Advancing the match-mismatch framework for large herbivores in the Arctic: Evaluating the evidence for a trophic mismatch in caribou

Climate-induced shifts in plant phenology may adversely affect animals that cannot or do not shift the timing of their reproductive cycle. The realized effect of potential trophic “mismatches” between a consumer and its food varies with the degree to which species rely on dietary income and stored capital. Large Arctic herbivores rely heavily on maternal capital to reproduce and give birth near the onset of the growing season but are they vulnerable to trophic mismatch? We evaluated the long-term changes in the temperatures and characteristics of the growing seasons (1970–2013), and compared growing conditions and dynamics of forage quality for caribou at peak parturition, peak lactation, and peak forage biomass, and plant senescence between two distinct time periods over 36 years (1977 and 2011–13). Despite advanced thaw dates (7−12 days earlier), increased growing season lengths (15−21 days longer), and consistent parturition dates, we found no decline in forage quality and therefore no evidence within this dataset for a trophic mismatch at peak parturition or peak lactation from 1977 to 2011–13. In Arctic ungulates that use stored capital for reproduction, reproductive demands are largely met by body stores deposited in the previous summer and autumn, which reduces potential adverse effects of any mismatch between food availability and timing of parturition. Climate-induced effects on forages growing in the summer and autumn ranges, however, do correspond with the demands of female caribou and their offspring to gain mass for the next reproductive cycle and winter. Therefore, we suggest the window of time to examine the match-mismatch framework in Arctic ungulates is not at parturition but in late summer-autumn, where the multiplier effects of small changes in forage quality are amplified by forage abundance, peak forage intake, and resultant mass gains in mother-offspring pairs.气候驱动的植物物候变化，可能对无法或不愿调整繁殖周期时间节点的动物造成不利影响。消费者与其食物之间潜在的营养级错配（trophic mismatch）的实际效应，取决于物种对食物摄入与储存资本的依赖程度。大型北极植食动物高度依赖母源资本（maternal capital）完成繁殖，并在生长季启动初期分娩，那么它们是否易受营养级错配的影响？本研究评估了1970—2013年温度与生长季特征的长期变化，并对比了36年间两个不同时间段（1977年与2011—2013年）内，北美驯鹿在分娩峰值期、泌乳峰值期、牧草生物量峰值期以及植物衰老期的生长条件与牧草质量动态。尽管解冻期提前7—12天、生长季长度延长15—21天，且驯鹿分娩日期保持稳定，但研究未发现牧草质量出现下降，因此本数据集未提供1977年至2011—2013年间分娩峰值期或泌乳峰值期存在营养级错配的证据。对于依靠储存资本繁殖的北极有蹄类动物（ungulates），其繁殖需求主要由前一年夏季和秋季积累的身体养分储备满足，这会降低食物可获得性与分娩时间错配可能带来的不利影响。不过，气候对夏秋季栖息地牧草的影响，确实与雌性北美驯鹿及其后代为下一繁殖周期和冬季积累体重的需求相契合。因此，我们认为在北极有蹄类动物中检验匹配-错配框架（match-mismatch framework）的时间窗口不应处于分娩期，而应在夏末至秋季：此时牧草质量小幅变化的倍增效应，会因牧草丰度、峰值牧草摄入量以及母子对由此获得的体重增长而被放大。