Water and nutrient supply for desert food webs: from autotrophs to top consumers within the framework of ecological stoichiómetry.

Abstract

Energy and material flow are essential to ecosystem functioning. Transfer of energy and nutrients within ecosystems is best described in terms of food web interactions among taxa in ecological comrnunities. Energy and material fiows and food web dynamics are related, and food web structure is influenced by limitations of energy and rnatter. Conversely, species interactions in food webs may also influence energy flow path and biomass production. Ecological stoichiometry examines how the balance of chemical elements in ecosysterns affects and is modified by organisms and how the flux of nutrients through food webs depends on organism, population and community processes. In terrestrial food webs the interactions among the component species are assumed to be constrained from the imbalance of nitrogen and phosphate on primary producers. Plants respond to variations on nutrient supply altering their elemental composition, whereas in the case of animals a constant elemental composition of their body is hypothesized. The main aim of this thesis was to characterize, for a desert case study, how nutrient supply will affect plant nutrient composition and growth, and how this, in tum, may affect food web stoichiometry. Our model system was the hyper-arid Atacama Desert of northern Chile, that contains many patchy coastal "fog ecosysterns", dominated by bromeliad-plants that obtain both nutrients and water from fog. These extreme ecosysterns support a food web that is sufficiently simple and conspicuous to systematic and empirically study the dynamic nature of nutrient supply and elemental stoichiometry, and the the direct and indirect causal relationships between nutrient constraints and mismatches between trophic levels. Our results did show that spatial and temporal variability in water and nutrient supply can have significant effects on plant stoichiometry. However, this study reveals that plant biomass nutrient content and C: N: P stoichiometry did not closely match environmental nutrient supply. However, under low nutrient conditions we found a decrease in plant nutrient content and an increase in C: nutrient and N: P ratios. P supply affects the elemental composition of plants, enhancing the growth rate, these provide further evidence consistent with the "Growth Rate Hyphotesis", and highlight a potential role of P limitation to autotroph's growth, especially in dry ecosystems. We found elemental imbalances at different trophic interactions, however, larger imbalances were found at producer - herbivore interface, especially in regard to nitrogen content. Consumers stoichiometry did show that the whole-body elemental composition varied among taxa. Sorne of this variation was related to trophic level as predators generally had higher N and P content, and lower C:N:P ratios than herbivores. There was sorne intra-specific variation on nitrogen and phosphorus content. These results do not support a common assumption in Ecological Stoichiometry that elemental composition is relatively constant within species. Although, we found stoichiometric differences at a producer leve!, elemental mismatches at plant - herbivore - invertebrate carnivore - vertebrate carnivore interfeces, and spatial differences in primary biomass production, this do not affected consurners nutrient content and secondary production. The consequences of nutrient supply on plant stoichiometry, growth rate, and consumers stoichiornetry highlight the main link between physiological and ecological processes and the consequences of nutrient limitation in terrestrial ecosysterns.