Linkage Rules for Plant–Pollinator Networks: Trait Complementarity or Exploitation Barriers?

Abstract

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.

A network model for plant–pollinator community assembly

“…The probability of a plant interacting with k pollinators (and vice versa) is assigned from an exponentially cut off power law, as is commonly observed in real ecosystems, with properties drawn from the literature (56). As in some other studies (31,33,34), we assign trait values to the species to characterize the beneficial or detrimental nature of their interactions. When trait values are used to determine whether interactions exist, recapitulation of real network properties is possible only if multiple dimensions of characteristics are considered (31).…”

“…As in some other studies (31,33,34), we assign trait values to the species to characterize the beneficial or detrimental nature of their interactions. When trait values are used to determine whether interactions exist, recapitulation of real network properties is possible only if multiple dimensions of characteristics are considered (31). Here, we choose an alternative route and impose the interactions by following a known degree distribution and use a single characteristic value to categorize interactions into three types.…”

“…[24][25][26] and have been instrumental in the development of recent community assembly models for food webs (4,6,(27)(28)(29)(30). In this work, we build on the growing literature on mutualistic network models (18,(31)(32)(33)(34)(35)(36)(37)(38) by introducing a dynamic Boolean network framework to simulate the process of repeated colonizations and extinctions inherent in the real process of community assembly.…”

Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks

“…Null model III additionally constraints this property: here, the number of links in the null model is equal to those of the observed network, as are the marginal totals. Constraining the number of links is analogous to some characteristic ecological and/or evolutionary processes, variously described as "trait mismatch", "morphological barriers" or "forbidden links" Stang et al 2006;Santamaría and Rodríguez-Gironés 2007;Stang et al 2007), and attributed to co-evolution of flowers and pollinators or hosts and parasitoids, respectively (but see Vázquez et al 2005). Thus, if null model I does not reproduce the observed data while null model III does, this may be interpreted as a real ecological property of the network, not a mere consequence of the fact that some species are more common than others.…”

“…Pollination networks have been described as being nested Santamaría and Rodríguez-Gironés 2007). In nested networks, pollinator communities on plants with few pollinators are subsets of plants with more pollinators.…”