Small mammal controls on the climate-driven range shift of woody plant species

Climate change is resulting in shifts in species' ranges as species inhabit new climatically suitable areas. A key factor affecting range-shifts is the interaction with predators. Small mammals, being primary seed predators and dispersers in forest ecosystems, may play a major role in determining which plant species will successfully expand and the rate at which range-shifts will occur. Plants dispersing seeds beyond the species' current range limits will encounter seed predators to which these seeds are novel; however, empirical studies of seed predator-novel seed interactions are lacking. The aims of our study were to: 1) quantify seed selection by small mammals presented with 'novel' seeds; 2) quantify the post-selection fate of 'novel' seeds; and 3) identify seed traits that affect seed selection and post-selection seed fate. We designed a field experiment exposing small mammal communities to novel seeds produced by plants expected to shift their ranges in response to climate change. We matched novel seeds with reference 'familiar' seeds and studied key steps defining interactions between small mammals and novel seeds. We found that the probability of selection of a novel seed varied among species and was, at times, higher than the selection probability of familiar seeds. Key traits that affected seed selection and the distance a seed was dispersed for caching were shell hardness and seed mass. We also found that 33% of dispersed seeds were cached in optimal germination sites (e.g. within fallen logs and buried under the leaf litter mat). Through seed emergence trials we found that emergence was higher for larger seeds, suggesting that the role of small mammals may be modulated by emergence rates. Our results suggest that the interaction between small mammals and novel seeds may have cascading effects on climate-induced plant range shifts and community composition.