Historically, scientists assumed that parasites don’t play a major role in regulating host populations. Interactions like predation and competition were thought to be more important controls on species abundances and distributions. To this day, we don’t have many concrete examples for parasites or pathogens that drove their host species extinct or substantially altered their host species’ distribution. Even in cases where we suspect that a species’ decline was due to a parasite or pathogen, the absence of long term data for the host and/or pathogen populations or the logistical difficulties associated with experimentally manipulating host and/or pathogen populations make it difficult for us to know for sure what is/was the true cause of decline. But for today, let’s ignore all those tricky examples, and focus instead on a really clear example where a pathogen has substantially altered the abundances and distributions of its host species. Prepare to absorb another really cool bat disease ecology paper.
Pseudogymnoascus destructans (Pd) is a cold-loving fungus that can hang out in cave soils or in bat hosts, where it sometimes causes the fuzzy white bat noses that gave the disease in bats its name: white nose syndrome. When bats are infected by the fungus, their natural torpor cycles are interrupted, causing them to rouse more often during the winter. Increased rousing events costs energy, and higher energy expenditure depletes bat fat reserves and can eventually lead to bat death. Huge mass mortality events have been observed in North America since white nose syndrome was first noted in a colony in New York in 2006. But notably, in Europe, no mass mortality events have been observed, even though bats in Europe are infected by the fungus in the wild.
We know that a lot of bats died, but did Pd appreciably change the abundances and/or distributions of North American bat species? Fortunately, we have long term data for many North American bat colonies, both before and after the introduction of Pd into North America. And comparing the numbers from before and after shows very clearly that North American bats have taken a huge hit, with abundances declining by an order of magnitude since the introduction of Pd (Frick et al. 2015). Interestingly, North American bat abundances now match the abundances of European bats (Frick et al. 2015), which have likely been coexisting with Pd for much longer. This suggests that low abundances will be the new norm in North America wherever Pd invades. This is all very sad for bats, but it is cool disease ecology!
Frick, W.F., et al. 2015. Disease alters macroecological patterns of North American bats. Global Ecology and Biogeography, 24(7): 741–749.