Pollination by Animals

Abstract

Most species of flowering plants depend on animals, such as bees, to move their pollen and enable sexual reproduction, and most animal pollinators, in turn, depend on flowers as sources of food or other materials. Although the interaction is mutualistic, benefitting both plants and animals, it is not cooperation, because the best ‘interests’ of the two partners differ. Animal pollination is critical for crop and natural ecosystems and plays central roles in plant ecology and evolution. In particular, selection imposed by animal pollinators is thought to have driven the evolution of much of the floral diversity we see today. Most plant–pollinator relationships are not specialised, one‐to‐one mutualisms – a fact that should make pollination networks somewhat resilient to species extinctions. Nevertheless, recent pollinator declines have given rise to concerns about loss of pollination services to crops and wild plant populations.

Key Concepts:

  • Most flowering plant species depend on animals to transfer pollen from anthers (the male organs of flowers) to stigmas (the female organs) in order to produce seeds.

  • Pollinating animals visit flowers to obtain resources: usually nectar or pollen, but sometimes oils, fragrances, resins or oviposition sites.

  • The divergent ‘interests’ of plants and pollinators in the interaction explain such phenomena as unrewarding flowers and nectar‐robbing insects.

  • Recent pollinator declines are causing concern about the maintenance of pollination services to crops and wild plants.

  • Pollination webs illustrating the linkages among plants and pollinators within a community show that reciprocally specialised interactions are rare.

  • Numerous studies show that pollinators can exert selection on plant traits, but selection by plants on pollinator traits has been harder to demonstrate.

  • Pollinators play an important role in the process of reproductive isolation and speciation in flowering plants.

Keywords: angiosperms; evolution; mutualism; pollination web; natural selection; specialisation

Figure 1.

Yuccas and yucca moths. (a) A female moth collects pollen with her mouthparts, after which she flies to a different plant (this is critical as yuccas are completely or partially self‐incompatible). (b) The female then oviposits into the ovary of a new flower, after which she places pollen on the stigma. (c) Larvae develop within a single fruit and consume some, but not all, of its seeds. The plants abort fruits into which too many eggs are laid, thus exerting some control over ‘cheaters’ that devour most or all the seeds. Reproduced from Riley ().

Figure 2.

A pollination web. Sampling of flower visitors over one fortnight in upland vegetation on the island of Mauritius, in the tropical Indian Ocean, yielded this web of interactions among pollinators (numbered along the top, with width of each coloured rectangle proportional to relative abundance of that species) and plants (along the bottom). Width of the wedge connecting animals to plants indicates the relative frequency of their interaction. The pollinators are coded as follows: red=Hymenoptera other than ants, magenta=Gekkonidae (one species, Phelsuma cepediana, blue‐tailed day gecko), blue=Diptera, dark green=Aves (one species, number 160, Zosterops mauritianus, grey white eye), green=Hemiptera, orange=Formicidae and yellow=Lepidoptera. The 32 plant species were visited by a mean of 3.1 pollinator species each, whereas the 28 pollinator species visited a mean of 2.6 plants each. This web is representative of pollination webs studied to date in that it contains a range of species from specialists to generalists, with specialists tending to interact with generalists rather than with other specialists. Reproduced from Kaiser ().

Figure 3.

Disruptive natural selection on width of the flower tube (corolla) in Ipomopsis. In rare years, hawkmoths join hummingbirds as pollinators, and selection favours the narrowest and widest corollas rather than intermediates. This situation recalls a scenario for pollinator‐mediated speciation without geographical isolation. There is no significant selection on corolla length. Reproduced from Campbell ().

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Further Reading

Bronstein JL, Alarcón R and Geber M (2006) The evolution of plant‐insect mutualisms. New Phytologist 172: 412–428.

Buchmann SL and Nabhan GP (1996) The Forgotten Pollinators. Washington, DC: Island Press.

Chittka L and Raine NE (2006) Recognition of flowers by pollinators. Current Opinion in Plant Biology 9: 428–435.

Chittka L and Thomson JD (eds) (2001) Cognitive Ecology of Pollination: Animal Behavior and Floral Evolution. Cambridge, UK: Cambridge University Press.

Harder LD and Barrett SCH (eds) (2006) Ecology and Evolution of Flowers. Oxford, UK: Oxford University Press.

Proctor M, Yeo P and Lack A (2003) The Natural History of Pollination. London, UK: HarperCollins.

Real LA (ed.) (1983) Pollination Biology. Orlando, FL: Academic Press.

Waser NM and Campbell DR (2004) Ecological speciation in flowering plants. In: Dieckmann U, Metz H, Doebeli M and Tautz D (eds) Adaptive Speciation, pp. 264–277. Cambridge, UK: Cambridge University Press.

Waser NM and Ollerton J (eds) (2006) Plant‐pollinator Interactions: From Specialization to Generalization. Chicago, IL: University of Chicago Press.

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Waser, Nickolas M, and Forrest, Jessica RK(Apr 2014) Pollination by Animals. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003163.pub3]