Constructing more informative plant-pollinator networks: visitation and pollen deposition networks in a heathland plant community

Abstract

Interaction networks are widely used as tools to understand plant-pollinator communities, and to examine potential threats to plant diversity and food security if the ecosystem service provided by pollinating animals declines. However, most networks to date are based on recording visits to flowers, rather than recording clearly defined effective pollination events. Here we provide the first networks that explicitly incorporate measures of pollinator effectiveness (PE) from pollen deposition on stigmas per visit, and pollinator importance (PI) as the product of PE and visit frequency. These more informative networks, here produced for a low diversity heathland habitat, reveal that plant-pollinator interactions are more specialized than shown in most previous studies. At the studied site, the specialization index [Formula: see text] was lower for the visitation network than the PE network, which was in turn lower than [Formula: see text] for the PI network. Our study shows that collecting PE data is feasible for community-level studies in low diversity communities and that including information about PE can change the structure of interaction networks. This could have important consequences for our understanding of threats to pollination systems.

Keywords: network; pollination; pollinator effectiveness; pollinator importance; specialization; species interaction.

Figure 1.

Single visit deposition (PE) of conspecific pollen: (a) pollen and nectar-foragers; (b) legitimate and robbing flower visitors. Median, interquartile range and outliers for individual visits are shown for each plant species. Significant differences marked with triangle.

Figure 2.

Bipartite networks illustrating (a) flower visitation, from stationary observations; (b) pollinator effectiveness of visitors (mean SVD); (c) pollinator importance of visitors, combining the data from (a) and (b). Key code, visitor group: 1, Bombus terrestris/lucorum; 2, Bombus pascuorum; 3, Bombus lapidarius; 4, Bombus jonellus; 5, Bombus hortorum; 6, Apis mellifera; 7, Halictidae; 8, other solitary bees; 9, large hoverflies; 10, Episyrphus; 11, Eupeodes; 12, small hoverflies; 13, Muscidae; 14, ants (Lasius); 15, Lepidoptera; 16, soldier fly.