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 vital rates 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. For instance, pregnant migratory females must balance energy expenditure between long-distance travel and gestation. Little is known, however, on 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 for over 35 years in northeastern Canada. Fall recruitment decreased with high precipitation experienced by adult females around estrus in previous fall (12% [95% confidence interval: 1-22]) and during gestation in winter (13% [3-21]), as well as with warm temperature (13% [3-23]) during gestation in winter and at calving (12% [2-21]). These environmental conditions experienced by both populations were positively correlated, suggesting these parameters could increase overall recruitment synchrony between nearby populations. We found no support for an effect of conditions experienced after birth, demonstrating that environmental conditions encountered from conception to calving are the strongest determinants of recruitment in migratory caribou. Our results highlight the importance of female condition throughout gestation with maternal allocation in fetal growth and 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.