Efectos ecológicos de la heterogeneidad espacial y predecibilidad en la distribución de los recursos carroñas y gremios de carroñeros

Resum

The ecological consequences of spatio-temporal distribution and abundance of trophic resources have received significant attention in ecology due to their predicted effects at the individual, population and community levels. Pulses are defined as occasional episodes of food abundance concentrated in space and time. Although these phenomena are common in many ecosystems we still have a limited understanding about their effects on the dynamics and functioning of guilds and vertebrate communities. To explore these issues we carried out research using carcasses and scavenger guilds as a study model. Carcasses are a paradigmatic pulsed resource that under natural conditions appears unpredictable. They are exploited by guilds of organisms ranging from bacteria to vultures and large carnivores. However, human activities (including certain conservation strategies) change the unpredictable conditions with foreseeable consequences in the structure and functioning of scavenger guilds. In this scenario, the main aims of this thesis were: 1) to explore the role of unpredictability (randomness) as a driver of both the diversity and the existence of facilitatory processes within an avian scavenger guild and 2) to detect the ecological consequences that occur when trophic resources become predictable, both for scavengers and for non-scavenger species. We explored the second goal using supplementary feeding stations (vulture restaurants) as our study model. Thus our results are not only of ecological significance, but can also be of great utility for the management and conservation of these species. We found that unpredictable (random) distribution of carcasses favours the diversity of scavengers and promotes the existence of intraguild facilitatory processes. Moreover, through a transcontinental approach we found that migrant scavengers change their spatial distribution patterns between the breeding (in Europe) and the wintering areas (in Africa) consistently following the degree of food aggregation. Moreover, we detect higher overlap in interspecific diets in Europe, where resources are scarce, clumped and predictable. The availability of long-lasting carcasses provokes the aggregation of large numbers of facultative scavengers, increasing the risk of predation on other vertebrates that live around these areas. Finally, we examined conservation consequences of the concentration of carcasses at supplementary feeding stations (vulture restaurants) promoted by current European sanitary legislation. We found that clumped and predictable food resources cause the co-occurrence of scavenger species at these points provoking an overlap in their diets, which is otherwise segregated. Moreover, supplementary feeding stations favour the gathering of individuals of the dominant species (the griffon vulture Gyps fulvus), which monopolize food to the detriment of small scavengers of conservation concern. To minimize these negative effects we provide guidelines for an appropriate design and propose that adaptive management is key for the most effective application of this widely used conservation tool. In conclusion, this study shows that changes in the distribution and predictability of food resources have important consequences for the structure and functioning of the scavenger guilds as well as other vertebrate communities not directly related to the exploitation of this resource. Studies such as this thesis, which deal with the issue from different perspectives (including those relating to conservation) are particularly relevant in understanding the phenomenon.