Pollinator Visits to Threatened Species Are Restored Following Invasive Plant Removal

Abstract

An indirect consequence of plant invasions is the disruption of native plant-pollinator interactions. We examined effects of invasive baby’s breath (Gypsophila paniculata) and spotted knapweed (Centaurea maculosa) on floral visitors to federally threatened Pitcher’s thistle (Cirsium pitcheri) in Lake Michigan dunes. In sweep net surveys, abundances of pollinator taxa were five times higher in invaded than in naturally invader-free sites. However, plot-level G. paniculata removal treatments increased pollinator visits to C. pitcheri relative to invaded plots and restored visitation to levels found in naturally uninvaded plots. Invader removal also increased native plant species richness, which was positively correlated with pollinator visitation to C. pitcheri, suggesting an indirect effect on pollinators mediated through invader-altered plant composition. In temporary floral arrays, the rate of pollinator visitation to C. pitcheri was not affected by neighbor plant species identity. However, compared with native Monarda punctata, invasive C. maculosa attracted more total pollinators to the array but reduced the proportion of total visits that were to C. pitcheri and increased pollinator movements between plant species. While both G. paniculata and C. maculosa appear to act as magnet species by attracting more pollinators at the plot level, these invaders have the potential to reduce reproduction of C. pitcheri by decreasing pollinator visits and increasing interspecific pollen transfer.