Distance-dependent effects of invasive Lupinus polyphyllus on pollination and reproductive success of two native herbs

Abstract

A comprehensive understanding of the effects of invasive plants on native species requires identification of both the mechanisms of interaction and the spatial scale over which they act. Indirect interactions involving mobile organisms such as pollinators are likely to be scale-dependent, yet most studies examining effects of invasive species on pollination of native plants have considered effects across a single distance between interacting species. We examined the effects of the invasive herb Lupinus polyphyllus on pollination of two native herbs using multiple distances between the invasive and the natives. We recorded pollinator visitation and seed production in the native herbs Lotus corniculatus and Lychnis viscaria at 0, 5m or 200m away from L. polyphyllus. To reduce the influence of confounding factors, we used experimentally established populations of the invasive and potted individuals of the natives. In the immediate vicinity to L. polyphyllus, visitation to L. corniculatus was higher than 200m away, and seed production per flower was higher than 5m and 200m away. In L. viscaria, bumblebee visitation was higher adjacent to L. polyphyllus than 5m and 200m away, but total pollinator visitation and reproductive success did not vary with distance. The results indicate that L. polyphyllus facilitates pollination of the native plants, and that this occurs at a very local spatial scale as effects dropped off already at a distance of 5m. Presence of L. polyphyllus could benefit both pollinators and pollination of native herbs, and these positive effects should be considered along with likely negative effects due to resource competition. Moreover, the results illustrate the necessity to consider scale-dependent effects when assessing the impact of invasive flowering plants on native pollination interactions.