be associated with a high proportion of first‐season breeders which often show a principally lower reproductive performance. We tested for the existence of the latter mechanism as well as for density‐dependent individual changes of reproductive effort in a population of European rabbits living in a homogeneous grassland habitat. The study was conducted over a period of eleven years. Overall, a strong relationship between mean reproductive rates and the breeding density of females was apparent. All necessary conditions for the presence of a density‐dependent effect caused by age‐dependent reproduction were fulfilled: Fluctuations of breeding density were paralleled by variations in the proportion of one‐year‐old females. These one‐year‐old, first‐season breeders showed a consistently lower reproductive performance than older females, which might be caused by their lower body mass and their lower social rank. However, we also found strong evidence for density‐dependent response of individual reproductive effort: Individual changes in fecundity over successive years were explained by changes in the breeding density of females. The results suggest that density dependence of reproduction in European rabbits is due to an interaction of age‐dependent reproductive performance together with short‐term fluctuations in breeding density, and a density‐dependent, individual based response of reproductive rates. We further conclude that the lower reproductive performance of first‐season breeders in age‐structured animal populations may contribute substantially to interannual, and under particular circumstances to density‐dependent variations of mean reproductive rates.