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, Sanchez and Guitian 1994). It is believed that nectar robbers may also bring about a decrease in the number of flowers that are visited by genuine pollinators, which leads to an increase in foraging distances, in turn leading to an increase in genetic variability through out-crossing (Cushman and Beattie 1991). In studies put forth by (Guitian, Sanchez and Guitian 1994) it was observed that there was a positive effect on the seed set of Petrocoptis grandiflora due to robbing by carpenter bees. There was also evidence to suggest that bumblebees had a positive effect on the seed set of Anthyllis vulneraria according to (Navarro 2000). However, in some plants, there may be differences in the efficiency of how visitors are at depositing pollen. It all depends on the relationship that exists between plant and pollinator (Navarro 2000). Bumblebees, (Bombus sonorus) for example robs Vicia faba and bluebells but with no important effect on female plant fitness (Newton and Hill 1983); (Morris and F 1996). In Honeybees, (Apis mellifera), and bumblebees (Bombus sonorus) there are differences in their effectiveness at depositing pollen; bumblebees are very effective at depositing measurable pollen, while honeybees generally are not. Additionally, honeybees can often be costly to the plant by causing stigmas to close, preventing later pollen deposition (Richardson 2004). The influence of nectar robbing on plant fitness is a complicated aspect (Maloof and Inouye 2000). However, it is important to understand the effects that nectar robbers have on the plants they rob and their legitimate pollinators. This is especially important when considering the excessive rate of robbing that some plant populations experience and the effect on legitimate pollinators (Zhang, Wang and Guo 2007). Before one can delve into the influences of nectar robbing, it might be worth it to take a look at why is it that animals steal nectar from and feed on flowers in an illegitimate fashion. It was proposed that animals may have become nectar robbers as it was a lot easier and efficient and they save energy when getting nectar form flowers. Another point is that animals tend be nectar robbers because of the difference in morphology of their mouthparts to that of the structure of the flower (Zhang, Wang and Guo 2007). It is sometimes common for plants that offer rewards (food, nectar, pollen), to develop specific adaptations where only legitimate pollinators can receive their reward. This is most commonly seen in the structure of flowers that attempt to restrict or limit access to potential non-pollinators (Stout, Allen and Goulson 2000). This can lead to the exploitation of these flowers by nectar robbers in order to receive the nectar (reward) (Roubik 1982). For example, Linaria vulgaris, the common yellow toadflax, produces flowers that have a long spur in which nectar collects, making it so that only bumblebees with long tongues can have access to the nectar when they enter from on top of the corolla. This makes it harder for short-tongued bumblebees to get to the nectar so they resort to cutting a hole in the base of the flower in order to reach the nectar (Stout, Allen and Goulson 2000). There seems to a relationship between the size of the host flower that is being robbed and the nectar robbers. What does this mean' Depending on the size of the flower, large robbers for example bumblebees and carpenter bees can sometimes have neutral or positive effects on the host (Zhang, Wang and Guo 2007). It was concluded that because the bumblebees and carpenter bees have relatively large bodies they are able to touch the sexual organs of the flowers with their abdomen thereby causing pollination, these type of nectar robbers are sometimes called robber-like pollinators (Higashi, Ohara and Matsuo 1988). Small-bodied nectar robbers like the Trigona bees does not cause pollination. According to (Roubik 1982) Trigona bees would have a negative impact on plant fitness because by robbing nectar they may drove away potential pollinators. Even though some nectar robbers sometimes pollinate flowers, they can sometimes change the function of real pollinators into secondary nectar robbers, where instead of pollinating the flower when getting nectar they would instead get nectar from the holes that were created by these primary nectar robbers (Inouye 1980). They can also put off visits from potential pollinators due to decrease in nectar availability because of robbing. Conversely, nectar robbers by robbing the flowers and decreasing nectar can cause genuine pollinators to increase the distance in which they forage therefore increasing the dispersal of pollen, the size of plant area and thereby decreasing the chance of geitonogamy (Maloof and Inouye 2000). Some studies have shown that nectar robbers have influences on plant fitness but have failed to show that the characteristics of some plants have influences on the cost of nectar robbing (Zhang, et al. 2009). Examples of some of these relationships are evident in some robber–plant systems. Bees are often a perfect example to look at when studying the relationship that exists between robber and plants. Bumblebees can be considered as both nectar robbers and pollinators; they can rob the adult flowers of Mertensia paniculata, for example, which contains nectar, but pollinate the younger ones that have only pollen (Morris and F 1996). A similar behavior is seen in carpenter bees where while robbing the ocotillo flower (Fouquieria splendens) they pierce a hole to receive nectar, but pollinate the same flower when collecting pollen (Waser 1979). Some examples where plants have influences in the way they are robbed can be seen in bees. Where the bumblebees are nectar robbers, they can have