A reconstruction of parasite burden reveals one century of climate-associated parasite decline

Long-term data allow ecologists to assess trajectories of population
abundance. Without this context, it is impossible to know whether a taxon
is thriving or declining to extinction. For parasites of wildlife, there
are few long-term data – a gap that creates an impediment to managing
parasite biodiversity and infectious threats in a changing world. We
produced a century-scale time series of metazoan parasite abundance and
used it to test whether parasitism is changing in Puget Sound, USA and, if
so, why. We performed parasitological dissection of fluid-preserved
specimens held in natural history collections for eight fish species
collected between 1880 and 2019. We found that parasite taxa using three
or more obligately required host species – a group that comprised 52% of
the parasite taxa we detected – declined in abundance at a rate of 10.9%
per decade, whereas no change in abundance was detected for parasites
using one or two obligately required host species. We tested several
potential mechanisms for the decline in 3+-host parasites and found that
parasite abundance was negatively correlated with sea surface temperature,
diminishing at a rate of 38% for every 1°C increase. Although the
temperature effect was strong, it did not explain all variability in
parasite burden, suggesting that other factors may also have contributed
to the long-term declines we observed. These data document one century of
climate-associated parasite decline in Puget Sound – a massive loss of
biodiversity, undetected until now.