Experiment Two
Given that the top-down effects of crayfish were so strong, essentially
eliminating juvenile snails, we were concerned that the top-down effects
of crayfish were masking any positive effects of herbicides on snail
production and schistosomiasis risk. Consequently, we conducted a
follow-up experiment where we crossed the same herbicide exposures with
the presence and absence of crayfish. However, using five crayfish led
to complete depredation of snails in the predator tanks, reducing our
number of tanks with live snails in half (n = 32). Subsequently,
we found that little variance in Bi. glabrata biomass was
explained by treatment (Table S4), and likelihood ratio tests did not
detect significant herbicide effects in our nested models (Table S5). An
interaction term between crayfish presence and herbicide type for
infected, uninfected, and total Bi. glabrata biomass, after model
selection (Tables S11-S13), showed no significant interaction between
crayfish presence/absence and herbicides (Table 2). Focusing on tanks
without crayfish predators, the herbicides atrazine and acetochlor were
associated with increases in the average biomass of infected snails (55
%) and total snails (131 %), respectively (Table S13; Fig. 3a,3c). In
contrast, the herbicide alachlor was associated with a decrease in
uninfected snail biomass (126%) in tanks without predators (Table S14;
Fig. 3b). A Pearson’s correlation between snail biomass and infected
snail biomass suggested a significant positive association (rho =
0.67, t30 =4.96, p < 0.001; Fig.
1c). Moreover, infected snail biomass was a significant positive
predictor of total cercariae in tanks (Fig. 1d).