Seed size variation and coexistence: testing a novel hypothesis

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Plant species compete for nutrients, water and light, yet differ in competitive ability. Ecological theory predicts that the superior competitor should drive inferior competitors to local extinction (i.e. the best fighter wins!) Yet, plant communities contain many species with varying competitive abilities, so a central question in ecology is: How do competing species coexist in nature? For example, co-occurring plant species differ dramatically in seed size, and larger seeded species are often better competitors and also sometimes better at tolerating stressful conditions than smaller seeded species. Given these inequalities, how is seed size variation maintained? A leading theory is that small-seeded species have better dispersal abilities than large-seeded species, enabling them to reach local sites where large-seeded species are absent. This theory, however, ignores the fact that large-seeded species often have substantially more of their seeds eaten by rodents than do small-seeded species. Thus, this research will experimentally test the novel idea that large and small-seeded species coexist because rodent seed predators preferentially feed on large seeded species, thereby countering the imbalance in competitive ability among these species based on their seed size. This research will generate a deeper understanding of the mechanisms that promote coexistence and a fuller appreciation for the ecological forces that determine the relative abundance and diversity of species in communities. This work will also be used to increase public understanding of grassland ecology and enhance knowledge of how plant communities are organized, which is essential for effectively managing these habitats. Seed size and number have long been considered central to coexistence strategies, yet empirical evidence showing how these traits are shaped by specific coexistence mechanisms remains scarce. Within sites, seeds of 18 co-occurring forb species (9 small-seeded, 9 large-seeded) will be added to plots with and without resident bunchgrass competitors to test how species vary in competitive tolerance. Across 12 sites that vary in productivity and other abiotic factors, seeds of the same focal species will be added to plots at different densities where competition from resident dominant bunchgrasses and rodent seed predation are manipulated. This will allow an evaluation of how competition, seed predation, and ability to handle stressful abiotic conditions influences recruitment functions of species with different seed sizes. It will also enable an exploration of whether seed size is part of a suite of correlated traits that form "syndromes" that predict demographic performance under distinct biotic and/or abiotic conditions.