Escape in space: Effects of density and distance from invasive vegetation on post-dispersal seed-consumption of congeneric lupines

Background/Question/Methods

Lupinus tidestromii is an endangered, coastal dune plant that is threatened by habitat loss and consumption by native deer mice, Peromyscus maniculatus, whose density is inflated by the presence of the invasive beach grass, Ammophila arenaria. In several populations, L. tidestromii, a low-growing perennial herb, co-occurs with its widespread congener, L. chamissonis, a long-lived shrub. Due to their differing architectures and life-histories, L. tidestromii is disproportionately affected by pre-dispersal seed consumption. L. chamissonis may be relatively more affected by post-dispersal seed consumption because its seeds are larger and more numerous. To determine if the two species differed in their rates of post-dispersal seed consumption we established a field experiment in the largest population of L. tidestromii. We installed 64 wire mesh cages in two blocks and applied the following treatments: predation level (small mammals excluded or allowed), density (high versus low), distance to beachgrass (near versus far), and species (rare versus common). Seeds were sown in August 2009, germination was recorded every three weeks between January to June 2010, and remaining seeds were sifted and counted in July 2010. We estimated seed consumption as the proportion of seeds not accounted for by these methods.

Results/Conclusions

We found that post-dispersal seed consumption was higher in the high density treatment suggesting that mice are motivated by seed density. We found a significant distance by species interaction effect: while L. chamissonis experiences high consumption both near and far from beach grass, L. tidestromii experiences reduced predation with increased distance from A. arenaria (an escape in space). This may be because mice risk traveling further for large L. chamissonis seeds. Population models are needed to evaluate how observed differences between the species in pre- and post-dispersal seed consumption translate into differences in overall reproductive success and population growth rates. Our results are evaluated in light of a current restoration project that will remove A. arenaria from the native dune community at our study site.