Invasive species reduces parasite prevalence and neutralizes negative environmental effects on parasitism in a native mosquito.

Invasive species research often focuses on direct effects of invasion on native ecosystems and less so on complex effects such as those influencing host-parasite interactions. However, invaders could have important effects on native host-parasite dynamics. Where infectious stages are ubiquitous and native host-pathogen specificity is strong, invasive less-competent hosts may reduce the pool of infectious stages, effectively reducing native host-parasite encounter rate. Alternatively, invasive species could alter transmission via changes in native species abundance. Biotic and abiotic environmental factors can also impact disease dynamics by altering host or parasite condition. However, little is known about potential interactive effects of invasion and environmental context on native species disease dynamics. Moreover, experimental examinations of the mechanisms driving dilution effects are limited, but serve to provide tests of predictions leading to diversity-disease relationships. Using field and laboratory experiments, we tested competing hypotheses that an invasive species reduces the prevalence of a native parasite in its host by removing infectious propagules from the environment or by reducing native host abundance. In addition, we evaluated the role of detritus quantity as a resource base in mediating effects of the invasive species. Native parasite prevalence was reduced when the invasive species was present. Prevalence was also higher in high detritus habitats, although this effect was lost when the invasive species was present. The invasive species significantly reduced infectious propagules from the aquatic habitats. Presence of the invasive species had no effect on the native species abundance; thus, the reduction in parasitism was not due to changes in host density but through a reduction in infectious propagule encounters. We conclude that an invasive species can facilitate a native species by reducing parasite prevalence via a dilution effect and that these effects can be modified by resource level. Reductions in parasitism may have ripple effects throughout the community, altering the strength of competitive interactions, predation rates or coinfection with other pathogens. We advocate considering potential positive effects of invasive species on recipient communities, in addition to effects of invasions on host-parasite interactions to gain a broader understanding of the complex consequences of invasion.

Landscape Physiognomy Influences Abundance of the Lone Star Tick, Amblyomma americanum (Ixodida: Ixodidae), in Ozark Forests.

The lone star tick, Amblyomma americanum Linnaeus (Ixodida: Ixodidae), is emerging as an important human disease vector in the United States. While some recent studies have modeled broad-scale (regional or county-level) distribution patterns of A. americanum, less is known about how local-scale habitat characteristics drive A. americanum abundance. Such local-scale information is vital to identify targets for tick population control measures within land management units. We investigated how habitat features predict host-seeking A. americanum adult and nymph abundance within a 12-ha oak-hickory forest plot in the Missouri Ozarks. We trapped ticks using CO2-baited traps at 40 evenly spaced locations for three 24-h periods during the summer of 2015, and we measured biotic and abiotic variables surrounding each location. Of 2,008 A. americanum captured, 1,009 were nymphs, and 999 were adults. We observed spatial heterogeneity in local tick abundance (min = 0 ticks, max = 112 ticks, mean = 16.7 ticks per trap night). Using generalized linear mixed models, we found that both nymphs and adults had greater abundance in valleys as well as on northern-facing aspects. Moreover, nymph abundance was negatively related to temperature variance, while adult abundance had a negative relationship with elevation. These results demonstrate that managers in this region may be able to predict local tick abundance through simple physiognomic factors and use these parameters for targeted management action.