Data from: Rodent seed predators and a dominant grass competitor affect coexistence of co-occurring forb species that vary in seed size

1. Propagule size and number often vary by several orders of magnitude among co-occurring plant species. Explaining the maintenance of this variation and understanding how propagule size contributes to coexistence remains a central challenge for community ecologists. The dominant paradigm is that a competition-colonization trade-off maintains interspecific variation in seed size, but empirical support is limited and other coexistence mechanisms, such as size-dependent seed predation, have not been examined.
2. We examined how seed size, fecundity, and other functional traits of 18 co-occurring perennial forbs trade-off with both vulnerability to rodent seed predation and competitive response to a community dominant perennial bunchgrass. We added seeds of these species to 1 m2 plots at 10 sites where we factorially manipulated rodent seed predation and competition from the community dominant, Festuca campestris. For a given plot, seeds of each species were added at densities that reflected the fecundity (i.e. per capita seed production) for each species. Moreover, we varied total seed density among plots by adding seeds at one of five relative densities (0, 0.25. 0.5, 1.0 and 1.5 times each species’ fecundity) to examine how fecundity affected overall recruitment for each species.
3. There was a trade-off between seed size and fecundity, as expected, but larger seed size was also associated with greater plant height, lower C/N ratios, and lower water use efficiency, suggesting that seed size represented a “resource acquisitive” trait syndrome (as quantified by trait principal component scores).
4. In the field experiment, the suppressive effects of seed predation on seed recruitment rate increased with increasing seed size. In contrast, smaller-seeded species with less resource acquisitive traits were more negatively affected by competition from F. campestris than were species with more resource acquisitive traits.
5. Synthesis. While the competition-colonization trade-off has been the predominant mechanism thought to maintain coexistence among species that vary in fecundity and seed size, our work suggests that susceptibility to rodent seed predation and competitive response to community dominants represent alternative mechanisms that can differentially influence plant recruitment of species based on their seed size and associated traits.

1. 同域共存的植物物种间，繁殖体（propagule）大小与数量通常存在数个数量级的差异。阐明该变异的维持机制，并解析繁殖体大小如何促进物种共存，至今仍是群落生态学家面临的核心挑战。当前主流范式认为，竞争-定植权衡（competition-colonization trade-off）维持了种子大小的种间变异，但相关实证支持十分有限，而其他共存机制（例如依赖种子大小的种子捕食）尚未得到研究。
2. 本研究针对18种共存的多年生杂类草（perennial forbs），探究了其种子大小、繁殖力及其他功能性状，分别与啮齿类种子捕食易感性、以及对群落优势多年生丛生禾草的竞争响应之间的权衡关系。我们将各物种的种子播撒至10个样地的1平方米样方中，在每个样地内采用析因实验设计，操控啮齿类种子捕食与群落优势种草原羊茅（Festuca campestris）带来的竞争压力。在单个样方中，各物种的种子播撒密度依据其各自的繁殖力（即单株种子产量）设定。此外，我们通过设置五种相对种子播撒密度（分别为各物种繁殖力的0、0.25、0.5、1.0及1.5倍），调整各样地间的总种子播撒密度，以探究繁殖力对各物种总种子补充更新率的影响。
3. 正如预期，种子大小与繁殖力之间存在权衡关系；同时，更大的种子尺寸同时与更高的植株高度、更低的C/N比及更低的水分利用效率显著相关，这表明种子大小可作为“资源获取型（resource acquisitive）”性状综合征的量化表征（通过性状主成分得分进行量化）。
4. 在野外实验中，种子捕食对种子补充更新率的抑制作用随种子尺寸增大而增强。与之相反，种子更小且资源获取型性状较弱的物种，相较于资源获取型性状较强的物种，受到草原羊茅竞争带来的负面影响更为显著。
5. 综合结论：尽管竞争-定植权衡（competition-colonization trade-off）一直被认为是维持繁殖力与种子大小存在变异的物种间共存的主流机制，但本研究表明，对啮齿类种子捕食的易感性以及对群落优势种的竞争响应，可作为替代共存机制，依据物种的种子大小及相关性状差异，对不同物种的植物补充更新产生差异化影响。