Sazonalidade e tempo de desenvolvimento larval de cinco espécies de hilídeos (Anura) no bioma Pampa

Abstract

The development, growth and survival of tadpoles varies widely along aquatic environments, depending on factors such as competition, temperature, pH and hydroperiod. Species are expected to adjust their rates of development because of these factors, which may lead to a variation, for example, in the reduction of the larval period as an evolutionary response in order to avoid unfavorable conditions. Thus, the analysis of age structure of populations is of high importance, since it can be used to determine many demographic characteristics, such as reproduction rate, growth, mortality and longevity, under specific environmental conditions. However, few studies on the larval development of Neotropical anurans have been conducted so far. The objective of the present work was to describe the patterns of temporal distribution, age structure and recruitment period (larval and juvenile) of the larvae of five hylid species (Julianus uruguayus, Scinax fuscovarius, S. granulatus, S. nasicus e S. squalirostris) which co-occur in the Pampa biome. Tadpoles were collected monthly, over a year, in 21 ponds (eight temporary and 13 permanent) in two municipalities: Santa Maria (Depressão Central) and Caçapava do Sul (Serra do Sudeste). The occurrence of a seasonal pattern in the circanual distribution of tadpoles was tested through circular statistical analysis. Size and development stages were used to describe the species age structure. These data were graphically distributed in classes over the sampling months for graphical inspection of recruitment events (larval and juvenile). For each species, was tested if the body length of tadpole varied between temporary and permanent ponds. Tadpoles of the five species used the environments seasonally, with species distributed in groups: one typical of the end of the winter until the middle of the spring, and a second group associated to the period of spring and summer. Seasonal distribution pattern was expected, and followed as reported for communities in austral region from Brazil. The temporal distribution occurred without overlapping at the peak of larvae abundance, characterizing a process of temporal succession within the more favorable occurrence period. This segregation observed can occur due to phylogenetically close species tend to differentiate ecologically in order to reduce intraspecific competition. Alternatively (or complementary), the species distribution seems to follow a temperature gradient at which tadpoles are exposed over time, suggesting that ecophysiological limitations explain the observed segregation. The larval development time for the five species ranged two months, without distinction between temporary and permanent environments, contrary to the expected and portrayed in some studies with other species. The hypothesis is that the desiccation risk in the temporary ponds was low, and there was not enough selective pressure on the species. Another hypothesis that can explain the observed is the high predation in permanent ponds, causing a decrease in metamorphosis time. The number of recruitments (larval and juvenile) was influenced by pond hydroperiod, evidencing that in permanent environments there is a greater support for successive breeding events.