Have We Been Underestimating Earthworm Numbers in Grazed Grasslands? Earthworm Hubs Underneath Cattle Dung Pats.

The assessment of earthworm populations (Annelida, Lumbricidae) in grazed grasslands by random sampling design is assumed to produce reliable abundance estimates. However, we hypothesized that cattle dung pats influence the spatial distribution of earthworms, generating temporary aggregations. Therefore simple-random or adaptive pat-avoiding sampling applications would underestimate the abundance and possibly species richness in relation to certain species. The study took place in spring 2015 on a 1 ha permanent dairy grassland in SE Ireland. During 10 weeks samples were taken every second week directly under dung pats (Treatment) (n=5) and 2 m away from pats (Control) (n=5). Earthworms were extracted by hand sorting of soil blocks (30 cm x 30 cm x 25 cm deep), preserved, counted, weighed and identified to species level. Earthworm abundance peaked at 222 individuals per block in the Treatment and plummeted to 45 in adjacent Control. Dung-associated abundances averaging 2.25 times that of the Control increased over time, starting at a mean (n=5) of 101 individuals per block in the first sampling week, peaked at 173 individuals eight weeks after grazing and decreasing to 105 thereafter. Earthworms of the epigeic ecological group (surface and litter dwellers) were mainly attracted to dung pats, with negligible numbers in Control. Endogeic species (mineral soil dwellers) divided into attracted and not attracted species. Of the more sedentary anecic species (deep burrowers), only one species was attracted. A significantly higher abundance of certain species under cattle dung pats forming temporary ‘hubs’ of earthworm activity suggests that the attractiveness of the dung pat environment changes as the dung ages, perhaps through an initial high ammonium content and diminishing nutrient availability later on. In the light of these findings, earthworm sampling approaches may have to be reviewed for pastures with dung pats from large herbivores present. The observed intensive activity of earthworms under dung pats probably accelerates soil structural processes influencing soil physical quality. These effects are currently being investigated in the second phase of this project.