From the research we've discussed on temperature-dependent sex determination, we all have an understanding of how terrifying the implications of global warming are on the conservation of our planet's biodiversity, especially in ectotherms and other species heavily affected by changes in temperature. One paper from the University of Connecticut has predicted that climate change will result in the extinction of at least 8% of species globally (Urban, 2015). Beyond species extinction, climate change has and will continue to cause habitat shifts, habitat loss, and countless other detrimental effects. One such effect is a change in the dynamics of infectious disease and parasite behavior. I'll be examining one parasite in particular, and how climate change has affected its' developmental biology in favor of the parasite but at the cost of its' main host species.
Muskoxen are ruminants native to the Canadian Arctic. They are a keystone species for the maintenance and vitality of the remote communities spread throughout the barren wasteland of the Yukon, Northwest Territories, and Nunavut. From 1988-1994, muskoxen populations saw a massive decline of 50% of their population; investigation of some of the dead individuals showed rampant infection of a new species of nematode, which was named Umingmakstrongylus pallikuukensis (Kutz, 2001). Further study of this nematode yielded some interesting details. It displays a complex lifecycle; its intermediate host is a gastropod while the muskoxen are its definitive host. Developing U. pallikuukensis larvae must infect one of the gastropod species that serve as an intermediate host in order to progress to the stage three larvae capable of infecting muskoxen. In muskoxen, the nematode makes its way to the lungs and begins to encyst and reproduce; this behavior gave the nematode its common name, lungworm.
This close association between lungworm and muskoxen seemed to signify that they had been coevolving in their host-parasite relationship for a long time (Kafle, 2020). However, these declines, which seemed to be caused by the increased intensity of lungworm infection researchers saw in dead muskoxen, were only a recent development. Susan Kutz set out to find the answer to this puzzle, and discovered it in the phenotypic plasticity of lungworm development in gastropods. The lungworms displayed temperature-dependent larval development; as conditions around them grew warmer, their progression between larval stages picked up as well. This is because of their ectothermic intermediate hosts; as global warming, which has a particularly intense effect in the arctics, caused temperatures to rise, the temperatures inside of the gastropods hosting lungworms also rose, and the lungworms were able to develop faster than ever before.
Susan Kutz in her 2001 paper developed a degree-day model to estimate larval development time, which determined the number of days in a year whose average temperature was above the calculated temperature at which larval development was zero (Kutz, 2005). She determined this temperature to be 8.5 degrees Celsius in lungworms and found that they needed at least 167 of these degree days to complete their life cycle. Using this information, she looked back at the average daily temperatures throughout the late 1900s. Prior to 1988, lungworms only had a total of three separate years where there were enough degree days for them to complete their lifecycle; after 1988, it was rarer for there to be years when there weren't enough degree days. Out of the years between 1988 and 2005, 12-13 would've been suitable for lungworm development. This explained why lungworms hadn't caused any serious declines in muskoxen populations until recently and allowed the recent decline to be attributed to global warming.
In class, we have looked at several species showing clear evidence for the destructivity of climate change. Species that are especially sensitive to temperature seem to be in the most danger, as was shown in the feminization of green turtles and sex ratio and habitat shifts of American alligators. By expanding the scope of the effects of climate change on development biology to include potential changes to parasite dynamics, light is shed on declines of species like the muskoxen, who are not quite as sensitive to temperature yet suffer from the consequences of global warming all the same.
Works Cited
Urban, M. C. (2015). Accelerating extinction risk from climate change. Science, 348(6234), 571-573.
Kutz, S. J., Hoberg, E. P., & Polley, L. (2001). A new lungworm in muskoxen: an exploration in Arctic parasitology. Trends in Parasitology, 17(6), 276-280.
Kutz, S. J., Hoberg, E. P., Polley, L., & Jenkins, E. J. (2005). Global warming is changing the dynamics of Arctic host–parasite systems. Proceedings of the Royal Society B: Biological Sciences, 272(1581), 2571-2576.
Kafle, P., Peller, P., Massolo, A., Hoberg, E., Leclerc, L. M., Tomaselli, M., & Kutz, S. (2020). Range expansion of muskox lungworms track rapid arctic warming: implications for geographic colonization under climate forcing. Scientific reports, 10(1), 1-14.
Woah! Super interesting topic. I researched a similar theme of the effect of climate change on a host-parasitoid interaction. however, the opposite effect was observed: the parasitoid had a lower thermal tolerance than its host. The contrast between these 2 papers just shows how each system is unique in its response to climate change. We really don't know what to expect with dynamic relations, like that between a host and parasite.
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