Division of Biological Sciences, University of Montana, Missoula, Montana, 59812, USA.
Rocky Mountain Research Station, U.S. Department of Agriculture Forest Service, Missoula, Montana, 59801, USA.
Ecology. 2019 Dec;100(12):e02848. doi: 10.1002/ecy.2848. Epub 2019 Aug 22.
Understanding controls on recruitment is critical to predicting community assembly, diversity, and coexistence. Theory posits that at mean fecundity, recruitment of highly fecund small-seeded plants should be primarily microsite limited, which is indicated by a saturating recruitment function. In contrast, species that produce fewer large seeds are more likely to be seed-limited, which is characterized by a linear recruitment function. If these patterns hold in nature, seed predation that disproportionately affects larger-seeded species can limit their establishment. We tested these predictions by comparing recruitment functions among 16 co-occurring perennial forb species that vary by over two orders of magnitude in seed size. We also assessed how postdispersal seed predation by mice influenced recruitment. We added seeds at densities from zero to three times natural fecundity of each species to undisturbed plots and examined spatial variation in recruitment by conducting experiments across 10 grassland sites that varied in productivity and resource availability. Consistent across two replicated years, most species had linear recruitment functions across the range of added seed densities, indicative of seed-limited recruitment. Depending on year, the recruitment functions of only 19-37% of target species saturated near their average fecundity, and this was not associated with seed size. Recruitment was strongly inhibited by rodent seed predation for large-seeded species but not for smaller-seeded species. Proportional recruitment was more sensitive to spatial variation in recruitment conditions across sites for some small-seeded species than for large-seeded species. These results contradict the common belief that highly fecund small-seeded species suffer from microsite-limited recruitment. Rather, they imply that, at least episodically, recruitment can be strongly correlated to plant fecundity. However, proportional recruitment of small-seeded species was inhibited at productive sites to a greater extent than large-seeded species. Results also show that in a system where the dominant granivore prefers larger seeds, low-fecundity large-seeded species can suffer from even greater seed-limited recruitment than would occur in the absence of predators.
理解繁殖的控制对于预测群落组装、多样性和共存至关重要。理论认为，在平均繁殖力下，高度繁殖的小种子植物的繁殖应该主要受到微生境的限制，这表现为饱和的繁殖函数。相比之下，产生较少大种子的物种更可能受到种子限制，这表现为线性繁殖函数。如果这些模式在自然界中成立，那么不成比例地影响大种子物种的种子捕食可能会限制它们的建立。我们通过比较 16 种同时存在的多年生草本植物物种之间的繁殖函数来检验这些预测，这些物种的种子大小差异超过两个数量级。我们还评估了老鼠在种子散布后的种子捕食如何影响繁殖。我们在未受干扰的斑块中以每个物种自然繁殖力的零到三倍的密度添加种子，并通过在 10 个草原地点进行实验来评估繁殖的空间变化，这些地点在生产力和资源可用性上有所不同。在两年的两个重复实验中，大多数物种在添加种子密度的范围内都表现出线性繁殖函数，表明繁殖受到种子的限制。根据年份的不同，只有 19-37%的目标物种的繁殖函数在其平均繁殖力附近饱和，而这与种子大小无关。对于大种子物种，啮齿动物的种子捕食强烈抑制了繁殖，但对于小种子物种则没有。对于一些小种子物种，相对于大种子物种，对空间上不同的繁殖条件的比例繁殖对站点的变化更为敏感。这些结果与高度繁殖的小种子物种遭受微生境限制的普遍观点相矛盾。相反，它们表明，至少在某些情况下，繁殖可以与植物繁殖力密切相关。然而，小种子物种的比例繁殖在生产力较高的地点受到的抑制比大种子物种更为严重。结果还表明，在一个优势食草动物更喜欢大种子的系统中，低繁殖力的大种子物种可能会受到比没有捕食者时更大的种子限制繁殖的影响。
