title: Conditions favouring hard seededness as a dispersal and predator escape strategy.
abstract: The water-impermeable seed coat of 'hard' seeds is commonly considered a dormancy trait. Seed smell is, however, strongly correlated with seed water content, and hard seeds are therefore olfactionally cryptic to foraging rodents. This is the rationale for the crypsis hypothesis, which proposes that the primary functions of hard seeds are to reduce seed predation and promote rodent seed dispersal. We use a mechanistic model to describe seed survival success of plants with different dimorphic soft and hard seed strategies. The model is based on established empirical-ecological relationships of moisture requirements for germination and benefits of seed dispersal, and on experimentally demonstrated relationships between seed volatile emission, predation and predator escape. We find that water-impermeable seed coats can reduce seed predation under a wide range of natural humidity conditions. Plants with rodent dispersed seeds benefit from producing dimorphic soft and hard seeds at ratios where the anti-predator advantages of hard seeds are balanced by the dispersal benefits gained by producing some soft seeds. The seed pathway predicted from the model is similar to those of experimental seed-tracking studies. This validates the relevance and realism of the ecological mechanisms and relationships incorporated in the model.Synthesis. Rodent seed predators are often also important seed dispersers and have the potential to exert strong selective pressures on seeds to evolve methods of avoiding detection, and hard seeds seem to do just that. This work suggests that water-impermeable hard seeds may evolve in the absence of a dormancy function and that optimal seed survival in many environments with rodent seed predators is obtained by plants having a dimorphic soft and hard seed strategy.
authors.affiliation: Department of Biology, University of Bergen Postboks 7803, 5020, Bergen, Norway.
authors.forename: Torbjørn R
authors.initials: TR
authors.lastname: Paulsen
authors.affiliation: Department of Biology, University of Bergen Postboks 7803, 5020, Bergen, Norway.
authors.forename: Göran
authors.initials: G
authors.lastname: Högstedt
authors.affiliation: Department of Animal and Plant Sciences, University of Sheffield Sheffield, S10 2TN, UK.
authors.forename: Ken
authors.initials: K
authors.lastname: Thompson
authors.affiliation: Department of Biology, University of Bergen Postboks 7803, 5020, Bergen, Norway.
authors.forename: Vigdis
authors.initials: V
authors.lastname: Vandvik
authors.affiliation: Department of Biology, University of Bergen Postboks 7803, 5020, Bergen, Norway.
authors.forename: Sigrunn
authors.initials: S
authors.lastname: Eliassen
authors.affiliation: Department of Biology, University of Bergen Postboks 7803, 5020, Bergen, Norway.
authors.forename: Michelle
authors.initials: M
authors.lastname: Leishman
keywords: dispersal; granivore; mechanistic model; olfaction; physical dormancy; rodent; seed predation