Assessing Nutritional Resources for Dung Beetles - Optimising Ecosystem Services

Abstract

<p>In terrestrial systems, dung beetles mediate several important ecological processes (e.g. nutrient cycling, bioturbation, soil friability and seed dispersal). After finding a dung source, dung beetles quickly relocate the manure, either by burying it under the soil, or rolling it away from the dung pat for feeding and nest building. Dung is the only source of nutrition for most species of dung beetles. Animal dung consists of complex organic compounds produced from semi-digested and wasted feed, along with simple organic and inorganic compounds generated in the intestinal tract of animals. Dung nutrients are important in dung beetle development, growth and reproduction. The nature of cattle dung as a food resource for dung beetles is poorly understood, despite it being a vital resource for these ecologically important insects. Therefore, the current study assessed dung quality parameters in cattle dung over 12 months to evaluate any differences in chemical and physical attributes of dung, and to predict the nutritional availability for dung beetles throughout the year. Dung collected during the warmer parts of the year has a high moisture content, with significant variation in dung moisture observed over the 12 months. The measured chemical characteristics of cattle dung also showed variation across the seasons, but there were no uniform trends obtained (i.e. each parameter showed a different trend and no generalized trends could be seen). I also examined the relationship between measured dung quality parameters and beetle performance by evaluating the reproductive output of adult <i>Onthophagus binodis</i> Thunberg when fed dung collected during four different months (chosen to represent the four seasons). There was a clear relationship between variation in some mineral elements in the cattle dung (N, P, Mg, Cu, Mn), and the reproductive output of beetles.</p> <p>Dung beetle distribution may be influenced by the type of dung available, and the ability of dung beetles to detect and select different resource types. Here, I monitored volatile compounds present in cattle dung collected over 12 months to investigate the abundance of these volatile compounds across the year, and the role of these volatiles in the attractiveness of dung to beetles. I used gas chromatography mass spectroscopy (GCMS) to record the volatiles emitted from cattle dung that were collected by solid-phase micro extraction sampling (SPME). Dung volatiles were dominated by terpenes, alkanes, ketones, phenolic compounds, volatile fatty acids, sulfurous compounds, aldehydes and carboxylic acids, methyl sulphides, and indole family compounds. Dung volatiles were distributed with varying abundances over 12 months, as well as over the five dung ages examined for each month (0 hr, 2 hrs, 4 hrs, 6 hrs, and 8 hrs old).</p> <p>Finally, to investigate the role of volatile compounds as olfactory cues for dung beetles during resource selection, the antennal responses of the dung beetle<i>O. binodis</i> towards the profile of volatile organic compounds (VOCs) of cattle dung were examined. Antennal responses were recorded in electroantennogram (EAG) profiles by using coupled gas chromatography (GC)/electroantennogram detection (EAD). Eight compounds from cattle dung emissions (phenol, p-cresol, alpha-pinene, D-limonene, eucalyptol, toluene, xylene, and indole) were found to generate olfactory responses in<i>O. binodis</i>. These eight compounds could be responsible for the attraction of dung beetles towards dung resources. The current findings highlights the links between dung moisture content, nutritional quality of dung, dung volatiles preferred by dung beetles and dung beetles’ reproductive capacities.</p>