a positive effect on the plant Anthyllis vulneraria but a negative effect on Ipomopsis aggregata fitness (Navarro 2000), (Irwin and Brody 1999). Carpenter bees had a positive effect on the fitness of Fouquieria splendens and Petrocoptis grandiflora as nectar robbers but a negative effect on the fitness of Glechoma longituba (Guitian, Sanchez and Guitian 1994), (Zhang, et al. 2009), (Zhang, Wang and Guo 2007). Bees however are not the only nectar robbers to have an influence on the seed set of plants. Some bird species, insects, such as ants, moths etc and even some rodents such as squirrels have an effect on the seed set of flowering plants. Recent studies have shown that the striped squirrel (Tamiops Swinhoei hainanus) as a nectar robber may have an effect on the seed set of (Alpinia kwangsiensis) ginger (Deng, et al. 2004). The effect from nectar robbing by striped squirrel on the ginger flower is said to be a negative one, as they damage the styles of the flower when collecting nectar without pollinating it hereby reducing the seed set of the plant. The damage left to plant can usually be quite severe due to the fact that after the squirrel have robbed the plant they may be robbed again by secondary nectar robbing insects such as ants which dramatically decreases the number of potential pollinators such the Trigona bee (Deng, et al. 2004). There are many types of nectar robbing birds. However, birds are also sometimes considered as robber-like pollinators. An example of this is a genus of birds known as flower piercers (Diglossa), where while robbing the plant Tristerix longebracteatus of nectar, it was concluded that they actually pollinate while robbing as the fruit set of the plant remained high (Maloof and Inouye 2000) As you can see, it is quite complicated to generalize the effects that nectar robbing has on plant fitness in different systems. Nectar robbing can extremely influence the patterns of nectar availability, and this change may affect flower attractiveness and floral visitor’s behavior (Castro, Silveira and Navarro 2008) but it all depends on the plant and robber. The changes are place into three categories, as previously mentioned where there is a positive effect, neutral effect or a negative effect on plant fitness. Where there is a neutral effect, the behavior of nectar robbers may destroy or damage the corollas of flowers, but they do not touch the sexual organs or damage the ovules. Their behavior has no effect the fruit or seed sets of the flowering plant (Zhang, Wang and Guo 2007). When there is a positive effect, nectar robbers usually acts as pollinators and pollinate the plant or change the behavior of legitimate pollinators, which in turns enhance fitness through the increase of seed or fruit set and out-crossing (Maloof and Inouye 2000). This of course does not happen in every plant-robber-pollinator system. The negative effects of nectar robbing tend to be the most common. Where the effects usually include a reduction in seed and fruit set, damage to sexual organs and other parts of the flower and changes in the behavior of legitimate pollinators (Navarro 2000), (Irwin and Brody 1999), (Maloof and Inouye 2000), (Deng, et al. 2004).   Figure 1: Diagram showing a Trigona robbing nectar through a hole it has created in a flower Figure 2: Diagram of a striped squirrel robbing a ginger plant of nectar. Works Cited Castro, Sı´lvia, Paulo Silveira, and Luis Navarro. "Consequences of nectar robbing for the fitness." Plant Ecology, 2008. Cushman, J. H, and A. J. Beattie. "Mutualisms: assessing the benefits to host and visitors." Trends in Ecology and Evolution 6, 1991. Deng, Xao-Bao, Pan-Yu Ren, Jiang-Yun Gao, and Qing-Jun Li. "The Striped Squirrel (Tamiops swinhoei hainanus) as a Nectar Robber of Ginger (Alpinia kwangsiensis)." Biotropica, 2004. Guitian, J, J M Sanchez, and P Guitian. "Pollination ecology of Petrocoptis grandiflora Rothm. (Caryophyllaceae): a species endemic to the north west part of the Iberian Peninsula." Botanical Journal of the Linnean Society, 1994. Higashi, S, H A Ohara, and K Matsuo. "Robber-like pollinators: overwintered queen bumblebees foraging on Corydalis ambigua." Ecological Entomology, 1988. Inouye, D W. "The terminology of floral larceny." Ecology, 1980. Irwin, R E, and A K Brody. "Nectar-robbing bumblebees reduce the fitness of Ipomopsis aggregata ." Ecology, 1999. Maloof, Joan E, and David W Inouye. "Are Nectar robbers cheaters or mutualist'" Ecology, 2000. Morris, and W F. "Mutualism denied' Nectar-robbing bumble bees do not reduce female or male success of bluebells." Ecology, 1996. Navarro, Luis. "Pollination ecology of Anthyllis vulneraria subsp. vulgaris (Fabaceae): nectar robbers as pollinators." American Journal of Botany, 2000. Newton, SD, and GD Hill. "Robbing of field bean flowers by the short-tongued bumble bee Bombus terrestris L." Journal of Apicultural Research, 1983. Richardson, Sarah C. "Are nectar-robbers mutualists or antagonists'" Oecologia, 2004. Roubik, D W. "The ecological impact of nectar robbing bees and pollinating hummingbirds on a tropical shrub." Ecology, 1982. Stout, Jane C, John A Allen, and Dave Goulson. "Nectar robbing, forager efficiency and seed set: Bumblebees foraging on the self incompatible plant Linaria vulgaris (Scrophulariaceae) ." Acta Oecologica, 2000. Waser, N M. "Pollinator availability as a determinant of flowering time in ocotillo (Fouquieria splendens)." Oecologia, 1979. Zhang, Yan-Wen, Qian Yu, Ji-Min Zhao, and You-Hao Guo. "Differential effects of nectar robbing by the same bumble-bee species on three sympatric Corydalis species with varied mating systems." Annals of Botany, 2009. Zhang, Yan-wen, Yong Wang, and You-hao Guo. "Effects of nectar-robbing on plant reproduction and evolution." Frontiers of Biology in China, 2007.