Response of American Toads and Their Invertebrate Prey to Experimentally Elevated Soil pH
Acidification can have broad effects on forest ecosystems, ranging from consequences for individual organisms to alterations in trophic dynamics. While the effects of acidification on certain aspects of forest ecosystems have been well studied, less is known about the influence of soil acidification on the forest floor food web that includes amphibians and invertebrates. We investigated the effects of acidification on the American Toad (Anaxyrus americanus) and its interaction with the forest floor invertebrate community. We evaluated survival, growth, and diet of newly metamorphosed toads placed in terrestrial enclosures in forest plots with either experimentally elevated soil pH or untreated, acidified soils. We also conducted invertebrate pitfall sampling in these two soil pH types to evaluate the trophic interactions between toads and invertebrates. Toad mass after 90 days tended to be larger in plots with elevated soil pH, although survival and diet did not differ between soil pH types. We found no effect of soil pH on invertebrate abundances nor overall invertebrate diversity. We also found no evidence that toads exhibited top-down control of the invertebrate community. Collectively, our results indicate that acidified soils did not affect forest floor trophic dynamics. The presence of temporary enclosures we constructed, however, significantly reduced invertebrate abundances and overall diversity. Thus, the strong effect these structures can have on invertebrate communities should be considered when used in future studies.
Schematic demonstrating the experimental design, location of forest plots within the three study forests, and location of 1 m2 subplots within a forest plot. (A) General layout of the experiment among our three scales of interest: study forest, forest plot, and subplot. (B) Typical arrangement of six forest plots within each of the three study forests. Within each forest, white rectangles represent the three forest plots with untreated, acidified soils while lined rectangles represent the three lime-treated, elevated soil pH forest plots. (C) An enlarged example from (B) of one possible random arrangement of the three 1 m2 subplots (light gray boxes; not drawn to scale) located along the perimeter of a forest plot with untreated, acidified soil. Each 1 m2 subplot in each forest plot was assigned to one of three enclosure treatments: a) no enclosure, b) enclosure without American Toads, and c) enclosure with four American Toads.
American Toad survival (proportion of initial toads located, n = 4 toads per enclosure initially) and body mass over 90 days within enclosures located in forest plots with elevated soil pH (circles and solid line) and forest plots with acidified soils (triangles and dashed line). (A) Proportion of American Toads located in each enclosure within each soil pH treatment after 25, 60, and 90 days. Data are mean per soil pH treatment at each census period ± 1 s.e. (B) American Toad mass in each enclosure within each soil pH treatment at the start of the study and after 25, 60, and 90 days. Mean toad mass was calculated per enclosure at each census period. Data are mean per soil pH treatment at each census period ± 1 s.e.
Non-metric multidimensional scaling (NMDS) ordination of invertebrate community composition on day 0 (A) and day 91 (B). Soil pH treatment group scores and standard errors are represented by symbol type and line type (sites with elevated soil pH—solid shapes, solid line; sites with acidified soils—open shapes, dashed line). Forests are represented by symbol shape (CWRU Farm—circle; Schoop Forest—triangle; Pierson Creek Forest—square).
Invertebrate abundances collected on day 0 (A) and day 91 (B) from subplots with three different enclosure treatments (NE = No Enclosures: no enclosures present; NT = No Toads: enclosures present and no toads; T = Toads: enclosures present with toads). Data are mean ± 1 s.e. For each invertebrate grouping where the overall model was significant, enclosure types with the same letter were not different at P < 0.05 from post hoc tests.
Contributor Notes
Associate Editor: C. Bevier.