Abstract
BackgroundIt has been suggested that increasing biodiversity, specifically host diversity, reduces pathogen and parasite transmission amongst wildlife (causing a “dilution effect”), whereby transmission amongst efficient reservoir hosts, (e.g. Peromyscus spp. mice for the agent of Lyme disease Borrelia burgdorferi) is reduced by the presence of other less efficient host species. If so, then increasing biodiversity should inhibit pathogen and parasite invasion.MethodsWe investigated this hypothesis by studying invasion of B. burgdorferi and its tick vector Ixodes scapularis in 71 field sites in southeastern Canada. Indices of trapped rodent host diversity, and of biodiversity of the wider community, were investigated as variables explaining the numbers of I. scapularis collected and B. burgdorferi infection in these ticks. A wide range of alternative environmental explanatory variables were also considered.ResultsThe observation of low I. scapularis abundance and low B. burgdorferi infection prevalence in sites where I. scapularis were detected was consistent with early-stage invasion of the vector. There were significant associations between the abundance of ticks and season, year of study and ambient temperature. Abundance of host-seeking larvae was significantly associated with deer density, and abundance of host-seeking larvae and nymphs were positively associated with litter layer depth. Larval host infestations were lower where the relative proportion of non-Peromyscus spp. was high. Infestations of hosts with nymphs were lower when host species richness was higher, but overall nymphal abundance increased with species richness because Peromyscus spp. mouse abundance and host species richness were positively correlated. Nymphal infestations of hosts were lower where tree species richness was higher. B. burgdorferi infection prevalence in ticks varied significantly with an index of rates of migratory bird-borne vector and pathogen invasion.ConclusionsI. scapularis abundance and B. burgdorferi prevalence varied with explanatory variables in patterns consistent with the known biology of these species in general, and in the study region in particular. The evidence for a negative effect of host biodiversity on I. scapularis invasion was mixed. However, some evidence suggests that community biodiversity beyond just host diversity may have direct or indirect inhibitory effects on parasite invasion that warrant further study.
Highlights
Introduction by terrestrial hostsWe do not have any indices of rates of movement of I. scapularis or B. burgdorferi by terrestrial hosts
We investigated possible effects of host diversity, but we considered the more holistic view that biodiversity may impact on tick survival and pathogen transmission cycles by mechanisms involving competitors, predators and pathogens/parasites that are independent of host species diversity
Increasing host diversity was associated with reduced nymphal tick infestations of rodent hosts, which at first sight could be interpreted as evidence of a dilution effect inhibiting invasion of I. scapularis
Summary
Introduction by terrestrial hostsWe do not have any indices of rates of movement of I. scapularis or B. burgdorferi by terrestrial hosts. The risk of Lyme disease is increasing in southern parts of eastern and central Canada due to I. scapularis ticks that are expanding their geographic range northwards through dispersion by songbirds during spring migration and facilitation of the establishment of tick populations by a warming climate [5,6,7] These factors are likely common drivers of tick species range expansion in temperate zones, when combined [8]. Ixodes scapularis ticks, followed by B. burgdorferi, are currently invading southeastern and south central Canada offering us a unique opportunity to investigate whether or not biodiversity has an effect on the invasion of these species
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