Refugia, connectivity, and transmission

When populations become small, their probability of extinction typically goes up, because demographic and environmental stochasticity are more likely to set the population on an irreversible decline. However, when one population goes locally extinct, a species is not necessarily lost; the area might be re-colonized by migrants from a different population later, if other populations exist. Metapopulation theory tells us that a balance between population extinctions and re-colonizations in heterogeneous patches that are linked by dispersal can allow a species to persist regionally, even when it goes extinct locally.

What happens when we add infectious diseases into our host metapopulation model? Connectivity might be detrimental to regional persistence when infectious diseases are introduced, because pathogens in one population can invade the other populations via host dispersal, whereas pathogens are limited to a single population when populations aren’t linked via dispersal. Or…not?

Heard et al. (2015) recently published a quite complicated and fancy metapopulation model that suggests that connectivity actually increases the probability of metapopulation persistence in an Australian frog species endangered by the fungal pathogen (Bd), which causes the disease chytridiomycosis. From survey work, they knew that the prevalence of Bd in growling glass frogs was lower in warm and/or salty wetlands. They could also show that the probability of a local extinction in any given frog population increased with the prevalence of Bd in the frog population. By linking local extinction risk to Bd prevalence and microclimate, they could create metapopulation models using known dispersal distances for the growling glass frog, and they could run simulations regarding metapopulation persistence under scenarios where they eliminated frog dispersal among populations or not. They had two important findings. First, if you ignore the fact that Bd prevalence varies with microclimate, the probability of metapopulation persistance is predicted to be much lower than it actually is. The warm and/or salty wetlands act as important low Bd prevalence frog population refugia that can seed the other populations in a metapopulation when they go locally extinct, such that microclimate variability increases persistence. Second, “re-seeding” can only happen if dispersal occurs among populations, so connectivity increases metapopulation persistence in this system.

One lingering question is whether this system is a good example of the role of connectivity in all host metapopulations plagued by infectious diseases. Heard et al. (2015) argue that Bd is basically everywhere already – and there to stay – because it can be maintained in both environmental reservoirs and reservoir hosts. Therefore, they suggest that for growling glass frogs, dispersal of hosts among populations doesn’t really play a role in disease dynamics. In systems where the pathogen is not yet widespread (i.e., regions currently being invaded), where reservoirs are less likely to provide long-term maintenance of the pathogen, or where dispersal of reservoir hosts is particularly important to pathogen spread, host dispersal could start to have detrimental impacts on long-term metapopulation persistence. This is cool stuff that deserves more attention!

Reference:

Heard, G.W., C.D. Thomas, J.A. Hodgson, M.P. Scroggie, D.S. Ramsey, and N. Clemann. 2015. Refugia and connectivity sustain amphibian metapopulations afflicted by disease. Ecology Letters, 18: 853–863.

Daphnia and Chytrid

As I’ve mentioned previously, I really like predators of parasites (like this and this).  And I’m interested in how predators of parasites alter parasite transmission and foodwebs (like this and this).  Today, I’m blogging about how Daphnia can be predators of Bd, the causative agent of chytridiomycosis in amphibians, and how that predation can reduce transmission of Bd to tadpoles.  The scientific community is currently desperately looking for ways to reduce transmission of Bd, because chytrid is causing extinction of amphibian species all over the world.  Daphnia are being considered as one potential biocontrol option.

The paper that I just read was by Searle et al. (2013) – it’s an Ecology and Evolution paper that you can find here FO FREE.  Searle et al. (2013) explored how Daphnia (=predator) density, algae (=alternative prey) density, and grazing period affected the presence and abundance of Bd in water samples and on exposed tadpoles.  They found that Daphnia reduced Bd both in water samples and on tadpoles, but the effect of Daphnia on Bd was context-dependent – the outcome depended on the Daphnia species, Daphnia abundance, algae abundance, and grazing period.

90% of my cartoons involve turning animals into superhero animals. Is that weird?

Here are three things I found really interesting about this paper:

1. Daphnia magna and Daphnia dentifera, which are different sizes, both reduced the amount of Bd spores in the water, but only D. magna reduced tadpole infection.  Searle et al. (2013) suggested that the two species had similar consumption rates of Bd, but that zoospores were filtered through D. magna more times and thus became less infectious in that treatment group.  Coooooool.
2. When Daphnia weren’t present, algae reduced the transmission of Bd.  But when Daphnia were present, the reverse happened – more algae resulted in higher Bd transmission.  There are some cool older papers that found that algae and habitat complexity can reduce the transmission of free-swimming aquatic parasites.   This brings up some interesting questions about transmission of parasites with free-living stages in eutrophic environments.
3. And finally, the amount of zoospores in water didn’t always relate to the presence/amount on tadpoles.  This suggests that at these Bd densities, there isn’t a dose-response effect on tadpole infection.  Interesting!!

Soooo… would Daphnia be a good biocontrol option?  Maybe.  Maybe not.  It depends on the context.  Biocontrol is tricky!

Reference:

C. L. Searle, J. R. Mendelson, L. E. Green, and M. A. Duffy.  2013.  Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles. Ecology and Evolution.  (Free pdf!)