Date Published: November 18, 2009
Publisher: Public Library of Science
Author(s): Sean P. Graham, Hassan K. Hassan, Nathan D. Burkett-Cadena, Craig Guyer, Thomas R. Unnasch, Jon Moen. http://doi.org/10.1371/journal.pone.0007873
Abstract: Determining the structure of ectoparasite-host networks will enable disease ecologists to better understand and predict the spread of vector-borne diseases. If these networks have consistent properties, then studying the structure of well-understood networks could lead to extrapolation of these properties to others, including those that support emerging pathogens. Borrowing a quantitative measure of network structure from studies of mutualistic relationships between plants and their pollinators, we analyzed 29 ectoparasite-vertebrate host networks—including three derived from molecular bloodmeal analysis of mosquito feeding patterns—using measures of nestedness to identify non-random interactions among species. We found significant nestedness in ectoparasite-vertebrate host lists for habitats ranging from tropical rainforests to polar environments. These networks showed non-random patterns of nesting, and did not differ significantly from published estimates of nestedness from mutualistic networks. Mutualistic and antagonistic networks appear to be organized similarly, with generalized ectoparasites interacting with hosts that attract many ectoparasites and more specialized ectoparasites usually interacting with these same “generalized” hosts. This finding has implications for understanding the network dynamics of vector-born pathogens. We suggest that nestedness (rather than random ectoparasite-host associations) can allow rapid transfer of pathogens throughout a network, and expand upon such concepts as the dilution effect, bridge vectors, and host switching in the context of nested ectoparasite-vertebrate host networks.
Partial Text: Increased focus on pathogens that emerge from their usual host populations and affect novel hosts (e.g., humans) is warranted. Several strategies have been used to characterize these pathogens, many of which stem from epidemiological paradigms , . Many studies focus on a single species acknowledged to play a key role in a pathogen’s transmission. Rarely do such studies consider vector-borne pathogen transmission cycles as properties of whole parasite-host networks , . The growing field of disease ecology has begun to resolve this discrepancy by merging the full repertoire of field and experimental research with epidemiological models . Numerous studies have demonstrated the utility of this approach to understand disease dynamics –, and these studies have become increasingly important due to the alarming increase in emerging pathogens in recent decades –.
Most of our networks (17 of 27) were more nested than the null model simulations using NODF (Table 1). There was no interaction between geographic area or species richness of a network and NODF nestedness (area: R2 = 0.11; p>0.05; richness: R2 = 0.14; p>0.05). A positive relationship was documented between nestedness (N) and species richness of ectoparasite-vertebrate host networks (R2 = 0.417; F = 18.777; p<0.0005). Collectively, these data suggest that the completeness of the matrix (in terms of number of species included in the analysis), rather than the size of the area studied, was an important factor influencing nestedness. We detected significant nestedness in most ectoparasite-vertebrate host networks. This indicates that specialized ectoparasites usually interact with hosts that attract many ectoparasites (‘generalist’ hosts), and generalist ectoparasites interact with these hosts as well as those that attract fewer ectoparasites (‘specialist’ hosts). Networks analyzed were as species rich (21–645 species), and were as geographically diverse (including networks from five continents) as previous attempts to characterize nestedness in other types of species interactions –. Despite the diversity of latitudinal, ecosystem, and taxonomic affiliations of these networks, considerable nestedness appears to be a consistent rule in ectoparasite-vertebrate host networks. Source: http://doi.org/10.1371/journal.pone.0007873