To understand how climate change will influence impacts of invasive species, we should consider potential for evolutionary changes as well as direct ecological impacts. Rapid evolutionary responses have been documented to climate change and invasive species. It is important to consider how interactive effects of these two forms of environmental change might alter evolution. We focus on plant-pollinator systems in addressing the potential for climate and invasive species to alter natural selection.
Invasive plants have the potential to influence pollinator-mediated selection experienced by a native plant by altering floral traits due to resource competition, changing behavioral preferences of pollinators, altering the pollinator visitor assemblage, or causing heterospecific pollen deposition. Reduced precipitation could influence selection by altering floral trait distributions, changing pollinator species composition, or making seed production less pollen-limited. Published research in plant systems on interactive effects of precipitation and invasion has emphasized potential impacts on growth or abundance, but there are many other potential mechanisms for impacts on pollination. These could operate by changing plant traits or by altering the third party, the pollinators. Neither ecological nor evolutionary impacts of those interactions have been tested.
We present experimental tests from three study systems of the impact of invasive species, water availability, or the combination on pollinator-mediated selection. For two study systems, we examined responses of pollinator visitation to experimental manipulations of the presence of an invasive species and availability of water. For the other system, we used long-term data to examine how the strength of selection depends on snowpack melt.
Removal of the invasive plant Brassica nigra led to selection favoring wider flowers in the southern California native Phacelia parryi, primarily by changing the behavior of introduced honeybees. Preliminary results also show interactive effects of B. nigra and water on selection for the trait of nectar production. In the mountains of Colorado, selection on floral length in Ipomopsis plants was weaker in years with earlier snowmelt, as variation in pollen receipt no longer translated into variation in seed production. Ongoing factorial manipulations of water availability and presence of the invasive Linaria vulgaris in Colorado test mechanisms for interactive effects on selection. Given the plethora of potential mechanisms, there is much still to be learned about interactive effects of water and invasive species on pollination. We encourage investigators of other kinds of species interactions to consider potential for evolutionary change when assessing impacts of climate and biological invasions.