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).