Nectar Robbing - Varying Effects of Nectar Robbing On Plant Fitness

Varying Effects of Nectar Robbing On Plant Fitness

Pollination systems are mostly mutualistic, meaning that the plant benefits from the pollinator's transport of male gametes and the pollinator benefits from a reward, like pollen or nectar. As nectar robbers utilize the rewards of the plant without being in direct contact with the reproductive parts of the flower, their behaviour is easily assumed to be cheating. However, there are examples in which the effect of robbery on the plant is neutral or even positive. In one of the most extreme examples, even when 80 percent of the flowers in a study site were robbed and it was shown that the robbers were not pollinating, neither the seed nor fruit set was negatively affected.

The effect of robbery on plant fitness depends on several issues. Firstly, it is not always clear that a nectar robber does not carry pollen. For example, nectar-robbing carpenter bees, bumble bees and some birds have been observed to take part in pollination. Pollination may take place when the body of the robber contacts the reproductive parts of the plant while it robs or during pollen collection which some bees are known to practice in concert with nectar robbing. Different types of robbing organisms affect the plant in different ways. The impact of Trigona bees (e.g. Trigona ferricauda) on the plant is almost always negative. This is probably due to their aggressive territorial behaviour which effectively evicts legitimate pollinators. In addition to evicting pollinators, nectar robbers may change the behaviour of legitimate pollinators in many other ways. As robbers reduce the amount of nectar available, pollinators may be forced to visit more flowers to fulfill their needs. The increased number of flowers visited and longer flight distances increase pollen flow and outcrossing, which is beneficial for the plant because it lessens inbreeding depression. This is, of course, only possible if the robber does not empty the flower completely. In this case, pollinators usually avoid the flower and the effect on plant fitness is clearly negative.

The response of different species of legitimate pollinators also varies. Some species, like the bumble bee Bombus appositus and many species of nectar-feeding birds can distinguish between robbed and unrobbed plants and minimize the energy used for foraging by avoiding the heavily robbed flowers. It is assumed that pollinating birds are better at this than insects, because of their higher sensory capability. The ways in which pollinators distinguish between robbed and unrobbed flowers have not been studied but they have been thought to be related to the damage on petal tissue after robbery or changes in nectar quality. If nectar robbing severely reduces the success of legitimate pollinators they may be able to switch their plant species.

Nectar robbing, especially by birds, can cause damage to the reproductive parts of a flower and thus diminish the fitness of a plant. In this case, the effect of robbery on a plant is direct. A good example of an indirect effect is the change in the behaviour of a legitimate pollinator, which either increases or decreases the fitness of a plant. There are both primary and secondary nectar robbers. The secondary robbers are those (e.g. flies and bees) that take advantage of the holes made by primary robbers.

In conclusion, the effect of robbing is positive if the robber also pollinates or increases the pollination by the legitimate pollinator. On the other hand, robbing is negative if the robber damages the reproductive parts of a plant or reduces the pollination success either by competing with the legitimate pollinator or by lessening the attractiveness of the flower. Distinguishing between a legitimate pollinator and a nectar robber can sometimes be extremely difficult.