Bees pollinating

A pollination network is a bipartite mutualistic network in which plants and pollinators are the nodes, and the pollination interactions form the links between these nodes.[1] The pollination network is bipartite as interactions only exist between two distinct, non-overlapping sets of species, but not within the set: a pollinator can never be pollinated, unlike in a predator-prey network where a predator can be depredated.[2] A pollination network is two-modal, i.e., it includes only links connecting plant and animal communities.[3]

Nested structure of pollination networks

A key feature of pollination networks is their nested design. A study of 52 mutualist networks (including plant-pollinator interactions and plant-seed disperser interactions) found that most of the networks were nested.[4] This means that the core of the network is made up of highly connected generalists (a pollinator that visits many different species of plant), while specialized species interact with a subset of the species that the generalists interact with (a pollinator that visits few species of plant, which are also visited by generalist pollinators).[5] As the number of interactions in a network increases, the degree of nestedness increases as well.[4] One property that results from nested structure of pollination networks is an asymmetry in specialization, where specialist species are often interacting with some of the most generalized species. This is in contrast to the idea of reciprocal specialization, where specialist pollinators interact with specialist plants.[6] Similar to the relationship between network complexity and network nestedness, the amount of asymmetry in specialization increases as the number of interactions increases.[6]

Modularity of networks

Another feature that is common in pollination networks is modularity. Modularity occurs when certain groups of species within a network are much more highly connected to each other than they are with the rest of the network, with weak interactions connecting different modules.[7][8] Within modules it has been shown that individual species play certain roles. Highly specialized species often only interact with individuals within their own module and are known as ‘peripheral species’; more generalized species can be thought of as ‘hubs’ within their own module, with interactions between many different species; there are also species which are very generalized which can act as ‘connectors’ between their own module and other modules.[7] A study of three separate networks, all of which showed modularity, revealed that hub species were always plants and not the insect pollinators.[8] Previous work has found that networks will become nested at a smaller size (number of species) than that where networks frequently become modular.[7]

Species loss and robustness to collapse

There is substantial interest into the robustness of pollination networks to species loss and collapse, especially due to anthropogenic factors such as habitat destruction. The structure of a network is thought to affect how long it is able to persist after species decline begins. In particular, the nested structure of networks has been shown to protect against complete destruction of the network, because the core group of generalists are the most robust to extinction by habitat loss.[9][10] Models specifically focused on the effects of habitat loss have shown that specialist species tend to go extinct first, while the last species to go extinct are the most generalized of the network.[9] Other studies focusing specifically on the removal of different types of species showed that species decline is the fastest when removing the most generalized species. However, there have been contrasting results on how rapidly decline occurs with removal of these species. One study showed that even at the fastest rate, the decline was still linear.[10] Another study revealed that with the removal of the most common pollinator species, the network showed a drastic collapse.[11] In addition to focusing on the removal of species themselves, other work has emphasized the importance of studying the loss of interactions, as this will often precede species loss and may well accelerate the rate at which extinction occurs.[12]

See also

References

  1. ^ Fonkalsrud, Sindre Interaction Patterns and Specialization in a Local and National Norwegian Pollination Network. University of Bergen. Spring 2014
  2. ^ Newman, M. (2009). Networks: an introduction, Oxford University Press.
  3. ^ Olesen,Jens M. ; Bascompte, Jordi; Dupont, Yoko L. ; Jordano, Pedro The modularity of pollination networks. PNAS, December 11, 2007 vol. 104 n° 50 19891–19896.
  4. ^ a b Bascompte, Jordi; Jordano, Pedro; Melián, Carlos J.; Olesen, Jens M. (5 August 2003). "The nested assembly of plant–animal mutualistic networks". Proceedings of the National Academy of Sciences. 100 (16): 9383–9387. Bibcode:2003PNAS..100.9383B. doi:10.1073/pnas.1633576100. ISSN 0027-8424. PMC 170927. PMID 12881488.
  5. ^ Nielsen, Anders; Bascompte, Jordi (1 September 2007). "Ecological networks, nestedness and sampling effort". Journal of Ecology. 95 (5): 1134–1141. doi:10.1111/j.1365-2745.2007.01271.x. hdl:10261/40160. ISSN 1365-2745.
  6. ^ a b Vázquez, Diego P.; Aizen, Marcelo A. (1 May 2004). "Asymmetric Specialization: A Pervasive Feature of Plant–Pollinator Interactions". Ecology. 85 (5): 1251–1257. doi:10.1890/03-3112. ISSN 1939-9170.
  7. ^ a b c Olesen, Jens M.; Bascompte, Jordi; Dupont, Yoko L.; Jordano, Pedro (11 December 2007). "The modularity of pollination networks". Proceedings of the National Academy of Sciences. 104 (50): 19891–19896. Bibcode:2007PNAS..10419891O. doi:10.1073/pnas.0706375104. ISSN 0027-8424. PMC 2148393. PMID 18056808.
  8. ^ a b Dupont, Yoko L.; Olesen, Jens M. (1 March 2009). "Ecological modules and roles of species in heathland plant–insect flower visitor networks". Journal of Animal Ecology. 78 (2): 346–353. doi:10.1111/j.1365-2656.2008.01501.x. ISSN 1365-2656. PMID 19021779.
  9. ^ a b Fortuna, Miguel A.; Bascompte, Jordi (1 March 2006). "Habitat loss and the structure of plant–animal mutualistic networks". Ecology Letters. 9 (3): 281–286. doi:10.1111/j.1461-0248.2005.00868.x. hdl:10261/41674. ISSN 1461-0248. PMID 16958893.
  10. ^ a b Memmott, Jane; Waser, Nickolas M.; Price, Mary V. (22 December 2004). "Tolerance of pollination networks to species extinctions". Proceedings of the Royal Society of London B: Biological Sciences. 271 (1557): 2605–2611. doi:10.1098/rspb.2004.2909. ISSN 0962-8452. PMC 1691904. PMID 15615687.
  11. ^ Kaiser-Bunbury, Christopher N.; Muff, Stefanie; Memmott, Jane; Müller, Christine B.; Caflisch, Amedeo (1 April 2010). "The robustness of pollination networks to the loss of species and interactions: a quantitative approach incorporating pollinator behaviour" (PDF). Ecology Letters. 13 (4): 442–452. doi:10.1111/j.1461-0248.2009.01437.x. ISSN 1461-0248. PMID 20100244.
  12. ^ Valiente-Banuet, Alfonso; Aizen, Marcelo A.; Alcántara, Julio M.; Arroyo, Juan; Cocucci, Andrea; Galetti, Mauro; García, María B.; García, Daniel; Gómez, José M.; Jordano, Pedro; Medel, Rodrigo; Navarro, Luis; Obeso, José R.; Oviedo, Ramona; Ramírez, Nelson; Rey, Pedro J.; Traveset, Anna; Verdú, Miguel; Zamora, Regino (1 March 2015). "Beyond species loss: the extinction of ecological interactions in a changing world". Functional Ecology. 29 (3): 299–307. doi:10.1111/1365-2435.12356. hdl:11336/22561. ISSN 1365-2435.

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