Connectedness in the plant-pollinator network
For the scenario above to result in
an ecosystem-wide effect,
pollination interactions must form a well-connected network, in which
particular plant species are pollinated by multiple pollinator species
and vice versa . This scenario is shown in Fig. 2a. The high level
of connectedness results in broad ranging competitive effects that can
propagate across the plant-pollinator community.
For example, if (Fig. 2a) Plant Species 1 produces more or less nectar
this will also affect Pollinator Species 3, even though it does not
visit Plant Species 1, via its competition for nectar with Pollinator
Species 2 that visits both Plant Species 1 and 3. Conversely, if plants
and pollinators are not well connected (Fig. 2b), then natural selection
for more or less nectar production and competition will still occur but
will be localized, and affect particular sub-sets of the pollination
community, but not the entire community and network. There are
specialized cases, such as in the fig plant-fig wasp mutualism, in which
pollination is largely as shown in Fig. 2b, even if in a few cases it
falls short of being completely specific (Molbo et al., 2003), in which
each plant species has only one pollinator wasp species (Janzen, 1979).
Empirical data on pollination networks indicate that species are
typically well connected (Memmott, 1999; Vizentin-Bugoni et al., 2018).
In addition many pollinators, including honey bees and bumble bees, are
generalists that visit many plant species (Biesmeijer & Slaa 2006;
Waser et al., 1996). Indeed, most plant species are visited by multiple
pollinator species (Waser et al., 1996, Ashworth et al., 2015).