El papel funcional de las larvas de anfibios en los ecosistemas acuáticos temporales

Resumen

The structure and dynamics of ecological communities is determined and regulated by several key processes that influence in the species distribution and abundance (e.g., selection, drift, speciation, and dispersal), and by ecological interactions as competition and predation. The relative importance of competition and predation will depend largely on the local assemblage of species, the type of predators, or the degree of niche segregation. Amphibians constitute an ideal system to address these questions, as they are key species in the trophic webs with a profound role in several ecological processes. Moreover, the effect of amphibian larvae in the aquatic systems and their trophic niche largely depend on their complex ecological interactions with competitors and predators, as they respond to them altering their morphology and behavior. As amphibians continue showing generalized dramatic and global population declines, it is essential to fully characterize the functional role of amphibians in the natural systems to understand the possible consequences of their local declines and extinction. The main objective of this thesis is to know the regulatory potential of amphibian larvae according to the environmental conditions, focusing on the research about their role over other trophic levels, and their interactions with the aquatic vegetation, comparing their effects in Mediterranean and Neotropical systems. By means of experiments with mesocosms, which simulate the environmental conditions in controlled conditions, I evaluated the effect of a guild of amphibian larvae from Doñana National Park on several biological and physicochemical parameters of the system. We detected an important effect of amphibian larvae on these parameters, mostly when at high larval density, causing a marked reduction of plants biomass, increasing water turbidity and several nutrients like the ammonia. Amphibian larvae also affected algae abundance, indirectly inferred by estimating chlorophyll concentration in the water, and they reduced zooplankton diversity. However, these changes were not produced as a consequence of all amphibian species, but they were caused mainly to the larvae of the western spadefoot toad (Pelobates cultripes). The larvae of this species are disproportionally larger than the other amphibian larvae of the area, and they consume a large amount of macrophyte biomass. This species by itself altered the zooplankton composition, as the presence of tadpoles of P. cultripes increased the relative proportion of copepods in respect to cladocerans. Urodeles (newts and salamanders) can also contribute to this bias in the zooplankton community, as they are efficient carnivorous that can considerably alter the zooplankton community composition. Apart from density, we also experimentally manipulated the presence of native and invasive predators, which were introduced either freely roaming or caged in the mesocosms to distinguish their non-consumptive predator effects. The direct consumptive effects of the invasive predator, the red swamp crayfish, were very harmful in the great survival reduction of amphibians as well as in the physicochemical characteristics of the system (e.g., marked increase in water turbidity and nutrients, pronounced reduction of the plants biomass). Using stable isotopes, we evaluated the trophic plasticity of the amphibian larvae community derived of the changes in the environmental conditions detected in the former experiment. We observed variations in the stable isotopic values for carbon (¿13C) and nitrogen (¿15N) among species and according to the treatments the amphibians were exposed to. High larval density, presence of P. cultripes and presence of red swamp crayfish produced a decrease in the ¿13C of amphibians, related to an increase in carbon-depleted resources in their diet such as detritus or zooplankton. Therefore, the opportunistic feeding of amphibians is widely conditioned by competition, whereas the indirect predator effects are negligible in this temperate system. However, in neotropical systems predators have a greater structuring role in the amphibian community, altering the relative trophic position of the different studied species in central Panama, and changing their growth rate and survival. Furthermore, the community composition also has a great effect in the trophic niche of the amphibian larvae as we distinguished changed in the ¿15N of some of the amphibian species. Since our results bring to light that anuran larvae of temporary aquatic systems are important primary consumers, and not only of phytoplankton or periphyton but also of aquatic macrophytes, this thesis also try to figure out the feeding preferences of different amphibian larvae regarding several species of macrophytes and charophytes. The western spadefoot toad is the greatest macrophyte consumer, showing special preference for those from the genus Callitriche. Herbivory by amphibian larvae can have an important role in these ecosystems, affecting the community composition of plants, their reproductive phenology or their seed dispersal. In fact, the role of amphibian larvae in the direct or indirect ¿through seed predators- seed dispersal of aquatic plants has been ignored until the present study, and therefore our data are the first observations concerning this. Apart from the seeds directly ingested from the plants, tadpoles can also consume the seeds buried in the seed bank while feeding on detritus, as it is a common resource for tadpoles. We observed that between 9 and 32% of the tadpoles, depending on the species, transported seeds or spores from the pond vegetation. Among them, P. cultripes was the species with greatest number of seeds. The preferred plant in the feeding experiment (Callitriche) was also the one from which we found in greatest number in the tadpole feces, whose germination rate was between 40 and 88% depending on the tadpole species. The results of this thesis reveal the important role of amphibian larvae on the complex community structure of aquatic systems. Thus, it is essential to widen our knowledge in respect to their functional role aiming to anticipate and understand the consequences of their pronounced and predictable population declines.