O PAPEL DO COMPORTAMENTO NA OCORRÊNCIA DE CASCATAS TRÓFICAS

Trophic cascades are well accepted mechanisms of top-down regulation in food webs, although its ultimate causes are still in debate. Trophic cascades can emerge through direct effects between populations, where predator consume their prey, decreasing the abundance of preys that affect lower trophic levels. However, trophic cascades can also emerge through indirect behaviorally mediated effects, so that the prey alter its foraging regime in response to predation risk. Such behavioral shifts affect prey foraging effort, reducing its impact on resources. This study presents a synthesis of empirical evidences that support behaviorally mediated indirect interactions as ultimate causes of trophic cascades in a wide range of ecosystems. It was observed that behavioral prey responses in the presence of predators are common in a wide array of ecosystems, strongly affecting patterns at the community and ecosystems levels. It's also noticed that the relative contribution of density or behaviorally mediated effects depends on the biotic or abiotic environmental context. As a conclusion, based on the empiral and theorectical data gathered until now, the use of the behavioral perspective on the food web theory is necessary as an attempt to elucidate the mechanisms that rule trophic cascades and the functioning and structure of ecological communitiesA cascata trófica é reconhecidamente um importante mecanismo na regulação em cadeias tróficas embora um grande debate exista a respeito dos mecanismos finais que determinam a ocorrência deste processo. Cascatas tróficas podem emergir a partir de efeitos diretos entre populações, no qual predadores consomem suas presas e, portanto, diminuindo a abundância de presas que influenciam níveis tróficos inferiores. Porém, as cascatas tróficas também podem emergir a partir de efeitos comportamentais indiretos, nos quais as presas alteram seu comportamento de forrageamento em resposta ao risco de predação. Estas mudanças comportamentais afetam o esforço de forrageamento da presa, diminuindo seu impacto sobre os recursos. Neste trabalho é apresentada uma síntese das evidências empíricas que suportam o papel das interações indiretas mediadas por mudanças comportamentais, como mecanismos causais da expressão de cascatas tróficas em diferentes ecossistemas. Observa-se que respostas comportamentais das presas em função da presença de predadores são comuns em uma grande variedade de ecossistemas afetando de forma significativa padrões observados no nível de comunidades e de ecossistemas. É também observado que a contribuição relativa dos efeitos mediados pela densidade ou por mudanças comportamentais pode variar dependendo do contexto biótico ou abiótico em que estas interações estão inseridas. Conclui-se como base no grande número de evidências levantadas que a incorporação da perspectiva comportamental na teoria de cadeias tróficas se faz necessária na tentativa de elucidar os mecanismos que controlam as cascatas tróficas bem como o funcionamento e a estrutura de comunidades ecológicas

Simulating land use changes, sediment yields, and pesticide use in the Upper Paraguay River Basin: Implications for conservation of the Pantanal wetland

As a consequence of accelerated and excessive use of pesticides in tropical regions, wilderness areas are under threat; this includes the Pantanal wetlands in the Upper Paraguay River Basin (UPRB). Using a Land Cover Land Use Change (LCLUC) modelling approach, we estimated the expected pesticide load in the Pantanal and the surrounding highlands region for 2050 under three potential scenarios: i) business as usual (BAU), ii) acceleration of anthropogenic changes (ACC), and iii) use of buffer zones around protected areas (BPA). The quantity of pesticides used in the UPRB is predicted to vary depending on the scenario, from an overall increase by as much as 7.4% in the UPRB in the BAU scenario (increasing by 38.5% in the floodplain and 6.6% in the highlands), to an increase of 11.2% in the UPRB (over current use) under the AAC scenario (increasing by 53.8% in the floodplain and 7.5% in the highlands). Much higher usage of pesticides is predicted in sub-basins with greater agricultural areas within major hydrographic basins. Changing the current trajectory of land management in the UPRB is a complex challenge. It will require a substantial shift from current practices, and will involve the implementation of a number of strategies, ranging from the development of new technologies to achieve changes in land use policies, to increasing dialogue between farmers, ranchers, the scientific community, and local or traditional communities through participatory learning processes and outreach

The several faces of fear: ecological consequences of predation risk in a lagoon model system

AIM: The aim of this study was to evaluate the role of predation risk on the occurrence of trophic cascades in a benthic food chain, and detect if the ecological consequences of predation risk can reverberate in patterns observed across different hierarchical scales, such as prey size, prey growth efficiency and nutrient recycling patterns. METHODS: The model system used in the present experiment consisted of a simple linear food chain comprising a predator, a consumer and periphyton as basal resources. For 2 weeks, we manipulated predation risk using caged predators, incapable of killing their prey, across twelve outdoor mesocosms, simulating natural lagoon conditions. RESULTS: Our results showed that predation risk can be responsible for the occurrence of a trophic cascade and the strength of the cascade is proportional to the intensity of risk. Predation risk can also negatively influence prey biomass and growth efficiency as well as affect nutrient recycling patterns by altering prey nutrient excretion rates. Through a simple mathematical formulation, we attempted to show that individual-level experimental results can be generalized to natural populations if evolutionary constraints to prey fitness can be reproduced in experimental conditions. CONCLUSIONS: Our results corroborate to integrate ecosystem dynamics with animal behavior, highlighting that not only bottom-up but also top-down mechanisms are responsible for determining ecosystem properties. We ultimately claim that prey adaptive foraging may serve to integrate ecosystem and evolutionary ecology, resulting in the development of a more robust and predictive theory of the functioning of aquatic ecosystems

Effects of food web structure and resource subsidies on the patterns and mechanisms of temporal coherence in a tropical coastal lagoon: an experimental mesocosm approach

AIM: The study of the patterns and mechanisms of temporal coherence of ecological variables among lakes has become an important area of limnology. However, no study to date has experimentally tested whether and how resource subsidies and food web configuration affect the patterns and mechanisms of temporal coherence of limnological variables. We conducted a field mesocosm experiment to test the following hypotheses: (i) nutrient enrichment would reduce the temporal coherence of system variables; (ii) fish predation would enhance the temporal coherence of system variables; and (iii) the strength of temporal coherence decreases from physical (water transparency), to chemical (dissolved oxygen concentration [DO]) to biological variables (total zooplankton biomass). METHODS: For 11 weeks, we manipulated fish presence and nutrient (N and P) concentration in a 2 × 2 factorial design in sixteen within-lake enclosures installed in a tropical coastal lagoon. Coherence was estimated by pair-to-pair Pearson's moment correlations of the temporal trajectories of each response variable among enclosures of the same treatment. RESULTS: Fish presence only enhanced the temporal coherence of zooplankton biomass, whereas contrary to our expectations, nutrient addition enhanced the temporal coherence of [DO]. The strength of the individual effects of fish and nutrients on temporal coherence was affected by variable identity, but this variation did not occur in a consistent pattern across variables. However, the interactive effects of fish and nutrients on the temporal coherence of the three variables monitored were not statistically significant. CONCLUSIONS: Our results indicate that local factors, such as fish presence and nutrient availability, may affect the temporal coherence of several system variables, but these effects are better predicted by the strength of direct interactions between the local factor and the variable than by the identity of the variable itself. We conclude that eutrophication and overfishing may alter the coupling of spatial and temporal dynamics of some ecosystem variables

Early stage litter decomposition across biomes

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVINInternational audienceThrough litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging fro