Abstract
BackgroundGrazed grassland management regimes can have various effects on soil fauna. For example, effects on earthworms can be negative through compaction induced by grazing animals, or positive mediated by increases in sward productivity and cattle dung pats providing a food source. Knowledge gaps exist in relation to the behaviour of different earthworm species i.e. their movement towards and aggregation under dung pats, the legacy effects of pats and the spatial area of recruitment. The present study addressed these knowledge gaps in field experiments, over 2 years, using natural and simulated dung pats on two permanent, intensively grazed pastures in Ireland.ResultsDung pats strongly affected spatial earthworm distribution, with up to four times more earthworms aggregating beneath pats, than in the control locations away from pats. In these earthworm communities comprising 11 species, temporally different aggregation and dispersal patterns were observed, including absence of individual species from control locations, but no clear successional responses. Epigeic species in general, but also certain species of the anecic and endogeic groups were aggregating under dung. Sampling after complete dung pat disappearance (27 weeks after application) suggested an absence of a dung pat legacy effect on earthworm communities. Based on species distributions, the maximum size of the recruitment area from which earthworms moved to pats was estimated to be 3.8 m2 per dung pat. Since actual grazing over 6 weeks would result in the deposition of about 300 dung pats per ha, it is estimated that a surface area of 1140 m2 or about 11% of the total grazing area can be influenced by dung pats in a given grazing period.ConclusionsThis study showed that the presence of dung pats in pastures creates temporary hot spots in spatial earthworm species distribution, which changes over time. The findings highlight the importance of considering dung pats, temporally and spatially, when sampling earthworms in grazed pastures. Published comparisons of grazed and cut grasslands probably reached incorrect conclusions by ignoring or deliberately avoiding dung pats. Furthermore, the observed intense aggregation of earthworms beneath dung pats suggests that earthworm functions need to be assessed separately at these hot spots.
Highlights
Grazed grassland management regimes can have various effects on soil fauna
The earthworm abundance peak in dung pat (DP) treatment reached an average of 173 ± 37 individuals per sampling area (0.09 m2) in Experiment 1, which translated into a population equivalent of 1900 individuals m−2, and 360 ± 60 (Experiment 2) equivalent to 4000 m−2
The earthworm biomass peak reached an average of 40 ± 10 g per sampling area, which translated into a population equivalent of 444 g m−2 (Experiment 1) and 106 g ± 20 per sampling area (Experiment 2) equivalent to 1182 g m−2
Summary
Grazed grassland management regimes can have various effects on soil fauna. For example, effects on earthworms can be negative through compaction induced by grazing animals, or positive mediated by increases in sward productivity and cattle dung pats providing a food source. In many parts of the world pasture based animal agriculture aims to increase both outputs and efficiency through intensification [3, 4]. Bacher et al BMC Ecol (2018) 18:59 their dung on natural grasslands [14,15,16] Their droppings are important for dung fauna, including earthworms, and associated ecological processes such as bioturbation, nutrient cycling and decomposition of organic matter in these natural ecosystems are probably similar to those found in managed ecosystems. James [16] quantified the aggregation of earthworms under bison dung pats in the Tallgrass prairie in Kansas, USA
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