Species invasions are one of the biggest threats to biodiversity worldwide. Invasive plants often use mutualisms to establish and spread in their new habitats. These plants tend to be well-integrated into plant-pollinator networks in terms of being visited by resident pollinators similarly or more frequently than the native plants. However, the long-term persistence of non-native plants not only depends on the visits they receive but on the quality of those visits and the resulting reproductive success. These other two key factors for plant invasion success (i.e., visit quality and reproductive success) have been rarely studied in the context of plant-pollinator networks.
We use a network dynamic model that accounts for population dynamics of plants and pollinators, quality and quantity of pollinator visits, the dynamics of floral rewards, and pollinators’ adaptive foraging, to identify the mechanisms by which non-native plants invade and impact plant-pollinator networks. We simulate the introduction of different types of non-native plants into thousands of networks varying in structure. The introduced non-natives vary in their production of floral rewards, ability to attach pollen on pollinators, and their level of specificity.
We found the counterintuitive result that introduced plants visited by fewer pollinators were more successful at securing the high-quality visits necessary for invading. This is because: a) for a given quantity of visits required to deplete the introduced rewards to their equilibrium level, more foraging effort can be assigned by pollinators of lower abundance, and b) the higher the foraging effort a pollinator assigns to the introduced plant, the higher the quality of visits the pollinator performs to that plant.
Native pollinators increased their abundance with the plant invasions, but the reallocated foraging effort concentrated on invaders reduced the quality and quantity of visits to native plants and made the network’s visitation structure more modular and nested. These effects of plant invasions on natives decreased with the number of native plant species in the network. Interestingly, the significant structural changes to visitation only caused a minimal decline in native plant abundance and no extinctions were observed. This supports previous calls including ours to evaluate the impact of invasive plants not only on visitation rates and network structure, but also on the demographics of native plants, which depend on other processes beyond pollination including seed production and recruitment.