Dung beetle-mammal associations: methods, research trends and future directions

Mammal dung-dung beetle trophic networks: an improved method based on gut-content DNA

Background

Dung beetles provide many important ecosystem services, including dung decomposition, pathogen control, soil aeration, and secondary seed dispersal. Yet, the biology of most dung beetles remains unknown. Natural diets are poorly studied, partly because previous research has focused on choice or attraction experiments using few, easily accessible dung types from zoo animals, farm animals, or humans. This way, many links within natural food webs have certainly been missed. In this work, we aimed to establish a protocol to analyze the natural diets of dung beetles using DNA gut barcoding.

Methods

First, the feasibility of gut-content DNA extraction and amplification of 12s rDNA from six different mammal dung types was tested in the laboratory. We then applied the method to beetles caught in pitfall traps in Ecuador and Germany by using 12s rDNA primers. For a subset of the dung beetles caught in the Ecuador sampling, we also used 16s rDNA primers to see if these would improve the number of species we could identify. We predicted the likelihood of amplifying DNA using gut fullness, DNA concentration, PCR primer, collection method, and beetle species as predictor variables in a dominance analysis. Based on the gut barcodes, we generated a dung beetle-mammal network for both field sites (Ecuador and Germany) and analyzed the levels of network specificity.

Results

We successfully amplified mammal DNA from dung beetle gut contents for 128 specimens, which included such prominent species as Panthera onca (jaguar) and Puma concolor (puma). The overall success rate of DNA amplification was 53%. The best predictors for amplification success were gut fullness and DNA concentration, suggesting the success rate can be increased by focusing on beetles with a full gut. The mammal dung-dung beetle networks differed from purely random network models and showed a moderate degree of network specialization (H2': Ecuador = 0.49; Germany = 0.41).

Conclusion

We here present a reliable method of extracting and amplifying gut-content DNA from dung beetles. Identifying mammal dung via DNA reference libraries, we created mammal dung-dung beetle trophic networks. This has benefits over previous methods because we inventoried the natural mammal dung resources of dung beetles instead of using artificial mammal baits. Our results revealed higher levels of specialization than expected and more rodent DNA than expected in Germany, suggesting that the presented method provides more detailed insights into mammal dung-dung beetle networks. In addition, the method could have applications for mammal monitoring in many ecosystems.

The joint effects of forest habitat area and fragmentation on dung beetles

Habitat loss and habitat fragmentation usually occur together, at the same time and place. However, while there is a consensus that habitat loss is the preeminent threat to biodiversity, the effects of fragmentation are contentious. Some argue that habitat fragmentation is not bad for biodiversity, and even that it is good. Generally, the studies that find no harm or positive outcomes of fragmentation invariably assume that it is independent of habitat loss. However, dissociating the effects of habitat fragmentation from habitat loss is questionable because the two are essentially coupled. Accordingly, we evaluated how forest area and fragmentation (via edge effects) influenced dung beetles per se, and through their effects on the abundance of mammals, using structural equation modeling (SEM). Dung beetles are very sensitive to forest habitat loss and fragmentation and to changes in the abundance of mammals on which they depend for dung. Our study area was in the Tana River, Kenya, where forest fragments are depauperated of mammals except for two endemic species of monkeys. We mapped 12 forests, counted the resident monkeys, and sampled 113,955 beetles from 288 plots. Most of the 87 species of beetles found were small tunnellers. After implementing a fully latent Structural Regression SEM, the optimal model explained a significant 26% of the variance in abundance, and 89% of diversity. The main drivers of beetle abundance were positive, direct, effects of forest area and number of monkeys, and negative edge effects. The main drivers of diversity were the direct effects of the beetle abundance, indirect effects of forest area and abundance of mammals, and indirect negative edge effects. Thus, forest area, fragmentation (via edge effects), and the number of monkeys jointly influenced the abundance and diversity of the beetles directly and indirectly.

Bait attractiveness changes community metrics in dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae)

Species relative abundance (SRA) is an essential attribute of biotic communities, which can provide an accurate description of community structure. However, the sampling method used may have a direct influence on SRA quantification, since the use of attractants (e.g., baits, light, and pheromones) can introduce additional sources of variation in trap performance. We tested how sampling aided by baits affect community data and therefore alter derived metrics. We tested our hypothesis on dung beetles using data from flight interception traps (FITs) as a baseline to evaluate baited pitfall trap performance. Our objective was to assess the effect of bait attractiveness on estimates of SRA and assemblage metrics when sampled by pitfall traps baited with human feces.Dung beetles were sampled at three terra firme primary forest sites in the Brazilian Amazon. To achieve our objective, we (i) identified species with variable levels of attraction to pitfall baited with human feces; (ii) assessed differences in SRA; and (iii) assessed the effect of bait on the most commonly used diversity metrics derived from relative abundance (Shannon and Simpson indices). We identified species less and highly attracted to the baits used, because most attracted species showed greater relative abundances within baited pitfall traps samples compared with our baseline. Assemblages sampled by baited pitfall traps tend to show lower diversity and higher dominance than those sampled by unbaited FITs. Our findings suggest that for ecological questions focused on species relative abundance, baited pitfall traps may lead to inaccurate conclusions regarding assemblage structure. Although tested on dung beetles, we suggest that the same effect could be observed for other insect taxa that are also sampled with baited traps. We highlight a need for further studies on other groups to elucidate any potential effects of using baits.

