Data from: Scale-dependent shifts in the species composition of flower visitors with changing floral density

Responses of flower-visiting animals to floral density can alter interactions between plants, influencing a variety of biological processes, including plant population dynamics and the evolution of flowering phenology. Many studies have found effects of floral or plant density on pollinator visitation rates at patch scales, but little is known about responses of flower visitors to floral densities at larger scales. Here, I present data from an observational field study in which I measured the effects of floral density on visitation to the annual composite Holocarpha virgata at both patch (4 m(2)) and site (12.6 ha) spatial scales. The species composition of flower visitors changed with floral density, and did so in different ways at the two scales. At the site scale, average floral density within patches of H. virgata or within patches of all summer-flowering species combined had a significant positive effect on per-flowerhead visitation by the long-horned bee Melissodes lupina and no significant effects on visitation by any other taxa. At the patch scale, per-flowerhead visitation by honeybees significantly increased whereas visitation by M. lupina often decreased with increasing floral density. For both species, responses to patch-scale floral density were strongest when site-scale floral density was high. The scale-dependence of flower visitor responses to floral density and the interactions between site- and patch-scale effects of floral density observed in this study underscore the importance of improving our understanding of pollinators' responses to floral density at population scales.

访花动物对花卉密度的响应可改变植物间的相互作用，进而影响包括植物种群动态、开花物候演化在内的诸多生物过程。诸多研究已在斑块尺度上探明了花卉或植物密度对传粉者访花频率的影响，但针对大空间尺度下访花动物对花卉密度的响应，目前仍所知有限。本研究基于野外观测实验获取相关数据，分别在斑块（4 m²）与样地（12.6 ha）两个空间尺度下，测定了花卉密度对一年生菊科植物Holocarpha virgata的访花情况所产生的效应。访花者的物种组成随花卉密度发生变化，且在两种尺度下呈现出截然不同的变化模式。在样地尺度上，Holocarpha virgata斑块或所有夏季开花物种合并斑块内的平均花卉密度，对长角蜂Melissodes lupina的单头花访花频率具有显著正向影响，而对其余类群的访花频率无显著作用。在斑块尺度上，蜜蜂的单头花访花频率随花卉密度升高显著提升，而Melissodes lupina的访花频率则多随密度增加呈下降趋势。当样地尺度的花卉密度处于较高水平时，两种蜂类对斑块尺度花卉密度的响应强度均达到峰值。本研究发现访花动物对花卉密度的响应存在尺度依赖性，同时证实了花卉密度在样地与斑块尺度上的效应存在交互作用，这凸显了在种群尺度上加深对传粉者响应花卉密度机制理解的重要性。