Contrasting effects of invasive plants in plant–pollinator networks

Abstract

The structural organization of mutualism networks, typified by interspecific positive interactions, is important to maintain community diversity. However, there is little information available about the effect of introduced species on the structure of such networks. We compared uninvaded and invaded ecological communities, to examine how two species of invasive plants with large and showy flowers (Carpobrotus affine acinaciformis and Opuntia stricta) affect the structure of Mediterranean plant-pollinator networks. To attribute differences in pollination to the direct presence of the invasive species, areas were surveyed that contained similar native plant species cover, diversity and floral composition, with or without the invaders. Both invasive plant species received significantly more pollinator visits than any native species and invaders interacted strongly with pollinators. Overall, the pollinator community richness was similar in invaded and uninvaded plots, and only a few generalist pollinators visited invasive species exclusively. Invasive plants acted as pollination super generalists. The two species studied were visited by 43% and 31% of the total insect taxa in the community, respectively, suggesting they play a central role in the plant-pollinator networks. Carpobrotus and Opuntia had contrasting effects on pollinator visitation rates to native plants: Carpobrotus facilitated the visit of pollinators to native species, whereas Opuntia competed for pollinators with native species, increasing the nestedness of the plant-pollinator network. These results indicate that the introduction of a new species to a community can have important consequences for the structure of the plant-pollinator network.

Ecological impacts of invasive alien species on bees

“…For example, Lopezaraiza-Mikel et al (2007) could find no suitable un-invaded control sites for comparison with their invaded and experimentally cleared sites. Bartomeus et al (2008b) found similar pollinator community richness in invaded and uninvaded plots, but these were only separated by around 300 m. Hence conclusions about the impacts of the invader on the native community are difficult to draw since it is difficult to measure the true native community. In addition, if displacement of native forage resources causes native specialist bees to disappear, there may be a time-lag during which this loss occurs, which may be difficult to detect via short-term alien removal experiments.…”

“…For example, Carpobrotus spp. in California (Aigner, 2004), the Balearic Islands (Moragues and Traveset, 2005), and north-eastern Spain (Bartomeus et al, 2008b) have varying impacts on different taxa of native plants (Tab. I).…”

Patterns of introduced species interactions affect multiple aspects of network structure in plant–pollinator communities

“…In contrast, the model that performed the worst was the competitive model. This demonstrates that I. glandulifera is more likely to add interactions to the community than to remove them (Chittka and Schu¨rkens 2001, Memmott and Waser 2002, Aizen et al 2008, Bartomeus et al 2008). …”

“…Although invasive mutualists seem able to alter the 6 E-mail: lrusso@cornell.edu network structure of resident communities, the results of comparative studies are sometimes contradictory. For example, invasive mutualists can increase the nestedness of networks they invade (Bartomeus et al 2008), but decrease connectance among resident species (Aizen et al 2008). Other studies find little or no apparent impact of a species invasion on network structure (e.g., Vila´et al 2009, Carvalheiro et al 2011.…”

Exotic Plant Species in the Mediterranean Biome: A Reflection of Cultural and Historical Relationships

“…Moreover, facilitative effects on the reproductive success of co-flowering native plants have been reported in the presence of the invasive weed Oxalis pescaprae [150]. Likewise, it has been shown that litter of invasive exotic plants facilitated growth of the dominant native plants by altering soil moisture in Californian shrublands [151]. Additionally, in some cases, the presence of exotic plant species facilitated the visit of pollinators to native species [152].…”