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Glucocorticoids play a key role in mediating stress responses in vertebrates. Corticosterone (CORT) is the main glucocorticoid produced in amphibians, birds, and reptiles, and regulates several metabolic functions. The most common methods for quantifying CORT are competitive binding immunoassays: radioimmunoassay (RIA) and enzyme immunoassay (EIA). RIA has been broadly used since the 1980’s but it requires radioactivity. Commercial EIA kits permit quantifying hormone levels without radioactivity although the requirement for a larger sample volume may be a strong limitation for measurements involving larval amphibians. Here we used Xenopus laevis tadpoles to compare the performance of three commonly used procedures for determination of CORT: RIA on a chloroform extract of whole-body homogenate, EIA on plasma, and EIA on supernatant of whole-body homogenate. We treated tadpoles with exogenous CORT at 0, 25, 50, and 100 nM. RIA could distinguish between 0 and 25 nM, and EIA on plasma between 0 and 50 nM, whereas whole-body homogenate EIA only detected significant differences between 0 and 100 nM. Each procedure presents advantages and disadvantages regarding sensitivity, the use of radioactivity, sample size, handling time, and economic cost. RIA is preferred when studying small-bodied animals from which blood samples cannot be obtained. When CORT level differences are intermediate and blood sampling is possible, EIA on plasma is a good non-radioactive alternative. EIA on whole-body homogenates may be useful to assess qualitative changes in CORT levels when considerable differences are expected. Finally, we discuss our findings in the context of previous studies on CORT in amphibians. [ABSTRACT FROM AUTHOR]

Connections between consumers and resources in food webs are complex and affect the structure and functioning of ecosystems. We assessed the influence of amphibians as consumers on the structure and functioning of temporary ponds, determining their impact on macrophyte abundance, zooplankton diversity and water chemistry., The effect of amphibians may be modulated by interactions with predators or competitors that alter tadpole density or behaviour. Therefore, we also investigated the consumptive and non-consumptive effects of native and invasive predators on amphibian larvae and pond ecosystems., High amphibian density decreased macrophyte biomass and zooplankton diversity and increased water turbidity and nutrient content. These effects were largely attributable to tadpoles of the largest herbivorous species, spadefoot toads ( Pelobates cultripes). In the absence of spadefoot toads, amphibians unexpectedly affected plant biomass positively., Invasive crayfish ( Procambarus clarkii) altered community structure in similar ways to high densities of amphibians and caused greater mortality of amphibian larvae than did native predators., The high herbivorous impact of spadefoot toads and invasive crayfish carried over to the following hydrological cycle. [ABSTRACT FROM AUTHOR]