Toward a standardized methodology for sampling dung beetles (Coleoptera: Scarabaeinae) in the Neotropics: A critical review

Introduction

The standardization of sampling protocols is imperative for robustly studying any taxonomic group. Replicable methods allow the comparison of data between different spatial and temporal studies. In the case of dung beetles, one of the best-studied indicator groups in analyses of environmental disturbance, a wide range of collection methodologies are used, from basic pitfall traps to more complex or complementary methods such as mini-Winkler extractor. Also, different types of attractive baits, sampling effort, durations, and designs are used in dung beetle studies. Variations in methodological approaches are particularly noted in the Neotropics, which may be related to the vast number of biological strategies and behavior of dung beetles that inhabit this region. A lack of methodological unification for the Neotropical region makes a cross-sectional analysis of the information impossible.

Methods

We performed a compilation and analytical review of the existing literature for dung beetle sampling in the Neotropics, discussing the most used methodologies, their advantages and disadvantages, and specific cases in which particular models are more efficient.

Results

Pitfall traps baited with human excrement are the most common sampling method, but there is a wide range of models and variations in the structure of this trap. The complementary effect generated by flight interception traps, light traps, and direct collections, particularly within microhabitats, is exciting for the potential of finding new species. Some methodologies, such as mini-Winkler extractor, fogging, or very specific baits, are infrequently used.

Discussion

There was a lack of inclusion of spatial and temporal variation among studies. Therefore, it is necessary to consider broader sampling windows, which include different spatial scales, seasons, and years. Finally, we propose a standard protocol for sampling dung beetles in the Neotropics, depending on each objective, and including a basic methodology for obtaining complete local inventories.

A trait-based framework for dung beetle functional ecology

Traits are key for understanding the environmental responses and ecological roles of organisms. Trait approaches to functional ecology are well established for plants, whereas consistent frameworks for animal groups are less developed. Here we suggest a framework for the study of the functional ecology of animals from a trait-based response-effect approach, using dung beetles as model system. Dung beetles are a key group of decomposers that are important for many ecosystem processes. The lack of a trait-based framework tailored to this group has limited the use of traits in dung beetle functional ecology. We review which dung beetle traits respond to the environment and affect ecosystem processes, covering the wide range of spatial, temporal and biological scales at which they are involved. Dung beetles show trait-based responses to variation in temperature, water, soil properties, trophic resources, light, vegetation structure, competition, predation and parasitism. Dung beetles' influence on ecosystem processes includes trait-mediated effects on nutrient cycling, bioturbation, plant growth, seed dispersal, other dung-based organisms and parasite transmission, as well as some cases of pollination and predation. We identify 66 dung beetle traits that are either response or effect traits, or both, pertaining to six main categories: morphology, feeding, reproduction, physiology, activity and movement. Several traits pertain to more than one category, in particular dung relocation behaviour during nesting or feeding. We also identify 136 trait-response and 77 trait-effect relationships in dung beetles. No response to environmental stressors nor effect over ecological processes were related with traits of a single category. This highlights the interrelationship between the traits shaping body-plans, the multi-functionality of traits, and their role linking responses to the environment and effects on the ecosystem. Despite current developments in dung beetle functional ecology, many knowledge gaps remain, and there are biases towards certain traits, functions, taxonomic groups and regions. Our framework provides the foundations for the thorough development of trait-based dung beetle ecology. It also serves as an example framework for other taxa.

