The impact of an invasive species (Lythrum salicaria) on pollination and reproduction of a native species (L. alatum)

Invasive species are regarded as a major cause of reduction in global biodiversity, ranked second only to habitat destruction. Lythrum salicaria , a notorious wetland invasive, co-occurs with L. alatum , the most common native Lythrum in North America. Where these species co-occur, they are similar in height, bloom concurrently for approximately six weeks, and share pollinators. This research asks four questions: (1) Is seed set in L. alatum reduced when L. salicaria is nearby? (2) Do visit quantity and visit quality play roles in seed set reduction in L. alatum? (3) Does the presence of L. salicaria pollen on L. alatum stigmas reduce seed set? and, (4) Does seed set reduction occur even when L. alatum is at a distance from L. salicaria ?; The first and second questions were addressed using experimental arrays of plants with eight, sixteen and 24 L. alatum or 8:8 and 8:16 L. alatum:L. salicaria treatments. Seed set, visitation rate, and interspecies movements were determined within the experimental arrays for two growing seasons. Lythrum alatum suffers significantly reduced seed set in the presence of L. salicaria . The pollination visitation rate in L. alatum is significantly reduced by 33% when L. salicaria is nearby. In mixed treatments, pollinators readily move between species. Using fluorescent dye I confirmed that interspecies movement resulted in pollen from L. salicaria anthers reaching L. alatum stigmas.; To answer question 3, I handpollinated L. alatum stigmas with 100% legitimate pollen, a 50/50% mixed pollen load and 100% L. salicaria pollen. The mixed loads reduced seed set in L. alatum by 28.8% with a few of the 100% L. salicaria pollinations resulting in putative hybrid formation. Using epifluorescence microscopy, I determined that ovule usurpation is not a cause of this reduction. With regard to question four, potted plants of L. alatum placed at increasing distance from L. salicaria populations showed a trend toward increased seed set with distance from L. salicaria . The impact of invasive species on the native flora may be much greater than previously anticipated when competition for pollination and/or pollination quality are considered.