Sandy seeds: Armour or invisibility cloak? Mucilage‐bound sand physically protects seeds from rodents and invertebrates

Abstract

Abstract Seeds represent a stage of a plant's life cycle that is extremely vulnerable to predation, which unlike most vegetative herbivory, is fatal to the individual. As such, understanding the distribution and abundance of plants may rely on understanding seed defences: characteristics that make seeds more difficult for granivores to locate, less beneficial for granivores to consume or both. Seeds that produce mucilage are widespread, found across many families and species. Although short‐term (single day) studies indicate that mucilage may be a fundamental seed defence found throughout the plant kingdom, it is not clear to what extent mucilage provides long‐term benefits to seeds by reducing granivory. Moreover, it is not clear whether this long‐term defence occurs because mucilage reduces seed apparency as substrate‐coated seeds are more difficult to detect, whether substrate‐coated seeds are less valuable or both. In the first field experiment, we factorially manipulated sand coatings, background sand colour and granivore community using feeding depots to test the mechanistic basis of mucilage‐bound sand as a seed defence against diverse granivores. We found that the sand coating significantly extended chances of seeds remaining over the 101‐day trial, during which rodents were the primary granivores, and our coloration manipulation suggests that the effect was almost entirely physical. We found that whether a seed's sand coating matched its background did not greatly affect chance of removal, leaving the crypsis mechanism unsupported. We conducted a follow‐up field experiment to test the background‐matching hypothesis in more detail, using 20 sand colours and two colours of flax seeds. We again found no support for crypsis, corroborating the finding that mucilage‐bound sand provides a primarily physical defensive benefit. Synthesis. The totality of the results from this and previous studies across granivore taxa and plant species suggests that seed mucilage is a widespread and highly effective physical defence employed by many plants against diverse granivores in many environments