Conservation of migratory species at risk: environmental conditions experienced from pre‐gestation to parturition affect fall recruitment in migratory caribou

Annual variations in environmental conditions can strongly affect demographic rates of wild populations, such as survival and recruitment. These effects are likely to be exacerbated in highly seasonal environments and in species facing substantial energetic needs during specific seasons. In migratory mammals, for instance, females must contend with the energy expenditures required for long‐distance travel as well as those needed to complete gestation. Little is known, however, about the environmental factors influencing recruitment in species exhibiting long‐distance migrations. We aimed to fill this gap by evaluating the effect of environmental conditions from pre‐gestation to weaning on fall recruitment of two migratory caribou Rangifer tarandus herds followed over nearly three decades in northeastern Canada. Recruitment decreased with high precipitation experienced by adult females around estrus in previous fall and during gestation in winter, as well as with warm temperatures during gestation in winter and at calving. These environmental conditions experienced by both populations were positively correlated, suggesting they could increase overall recruitment synchrony between nearby populations. We found no evidence for an effect of conditions experienced after birth, suggesting that environmental conditions encountered from conception to calving are likely the strongest determinants of recruitment in migratory caribou. Results of this population‐level analysis highlight the importance of female condition throughout gestation with maternal allocation in fetal growth and the newborn calf playing a major role in recruitment. Synchrony observed in recruitment between herds also underlines the importance of considering multiple populations facing similar environmental conditions in conservation strategies as they may exhibit simultaneous fluctuations in abundance.