Understanding Linkage Rules in Plant-Pollinator Networks by Using Hierarchical Models That Incorporate Pollinator Detectability and Plant Traits

Abstract

The analysis of mutualistic networks has become a central tool in answering theoretical and applied questions regarding our understanding of ecological processes. Significant gaps in knowledge do however need to be bridged in order to effectively and accurately be able to describe networks. Main concern are the incorporation of species level information, accounting for sampling limitations and understanding linkage rules. Here I propose a simple method to combine plant pollinator effort-limited sampling with information about plant community to gain understanding of what drives linkage rules, while accounting for possible undetected linkages. I use hierarchical models to estimate the probability of detection of each plant-pollinator interaction in 12 Mediterranean plant-pollinator networks. As it is possible to incorporate plant traits as co-variables in the models, this method has the potential to be used for predictive purposes, such as identifying undetected links among existing species, as well as potential interactions with new plant species. Results show that pollinator detectability is very skewed and usually low. Nevertheless, 84% of the models are enhanced by the inclusion of co-variables, with flower abundance and inflorescence type being the most commonly retained co-variables. The predicted networks increase network Connectance by 13%, but not Nestedness, which is known to be robust to sampling effects. However, 46% of the pollinator interactions in the studied networks comprised a single observation and hence could not be modeled. The hierarchical modeling approach suggested here is highly flexible and can be used on binary or frequency networks, accommodate different observers or include collection day weather variables as confounding factors. An R script is provided for a rapid adoption of this method.

A common framework for identifying linkage rules across different types of interactions

“…Specifically, sampling completeness is rarely achieved when collecting interaction networks (Chacoff et al . ; Bartomeus ), and hence, some unobserved interactions may indeed occur (i.e. false absence of interactions).…”

“…Rohr et al . ; Crea, Ali & Rader ) and requires methods to estimate absences (Bartomeus ) or the development of model‐fitting procedures based on observed interactions only.…”

“…As an alternative, recent attempts to understand trait matching by statistically modelling empirical data are promising (e.g. models incorporating imperfect detectability: Bartomeus ; fourth corner analysis: Dehling et al . ; linear models: González‐Castro et al .…”

The potential for indirect effects between co‐flowering plants via shared pollinators depends on resource abundance, accessibility and relatedness

“…Plants offering more resources are likely to be visited by more pollinators (e.g. Kunin ; Cartar ; Bartomeus ). We therefore expect greater resource availability (flower abundance or floral resource quality) of a plant species to result in greater influence on the pollination of co‐flowering plant species (hypothesis 2).…”

An a posteriori measure of network modularity

“…The existence of interactions in ecological systems involves a large family of processes, ranging from abudance related 25, 26 (abundant species are more likely to interact together) to trait related 27 (pollination depends on the flower and insect having compatible morphologies, predators are constrained by the body-size of their preys). The interaction within these different families of mechanisms will drive heterogeneity in interaction strength 28 .…”