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

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 upon understanding seed defenses: characteristics of seeds that make them more difficult for granivores to locate, make them less beneficial for granivores to consume, or both. Seeds that produce mucilage are widespread and found across many families and species. Although short-term (single-day) studies indicate that mucilage may be a fundamental seed defense found throughout the plant kingdom, it is not clear whether mucilage provides long-term benefits to seeds via reducing granivory. Moreover, it is not clear whether this long-term defense occurs because mucus reduces seed apparency because substrate-coated seeds are more difficult to detect, because substrate-coated seeds are of less value, or both. In this field experiment, we factorially manipulated sand coatings, background sand color, and granivore community using feeding depots in order to test the mechanistic basis of mucilage-bound sand as a seed defense against diverse granivores. We found that the sand coating significantly extended seed survival 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 or not a seed’s sand coating matched its background did not greatly affect seed survival, leaving the crypsis mechanism unsupported. A follow-up experiment to test the background-matching hypothesis in more detail, using 20 sand colors and two colors of flax seeds again found no support for crypsis, corroborating the finding that mucilage-bound sand provides a primarily physical defensive benefit. The totality of the results from this and previous studies across granivore taxa and plant species suggests this highly effective defense may be ubiquitous.

种子是植物生命周期中极易遭受捕食的阶段，且与多数营养组织的植食性取食不同，种子被捕食会直接导致个体死亡。因此，要明晰植物的分布与种群丰度，需先理解种子防御机制：即令食种子动物（granivore）难以定位种子、降低其取食收益，或同时兼具两种效果的种子特征。产黏液（mucilage）的种子广泛分布于众多植物科与物种中。尽管短期（单日）研究表明，黏液或许是遍布整个植物界的基础种子防御手段，但目前尚不明确黏液是否能通过降低食种子动物的取食压力，为种子带来长期生存优势。此外，目前仍不清楚这类长期防御效应的产生机制：究竟是基质包裹使种子更难被发现，从而降低了其被识别的显见性，还是包裹后的种子营养价值下降，抑或是两种因素共同作用。本研究通过取食站开展野外析因实验，对种子的沙质包裹层、背景沙色以及食种子动物群落进行操控，以验证黏附黏液的沙粒作为种子防御手段对抗多样食种子动物的作用机制。实验结果显示，在为期101天的试验周期内（此间啮齿类为主要食种子动物），沙质包裹层显著提升了种子存活率；而颜色操控实验的结果表明，该防御效应几乎完全源于物理作用。我们发现，种子的沙质包裹层是否与背景环境颜色匹配，对种子存活率并无显著影响，这一结果不支持隐蔽拟态（crypsis）的作用机制假说。后续实验进一步细化验证背景匹配假说：研究人员选用20种不同颜色的沙粒与两种颜色的亚麻籽开展实验，仍未发现支持隐蔽拟态的证据，从而印证了黏附黏液的沙粒主要通过物理作用实现防御的结论。综合本研究与过往针对不同食种子动物类群和植物物种的研究结果，这种高效的种子防御手段或许广泛存在于植物界中。