Biology Department, 572 Rivers Street, Appalachian State University, Boone, North Carolina 28608, USA.
Ecology. 2012 Jul;93(7):1581-92. doi: 10.1890/11-1271.1.
Partner abundance affects costs and benefits in obligate mutualisms, but its role in facultative partnerships is less clear. We address this gap in a pollination web consisting of two clovers (Trifolium) that differ in specialization on a bumble bee pollinator Bombus balteatus. We examine how pollination niche breadth affects plant responses to pollinator abundance, comparing early-flowering (specialized) and late-flowering (generalized) cohorts of T. parryi and early T. parryi to T. dasyphyllum, a pollination generalist. Co-pollinators disrupt the link between B. halteatus visitation and pollination rate for both clovers. Only for early-flowering T. parryi do visitation, pollination, and seed set increase with density of B. balteatus. Bumble bee density also alters timing of seed germination in T. parryi, with seeds from plants receiving augmented B. balteatus germinating sooner than seeds of open-pollinated counterparts. Benefits saturate at intermediate bumble bee densities. Despite strong effects of B. balteatus density on individual plant fitness components, population models suggest little impact of B. balteatus density on lamda in T. parryi or T. dasyphyllum. Findings show that functional redundancy in a pollinator guild mediates host-plant responses to partner density. Unexpected effects of pollinator density on life history schedule have implications for recruitment under pollinator decline.
伙伴丰度影响专性互利共生的成本和收益，但在兼性伙伴关系中其作用尚不明确。我们在一个由两种三叶草（Trifolium）组成的传粉网络中解决了这一空白，这两种三叶草在熊蜂传粉者 Bombus balteatus 上的专业化程度不同。我们研究了传粉生态位宽度如何影响植物对传粉者丰度的反应，比较了早花（专业化）和晚花（泛化）T. parryi 以及早 T. parryi 与 T. dasyphyllum 的植物反应，后者是一种传粉泛化种。共同传粉者破坏了 B. halteatus 访问与两种三叶草传粉率之间的联系。只有早花 T. parryi 的访问、传粉和结实率随 B. balteatus 密度的增加而增加。熊蜂密度也改变了 T. parryi 中种子萌发的时间，接受增强的 B. balteatus 的植物的种子比开放授粉对照植物的种子更早萌发。收益在中等熊蜂密度下饱和。尽管 B. balteatus 密度对个体植物适合度成分有强烈影响，但种群模型表明 B. balteatus 密度对 T. parryi 或 T. dasyphyllum 的 lambda 影响很小。研究结果表明，传粉者群体中的功能冗余介导了宿主植物对伙伴密度的反应。传粉者密度对生活史时间表的意外影响对传粉者减少下的繁殖有影响。
