1. Despite theoretical advances, the ecological factors and functional traits that enable species varying in seed size and fecundity to coexist remain unclear. Given inherent fecundity advantages, why don't small‐seeded species dominate communities?
2. In perennial grasslands, we evaluated whether small‐seeded species are less tolerant of competition from the community dominant bunchgrass than large‐seeded species, but also less vulnerable to seed predation by mice. We also explored whether trade‐offs involving competitive tolerance include two other functional traits, height and leaf mass per area (LMA). We added seeds of 17 forb species to plots where bunchgrass competition and rodent seed predation were manipulated across sites varying in bunchgrass productivity and thus competitive intensity. Seeds were added at densities mimicking interspecific variation in fecundity among target species.
3. Standardizing for differences in fecundity (i.e. seed input, which enabled us to evaluate inherent interspecific differences in susceptibility to biotic interactions), bunchgrass competition more greatly reduced recruitment and establishment of small‐ versus large‐seeded species, whereas rodent seed predation more greatly reduced the recruitment of large‐ versus small‐seeded species. Plant height and LMA were unrelated to the competition effect size.
4. Small‐seeded species abundance decreased across sites increasing in bunchgrass productivity, whereas this was not the case for large‐seeded species. For adult plants but not seedlings, community‐weighted functional trait means (CWM) for seed size, height and LMA increased in plots with versus without bunchgrass competition and the CWM for seed size and height also increased at sites with greater bunchgrass productivity (for adults only). In contrast, rodent seed predation had no significant effects on CWM seed size.
5. At the end of the experiment, adult abundance positively correlated with plant fecundity in plots lacking bunchgrass, indicating the inherent advantages accrued to high fecundity small‐seeded species. However, with bunchgrass competition, abundances were equalized across species due to reduced competitive tolerance of high fecundity small‐seeded species.
6. Synthesis. Our results suggest that coexistence among subordinate forb species varying in seed size and fecundity is in‐part due to a trade‐off involving competitive tolerance and fecundity, mediated by seed size and associated functional traits.

中文翻译：

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种子大小和生物相互作用之间的权衡有助于繁殖力不同的共生物种的共存

1. 尽管在理论上取得了进步，但尚不清楚使种子大小和繁殖力不同的物种共存的生态因素和功能特征。鉴于固有的繁殖力优势，为什么小种子物种不能统治社区？
2. 在多年生草地上，我们评估了小种子物种是否比大种子物种更不耐受来自社区优势束草的竞争，而且是否更不容易受到小鼠的种子捕食。我们还探讨了涉及竞争耐受性的权衡是否包括其他两个功能性状，即身高和单位面积叶片质量（LMA）。我们在田间添加了17种Forb物种的种子，在不同草丛生产力和竞争强度的不同地点进行了草丛竞争和啮齿动物种子捕食的操作。以模拟目标物种间繁殖力种间差异的密度添加种子。
3. 对繁殖力差异进行标准化（即种子输入，这使我们能够评估生物相互作用的内在固有差异），束草竞争更大程度地减少了小种子和大种子物种的招募和建立，而啮齿动物的种子捕食则大大减少了大种子和小种子物种的招募。株高和LMA与竞争效应的大小无关。
4. 小种子物种的丰度在不同地点降低，束草生产力提高，而大种子物种却并非如此。对于成年植物但不包括幼苗，在有无束草竞争的地块中，种子大小，高度和LMA的社区加权功能性状均值（CWM）有所增加，在束草生产力更高的地方，种子大小和高度的CWM也有所增加（对于成年人只要）。相比之下，啮齿动物的种子捕食对CWM种子大小没有显着影响。
5. 在实验结束时，成年丰度与缺乏束草地块的植物繁殖力呈正相关，表明高繁殖力小种子物种具有固有优势。但是，在束草竞争中，由于高繁殖力小种子物种的竞争耐受性降低，物种间的丰度均等。
6. 综合。我们的结果表明，种子大小和繁殖力不同的次生福布斯物种之间的共存部分是由于种子大小和相关功能性状介导的涉及竞争耐受力和繁殖力的权衡。