Urbanization and translocation disrupt the relationship between host density and parasite abundance

1.) The species interactions that structure natural communities are
increasingly disrupted by radical habitat change resulting from the
widespread processes of urbanization and species translocations. Although
many species are disadvantaged by these changes, others thrive in these
new environments, achieving densities exceeding those found in natural
habitats. Often the same species that benefit from urbanization are
successful invaders in introduced habitats, suggesting that similar
processes promote these species in both environments. 2.) Both processes
may especially benefit certain species by modifying their interactions
with harmful parasites (“enemy release”). To detect such modifications, we
first need to identify the mechanisms underlying host-parasite
associations in natural populations, then test whether they are disrupted
in cities and introduced habitats. 3.) We studied the interaction between
the cane toad (Rhinella marina), a globally invasive species native to
South America, and its Amblyomma ticks. Our field study of 642 cane toads
across 46 sites within their native range in French Guiana revealed that
56% of toads carried ticks, and that toads with ticks were in poor body
condition relative to uninfected conspecifics. Across natural and
disturbed habitats, tick prevalence and abundance increased with toad
density, but this association was disrupted in the urban environment,
where tick abundance remained low even where toad densities were high, and
prevalence decreased with density. 4.) Reductions in the abundance of
ticks in urban habitats may be attributable to pesticides (which are
sprayed for mosquito control, but are also lethal to ticks), and our
literature review shows that tick abundance is generally lower in cane
toads from urban habitats across South America. In the invasive range,
ticks were either absent (in 1,960 toads from Puerto Rico, Hawai’i, Japan,
and Australia) or less abundant (in Florida and the Caribbean; literature
review). 5.) The positive relationship between host density and parasite
abundance is thought to be a key mechanism through which parasites
regulate host populations; anthropogenic processes that disrupt this
relationship may allow populations in urban and introduced habitats to
persist at densities that would otherwise lead to severe impacts from
parasites.