Ecological Aspects of Vertebrate Scavenging in Central Indian Forests

Abstract

Scavengers assist in the cycling of nutrients in the ecosystem. However, few studies have examined the relative importance of microbes, invertebrate and vertebrate scavengers on the decomposition of carrion. Biotic and abiotic factors have both been known to affect scavenger communities. Environmental factors such as region, climate, season, habitat type, along with resource-specific factors such as carcass size influence the species which feed on a carcass as well as the time to detection of the carcass and carcass persistence time. Size in particular plays an important role in the species feeding on a carcass, with larger vertebrate scavengers more prevalent at larger carcasses. Vultures are the only vertebrates which are obligate scavengers, relying on carrion alone as a food source. Other than vultures, all vertebrate scavengers are facultative scavengers. This includes most mammalian carnivores, which hunt for prey but also make use of carcass availability. This increases the interaction routes linking the processes of scavenging and predation, increasing stability of the food web. Vultures in particular, as the only obligate vertebrate scavengers, seem to play a special role in scavenger communities. Exclusion of vultures from carcasses has been linked with longer decomposition time, more intra-species contacts at carcasses (potentially leading to increased disease spread), and increased number of species feeding at the carcass. I carried out this study to better understand the interactions between scavengers and carrion, and to observe, if any, the effect vultures and carcass size have on scavenger communities. The study was carried out in two protected areas in Madhya Pradesh – Kanha Tiger Reserve and Panna Tiger Reserve – from December 2018 to April 2019. Carcasses of animals – cattle, chital, and sambar – killed by wild predators, as well as fresh carcasses (goats and chickens) were monitored using infrared camera traps. I also carried out an experiment to observe the comparative rate at which vertebrate, invertebrates and microbes consume carrion biomass using chicken carcasses and different treatments. Vertebrate scavengers were found to remove carrion at the highest rate, at 99% biomass per day, followed by invertebrates (4.9% per day), and microbes (2.9% biomass removed per day). Despite greater vulture presence in Panna Tiger Reserve, and a relatively low vulture population in Kanha, Analysis of Similarity could not find a significant difference in the vertebrate scavenger species assemblage (the number of species and the relative abundances of those species) that visited monitored carcasses between the two study sites. Carcass size also did not significantly affect which species visiting the carcasses. I carried out occupancy modelling to estimate the probability of detecting a carcass by individual species of the carcass. Covariates which were found to affect detection probability were canopy cover, initial age of carcass, initial weight of carcass, horizontal cover, and vulture presence at the carcass. The use of occupancy modelling for estimating detection probability of carrion for different vertebrate scavengers is a unique approach, and with more data can be highly informative of the patterns and processes that govern the relationship between species and carrion. Detection corrected, model inferred occupancy gave significant improvement over the naïve occupancy estimate for all species, suggesting that carcass detection by scavengers or low abundance of scavengers was a limiting factor for visits of carcasses by vertebrate scavengers.