Effects of dung beetle activity on tropical forest plants

Dung beetles are recognized as providers of important ecosystem functions, most of which are derived from the removal of vertebrate dung from the soil surface. These insects occur in nearly all terrestrial biomes but are most diverse in the humid tropics. Several of the ecological functions attributed to dung beetles are related to their direct and indirect interactions with plants. Among these functions, the secondary dispersal of seeds defecated by mammals has received the most attention in tropical forests. Nonetheless, while several aspects of secondary seed dispersal by dung beetles are relatively well understood, others remain understudied or have not been addressed at all. Thus, a broad generalization about the effects of secondary seed dispersal by dung beetles on plant fitness remains somewhat elusive. Furthermore, other effects of dung beetle activity on tropical plants have received very little attention. A few studies have shown that through their behaviors of dung burial and soil-excavation, dung beetles can shape seed bank structure and dynamics. Also, though numerous greenhouse studies and field experiments in agricultural lands and temperate grasslands have shown that dung beetle activity increases plant nutrient uptake and yield, it is uncertain whether such effects are common in tropical forests. Here, we review and synthesize our current knowledge on how dung beetles affect tropical forest plants by dispersing defecated seeds, shaping the structure and dynamics of seed banks, and influencing the performance of understory seedlings. We focus on the Neotropics, where most studies on the effects of dung beetles on tropical forest plants have been carried out, but we also show results from other regions and biomes, to present a more general picture of these beetle-plant interactions. Throughout the review we emphasize aspects that need more research to allow generalizations and point out those questions that remain unanswered. We hope that this review will stimulate more research about the fascinating interactions between dung beetles and plants in tropical ecosystems.

Human Disturbance Affects Dung Beetle Assemblages in French Guiana Forests

French Guiana forests are threatened by increasing human activity such as infrastructure development, facilitating access to the forest and, therefore, logging, mining, farming and hunting. To highlight the impact of human pressure on the forest fauna, dung beetle assemblage was analyzed near Saint-Georges-de-l’Oyapock and compared with other sites in French Guiana, considering the distance to the main city and forest cover loss as proxies of human activities. Hill numbers and beta diversity were calculated. Non-metric multidimensional scaling and redundancy analyses were carried out to disentangle the effect of the distance to the nearest city and forest cover loss as proxies of human pressure, but also temperature and rainfall as proxies of climatic variations on dung beetle assemblage. Species richness increased significantly with the distance to the nearest city and decreasing forest cover loss. Assemblage structure varied among sites mainly with distance to the nearest city but also with rainfall. It varied also with forest cover loss, but not significantly. This study showed that human disturbances and climatic conditions, even if represented by proxies, affected dung beetle assemblage structures in French Guiana forests.

Density-dependent ecosystem service delivery under shifting temperatures by dung beetles

Increases in the frequency and magnitude of suboptimal temperatures as a result of climate change are subjecting insects to unprecedented stresses. This may negatively affect their fitness and the efficiency of their ecosystem service provision. Dung beetles are ecosystem service providers: through feeding on and burying dung, they facilitate nutrient recycling, secondary seed dispersal, parasite control, soil bioturbation and dung decomposition. As such, prediction of how dung beetles respond to multiple anthropogenic environmental changes is critical for the conservation of ecosystem services. Here, we quantified ecosystem services via dung utilisation and dung ball production in three telecoprid species: Allogymnopleurus indigaceous, Scarabaeus zambezianus and Khepher prodigiosus. We examined ecosystem service efficiency factorially under different beetle densities towards different dung masses and under three temperature treatments (21 °C, 28 °C and 35 °C). Khepher prodigiosus, exhibited greatest dung utilisation efficiency overall across dung masses, compared to both S. zambezianus and A. indigaceous. Dung removal was exhibited under all the tested temperatures by all tested species, and therefore the sub-optimal temperatures employed here did not fully inhibit ecosystem service delivery. However, emergent effects among temperatures, beetle species and beetle density further affected removal efficiency: S. zambezianus and A. indigaceous utilisation increased with both warming and beetle density, whereas K. prodigiosus performance was less temperature- and density-dependent. Beetles also tended to exhibit positive density-dependence as dung supply increased. The numbers of dung balls produced differed across species, and increased with temperature and densities, with S. zambezianus producing significantly most balls overall. Our study provides novel evidence for differential density-dependent ecosystem service delivery among species across stressful temperature regimes and emergent effects for dung mass utilisation. This information is essential for biodiversity-ecosystem-function and is critical for the conservation of functionally efficacious species, with implications for natural capital conservation policy in rapidly changing environments.

Dung beetle community assemblages in a southern African landscape: niche overlap between domestic and wild herbivore dung

