Figure 1.
A conceptual model of the effects of hosts’ density and competency on parasite transmission, our experimental design depicted graphically, and some predictions.
A) If we imagine starting with a single host species J, and add to it a second host species K, the horizontal axis describes the range of possibilities in terms of their eventual densities. For example, resource competition or predator-prey interactions might lead to K extirpating J (“replacement”), or coexisting at reduced densities (“substitutive”). If J and K are largely independent, the introduction of K might leave the density of J unchanged (“additive”). If J and K are mutualists directly or indirectly, J may increase in abundance after the introduction of K (“synergism”). The y-axis describes the range of potential differences in host competency between species J and K. The graphical space thus represents all possible combinations of density effects and differences in host competency. The color indicates expected changes (based on qualitative, conceptual understanding) to total parasite transmission, given the type of community assembly and host competency. Black arrows represent situations where species K could have particularly strong effects on infection in species J through encounter reduction, that is, by absorbing parasites that otherwise would have intercepted species J individuals. B) Rana tadpole and snail host/decoy density in each treatment. Comparing R16, R24, and R48 tests the effects of tapdole density. Comparing R16 vs. RHP, and R24 vs. RH or RP, tests for host diversity effects in communities that assemble additively. Comparing R48 vs. RHP or RH or RP tests for host diversity effects in communities that assemble substitutively. C) The graphical space and background color are as before in panel A, but now superimposed are the experimental situations created: substitutive and additive community assembly for RO (where alternative species are incompetent hosts), and for ETa (where alternative species are fairly competent hosts). This graphical approach gives us clear a priori expectations for the results of the experimental design. Asterisks indicate where experimental results coincide with these expectations.
Figure 2.
Counts of E. trivolvis metacerariae versus tadpole mass (log scaled), for the six tadpole density and host diversity treatments.
Dots are observed infection intensities in individual tadpoles, thick lines are the predicted (from NB GLMM model) infection intensity for an animal of a given mass in that treatment, and thin lines represent 95% Bayesian credible intervals. Larger animals were more likely to be infected, but the rate of increased infection with size depended on treatment. Note that per-capita ETa infection increased with Rana density (R16 vs. R24 vs. R48), but was not affected by the addition of alternative hosts (RHP vs. R16, RH & RP vs. R24).
Figure 3.
E. trivolvis infection by host species, and in total, for each treatment. Means plus one standard error are reported.
Shades of purple represent the individual host taxa, while the wider gray bars represent the total of multi-species communities. Total infection increased with Rana density (R16 vs. R24 vs. R48). There was more total infection in diverse communities when host densities were additive (R16 vs. RHP or R24 vs. RH/RP), but equal infection in diverse communities when host densities were substitutive (RHP/RH/RP vs. R48).
Figure 4.
R. ondatrae infection in each of the six tadpole density and host/decoy treatments.
Each point represents the total number of metacercariae among all the animals in a single tank. The high tank to tank variability within each treatment made detecting any potential treatment differences difficult.