Parasitic Castrators: Population-level Effects

For background info about parasitic castrators, check out the previous two posts, here and here.

As promised, this week, I’m going to talk about the effect(s) of parasitic castrators on host populations.  I’m specifically going to talk about trematode parasites (Halipegus occidualis) that castrate their snail hosts.  As a motivating statistic, 60% of snails in Charlie’s Pond (the pond studied in Negovetich and Esch 2008) may be castrated by H. occidualis trematodes.  That’s 60% of the snail population that can’t reproduce!!!  It would be hard to imagine castration at that scale NOT having an effect on snail population dynamics.

Whenever math and snails happen in the same paper, I am very happy.  In this paper, Negovetich and Esch (2008) used size-structured population matrix models to look at snail population growth.  They took parameter estimates for the size-specific growth rates, fecundity, and survival rates of snails from a previous field study.  They also took the natural prevalence of trematode infection (60%) and the natural yearly probability that overwintered snails cleared their parasite infections (35%) from the previous field study.

They calculated the natural snail population growth rate (λ) by finding the dominant eigenvalue of their annual matrix, just like you’d do for any matrix model.  Using the natural trematode prevalence and clearing rate, λ was 1.55.  They also calculated a hypothetical snail population growth rate for when there was 0% trematode infection – in that case, λ = 2.41.  So, having the natural levels of trematode infection resulted in a 36% decrease in snail population growth rates!!

I’d vote for Gary.

Negovetich and Esch (2008) also used sensitivity and elasticity analysis to see how changing the various rate parameters – fecundity, survival, growth – would affect the snail population growth rate.  They found that decreasing the size that snails reached reproductive maturity was the most effective way to increase the snail population growth rate.  You might remember from the previous posts that decreasing the size at reproductive maturity is one way to get fecundity compensation.

So, here’s where I quibble with the authors a bit.  They point out that that decreased size at reproductive maturity (=fecundity compensation) has been observed in several systems with snails and castrating trematodes.  Very cool!  But Negovetich and Esch (2008) also imply that these decreases are destined to evolve because they’re the best way to increase snail population growth rates according to this model.  And maybe if snails don’t do something to increase their population growth rate, the large-scale parasitic castration will lead to extinction.  I don’t like this explanation because selection acts at the individual level – the snail population isn’t deciding as a whole to reproduce at smaller sizes.

As we’ve talked about previously, individuals may reproduce earlier when they’re infected either because they know they’re about to be castrated or because the parasite’s manipulation of host resource allocation causes the snails to reach reproductive maturity sooner than the snails normally would.  There’s clearly some fascinating future work to be had on this topic – what’s the mechanism behind earlier snail reproduction in the presence of castrating trematodes?!  I MUST KNOW.

Until then, check out this paper!

Reference:

Negovetich, N.J., and G.W. Esch. 2008. Quantitative Estimation of the Cost of Parasitic Castration in a Helisoma anceps Population Using a Matrix Population Model.  Journal of Parasitology, 94(5):1022-30.