The invasive aquatic plant Ludwigia grandiflora affects pollinator visitants to a native plant at high abundances

Iris Stiers,Ludwig Triest,Kjell Coussement

doi:10.3391/ai.2014.9.3.10

Abstract

The presence of an invasive species can either have a negative effect on pollination of natives by competing for pollinators or a positive effect since they may act as ‘magnet’ species facilitating pollinator visits to co-flowering species. We studied the plant-pollinator interactions for Ludwigia grandiflora, a highly invasive aquatic weed in Europe. First, a food web approach was used in one field site and our results showed an integration of L. grandiflora into the native plant-pollinator network with a dominance of L. grandiflora in terms of frequency of pollinator visits. Second, an experiment was designed to identify the pollinator guild of invasive L. grandiflora and native Lythrum salicaria and to measure interspecific pollinator switching. We also estimated the pollinator-mediated effect of high relative abundance of L. grandiflora (% cover of L. grandiflora) on L. salicaria plants. We monitored species composition, abundance and foraging behaviour of pollinators on L. salicaria. In addition, we assessed seed set per fruit of L. salicaria. Competition for pollinator services between invasive L. grandiflora and native L. salicaria seems minor as there was no evidence for decreased pollinator visitation or seed set of L. salicaria. On the contrary, more pollinators were recorded on L. salicaria plants when the cover of L. grandiflora was high compared to the control plants thereby indicating a facilitation effect, however this was not reflected in seed set. Despite the fact that L. grandiflora is well integrated in the native plant-pollinator network and highly attractive to pollinators, there was no evidence of negative impact of L. grandiflora on pollination of a native plant.

Highlights

Pollination is one of the most important mechanisms contributing to biodiversity conservation and, like many ecosystem services, it can be influenced by alien invasive plants (Vilà et al 2010)
We address the following questions: 1) Is alien L. grandiflora integrated in the native plant-pollinator network? 2) What is the degree of pollinator overlap between invasive L. grandiflora and native Lythrum salicaria? 3) Does the high abundance of L. grandiflora influence pollinator visits and pollinator foraging behaviour on native L. salicaria and is this reflected in seed set? 4) Do pollinators move frequently between the two species?
A total of 1992 interactions were recorded among seven species of flowering plants (Ludwigia grandiflora, Lythrum salicaria, Alisma plantagoaquatica, Myosotis scorpioides, Scutellaria galericulata, Mentha aquatica and Lotus corniculatus) and six categories of pollinators (Figure 3)

Summary

Introduction

Pollination is one of the most important mechanisms contributing to biodiversity conservation and, like many ecosystem services, it can be influenced by alien invasive plants (Vilà et al 2010). Recent studies have included effects of density of invasive species to try and disentangle the impacts of showy invaders on floral visitation and/or reproductive output of natives (Muñoz and Cavieres 2008; Flanagan et al 2010; McKinney 2010). The majority of these studies showed that the presence of an invasive species lowered the visitation rate and/or seed output of the native only at high abundance. A high degree of pollinator sharing can lead to interspecific pollen transfer when pollinators switch between flowers of different species during single foraging bouts or flights This can lead to heterospecific pollen deposition (HPD) and⁄or conspecific pollen loss (CPL) (Morales and Traveset 2008). Jakobsson et al (2008) monitored HPD from invasive Carpobrotus spp. to natives and found a reduced seed production of the native Helichrysum stoechas probably due to stigma clogging

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