Shared habitat preference leads to higher-than-expected disease risk in Baltimore checkerspot butterflies

Background/Question/Methods

: It is widely accepted that there is a relationship between population density and the rate of disease spread throughout a focal population. This concept is often considered to apply interchangeably when considering the relationship between population size and disease transmission: A given population of organisms will be expected to experience a greater degree of intraspecific contact as it increases. While this is true as a broad generalization, it ignores is an important nuance: different species show varying levels of discretion when deciding what portions of available habitat to occupy. If individuals in a population each show a preference for higher-quality habitat patches, they will be more clustered than expected by chance. Changes in density will in turn influence the rate of intraspecific contact and resulting disease transmission. Con-specific clustering is likely to have a strong influence on the minimum population size required to sustain a pathogen. Here we use the Baltimore checkerspot system to examine the relationship between the habitat-use decisions made by ovipositing females, the spatial distribution of their offspring’s natal sites, and the expected degree of contact within a population at different sizes. Baltimore checkerspots in the Northeastern US were recently discovered to be host to a pathogenic densovirus (JcDNV). JcDNV is primarily horizontally transmitted, and has been shown to result in substantial mortality in infected larvae. This work uses empirical larval movement parameters to estimate dispersal from observed natal sites and the resulting contact between neighboring natal sites. Specifically, we ask: • Do larval natal nest sites exhibit an even, random, or clustered distribution within available habitat? • How far are larvae expected to disperse from natal sites during their development? • Given the expected larval dispersal, does nest distribution influence larval contact and potential for disease transmission? • How does the degree of predicted contact change in relation to population size within a site? Is there a consistent trend across all observed populations, or does the relationship between population size and intraspecific contact change from site to site?

Results/Conclusions

Preliminary findings indicate that female checkerspots share similar habitat preferences, as indicated by significant spatial clustering in larval natal nest sites. Average larval dispersal prior to pupation is estimated to be <20 m. There is a strong influence of natal nest clustering on estimated intraspecific contact, and greater-than-expected contact at small population sizes.