Warming-Assisted Rapid Evolution of a Parasitic Host by Craig D. Idso of Cato.
"In 1980, heated water from a nuclear power plant in Forsmark,
Sweden (60.42°N, 18.17°E) began to be discharged into Biotest Lake, an
artificial semi-enclosed lake in the Baltic Sea created in 1977 that is
adjacent to the power plant and covers an area of 0.9 km2 with a mean
depth of 2.5 m. The heated water has raised the temperature of the lake
by 6-10°C compared to the surrounding Baltic Sea, but aside from this
temperature difference, the physical conditions between the lake and the
sea are very similar.
A few years after the power plant began operation, scientists
conducted a study to determine the effect of the lake’s increased
temperatures on the host-parasite dynamics between a fish parasite, the
eyefluke (Diplostomum baeri), and its intermediate host, European perch (Perca fluviatilis).
That analysis, performed in 1986 and 1987, revealed that perch in
Biotest Lake experienced a higher degree of parasite infection compared
to perch living in the cooler confines of the surrounding Baltic Sea
(Höglund and Thulin, 1990), which finding is consistent with climate
alarmist concerns that rising temperatures may lead to an increase in
infectious diseases.
Fast forward to the present, however, and a much different ending to the story is observed.
Nearly three decades later, Mateos-Gonzales et al. (2015)
returned to Biotest Lake and reexamined the very same host-parasite
dynamic to learn what, if anything, had changed in the intervening time
period. According to the team of researchers, Biotest Lake “provides an
excellent opportunity to study the effect of a drastically changed
environmental factor, water temperature, on the evolution of
host-parasite interactions, in a single population recently split into
two.” Specifically, it was their aim “to examine if the altered
conditions have produced a change in prevalence and/or intensity of
infection, and if these potential variations in infection have led to
(or might have been caused by) a difference in parasite resistance.”
To accomplish their objective, Mateos-Gonzales et al.
compared the prevalence and intensity of parasitic infection in perch
populations growing in warmer Biotest Lake versus the natural population
from the surrounding cooler Baltic Sea in 2013 and 2014. They also
conducted a controlled laboratory experiment in which they exposed perch
from both locations to D. baeri, comparing their infection rates.
The field results indicated that fish from the warmed Biotest Lake had a much lower
parasite infection rate than fish from the Baltic Sea. In fact, the
authors report that the “intensity of infection in Baltic fish was on
average 7.2 times higher than in the corresponding Biotest
fish” (italics added, see figure below). In addition, Baltic fish were
found to acquire “slightly more parasites as they age,” whereas Biotest
fish did not.
With respect to the laboratory tests, Mateos-Gonzales et al.
report that exposure to parasites “did not have an effect in fish from
the Biotest Lake, but it did in fish from the Baltic Sea,” increasing
their intensity of infection by nearly 40 percent.
Infection intensity of the parasite
D. baeri in juvenile perch from the Baltic Sea (left panel) and Biotest
Lake (right panel). The line indicates best-fit and the shaded area
represents the 95% confidence interval. Adapted from Mateos-Gonzales et
al. (2015).
In discussing their findings, Mateos-Gonzales et al. write
they present “a dramatic contrast” to those reported nearly three
decades earlier when Biotest fish were infected at a rate of “almost
twice” that of Baltic fish. Compared to 1986/87, the intensity of
parasitic infection in Biotest fish has fallen almost 80%, whereas it
has decreased only slightly in Baltic fish. Consequently, the authors
conclude their results illustrate “how an increased temperature has
potentially aided a dramatic change in host-parasite dynamics,” and that
change, it might be added, has clearly been for the better.
Furthermore, Mateos-Gonzales et al. note this rapid and surprising
reversal of fortunes has “direct implications for consequences of global
climate change, as they show that fast environmental changes can lead
to equally rapid evolutionary responses.” Indeed they can; and such
responses need to be included in predictive models that “highlight the
importance of empirical research in order to validate future
projections” of host-parasite interactions in a world of rising
temperatures. Clearly, the future outlook on host-parasite interactions
has the potential to be much more favorable than climate alarmists often
make it out to be.
References Höglund, J. and Thulin, J. 1990. The
epidemiology of the metacercariae of Diplostomum baeri and D. spathaceum
in perch (Perca fluviatilis) from the warm water effluent of a nuclear
power station. J Helminthol. 64: 139-150.
Mateos-Gonzalez, F., Sundström, L.F., Schmid, M. and Björklund, M.
2015. Rapid evolution of parasite resistance in a warmer environment:
Insights from a large scale field experiment. PLOS ONE 10: e0128860,
doi:10.1371/journal.pone.0128860."
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