Landscape connectivity, habitat isolation, and tick-borne pathogen ecology

Habitat loss and forest fragmentation are often linked to increased pathogen transmission, but the extent to which habitat isolation and landscape connectivity affect disease dynamics through movement of disease vectors and reservoir hosts has not been well examined. Tick-borne diseases are the most prevalent vector-borne diseases in the US and on the West Coast, Ixodes pacificus is one of the most epidemiologically important vectors. We investigated the impacts of habitat fragmentation on pathogens transmitted by I. pacificus and sought to disentangle the effects of wildlife communities and landscape metrics predictive of pathogen diversity, prevalence, and distribution. We collected pathogen data for four co-occurring bacteria transmitted by I. pacificus and measured wildlife parameters. We also used spatial data and cost-distance analysis integrating expert opinions to assess landscape metrics of habitat fragmentation. We found that landscape metrics were significant predictors of tick density and pathogen prevalence. However, wildlife variables were essential when predicting the prevalence and distribution of pathogens reliant on wildlife reservoir hosts for maintenance. We found that landscape structure was an informative predictor of tick-borne pathogen richness in an urban matrix. Our work highlights the implications of large-scale land management on human disease risk.

Methods
Location: California, U.S.A.

Time Period: 2016-2021

Major taxa studied: Ixodes pacificus, zoonotic bacterial pathogens, large vertebrate and rodent communities

Methods:

We collected pathogen data for four co-occurring bacteria transmitted by I. pacificus; Borrelia burgdorferi sensu stricto, Borrelia bissettiae, Borrelia miyamotoi, and Rickettsia tillamookensis. Additionally we measured wildlife and landscape variables for 19 sites in northern California over six years. Methods involved surveys of host-seeking ticks, wildlife camera trapping, and mark-recapture rodent surveys. We used spatial data and cost-distance analysis integrating expert opinions to assess landscape metrics of habitat fragmentation.

Results:

Landscape metrics, i.e., habitat size, measurements of surrounding green space, and habitat isolation, are significant predictors for tick density and pathogen prevalence. However, wildlife variables, i.e., wildlife, predator, and rodent diversity, also contribute to understanding the prevalence and distribution of pathogens which rely on wildlife reservoir hosts for maintenance. In our study, we found minimal benefit to the use of cost-distance over simpler habitat isolation metrics.

Main Conclusions:

A modification to island biogeography theory can be used to predict tick-borne pathogen richness for habitat patches within an urban matrix. Our work highlights the importance of considering the impacts of large-scale land management on human disease risk.