Invasive, alien species are frequently considered superlative competitors, negatively impacting native species, usually through competition for nutrients, water, and/or space. Competition for pollination is a neglected factor which may affect the competitive ability of self-incompatible native plant species. Lythrum salicaria (purple loosestrife) is an invasive, alien plant that is sympatric with the native L. alatum (winged loosestrife) throughout their northern range. These self-incompatible, heterostylous species rely on pollinators for seed set. We used a randomized block design to determine if L. salicaria affects seed set in L. alatum and if pollinators visit L. salicaria more frequently when both species are present. This experiment was utilized in two ways. First, seed set for each plant was determined based on a sub-sample of 15 fruits per plant at the end of the growing season. These data indicate that presence of L. salicaria significantly decreases seed set in L. alatum. The second aspect of our experiment involved the observation of pollinator visitation for each array. Three rounds of observation were conducted at 2-3 week intervals. During each round every array was observed for 3-4 15-minute periods. These data indicate that pollinator visitation was significantly different for L. alatum when L. salicaria was present both in visitation rate and in number of approaches to a plant. Floral display and pollinator activity increased dramatically between early July and late August. Pollinators frequently moved between the two species and virtually all pollinators collected pollen rather than nectar. Our results indicate that the invasive L. salicaria can have strong negative impacts on native species simply by altering the pollination environment, and that invasive plants may have more wide-spread effects on natural communities than had previously been suspected.

Key words: invasive spedies, Lythrum alatum , Lythrum salicaria, pollination, pollinator visitation