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 was lower for the visitation network than the PE network, which was in turn lower than 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.
Highlights
Given current concerns over pollinator declines and the resultant impact on both food production and plant diversity, we need to understand how pollinator deficits could affect pollination services for both crops and wild plants [1,2,3]
Network was more specialized than both SV and pollinator effectiveness (PE) networks (H02 1⁄4 0:365; figure 2c). These results suggest that the addition of pollen deposition data (PE) to visitation data to give pollinator importance (PI) values can produce an increase in network specialization
We report values for pollen deposition onto stigmas for virtually all components of a plant –pollinator community and demonstrate that such data enhance the quality of flower visitor interaction studies by producing networks giving a more accurate estimate of PI
Summary
Given current concerns over pollinator declines and the resultant impact on both food production and plant diversity, we need to understand how pollinator deficits could affect pollination services for both crops and wild plants [1,2,3]. Most networks quantify plant –pollinator interactions as numbers of animal visits to flowers (‘visitation’ or ‘flower-visitor’ networks), though a few ‘pollen-transport’ networks demonstrate which visitors are potentially important pollinators, based on quantity and species composition of pollen loads carried [7,8] ( this pollen may have many fates other than deposition on stigmas [9]). The data were used to construct the following networks: (i) visitation (V) network, using the frequency of interaction between visitor groups and plant species. We used d0 to measure of species-level specialization, which measures the exclusivity of interactions that individual species take part in [48] This is the most biologically informative measure of visitor specialization in resource choice in a visitation network, and most relevant predictor of specialization in pollination for a plant in a PI network. Recent studies (e.g. [54]) indicate that nestedness may only be a secondary indicator of stability; the primary driver, degree distribution (linkage density), cannot be accurately calculated for small networks [55]
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