Do empty Juniperus communis seeds defend filled seeds against predation by Apodemus sylvaticus?

The juniper Juniperus communis, a dominant plant in the high mountains of SE Spain, produces a high proportion of empty seeds within well-developed cones. We tested the hypothesis that the production of empty seeds by jumper reduces seed predation by the woodmouse Apodemus sylvaticus, thereby benefitting the plant. We performed laboratory and field experiments to determine i) woodmouse discrimination ability between filled and empty seeds, and ii woodmouse response to changes in the proportion of empty versus filled seeds and in the seed density in seed clusters. In addition, we estimated, for six juniper populations over three years, whether plants or populations showing a higher proportion of empty seeds suffered reduced woodmouse predation. Experiments showed that woodmice can eventually discriminate externally between filled and empty seeds, but in most cases had to bite the seeds to identify and reject empty ones. The probability of predation for filled seeds was independent of changes in the proportion of empty versus filled seeds and in seed density per cluster. Seed predation suffered by plants in the field was unrelated to the proportion of empty seeds per plant for all populations and years. The presence of empty seeds did not benefit juniper against woodmouse predation, either in terms of individual reproductive output or in terms of offspring escape probability. Our study suggests that the effect of empty seeds on seed predation should be rare in plant-seed predator interactions where predators are polyphagous and eventually able to discriminate against empty seeds, and therefore suffer a low cost when coping with empty seeds.