Dung Beetles: From Ecosystem Engineers to Invasive Species

Abstract

<p>Ecosystem services are the benefits provided by the natural environment and species to human beings through a set of ecological functions. One group of ecosystem service providers is the dung beetles (Coleoptera: Scarabaeidae, Aphodiidae and Geotropidae). Dung beetles are regarded as ecosystem engineers because of the multiple ecosystem functions they provide while living, feeding and using dung as habitat and food for their offspring. Dung beetles can destroy dung pats through their dung handling, nesting and feeding, which can control dung-inhabiting pests, parasites and pathogens. Dung beetles recycle nutrients from the dung into the soil, improve soil physical and chemical properties, enhance pasture growth and disperse plant seeds. Exotic dung beetles have been introduced into at least eight countries globally, including Australia, for the biological control of the volume of dung produced by large introduced herbivores. In those countries where introductions have occurred, the native dung beetles were inefficient in dealing with the wet, big, exotic dung pats. In recent years, some introduced dung beetles have been reported as being invasive and causing native species displacement and local extinction.</p> <p>This thesis is based on a general introduction and synthesis, and then five research and review chapters documenting dung beetles from ecosystem engineers to invasive species. The chapters cover the critical review of global dung beetle introduction programmes, native and exotic dung beetle assemblages in grassland habitats, interpreting dung beetle guilds as ecological indicators and association of dung beetles with soil physical-chemical properties, and finally the global prospect of dung beetles being invasive species.</p> <p>In Chapter Two, I review the deliberate and accidental introduction of dung beetle species into the Antipodes (Australia and New Zealand) and North America (Canada and the United States of America). This chapter gives an overall assessment of dung beetle species in the regions and pinpoints the gaps and overlaps in ecosystem services. These regions share a similar context of livestock introduction, human settlements and dung beetle introductions. However, there is a lack of comparable studies that consider the relative status of native and exotic dung beetle species. I assess the historical perspectives of deliberate and accidental introductions and the current state of exotic and native dung beetle species. Dung beetles are known to control at least 15 different pests and parasites. In total, 37 deliberately and 47 accidentally introduced dung beetle species have become established in the Antipodes and North America. Across the Antipodes and North America, exotic dung beetles are the most abundant species in pasturelands. I elaborate on the risks, challenges, and opportunities associated with dung beetle introduction programmes. These findings will assist in designing future dung beetle introduction programmes in terms of integrating sustainable ecosystem services, environmental integrity, biodiversity and conservation.</p> <p>In Chapter Three, I describe the findings of dung beetle sampling undertaken in native and improved grasslands at eight locations along a biogeographical gradient from 385 to 1357 m ASL of Northern Tablelands, NSW, Australia. The abundant native dung beetle taxa co-evolved with marsupial dung were inefficient in cattle dung burial. Therefore, exotic dung beetles were introduced in Australia for controlling bush flies and livestock parasites along with efficient dung burial. In the context of almost half a century of exotic dung beetle introductions, I investigated the native and introduced dung beetle assemblages along this biogeographical gradient. Dung beetles were collected using standard pitfall traps baited with native wildlife (kangaroo) and introduced livestock (cattle and sheep) dungs across autumn, spring and summer seasons. Cattle dung and sheep dung attracted a higher number of dung beetle species, individuals and biomass as compared to kangaroo dung. Cattle dung attracted a higher number of introduced dung beetle species, individuals and biomass, while sheep dung attracted more native. However, kangaroo dung was not a preference for native or introduced dung beetles. In summer, a higher number of species, number of beetles and biomass was sampled as compared to spring and autumn. Introduced dung beetle diversity and biomass was higher in lower elevational sites, while native dung beetle diversity and biomass were higher on upper elevational sites. The details of native and exotic dung beetles community, assemblages and biodiversity are important to understand their potentials in handling exotic and native animal dung resources in the grasslands.</p> <p>Chapter Four investigates the structure and composition of dung beetle assemblages based on their nesting behaviour, diel activity, body size, and provenance of the species in the Northern Tablelands over three seasons. The analysis of dung beetle assemblages assesses ecosystem services, competition, and interaction among the species. I found that tunnellers were the predominant guild followed by dwellers and rollers. Regarding the diel groups, diurnal beetles were predominant, followed by nocturnal and crepuscular taxa. Based on body-size, small-sized predominated and introduced outnumbered native based on provenance, although these groupings are not mutually exclusive. This chapter indicates the gaps and overlaps in dung beetle assemblages across the Northern Tableland landscapes. In designing exotic dung beetle introduction programmes that complement the existing dung beetle fauna, landscape-level in-depth studies are recommended. </p> <p>In Chapter Five, I tested the hypothesis that dung beetle species occurrence and endemism has a strong association with habitat soil and its physical and chemical characteristics. To investigate this, I analysed dung beetle monitoring data and investigated their association with the physical (soil textures and soil moisture) and chemical (pH, organic carbon, nitrogen) soil properties. I use canonical correlation and principal component analyses to investigate the association between dung beetle (nesting behaviour, diel activity and provenance) and soil variables. I analysed dung beetle assemblages and established an association between species richness, abundance and biomass, and soil variables. Soil moisture has a positive correlation with native, tunneller and diurnal beetle abundance while it has a negative correlation with nocturnal abundance. Soil organic carbon has a positive correlation with diurnal abundance and pH has with crepuscular abundance. Except for soil moisture, there was no association between soil characteristics and species richness. Soil moisture, soil pH, and the proportion of silt, clay and organic carbon are significantly correlated with dung beetle biomass. In this chapter, I further discuss the probable association between soil characteristics and dung beetle assemblages—taking grassland habitats into account</p> <p>Chapter Six investigates dung beetles from invasion perspectives. Dung beetle introduction programmes provided exotic dung beetle species with an opportunity to cross natural barriers and spread beyond their native distribution range. Few dung beetle species became more successful invaders than others beyond their native range. Here, I assess what probable adaptation mechanisms and natural history traits enable these dung beetle species to be more successful invaders than others. Based on their spread, predominance, distribution and invasion reports, I have recognised four species, namely <i>Digitonthophagus gazella, Onthophagus taurus, Euoniticellus intermedius</i> and <i>Aphodius fimetarius</i> as invasive dung beetles. I describe which of the natural history traits of these species have perhaps increased their fitness, making them successful invaders. I recommend an in-depth understanding of exotic species and a baseline of native dung beetle assemblages within the design and before the implementation of introduction programmes.</p>