A methodological step forward to quantify past population size of large herbivores and the impact of their disturbance regime

Background/Question/Methods

Large herbivores, whether domesticated or wild, are increasingly recognised as ecosystem engineers across most of the Earth’s biomes, both in cultural landscapes and wilderness areas. If present in sufficient density, the disturbance regime they generate on the long term is significant enough to deeply modify vegetation structure and composition. Despite this recognition, there are no methodologies currently available to estimate long time series (>50 years) of large herbivore density, and study their past, and potentially future, effect upon ecosystem dynamics.

We are developing a novel paleoecological methodology that facilitates the reconstruction of absolute past densities of large herbivores. This methodology complements existing tools either designed to identify the species present in the past or to obtain relative abundances without any calibration.

We analysed modern sediment samples from the New Forest national park, UK, and the Oostvaardersplassen nature reserve, The Netherlands. Matching the morphology of fungal spores in the sediments with those of known obligate, coprophilous species, we first ascertained a suite of spore types reliably bioindicative of large herbivore dung. We then compared their abundance at the Oostvaadersplassen with the average densities of large herbivores near the sampling point, based on weekly observations over a period of five years.

Results/Conclusions

A list of identifiable spore types originating from obligate or quasi-obligate coprophilous fungi taxa was established, the most common ones originating from the Sporormiaceae Munk, the Lasiosphaeriaceae Nannf., and the genus Sordaria Ces. & De Not. The abundance of these spore types in our pond sediments showed a positive and significant relationship with the abundance of herbivores (N=22, p<0.05). Furthermore, this relationship was improved when the visit frequency to the sampled ponds was taken into account. This, in conjunction with the estimated spore accumulation rates, allows the extrapolation of this relationship to be applied to other sites.

These results suggest that the accumulation rate of dung fungal spores is a reliable indicator of large herbivore population size, in particular within a short distance of the sampling point. In this respect the drinking water available to free ranging herbivores within a landscape, is an important factor determining their daily movements and thus the frequency of visitation to potential sampling points. Future studies using this quantitative reconstruction of past populations have the potential to make major contributions to our understanding of the long-term ecological impact of large herbivores, particularly in connection with wilderness restoration and re-wilding initiatives.