Last week, the first case of white nose syndrome on the West Coast was documented. USGS confirmed that a sick bat found by hikers in Washington was Pd positive. The fungus made the 1,300 mile jump to the West Coast much faster than predicted by models, and it’s unclear how transmission occurred.
There’s a really cool recent paper about white nose syndrome in bats that links temperature and humidity, bat fat stores, and arousal from torpor to predict the regions in the US where white nose syndrome should cause bat mortality. In short, it’s normal for bats to periodically arouse from torpor, and each arousal event uses up a huge chunk of the bats’ energy reserves for winter. Infection with Pd increases the number of arousal events, which increases bats’ energy expenditure and decreases the probability that bats will survive the whole winter on their energy reserves – especially in areas with long winters. Bigger bats with more fat reserves are more likely to survive, which might explain why bigger species like the big brown bat have experienced smaller declines than small species like the little brown bat.
That’s right, you guys. It’s not survival of the fittest. It’s survival of the FATTEST.
Also, remember how European bats are infected by Pd, but seem tolerant of their infections? Well, using the climate-body fat-arousal model, Hayman et al. (2015) showed that European bats should be able to make it through the winter just fine given their fat reserves, even when they’re infected by Pd. So now we know why European bats are doing so well, while North American bats (with relatively low energy reserves) are doing so poorly! Ahhhhhhhhhhhmazing.
I do apologize for any nightmares that this cartoon causes…
Hayman, T.S., J.R.C. Pulliam, J.C. Marshall, P.M. Cryan, and C.T. Webb. 2016. Environment, host, and fungal traits predict continental-scale white-nose syndrome in bats. Sci. Adv. 2: e1500831.
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.