A pollination network is a bipartite mutualistic network in which plants and

pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the maj ...

s are the

nodes In general, a node is a localized swelling (a "knot") or a point of intersection (a Vertex (graph theory), vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two ...

, and the

pollination Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds ...

interactions form the links between these nodes. The pollination network is bipartite as interactions only exist between two distinct, non-overlapping sets of

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

, but not within the set: a pollinator can never be pollinated, unlike in a predator-prey network where a predator can be depredated. A pollination network is two-modal, i.e., it includes only links connecting plant and animal communities.

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. 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). As the number of interactions in a network increases, the degree of nestedness increases as well. 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. Similar to the relationship between network complexity and network nestedness, the amount of asymmetry in specialization increases as the number of interactions increases.

Modularity of networks

Another feature that is common in pollination networks is

modularity Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a sy ...

. 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. 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. A study of three separate networks, all of which showed modularity, revealed that hub species were always plants and not the insect pollinators. Previous work has found that networks will become nested at a smaller size (number of species) than that where networks frequently become modular.

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 Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...

. 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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

by habitat loss. 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. 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. Another study revealed that with the removal of the most common pollinator species, the network showed a drastic collapse. 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.

Nested structure of pollination networks

Modularity of networks

Species loss and robustness to collapse

See also

References

Further reading