An Ecological Dipole in North American Mast-Eating Small Mammal Dynamics.

Ecological dipoles are the opposite patterns in animal and plant population indices over time in geographically distant areas. Ecological dipoles are recently described macroscale patterns in animal and plant population dynamics, and have been uncovered in mast seeding and in migration patterns of mast-eating birds at continental scales. It has been predicted that ecological dipoles occur in a range of taxa. We tested for patterns and drivers of spatiotemporal synchrony in the dynamics of hard-mast eating small mammal populations, along with co-located climate variables hypothesized to affect small mammal populations directly or indirectly through driving seed availability. We used box-trapping records from 2013 to 2022 for nine mast-eating small mammal species from 23 forested National Ecological Observatory Network (NEON) sites in the contiguous United States spanning distances over 4000 km. We found an ecological dipole in small mammal population dynamics, with a decay in synchrony as the distance between pairs of NEON sites increased, with anti-synchrony in small mammal dynamics at sites separated by > 2000 km. Site proximity, direct climate variables, and indirect climate variables (i.e., time-lagged responses to mast-seeding dynamics) were associated with the spatiotemporal synchrony in small mammal abundance. Our study supports the prediction that ecological dipoles may be a generalizable continental-scale pattern for multiple taxa.