What Impact Does Nectar-Robbing Have on the Seed-Set of Flowering Plants?

What impact does nectar-robbing have on the seed-set of flowering plants?
Plants that offer food may attract non-pollinators! Problems can arise from this such as nectar robbing. Nectar robbing occurs when an insect, animal or bird visits a flowering plant, robbing the nectar without touching any of the reproductive parts or pollinating it (Stout, Allen and Goulson 2000).
The relationship that exists between plants and genuine pollinators can be considered one of mutualism.   This is because plants depend on pollinators to transport male gametes and pollinators depend on plants for the resources they provide, like nectar. In this equation, each party both benefits from each other (mutualism) (Zhang, Wang and Guo 2007). In the plant-pollinator mutualism relationship, nectar robbers are often classified as cheaters as the name robber implies. This is because they rob the plant of the nectar it provides without pollinating it, that is, taking the reward without providing a service (Richardson 2004). Nectar robbers are insects, birds and some animals; they usually remove nectar from flowering plants by piercing or tearing a hole in the corolla (Cushman and Beattie 1991). See fig 1.
Nectar robbing can have both positive and negative impacts on the seed-set of flowering plants, and even neutral impacts. It all depends on the plant-pollinator relationship (Zhang, Wang and Guo 2007). Some negative impacts of nectar robbing is the removal of resources such as nectar and pollen, which may result in a decrease of visitation by pollinators, a decrease in seed set, damaging of floral tissues, the shortening of some flowers lifespan and in some cases can lead to the reduction in attractiveness to potential pollinators (Castro, Silveira and Navarro 2008). In some cases however, nectar robbers can have a positive impact on the reproductive success of flowering plants as their bodies sometimes come in contact with the sexual organs of the plant at some point in robbing (Guitian,...