Nestedness of ectoparasite-vertebrate host networks

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.

Figure 1. Box-plots of mean nestedness (N).

Comparison between previously published calculations of plant-pollinator/seed dispersal (mutualism) networks, food webs , and ectoparasite-vertebrate host networks (this study).

Figure 2. Regressions of nestedness (N; angular transformed) on species richness (log transformed).

Closed circles  =  ectoparasite-vertebrate host networks (this study); open circles  =  mutualisms (plant-pollinator and plant-seed disperser networks) ; closed triangles  =  food web networks .

Figure 3. Hypothetical network of perfect nestedness for 17 ectoparasites and 76 hosts.

Drawn using UCINET.

Figure 4. Hypothetical network of random interactions for 17 ectoparasites and 76 hosts.

Drawn using UCINET.

Figure 5. Observed network of mosquitoes and their vertebrate hosts.

Network from Tuskegee National Forest, Alabama, USA, representing 17 actual mosquito species and their 76 vertebrate hosts. Red dots represent mosquitoes and blue squares represent vertebrate hosts. Drawn using UCINET.