Large Herbivore Dung Decomposition: Effects on Nutrient Cycling in a Tropical Forest of Southern India
Large herbivores can selectively feed on nutrient rich resources, resulting in regular deposition of high quality organic matter in the form of dung. In tropical forests such as in southern India it is estimated that large herbivores contribute to several hundred kilograms of dung on a daily basis. Decomposition of this dung and its implications on nutrient cycling in an ecosystem, has recently become a subject of interest to ecologists across the globe. However, most of our understanding on dung decomposition and its implication come from agroecosystem studies conducted on cattle dung. The identity of primary insect communities involved in dung comminution and feeding is well documented, but little is known of the processes and their impact on nutrient and carbon dynamics. This thesis titled- Large Herbivore Dung Decomposition: Effects on Nutrient Cycling in a Tropical Forest of Southern India is based on dung decomposition experiments conducted on dung of three large herbivores cheetal (Axis axis), gaur (Bos gaurus) and elephant (Elephas maximus) in a tropical forest of the Western Ghats in Mudumalai Wildlife Sanctuary. We had set out with three primary objectives: 1) To identify the dung feeding insect communities, 2) Quantify the changes in dung composition during the decomposition and 3) Identify the impact of dung beetles on nutrient leachate and organic matter inclusion into the soil. To address these objectives, we carried out in-situ and ex-situ experiments in a tropical forest of southern India, the Mudumalai national park, for three large herbivores- elephant, gaur and cheetal, that constitute the major herbivore biomass of the region. We found two insect communities, termites and dung beetles, actively feeding on dung. However, their community composition, diversity and abundance varied with the age of the dung and the seasons. While small tunneller dung beetle were the most abundant insect community commonly found in all kinds of dung across seasons and various forests, termites were found to be very specific, found only in elephant dung in dry season. During the course of the experiments, we found that crude carbon is readily reduced but remains unaffected by seasons and across the three forest types (dry thorn, dry deciduous and moist deciduous) of the study area. We also analysed the recalcitrant component of carbon, lignin and easy to degrade, monosaccharides to understand what forms of carbon may be reduced in this process. Monosaccharides remained unaffected during the experiments, but lignin was readily reduced across habitats and seasons especially in dry conditions. The final experiment looking at the leachates from dung into the soil showed considerable difference between the herbivore dungs, but no effect of the dung beetle activity compared to the controls. To our knowledge, this study is among the first to use a comprehensive approach to study dung decomposition and its impact on nutrient and organic matter dynamics. It also helps in building a basic understanding of the direct role of large herbivores in cycling of nutrients and lay a foundation for future studies to better understand dung-soil dynamics.