Rareness and specialization in plant–pollinator networks

Abstract

Abstract.Most rare species appear to be specialists in plant-pollinator networks. This observation could result either from real ecological processes or from sampling artifacts. Several methods have been proposed to overcome these artifacts, but they have the limitation of being based on visitation data, causing interactions involving rare visitor species to remain undersampled. We propose the analysis of food composition in bee trap nests to assess the reliability of network specialization estimates. We compared data from a plant-pollinator network in the Monte Desert of Villavicencio Nature Reserve, Argentina, sampled by visit observation, and data from trap nests sampled at the same time and location. Our study shows that trap nest sampling was good for estimating rare species degree. The rare species in the networks appear to be more specialized than they really are, and the bias in the estimation of the species degree increases with the rareness. The low species degree of these rare species in the visitation networks results from insufficient sampling of the rare interactions, which could have important consequences for network structure.

Flower-visitor and pollen transport networks in a large city: structure and properties

“…For the CE model, the probability of a given plant and insect is a function of their specialization Guimaraes and Guimaraes 2006). As connectance may be prone to sampling bias (Nielsen and Bascompte 2007;Vazquez et al 2009;Blüthgen et al 2006;Blüthgen 2010;Dorado et al 2011), for assessing specialization of our visitation networks based on quantitative data (i.e., interaction frequencies), we calculated frequency-based indices H 2 and its standardized version H 2 0 , which relies on H 2max and H 2min values calculated from random interaction matrices. H 2 0 ranges between 0 and 1.0, and characterizes the degree of network specialization, respectively, from extreme generalization to perfect specialization (Blüthgen et al 2006).…”

“…In such cases, pollen analyses may add a valuable perspective to our knowledge of plantanimal networks (Forup and Memmott 2005;Gibson et al 2006;Forup et al 2008;Bosch et al 2009;Alarcon 2010;Devoto et al 2011;Popic et al 2013). Also, some traditionally used metrics for studying network properties, especially connectance, are said to be strongly dependent on sampling effort (Nielsen and Bascompte 2007;Vazquez et al 2009;Blüthgen et al 2006Blüthgen et al , 2008Blüthgen 2010;Dorado et al 2011), and adequate sampling of interaction diversity is labour intensive, so networks published to date may be largely undersampled (Chacoff et al 2012) and may not necessarily be ecologically meaningful (Ulrich 2009;Blüthgen 2010). Therefore, in purely technical terms, applying pollen datasets could enhance the resolution of the results as they allow us to record ''past flower visitation'' (Forup and Memmott 2005), and hence discover some existing links that usually go undetected, compensating, in some cases, lower sampling effort (Blüthgen 2010).…”

The strength of plant–pollinator interactions

“…Voucher specimens are stored at the Entomology Collection of the Instituto Argentino de Investigaciones de las Zonas Á ridas, CONICET, Mendoza, Argentina. Further details on trap nest methods are given in Dorado et al (2011).…”

“…We used only pollen, not nectar, for our predictor variables because only the former could be identified taxonomically from trap nest samples. Although the lack of consideration of nectar may bias our estimates of interaction strength, we believe this is a minor problem, as comparison of visitation and trap nest data suggests that these bee species use the same plant species as pollen and nectar sources (Dorado et al 2011). Furthermore, larvae of the study bee species feed mostly from pollen, and nectar is used only in small quantities.…”

Species Abundance, Not Diet Breadth, Drives the Persistence of the Most Linked Pollinators as Plant-Pollinator Networks Disassemble

“…Abundance can have a strong influence on observed linkage because, by chance alone, more abundant species are likely to be observed interacting with more partners. While this point is not lost on ecologists who do network analysis, it has nonetheless not been accounted for in most such analyses to date (Blüthgen et al 2008;Kleijn and Raemakers 2008;Dormann et al 2009;Vázquez et al 2009;Blüthgen 2010;Dorado et al 2011). Thus, our finding that the most linked species persist best could merely reflect the persistence of abundant species, which are expected to be at lower risk of extirpation (Davies et al 2000;Verdu and ValienteBanuet 2008).…”