Scavenger assemblages in the neotropicsecological and behavioral patterns

Resumen

Understanding the processes that allow several species depending on the same resource to coexist are fundamental in the maintenance of biodiversity, ultimately affecting ecosystem functions, and being a key issue in community ecology. The non-random structure of natural communities is driven by multiple biological, ecological, and evolutionary forces, and can be described by habitat utilization, resource availability, and the activity and interactions of the organisms that compose them. These interactions can be antagonistic (e.g., competition) or facilitative (e.g., mutualistic plant-pollinator). However, not all species within a network play the same ecological role or perform it with the same importance, since they will have different ecological traits. Competition between species that coexist within the same guild is particularly interesting because the similarity of their ecological niches increases competition strength. However, positive co-occurrence patterns within guilds may also emerge from facilitation processes, reducing competition and playing an important role in community structure. Carrion is an ephemeral and unpredictable resource in time and space. These characteristics allow a multitude of species to feed on the resource, even forming temporary mixed aggregations with high levels of competitive and facilitative interactions. Such dynamics make scavenger systems an ideal model for studying positive and negative processes across scales using diverse methodological approaches. This thesis focuses on describing the ecological factors and behavioral patterns that govern a scavenger community in the Neotropics, by the placement and monitoring of two carcass sizes in the field by using camera-trapping. Specifically this thesis aims to assess the following objectives: describe for the first time the scavenger community and identify the factors affecting scavenging efficiency in the Brazilian Cerrado (Chapter 3); infer interspecific processes of competition and facilitation through the study of species co-occurrence patterns in this Neotropical guild (Chapter 4); analyze possible information transmission cascades within this Neotropical scavenger community (Chapter 5); and determine factors driving temporal dynamics of scavenging successions (Chapter 6). In Chapter 3, we describe the vertebrate scavenger community composition of the Brazilian Cerrado, a biodiversity hotspot. In addition, we analyzed the effects of vegetation cover, time of carcass placement and carcass weight, on different variables related to community composition and efficiency. We documented a total of 19 vertebrate scavenging species, four species of vultures and 15 facultative scavengers. Carcass size was the most important factor affecting the scavenger assemblage and consumption patterns, while we did not find an effect of habitat or timing of carrion placement on scavenging patterns. The results show a highly diverse and efficient scavenging vertebrate community in the Brazilian Cerrado, and the need to preserve them in the face of the significant habitat transformations suffered by this biodiversity hotspot. In Chapter 4, we analyzed patterns of spatial and temporal co-occurrence between species, both qualitatively and quantitatively, and determined the activity patterns of the different scavenger species. Our results show complex competitive and facilitative relationships among scavenging species in the Brazilian Cerrado that are influenced by carcass size, and change depending on the spatial and temporal scale at which they are analyzed. The scavenger assemblages that consumed large and small carcasses were different, evidencing resource partitioning between obligate and facultative scavengers. Furthermore, as an alternative to reduce competition levels, most species showed differences in their scavenging patterns, in addition to a strong temporal segregation during carcass consumption. Regarding New World vultures, our results suggest a strong interference competition between species with clear differences in their ecological traits (e.g., size, social behavior). However, we also found evidence of facilitation processes between vulture species in the location and access to the interior of the carcasses. Our findings highlight the role of obligate scavengers both in competition and facilitation processes in this vertebrate scavenger community. To clarify the processes that result in associations between vulture species with different foraging efficiencies, in Chapter 5, we apply a survival-modelling strategy to determine the transmission of social information among different species during carrion location. The use of different senses (smell and sight) within this guild facilitates carcass location through the transmission of social information between species with different carrion foraging efficiencies. Vultures with a highly developed sense of smell play a key role in this process, as they are the first ones to arrive at the carcasses and their presence seems to serve as a visual cue for other species to locate the resource. Our study supports the local enhancement hypothesis within scavengers, whereby individuals locate carcasses by following foraging heterospecifics, also suggesting the importance of the sense of smell in the maintenance of the community structure. In Chapter 6, we perform the first in-depth analysis of the factors driving temporal dynamics of scavenging successions by using interspecific aggressions as a behavioral proxy of competition intensity. The results show that resource availability shapes behavioral interactions between species. Furthermore, facilitation was related to moments of higher tolerance (i.e., lower aggressiveness), thus reducing competition intensity and affecting community structure and dynamics. Our study highlights the importance of monitoring behaviors that are directly transferable to community function (e.g., those related to foraging and resource consumption), also considering the dynamics of succession over time. This novel framework evidences complex ephemeral successional processes characterized by a fluctuation in facilitation and competition intensity during the consumption of an unpredictable resource linked to key ecosystem processes. Finally, the general discussion (Chapter 7) addresses the implications of the findings obtained in the previous chapters, its contribution to the study of communities from a general perspective, and for scavenger assemblages, including conservation implications. Furthermore, we examine the limitations identified and outline avenues for future research aimed at comprehending the determinants of the balance between positive and negative processes among coexisting species.