Dung beetles provide important ecosystem functions in semiarid environments, improving the physiochemical characteristics of the soil through tunnelling and burying nutrient-rich dung. In sub-Saharan Africa, diverse indigenous mammal communities support highly abundant dung beetle populations in savannah ecosystems. However, the conversion of landscapes to livestock agriculture may result in changes in the abundance and diversity of wild mammal species. This is likely to have significant impacts on dung beetle communities, particularly because domestic livestock dung may be contaminated with toxic residues of veterinary parasiticides. The environmental impact is likely to be affected by the degree of niche overlap between the beetle communities that colonize cattle dung and those that colonize the dung of wild mammals. We compared dung beetle communities between a pristine national park habitat dominated by large wild herbivores, and a pastoral farming community dominated by domestic livestock. Diurnal dung beetles were attracted to cattle dung in greater abundance and diversity compared to elephant, zebra or giraffe dung. Nocturnal/crepuscular dung beetles were attracted to non-ruminant dung (elephant and zebra) in higher abundance compared to ruminant dung (cattle and giraffe). Although there were no clear trophic specializations, three diurnal species showed an association with cattle dung, whereas eight nocturnal/crepuscular species showed an association with non-ruminant (elephant and zebra) dung. Diurnal species may be at greater risk from the toxic effects of residues of veterinary parasiticides in domestic livestock dung. Although many species showed trophic associations with wild herbivore dung, these beetles can utilize a wide range of dung and will readily colonize cattle dung in the absence of other options. As more land is converted to livestock agriculture, the contamination of dung with toxic residues from veterinary parasiticides could therefore negatively impact the majority of dung beetle species.

Tropical forest dung beetle-mammal dung interaction networks remain similar across an environmental disturbance gradient

Conservation outcomes could be greatly enhanced if strategies addressing anthropogenic land-use change considered the impacts of these changes on entire communities as well as on individual species. Examining how species interactions change across gradients of habitat disturbance allows us to predict the cascading consequences of species extinctions and the response of ecological networks to environmental change. We conducted the first detailed study of changes in a commensalist network of mammals and dung beetles across an environmental disturbance gradient, from primary tropical forest to plantations, which varied in above-ground carbon density (ACD) and mammal communities. Mammal diversity changed only slightly across the gradient, remaining high even in oil palm plantations and fragmented forest. Dung beetle species richness, however, declined in response to lower ACD and was particularly low in plantations and the most disturbed forest sites. Three of the five network metrics (nestedness, network specialization, and functionality) were significantly affected by changes in dung beetle species richness and ACD, but mammal diversity was not an important predictor of network structure. Overall, the interaction networks remained structurally and functionally similar across the gradient, only becoming simplified (i.e., with fewer dung beetle species and fewer interactions) in the most disturbed sites. We suggest that the high diversity of mammals, even in disturbed forests, combined with the generalist feeding patterns of dung beetles, confer resilience to the commensalist dung beetle-mammal networks. This study highlights the importance of protecting logged and fragmented forests to maintain interaction networks and potentially prevent extinction cascades in human-modified systems.

From taxonomy to ecology: Entomologists are essential in environmental science. Comment on Zhou et al. (2019)

The present paper commented on Zhou et al. (2019) published on Science of the Total Environment. The authors measured the Pb concentration and bioaccumulation factor (BAF) of a few species of insects collected from Sichuan, China. However, based on the photos of the graphical abstract, we doubt the identification of the species. The biomass of the dung beetles was re-measured based on the specimens from Taiwan of the same species. The results are highly different than the authors, which positively affected the results and discussion. The experimental design should be based on actual food chains along with basal entomological knowledge. In conclusion, we recommend that studies on interactions between insects and the environment are conducted through collaboration with entomologists to ensure taxonomy and functional ecology are correctly identified.

Dung beetles as samplers of mammals in Malaysian Borneo-a test of high throughput metabarcoding of iDNA

Invertebrate-derived DNA (iDNA) sampling in biodiversity surveys is becoming increasingly widespread, with most terrestrial studies relying on DNA derived from the gut contents of blood-feeding invertebrates, such as leeches and mosquitoes. Dung beetles (superfamily Scarabaeoidea) primarily feed on the faecal matter of terrestrial vertebrates and offer several potential benefits over blood-feeding invertebrates as samplers of vertebrate DNA. Importantly, these beetles can be easily captured in large numbers using simple, inexpensive baited traps, are globally distributed, and occur in a wide range of habitats. To build on the few existing studies demonstrating the potential of dung beetles as sources of mammalian DNA, we subjected the large-bodied, Bornean dung beetle (Catharsius renaudpauliani) to a controlled feeding experiment. We analysed DNA from gut contents at different times after feeding using qPCR techniques. Here, we first describe the window of DNA persistence within a dung beetle digestive tract. We found that the ability to successfully amplify cattle DNA decayed over relatively short time periods, with DNA copy number decreasing by two orders of magnitude in just 6 h. In addition, we sampled communities of dung beetles from a lowland tropical rainforest in Sabah, Malaysia, in order to test whether it is possible to identify vertebrate sequences from dung beetle iDNA. We sequenced both the gut contents from large dung beetle species, as well as whole communities of smaller beetles. We successfully identified six mammalian species from our samples, including the bearded pig (Sus barbatus) and the sambar deer (Rusa unicolor)—both vulnerable species on the IUCN red list. Our results represent the first use of dung beetle iDNA to sample Southeast Asian vertebrate fauna, and highlight the potential for dung beetle iDNA to be used in future biodiversity monitoring surveys.