Do Flowering Plants Protect Themselves Against Nectar Robbers?
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 also have evolved through traits originally referred to pollination. Defences against nectar robbers have been thought to include 1) toxins and secondary compounds, 2) escape in time or space, 3) physical barriers and 4) 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.
Read more about this topic: Nectar Robbing
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