Fire legacies in Mediterranean streams: the key roles of the riparian canopy and the top predator on food webs

Wildfires are expected to increase in frequency and intensity because of climate change and changes in land use and management. In last decades, the research on fire effects on aquatic systems has grown, but it has been mainly conducted in the short- and mid-term (<5 years). Therefore, longer time frames are needed to assess fire effects on freshwater ecosystems. For instance, fire impacts on riparian and upland forests can be long-lived and wildfires often extirpate fish populations from streams, which may not recover due to barriers within the river network. Regarding fish extirpations, it is worth noting that freshwater fish are one of the most threatened fauna worldwide, especially the small-bodied species. In this context, the overarching goal of this PhD thesis was to investigate the long-term effects of a wildfire in Mediterranean streams. Specifically, this PhD thesis covered the indirect effects of two fire legacies: (1) the riparian canopy reduction and (2) the local extinction of the top predator in these streams, the endangered small-bodied fish Barbus meridionalis. The findings of this PhD thesis showed how the fire legacy in the riparian forest accelerated leaf-litter breakdown in an intermittent Mediterranean stream eight years after fire. The opening of the riparian forest canopy by fire increased light levels and water temperatures and reduced terrestrial-to-aquatic litter inputs. The increased water temperatures engendered by removal of canopy cover enhanced microbial mediated leaf breakdown. The reduction in leaf-litter inputs probably led to lower benthic organic matter levels, bringing to the observed increased shredder aggregation in leaf packs, thereby accelerating leaf breakdown rates. Our results demonstrated that the apex consumer was functionally irreplaceable, its local extinction led to the loss of an important functional role that resulted in major changes to the ecosystem's structure and function. Our mesocom experiment showed that Barbus meridionalis absence led to 'mesopredator release, and also to 'prey release despite intraguild predation, which contrasted with traditional food web theory. Top predator extirpation also changed whole macroinvertebrate community composition and increased total macroinvertebrate density. Regarding ecosystem function, periphyton primary production decreased in apex consumer absence. Moreover, we studied the feeding ecology of B. meridionalis from a functional perspective. Our results indicated that prey morphological and behavioral traits may explain prey vulnerability to predation. Specifically, the trait-based analysis showed that 10 of the 13 traits tested significantly influenced food choice (e.g. body size, concealment, locomotion, aggregation tendency, feeding habits). In addition, the leaf bags experiment confirmed that top predator absence enhanced leaf-litter breakdown, which was caused by the increase in shredder and scraper biomass in the absence of the predatory fish top-down control. Fish absence reduced leaf fungal biomass, but did not decrease microbially mediated leaf breakdown. These results suggested that leaf fungal biomass was stimulated from the bottom-up through nutrient recycling by the top predator. This PhD thesis demonstrated that past fires may have current influence on the structure and function of Mediterranean streams. Moreover, our findings evidenced that intermittent streams can be affected by the consequences of apex consumers’ extinctions, and that the loss of small-bodied top predators can lead to complex ecosystem changes. This PhD thesis interconnected several current topics in ecology research (i.e. fire effects on aquatic systems, top predator declines, and intermittent streams). Its relevance lies in the projected increase in fires in the Mediterranean region and in the current high extinction risk of small-bodied fish in freshwater ecosystems.