Climatically-dependent effects of defaunation and domestic grazing on larval tick survivorship.

Description

Abstract

1. The management of large-bodied mammalian hosts for controlling ticks and tick-borne infectious disease has yielded mixed success. Large-bodied mammals—especially ungulates—play an important role in maintaining tick-borne disease cycles as hosts for ticks. However, they likely also affect tick disease cycles indirectly, including by altering microclimates important for tick survivorship through grazing and trampling of vegetation. These indirect effects may be more important than previously recognized for tick survivorship, which has strong downstream effects on duration of host seeking behavior, tick abundance, and human exposure risk. 

2. Here, we use a large-scale field experiment in California to investigate the effects of two common forms of ungulate management—cattle addition and wild ungulate removal—on survivorship rates of two common tick species. Specifically, we employ an in situ larval tick survivorship experiment to isolate the indirect effects of ungulate management on tick survivorship rates and explore how these effects vary across climatic contexts. We then examine the extent to which any survivorship changes are mechanistically explained by changes in vegetation density and microclimate. 

3. We find ungulate management dramatically changes larval tick survivorship of both tick species, but that the effect varies across climatic contexts. In arid and intermediate contexts, total ungulate removal increases tick survivorship while cattle addition reduces survival. However, under mesic conditions, both cattle addition and complete ungulate removal increase tick survivorship. Our results strongly suggest that the effects of ungulate herbivores impact larval survivorship through their indirect effects on microclimate. Effects are stronger under more arid contexts because microclimate suitability plays a more prominent role in limiting survivorship times in these desiccating environments. 

4. Synthesis and applications. Our results suggest that livestock grazing, by altering understory vegetation and microclimate, can reduce habitat suitability for some tick species, potentially shifting disease risk, as larval survival to the nymphal stage is a critical parameter affecting TBD risk. These interactions are strongly climate-dependent, highlighting the need for integrated land and tick management strategies under increasing environmental change. Especially in drier environments, management of ungulate herbivores and targeted grazing to reduce understory vegetation may be an effective landscape intervention strategy for reducing tick encounters and disease risk mitigation.