Appendix A. Methods of experiments reviewed to examine effect of predator presence on individual size variation as a function of growth rate.
In the main text we presented a survey of the nonlethal effect of Anax spp. dragonfly larvae on the relative size variation of wood frog and green frog tadpoles, as measured by the coefficient of variation, CV (Fig. 5). The data come from five experiments, four published and the fifth very similar in design to one of those published. Note that although published, not all of the data required to compute the predator effect on size variation was reported. For example, in one study, only effects of tadpole density, and not predator presence, on size variation was reported, and in several studies, the predator effect on mean size, but not size variation, was reported. Below we summarize the experimental methods, and present the statistical analysis used for results presented in the main text.
Experiment summary: wood frog tadpoles
We presented data from three experiments examining wood frog tadpole growth. In all three experiments, data were collected after sufficient time had passed that the predator had little (< 10%) effect on mean mass (see main text for rational). Further, in all cases, higher tadpole density and reduced resource levels had the expected negative effect on mean growth rate.
The first experiment, performed in 300 l wading pools in 1999, examined the effect of caged Anax on wood frog growth at different competition levels established by varying resource levels and tadpole density (empty squares in Fig. 5a). Experimental details are described in Peacor and Werner 2004, in which predator effect on mean growth rate is examined, and in Peacor and Pfister 2006, in which the effect of competition (in predator absence) on size variation is examined. There was a “base” resource level/tadpole density treatment, and three manipulations that increased competition by either increasing tadpole density or reducing resource level. The “base” treatment received 80 wood frog tadpoles that were fed ground rabbit chow distributed evenly throughout each pool every other day at a rate of approximately 1.5 g per day. A high-density treatment was implemented with twice the initial tadpole density (160 tadpoles/pool) but with the same resource level as in the base treatment. Reduced resource level treatments were implemented by adding 1/4 and 1/8 the resource supply rate in the base treatment (and using the same tadpole density as in the base treatment). The second experiment (filled triangles in Fig. 5a) is unpublished but was set up with the same protocol as the previous experiment, except that we only varied wood frog tadpole density (and not resource level), using 40, 80, or 160 wood frogs in each pool. Resource level was ~ 1 g rabbit chow per day.
In the third experiment (open triangles in Fig. 5a) using wood frogs, the effect of caged Anax on wood frog growth was examined in larger experimental units (1100 l cattle tank mesocosms). Details of the experimental design are described in Schiasari et al. 2006, but without predator treatments. The effect of four caged Anax (see, e.g., Peacor and Werner 2000 for a description of the same predator manipulation and mesocosms preparation) was examined at two resource levels. In all three experiments there is a trend in the data for CV to be higher at higher growth rates (Fig. 5a). Linear regression was performed on the pooled data (9 points total), which indicated a positive effect of growth rate on size variation (F1,7 = 19.0, P = 0.003). We also tested the statistical significance of the positive effect of the predator at high growth rate, and the negative effect at low growth rate. Simple main effects (Neter et al. 1985) were examined to determine if predator presence affected size variation within particular treatments. Mean growth was fastest for the lowest density in the second experiment. In this case, the positive effect of the predator was statistically significant (F1,24 = 5.8, P = 0.025). Mean growth was the slowest in the low resource level treatment in the third experiment (Fig. 5a). In this case, the negative effect of the predator was statistically significant (F1,12 = 13.5, P = 0.003).
Experiment summary: green frog tadpoles
We presented data from two experiments examining green frog tadpole growth. In both experiments, data were collected after sufficient time had passed that the predator had little (< 10%) affect on mean mass (see main text for rational) with two exceptions described below. As in the wood frog experiments, tadpole density and resource levels had the expected effect of mean growth rate.In the first experiment (diamonds, in Fig. 5b) we examined the effect of caged Anax on green frog tadpole growth over a gradient of five small green frog densities. Details of the experiment are in Peacor and Werner 2000. Here we include the three highest green frog densities (160, 240, and 320) in which growth was approximately equal in predator presence and absence (note that including the other treatments at lower densities would only strengthen the pattern, and therefore it is not conservative to include them as in the case described below). Experiments were performed in cattle tank mesocosms, and the protocol was similar to that as in the third wood frog experiment. A difference was that nutrients were added to tanks through the experiment to foster periphyton growth as a resource supply.In the second experiment with green frogs, performed using the same venue as in the previous experiment with a similar protocol, we examined the effect of caged Anax on a green frog tadpole-snail community (squares in Fig. 5b). Details of the experimental protocol are in Werner and Peacor 2006. There were two nutrient addition levels, the higher leading to faster tadpole growth rates. In addition, we simulated predator removal of tadpoles by lowering tadpole density over time. Crossing these two manipulations led to four different scenarios with different growth rates, all of which were performed in the presence and absence of caged Anax. Note that our constraint of using data in which the predator had a small effect on mean size was met in the two treatments with the lowest growth rate that occurred in the no removal treatments. However, predator presence led to an increase in growth rate of 16% and 27% in the removal treatments that had the highest growth rates (this positive effect is likely due to an indirect positive effect on resource levels, Werner and Peacor 2006). We include these results because the predator leading to faster growth rate actually makes conclusions conservative, because in the absence of predators, faster growing tadpoles will have a lower CV (as seen for wood frogs above, Peacor and Pfister 2006, Peacor unpublished data on bull frog tadpoles).
In both experiments using green frog tadpoles there is a trend in the data for CV to be higher at higher growth rates (Fig. 5b). Linear regression was performed on the pooled data (7 points total), which indicated a positive effect of growth rate on size variation (F1,5 = 12.4, P = 0.017). In contrast to the wood frog data, simple main effects did not indicate that the predator had a significant effect at specific growth rates (P > 0.1 at both the lowest and highest growth rate).
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Peacor, S. D., and C. A. Pfister. 2006. Experimental and model analyses of the effects of competition on individual size variation in wood frog (Rana sylvatica) tadpoles. Journal of Animal Ecology 75:990–999.
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Peacor, S. D., and E. E. Werner. 2004. Context dependence of nonlethal effects of a predator on prey growth. Israel Journal of Zoology 50:139–167.
Schiesari, L., S. D. Peacor, and E. E. Werner. 2006. The growth-mortality tradeoff: evidence from anuran larvae and consequences for species distributions. Oecologia 149:194–202.
Werner, E. E., and S. D. Peacor. 2006. Interaction of lethal and non-lethal effects of a predator on a guild of herbivores mediated by system productivity. Ecology 87:347–361.