Nestedness in bipartite networks of Thuja plicata, Prunus laurocerasus and Buxus sempervirens and their pathogens

Abstract

The trade with cultivated plants is one of the major pathways for the introduction of invasive species, pathogens included. Based on network analysis, the present study aimed the interaction between several species of cultivated woody perennials found in gardening outlets and nurseries trading with ornamental species and their documented pathogens. Focal species of the host list were Thuja plicata, Buxus sempervirens and Prunus laurocerasus, the selection being based on reported bestselling figures. Bipartite, qualitative, undirected networks were constructed to incorporate woody perennials as hosts and their documented pathogens. The tested network properties were: connectance, node degree distribution, web asymmetry and nestedness. Cluster analysis using Euclidian distance and niche overlap index of Pianka were employed as additional pattern description metrics. The main network containing 33 host species and 112 pathogens was characterized by truncated power law distribution fitting the observed degree distribution of hosts and power law distribution fitting the observed degree distribution of pathogens, low connectance (C = 0.12), intermediate web asymmetry (W = 0.54) and high significant nestedness (N = 0.94). The network containing three focal hosts showed significant lower nestedness (N = 0.54), higher asymmetry (W = 0.94) and higher connectance (C = 0.38). Cluster analysis revealed the separation of focal species distinctly, the majority of other hosts merging in one cluster. Due to the prevalence of specialized pathogens the niche breadth was narrow, with small overlap in resources’ partition (Pianka index = 0.31). Our results showed that a random assembly of hosts (woody ornamentals displayed for sale in retail centers and nurseries) could harbor pathogens which attached in a non-random manner, generating a characteristic pathosystem, with distinctive topology. The possible implications of the study consisted in a new insight in invasive spread and the inclusion of new pathogens in local pathogen communities using network analysis as a powerful investigation tool.