Ecological Replacement of Peromyscus maniculatus gracilis by Peromyscus leucopus in Northern Michigan.

Abstract

Climate-induced ecological replacement of P. m. gracilis by P. leucopus has been documented with long-term studies in the Great Lakes Region. Gradually warming winters over the last 30-40 years in this area have facilitated northward range expansion of P. leucopus, and communities in which P. leucopus and P. m. gracilis co-occur have become characterized by strong numerical dominance of P. leucopus. I conducted a three year field study of syntopic P. leucopus and P. m. gracilis in northern Michigan to identify niche differences and investigate the mechanisms by which these differences facilitate ecological replacement. P. leucopus consistently experienced a longer breeding season than P. m. gracilis. P. leucopus and P. m. gracilis also differed in the survivorship of overwintered mice. This results in differences in the relative contributions of overwintered and young-of-year females to annual reproductive output, and may be a previously unreported niche difference between species. Ecological replacement of P. m. gracilis by P. leucopus results from the interaction between environmental conditions, niche differences, and species interactions. Abundance differed strongly in two out of three years, with P. leucopus outnumbering P. m. gracilis in 2011 and 2012. Increases in abundance of P. leucopus are well explained by enhanced survival conferred by increasingly warming conditions. Reproductive output in P. leucopus is highly dependent upon reproduction by overwintered females. Mild winters allow for increased abundance of breeding females in the spring and more favorable breeding conditions which facilitate successful early reproduction and lower mortality. The role of interspecific competition in ecological replacement in this species pair has not been previously investigated, and the role of competition in regulation of population growth remains unclear. Nevertheless, there was evidence that interspecific competition may influence replacement. Declining abundance of P. m. gracilis was caused by reduced reproduction by young-of-year females rather than by changes in survival, and these phenomena may be better explained by interspecific social interactions than by environmental conditions alone. Increasingly warm winters may shift the competitive balance in this community to favor P. leucopus by allowing this species to establish territories earlier than P. m. gracilis.