Invader complexes or generalist interactions? Seasonal effects of a disturbance gradient on plants and floral visitors

Abstract

Disturbances can facilitate the spread of exotic plants, which establish mutualisms with exotic bees, constituting invader complexes. However, a disturbance-mediated increase in flower resources can also promote native floral visitors due to the fact that plant-pollinator interactions are generalist. We experimentally tested these ideas in northern Patagonian woodlands. In three sites, we examined the effect of harvesting intensity (HI) (0%, 30%, 50%, 70% of biomass removal) on flower density (exotic: herbs; native: herbs, shrubs, trees) and floral visitor density (exotic: honey bees, bumblebees; native: bees and wasps, hoverflies, other flies, beetles, ants). For four years, we made observations throughout the flowering period to test seasonality. HI had the most substantial positive effect on native herbs, followed by shrubs, and it was null on trees. The effects on the floral density of exotic herbs depended on site. HI enhanced the density of exotic bumblebees, native bees and wasps, and hoverflies; but it had no effect on honey bees, and it showed variable effects on the other floral-visitor groups. Although seasonality was relevant, there was no strong interaction with HI. In general, HI enhanced the density of native and exotic floral visitors, which is better explained by generalist interactions rather than by invader complexes.