Effects of sampling completeness on the structure of plant–pollinator networks

Abstract

Plant-animal interaction networks provide important information on community organization. One of the most critical assumptions of network analysis is that the observed interaction patterns constitute an adequate sample of the set of interactions present in plant-animal communities. In spite of its importance, few studies have evaluated this assumption, and in consequence, there is no consensus on the sensitivity of network metrics to sampling methodological shortcomings. In this study we examined how variation in sampling completeness influences the estimation of six network metrics frequently used in the literature (connectance, nestedness, modularity, robustness to species loss, path length, and centralization). We analyzed data of 186 flowering plants and 336 pollinator species in 10 networks from a forest-fragmented system in central Chile. Using species-based accumulation curves, we estimated the deviation of network metrics in undersampled communities with respect to exhaustively sampled communities and the effect of network size and sampling evenness on network metrics. Our results indicate that: (1) most metrics were affected by sampling completeness but differed in their sensitivity to sampling effort; (2) nestedness, modularity, and robustness to species loss were less influenced by insufficient sampling than connectance, path length, and centralization; (3) robustness was mildly influenced by sampling evenness. These results caution studies that summarize information from databases with high, or unknown, heterogeneity in sampling effort per species and should stimulate researchers to report sampling intensity to standardize its effects in the search for broad patterns in plant-pollinator networks.

Sampling completeness in seed dispersal networks: When enough is enough

“…Asymptotic values of NODF were achieved earlier (day 8) than for Weighted NODF (day 16), but overall our results coincide with those of Rivera-Hutinel et al (2012) who found NODF to be relatively stable if at least 30% of the network had been sampled. However, the observed network was not significantly nested (Fig.…”

“…Additionally, these descriptors showed similar trends in all of our study sites, suggesting that such robustness to sample size is not network-specific but also that these descriptors are of limited informative value to compare seed dispersal networks. Previous studies on pollination networks found a tendency for an initial increase in Connectance towards an asymptote that might be reached at relatively low (Nielsen and Bascompte 2007) or higher (Rivera-Hutinel et al 2012) levels of sampling completeness. Our results suggest that, despite both Connectance and Weighted Connectance of seed dispersal networks may be stable even with a low sampling effort, they are of limited use if we want to compare at least similar seed dispersal networks.…”

“…Since rare species and links require a higher sampling effort to be recorded, the typically low evenness of species abundances alone can drive some observed properties of the structure of some networks (Vázquez & Aizen 2004). Network descriptors can be more or less sensitive to these sampling artefacts (Rivera-Hutinel et al 2012;Blüthgen 2010;Blüthgen et al 2008), thus its values may either reflect sample effects or real nature processes. The use of appropriate null models is a way to disentangle if the processes structuring the observed networks are mostly mathematical or biological (e.g.…”

“…Most network structure descriptors are affected by sampling effort to some degree (Blüthgen 2010;Blüthgen, Fründ, Vázquez, & Menzel 2008;Rivera-Hutinel, Bustamante, Marín, & Medel 2012), with qualitative indices being more sensitive to sample size than quantitative analogues (Banasek-Richter, Cattin, & Bersier 2004). Specifically, poor sampling underestimates the real diversity of links, truncating estimated trophic breath and leading to a biased network structure (Blüthgen et al 2008).…”

Reproductive consequences of flower damage in two contrasting habitats: The case of Viola portalesia (Violaceae) in Chile

“…Visitation rate was estimated by recording the number of pollinator visits to all fl owers within patches during 15 minutes focal censuses (see also Murúa et al 2010, Rivera-Hutinel et al 2012. We performed a total of 190 censuses (47.5 h of observation) in the native habitat and 108 censuses (27 h of observation) in the Pinus plantation All samplings were performed between 0900 and 1700 hours, always in sunny days.…”