Nectar robbing refers to the act by an animal, typically an insect or a bird, of removing nectar from a flowering plant, most often by drilling a hole in the corolla, other than by entering through the flower's opening. In this way animals without morphological adaptations required by the structure of the flower may access nectar. The animal may thus avoid touching the reproductive parts, and circumvent the mutualistic requirement of the plant-pollinator relationship. It has been suggested that flower visitors which neither damage nor pollinate the plant be called nectar thieves to distinguish them from nectar robbers. The term floral larcenist has been proposed to include both nectar robbers and nectar thieves.
Nectar robbers include some carpenter bees, bumblebees, stingless Trigona bees, wasps, ants, hummingbirds, and flowerpiercer birds (Diglossa). Nectar robbing has been recorded in a fruit bat. The squirrel Tamiops swinhoei hainanus robs nectar from the ginger plant Alpinia kwangsiensis.
Charles Darwin believed that nectar robbing always had a negative impact on the plant and his assumption was unquestioned into the late twentieth century. But the impact of nectar robbing on plants is less straightforward: only one third of studies reveal a negative impact on the plant, while others have shown either neutral or positive effects.
Effects 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, such as pollen or nectar. As nectar robbers receive the rewards without direct contact with the reproductive parts of the flower, their behaviour is easily assumed to be cheating. However, the effect of robbery on the plant is sometimes neutral or even positive. In an extreme example, when 80 percent of the flowers in a study site were robbed and the robbers did not pollinate, neither the seed nor fruit set were negatively affected.
The effect of robbery on plant fitness depends on several issues. Firstly, nectar robbers such as carpenter bees, bumble bees and some birds can pollinate flowers. 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 practice in concert with nectar robbing. The impact of Trigona bees (e.g. Trigona ferricauda) on the plant is almost always negative, probably because their aggressive territorial behaviour effectively evicts legitimate pollinators. Nectar robbers may change the behaviour of legitimate pollinators in other ways, such as by reducing the amount of nectar available. This may force pollinators 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 requires the robber not to 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 cost of foraging by avoiding the heavily robbed flowers. Pollinating birds may be better at this than insects, because of their higher sensory capability. Bees 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 damage 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. Secondary robbers are those (e.g. flies and bees) that take advantage of the holes made by primary robbers.
The effect of robbing is positive if the robber also pollinates or increases the pollination by the legitimate pollinator, and negative if the robber damages the reproductive parts of a plant or reduces 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 be difficult.
Pollination systems cause coevolution, as in the close relationships between figs and fig wasps as well as yuccas and yucca moths. If nectar robbers have an effect (direct or indirect) on a plant or pollinator fitness, they are part of the coevolution process. Where nectar robbing is detrimental to the plant, a plant species might evolve to minimize the traits that attract the robbers or develop some type of protective mechanism to hinder them. Another option is to try to neutralize negative effects of nectar robbers. Nectar robbers are adapted for more efficient nectar robbing: for instance, hummingbirds and Diglossa flowerpiercers have serrated bills that are thought to aid them in incising flower tissue for nectar robbing.
Nectar robbers may only get food in illegitimate ways because of the mismatch between the morphologies of their mouthparts and the floral structure; or they may rob nectar as a more energy-saving way to get nectar from flowers.
It is not completely clear how pollination mutualisms persist in the presence of cheating nectar robbers. Nevertheless, as exploitation is not always harmful for the plant, the relationship may be able to endure some cheating. Mutualism may simply confer a higher payoff than nectar robbing.
Defences in flowering plants
Even though there has not been much research on the defences evolved in plants against nectar robbers, the adaptations have been assumed to rise from traits used in interactions between plants and herbivores (especially florivores). Some defences may have evolved through traits originally referred to pollination. Defences against nectar robbers have been thought to include toxins and secondary compounds, escape in time or space, physical barriers and indirect defences.
Toxins and secondary compounds are likely to act as a defence against nectar robbing because they are often found in floral nectar or petal tissue. There is some evidence that secondary compounds in nectar only affect nectar robbers and not the pollinators. One example is a plant called Catalpa speciosa which produces nectar containing iridoid glycosides that deter nectar-thieving ants but not legitimate bee pollinators. Low sugar concentration in nectar may also deter nectar robbers without deterring pollinators because dilute nectar does not yield net energy profits for robbers.
If robbers and pollinators forage at different times of day, plants may produce nectar according to the active period of a legitimate pollinator. This is an example of a defence by escaping in time. Another way to use time in defence is to flower only for one day as a tropical shrub Pavonia dasypetala does to avoid the robbing Trigona bees. Escaping in space refers to a situation in which plant avoids being robbed by growing in a certain location like next to a plant which is more attractive to the robbers.
The last two methods of protection are physical barriers and indirect defence like symbionts. Tighly packed flowers and unfavourably sized corolla tubes, bract liquid moats and toughness of the corolla or sepal are barriers for some nectar robbers. A good example of an indirect defence is to attract symbiotic predators (like ants) by nectar or other rewards to scare away the robbers.
The term 'resistance' refers to the plant's ability to live and reproduce in spite of nectar robbers. This may happen, for example, by compensating the lost nectar by producing more. With the help of defence and resistance, mutualisms can persist even in the presence of cheaters.
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