Linkage rules for plant-pollinator networks:: Trait complementarity or exploitation barriers?

Recent attempts to examine the biological processes responsible for the general characteristics of mutualistic networks focus on two types of explanations: nonmatching biological attributes of species that prevent the occurrence of certain interactions ("forbidden links''), arising from trait complementarity in mutualist networks ( as compared to barriers to exploitation in antagonistic ones), and random interactions among individuals that are proportional to their abundances in the observed community ("neutrality hypothesis''). We explored the consequences that simple linkage rules based on the first two hypotheses ( complementarity of traits versus barriers to exploitation) had on the topology of plant-pollination networks. Independent of the linkage rules used, the inclusion of a small set of traits (two to four) sufficed to account for the complex topological patterns observed in real-world networks. Optimal performance was achieved by a "mixed model'' that combined rules that link plants and pollinators whose trait ranges overlap ("complementarity models'') and rules that link pollinators to flowers whose traits are below a pollinator-specific barrier value ("barrier models''). Deterrence of floral parasites (barrier model) is therefore at least as important as increasing pollination efficiency ( complementarity model) in the evolutionary shaping of plant-pollinator networks.