When the big partner disappears – ecological consequences of megafauna loss for dung beetlesSubmitted by editor on 10 September 2018.Get the paper!
Dung beetles – tireless workers behind the scenes
Dung beetles keep our ecosystems running by processing the left-overs of bigger animals. Services of high ecological and economic importance such as nutrient cycling and soil fertilization, bioturbation and aeration as well as secondary seed dispersal and pest control are provided by armies of these tireless but mainly unappreciated workers. Dung beetles therefore represent irreplaceable components of most terrestrial natural and agro-ecosystems.
Figure 1: Dung beetles provide numerous services of high ecological and economic importance by processing dung of large animals (Photo: Andreas Schweiger).
Megafauna and dung beetles – a long lasting partnership
Dung beetles co-evolved with large-sized animals since millennia, potentially at least since dinosaurs started to forage on angiosperms in the mid-Cretaceous and depend on the feces of big animals ever since. This “dookie-dependence” thereby scales with the body size of the dung producing and consuming animal. In other words, big dung beetles need big piles of poo which is produced by big animals, namely megafauna.
Figure 2: Large-sized dung beetles like Copris lunaris (L.) need big quantities of dung for breeding, thus, sensitively depend on large-sized, dung producing animals. The human-driven loss of megafauna and the concomitant downsizing of mammal communities during the last 50 millennia also reduced the abundance of these large-sized dung beetles in Europe and other parts of the world (Photo: Lars Brøndum).
When the big partners disappear – the ecological effects of megafauna loss
Until approx. 10,000 to 50,000 years ago, the world was dominated by animals of impressive body sizes that were important ecological partners for numerous species dependent on this megafauna. Once common, megafauna has been massively reduced in the past millennia, with human-induced downsizing still ongoing. About 60% of the Late Pleistocene megafauna species went extinct since these megafauna-rich times including numerous iconic examples like the wooly mammoth (Mammuthus primigenius), the straight-tusked elephant (Palaeoloxodon antiquus), the aurochs (Bos primigenius), and the giant deer (irish elk, Megaloceros giganteus). Nowadays, about 60% of the remaining large herbivores are classified as threatened by extinction.
These massive losses of large-sized animals would be expected to have had pronounced effects on the species dependent on megafauna as well as ecosystem functioning. However, empirical evidence for these concomitant, ecological effects of pre-historic megafauna losses has only recently begun accumulating with detrimental effects on commensal species being proposed as one of the major effects (see Galetti et al. 2018 for an extensive summary). Strong detrimental effects on dung beetles seem to be quite likely considering their close ecological and evolutionary association with large-sized animals. However, knowledge about effects of megafauna loss on dung beetles is mainly anecdotal and restricted to limited empirical evidence from short-term experimental studies for paleo records for individual dung beetle species.
To get a bigger picture about the effect of pre-historic megafauna losses on dung beetle community composition we analyzed Quaternary fossil records of dung beetle occurrence from Europe covering the last ~53,000 years. We quantified temporal changes in the body size distribution in dung beetle communities as a sensitive indicator for megafauna associated effects on beetle community composition, while also considering effects of other factors like climate. Our results show a strong down-sizing of European dung beetle communities over the last ~53,000 years. This down-sizing was not linear, but characterized by a weak decrease until the early Holocene, with a strong acceleration from 6-7000 years PB onwards. This acceleration of down-sizing thereby coincides with the final collapse of European wild megafauna and the start of major shifts in human agricultural land-use. In contrast, body size of non-coprophagous scarabids and ground beetles – two groups of beetles with no or weak relations to megafauna – increased towards the present with an acceleration coinciding with the onset of late-glacial warming (about 14,000 years BP), as expected from ectotherm-temperature relations.
The relevance of our study
Our study is the first showing that the human-induced downsizing of mammal communities can be coupled to a synchronous downsizing of dung beetle communities, on large spatial scale and over an extensive period of time. Interestingly, increasing availability of domesticated megafauna which coincided with the shifts in human agricultural land-use seem to have not compensated the detrimental effects of wild megafauna losses. Thus, other factors besides dung size like dung availability in the landscape or habitat structure seem to play additional, important roles for dung beetle community composition. Nevertheless, strong effects on mutualistic species like dung beetles has to be expected to happen as ecological consequences of the ongoing impoverishment of the global megafauna. Future nature restoration initiatives has to account for the often vacant, but reactivatable, ecological links between extirpated megafauna and interacting species, when planning and implementing the rewilding based on megafauna restoration. Such integrative rewilding initiatives will potentially be most successful in restoring biodiverse, self-functioning ecosystems – one of the major objectives for nature management in the Anthropocene.
Andreas Schweiger and Jens-Christian Svenning
Galetti, M. et al. 2018. Ecological and evolutionary legacy of megafauna extinction. – Biol. Rev. 93: 845–862.