Morphometric influence on pollinator sharing between native and invasive plant species along the north spit of Humboldt Bay, California

Although many studies have investigated the effect of invasive plants on native plant species, few studies have considered their effect on pollination success. Since invasive plants potentially compete with native plants for pollinator services, the first step in assessing the potential for competition is to determine the degree of overlap in pollinator assemblages. I examined the degree of pollinator sharing for three pairs of native and invasive plant species over two years across the North Spit of Humboldt Bay, California. Because the degree of morphological similarity might affect the potential for pollinator sharing, I chose pairs of species matched by floral morphology along a spectrum from open to highly specialized flowers. I found considerable potential for competition, as niche overlap values were frequently 70%; a niche overlap index is a measure of the percentage of a resource shared by two species. Morphometric similarity between flowers strongly influenced the degree of sharing, although plants with intermediate floral morphologies experienced more overlap with plants with open or specialized flowers than with the other intermediate plant species. However, morphology alone did not drive the potential for competition, as floral sequence and pollinator preference led to considerable variation in degree of pollinator sharing across years and sites.