Environmental variables linked to ponds characteristics and land use influenced the occurrence of tadpoles in the Brazilian Cerrado, with a strong phenotype”environment association in tadpoles. The most common tadpole morphological change was in body and tail shape, and in some cases we detected deviations in eye, nostril and mouth positions.
It is known that physical environment can exert an inductive and selective pressure on tadpole morphology (Van Buskirk and Relyea, 1998” Langerhans et al., 2007). Variation in morphology is known to be an important strategy for organisms to cope with diverse environmental conditions (Stearns, 1989” Scheiner, 1993). Our results highlight this interaction, demonstrating that some changes in tadpoles’ morphology are induced by certain environmental variables, such as presence of vegetation at the pond margins and land use change.
Pond dimensions and land use were environmental variables that were important in explaining the variation in morphological traits (PW1 and PW2). Tadpoles in larger ponds and with more intensive human activity had larger deviations in their eye, nostril and mouth positions. The presence of vegetation at the pond margins and inside ponds can provide different microhabitat for tadpoles and may be a structural factor that promotes niche partition among tadpoles of different anuran species. Microhabitat selection by tadpoles differs not only according to the position of the water column that they occupy (Eterovick & Barata, 2006), but also according to their association with different substrates. Thus, our results showed that in Brazilian Cerrado, higher levels of vegetation at pond margins favor tadpoles with low fins, as benthic species. Low levels of vegetation inside the pond margins favor tadpoles with high tail fins. Complex habitats with a lot of vegetation can be a challenge to tadpole locomotion and aquatic vegetation can favor ambush by invertebrate predators (e.g. larval dragonflies and water bugs” Nomura et al., 2011).
The percentage of land use within 500m of the pond perimeter provides an estimative of the human disturbance and conversion of native vegetation into pasture or agriculture. Moreover, intense use of pesticides can contaminate water pools through overspray in aerial application, lixiviation and overland flow (Goldsborough & Beck, 1989” Queiroz et al., 2011). Anthropogenic stressors can have adverse effects on organisms through effects on various individual traits (e.g. behavior and morphology” Teplitsky et al., 2005). Tadpoles are highly susceptible to contamination and their morphological traits could be affected by contaminants such as Glyphosate (Relyea, 2005” Costa & Nomura, 2016). Costa and Nomura (2016) evaluated the impacts of an agrochemical in tadpoles of Physalaemus cuvieri and found deviations in nostril”snout distance and eye width, which are morphological traits associated with tadpole sensory capabilities. Our results show deviations in eye, nostril and mouth positions, which may be due to anthropogenic stressors, and could affect an individual’s survival by reducing their competitive potential and increasing predation risks (Costa & Nomura, 2016).
Environmental variables are known to influence the occurrence of tadpoles (Van Buskirk, 2005). Vegetation types inside the pond, the pond dimensions and the pond bottom substrate reflect the availability of different microhabitats. Different types of vegetation and bottom substrate can offer site refuges and food for tadpoles, reducing the predation and competition rates (e.g. Rozas & Odum, 1988” Kopp et al., 2006). Moreover, vegetation inside the pond and vegetation at the pond margin can provide moisture, shelter and calling sites for adults. As tadpoles have little control of the habitat type that they will endure during metamorphosis (i.e. it is the adult that chooses the oviposition site) and their dispersion is limited (Altwegg & Reyer, 2003” Gr”zinger et al., 2012” Stein & Blaustein, 2015), the factors which influence tadpoles and adult distribution should be different. Adult distribution is affected by the spatial and terrestrial landscape (Resetarits et al., 2005” Wells, 2007), while tadpole distribution is affected by pond variables (e.g. size, vegetation, presence of predators” Ultsch et al., 1999). However, tadpole metacommunity structure can be influenced on a large scale by the physical characteristics of ponds (Provete et al., 2014). This suggests that adult reproductive behavior drives the tadpole occurrence dynamic, but on a local scale, microhabitat availability influences tadpole survival.
There is a lack of knowledge about amphibian diversity, abundance and occurrence in the Brazilian Cerrado (Valdujo et al. 2012), and our study provides additional information about environmental factors that contribute to the patterns of species occurrence. Furthermore, linking morphological and environmental traits help us to understand some ecological and evolutionary processes (e.g. niche partitioning, predation, competition) which may play important roles in tadpole assemblage structure. The Brazilian Cerrado is one of the global biodiversity hotspots and recent expansion of soybean culture and cattle ranching (Spera et al 2016) is threatening this biodiversity (Ribeiro & Walter, 2008). Our results indicate that there is more than one source of morphological variation in tadpoles, and should cause concern about the extensive use of agrochemicals linked to agricultural expansion. These chemicals can threaten tadpoles by changing their morphology, probably affecting their sensory capacity.