Invasive plants with large flowering displays have been shown to compete with native plants for pollinator services, often to the detriment of native plant fitness. In this study, we compare the pollinator communities and pollen deposited on stigmas of native plant species within and away from stands of the invasive alien plant, leafy spurge (Euphorbia esula) at a large natural area in North Dakota, USA. Specifically, we ask if infestation influences (1) visitation rates and taxonomic composition of visitors to native flowers, and (2) the amount of conspecific pollen, number of pollen species, and proportion of heterospecific pollen on stigmas of native plants. We observed visits to selected native species during May and June 2000 and 2001. Stigmas were collected from a subsample of the flowers within these plots, squashed, and the pollen identified and counted under a light microscope. Visitation varied between years and among species of native plants: infestation had mixed effects in 2000 but visitation, especially by halictids was always lower within infestations in 2001. Despite differences in visitation between years, we found significantly less conspecific pollen on stigmas from infested plots in six of eight cases; we never found significantly more conspecific pollen on stigmas from within infestations. Our results emphasize the temporal variability in plant-pollinator relations and the added complexity imposed by an invasive species that will always make prediction of effects difficult. Nonetheless, the consistently lower conspecific pollen counts on native stigmas within infestations, regardless of visitation, suggest the likelihood of negative effects.
Bibliographical noteFunding Information:
We gratefully acknowledge field assistance provided by A. Haas, G. Hanley, D. Lang, J. Larson, J. Plummer, P. Scherr and L. Van Riper and pollen counting by J. Larson and K. Jacobson. D. Yanega identified voucher specimens of bees. We are grateful for comments on a previous draft by S. Huerd, N. Jordan, R. Mitchell, C. Reed, N. Waser, two anonymous reviewers, and members of the Jordan and Larson lab groups, that significantly improved the manuscript. We thank staff at Theodore Roosevelt National Park who provided logistical support for this study and for many others over the years. Funding was provided by the US Geological Survey’s Park Oriented Biological Support Program and Northern Prairie Wildlife Research Center.
- Biological invasion
- Euphorbia esula
- Mixed-grass prairie
- Pollen on stigmas