Nectar Robbery by Native and Invasive Bumblebees Reduces Floral Rewards but Not Seed Production in Desfontainia...

Nectar robbery is common in hummingbird-pollinated plants and is often assumed to reduce plant reproductive success by depleting floral rewards and disrupting pollination. However, its quantitative effects on plant fitness remain poorly resolved, particularly in systems where native and invasive nectar robbers coexist. We evaluated nectar robbery by the native bumblebee Bombus dahlbomii and the invasive B. terrestris in the hummingbird-pollinated shrub Desfontainia fulgens in southern Chile. Flowering and nectar robbery were strongly synchronized, with robbery frequency peaking during maximum flower availability. Primary nectar robbing substantially reduced floral rewards: nectar standing crop in pierced flowers was approximately nine times lower than in intact flowers, whereas nectar sugar concentration showed only minor and statistically non-significant changes. Small ants and flies were found inside the flowers, suggesting that these insects may act primarily as nectar thieves rather than effective pollinators. Their abundance was not significantly associated with corolla perforation. Floral visitation intensity differed among visitor species and foraging strategies, with bumblebees visiting more flowers per plant during nectar robbing than during legitimate pollination. However, these observations describe foraging modes rather than demonstrating that robbing itself caused a behavioral shift. The hummingbird Sephanoides sephaniodes was observed exclusively as a legitimate pollinator. Experimental manipulations showed no negative effects of nectar robbery on seed production when pollinators were present. Flowers exposed to pollinators produced substantially more seeds than flowers from which pollinators were excluded, confirming strong pollinator dependence. In contrast, both the probability of seed production and the number of seeds produced per flower were similar between flowers exposed to pollinators with and without nectar robbers. These results indicate that nectar robbery can markedly erode floral rewards without translating into reduced female reproductive output.