Disease spread in small‐size directed trade networks: the role of hierarchical categories

Abstract

Summary1. Small-size, directed networks are relevant for many biological applications, from meta-populations to food webs, from transport flows to evolutionary trees, from epidemics within households to outbreaks of emerging plant pathogens (e.g. Phytophthora ramorum). However, little attention has been paid to dynamic processes in these networks. 2. In the horticultural trade, structural change in hierarchical categories, i.e. the proportion of producers, wholesalers and retailers, can influence the likelihood that plant epidemics will take place in such systems, but it is unclear how. 3. We model disease spread and establishment in directed networks of 100 nodes at four connectance levels in six network structures [local, small-world, random, and scale-free (SF) networks with positive, no, and negative correlation between in-and out-degree (number of incoming and outgoing links)], and study the role of hierarchical categories. 4. For non-SF networks, the correlation coefficient between number of incoming and outgoing links is negatively correlated with the proportion of producers and retailers, and positively correlated with the proportion of wholesalers. Given the previously reported negative correlation between the in-out degree correlation coefficient and the epidemic threshold, adding producers ⁄ retailers and removing wholesalers can contribute to making epidemics more difficult in non-SF networks. For SF networks these associations are not generally present, as in these structures epidemic development is driven by the presence of hubs, rather than the features of the majority of the nodes. 5. Synthesis and applications. despite the importance of trade movements of plants for plant epidemics and the emergence of new plant pathogens, little is known about the current contact structure of horticultural networks within and among nations, and about how this is changing. Such information is important for risk assessment and management in plant health regulation. This study thus calls for the long-term collection of data on the number and degree distribution of plant producers, wholesalers and retailers. Our results suggest that plant disease management should focus on the middle-tier of the nursery hierarchy. This result is likely to pertain also to other biological applications of small-size directed networks.

Seed exchange networks for agrobiodiversity conservation. A review

“…In the case of seed exchange, however, the connection of farmer x to farmer y does not necessarily imply the reverse connection (although it might in some cases). Similarly, most network models have treated individuals as either having a certain property or not, whereas there are many situations (including seed exchange) where a continuum between two states would be more realistic (Moslonka-Lefebvre et al 2009;Pautasso et al 2010c). For example, seed exchange does not just result in the presence or absence of a certain landrace, but in a proportion of farmers' seed belonging to that landrace.…”

Networks in Plant Epidemiology: From Genes to Landscapes, Countries, and Continents

“…Similar considerations apply to influencing hierarchical categories in networks of various structure and connectance level, i.e., the proportions of producers, wholesalers and retailers in a trade network. Other things being equal, for non-scale-free network structures adding wholesalers to a network is associated with an increase in the correlation among links in and out of nodes, and thus with a reduction in the epidemic threshold (100). The opposite result is obtained for the proportion of producers and retailers.…”

“…7) may have consequences not only for the likelihood of introduction of new pathogens and other organisms associated with plants (27,113,135), but also for gene flow and thus for the evolutionary potential of plant pathogens (70,81). Results from the modeling of disease spread in small-size networks call therefore for the long-term collection of data on the number and degree distribution of plant producers, wholesalers, and retailers in different nations (100). There is the need for comparable longterm data on the trade volumes in ornamental, horticultural, and agricultural crops among countries, regions, and individual firms.…”

Scientific Opinion on the Pest Risk Analysis onPhytophthora ramorumprepared by the FP6 project RAPRA

“…As the trade of plants susceptible to P. ramorum is a directed network (links from nurseries to garden centres do not imply the reverse connection), the risk of spreading the disease associated with different categories of business (producers, wholesalers, and retailers) is not the same. Simulations suggest that increasing the proportion of wholesalers (and reducing the proportion of producers and/or retailers) in plant trade networks with different structures is associated with a decrease in the epidemic threshold (Pautasso et al, 2010c). However, information on the structure of plant trade networks is mostly lacking.…”

“…Epidemiological modelling thus calls for the standardised and long-term collection of data on the contact structure of plant trade networks, i.e. not just trade volumes, but for example the frequency distribution in the number of incoming and outgoing links of trade players (producers, wholesalers, retailers) in various European regions at risk of inadvertent P. ramorum introduction (Pautasso et al, 2010c).…”