Conifer seed predation by terrestrial small mammals: A review of the patterns, implications, and limitations of top-down and bottom-up interactions

• Small mammal–conifer seed interactions have critical multi-trophic effects in forests.
• Terrestrial small mammals are the predominant post-dispersal conifer seed predators.
• Conifers utilize toxins, physical defenses, and mast seeding to reduce seed predation.
• Unlike deciduous forests, conifer masting has inconsistent effects on small mammals.
• Individual- and population-level adaptations by small mammals mitigate seed defenses.

Conifer seeds are an important component of the diet of many terrestrial small mammals, and seed predation by small mammals is often implicated as a major limiting factor in the recruitment and regeneration processes in coniferous forests. However, studies examining the effects of conifer seed quality and availability on individual small mammals and their populations are scarce. This interaction has important ecological implications at multiple trophic levels in forest ecosystems, given the considerable differences in seed defenses among associated conifer species, the prevalence of mast seeding as a pulsed resource in coniferous forests, and the persistent, cascading effects of seed–small mammal interactions observed in other forest systems. Here, I comprehensively review the patterns, implications, and limitations of the literature on post-dispersal seed predation by terrestrial small mammals (mice and voles of families Cricetidae and Muridae) in coniferous forests. This paper mainly focuses on North American forests, since studies on this topic are predominantly in this region, but relevant examples from Europe and Asia are also provided. I present a summary of the identities and impacts of competing conifer seed predators at both the pre- and post-dispersal stages. I also review the various physical, chemical, and population-level defensive strategies utilized by conifers to limit seed predation, and how the foraging and population dynamics of small mammals are affected by these defenses. Special emphasis is placed on the population responses of terrestrial small mammals to conifer mast seeding, a resource pulse–consumer interaction that can provide critical insights into the effects of resource availability on the regulation of behaviors, populations, and communities. I then discuss the behavioral, morphological, biochemical, and population-level mechanisms that seed predators use to overcome plant- and seed-defenses, focusing primarily on conifer seeds and terrestrial small mammals, but literature on other types of seeds and seed predators are included when relevant. Finally, I describe remaining key limitations and knowledge gaps in the current conifer–small mammal literature, and future research avenues that would broaden our understanding of the top-down and bottom-up interactions between conifer seeds and small mammals at basic, applied, and evolutionary levels.