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Ant-hill heterogeneity and grassland management

Citation

King, Timothy (2020), Ant-hill heterogeneity and grassland management, Dryad, Dataset, https://doi.org/10.5061/dryad.fqz612jrm

Abstract

1. In many grasslands, some ants act as ecological engineers to produce long-lasting soil structures which have a considerable influence on the patterns and dynamics of plant, vertebrate and invertebrate species. They promote species richness and diversity. 2. The yellow meadow ant, Lasius flavus, is the most abundant allogenic ecological engineer in grazed European grasslands, producing vegetated long-lasting mounds. Its ratio of influence to biomass is remarkably high. Grassland restoration projects frequently attempt to re-introduce grasslands on abandoned arable fields. When this ant does not colonise naturally it should be introduced. It probably limits the abundance of grasses in a similar manner to hemi-parasitic plant species. 3. Ant-hills make a distinctive contribution to grassland heterogeneity. Measurements on mounds in a single grassland over 45 years document the dynamics of the same 200+ ant-hills in volume, surface area and basal area. As the mounds aged they increased in size and took over a higher proportion of the grassland surface. Occupied mounds continued to grow, abandoned mounds decreased in volume and some disappeared entirely. 4. Four plant species favoured by the soil heaped by the ants were also monitored. Two woody perennials grew up through heaped soil and two short-lived species colonised its surface. As the mounds became occupied some of these species significantly increased, and when they were abandoned some decreased. 5. In a grassland, the ant-hill population provides a fluctuating subset of plant and animal species which are characteristic of temporary habitats. This seems likely to reduce the rate of local extinctions which might otherwise result from fluctuations in grazing pressure. In conservation settings, ant-hills should be introduced or maintained where possible, and considered in planning grassland maintenance and management.Minhyuk Seo

Methods

The progress of a single ant-hill population was measured by the same observer in 1970, 2007 and 2015 in the same calcareous grassland at Beacon Hill, Aston Rowant National Nature Reserve (UK NGR SU727972, 230-244 m altitude) at which the vegetation on the ant-hills has been investigated (e.g. King, 1977a, 1977b, 1977c, 2007). This plot (Figure 1) is described in detail in King 1977b. It was last ploughed about 1904, although it was a rabbit farm from 1935-1954 (local farmer, per sonal communication, 2009). The size of every fifth ant-hill in the northern-eastern half of this field, a total of 206, was measured in 1969. In 2007, the 333 ant-hills in 800 m2 of this area were all mapped and their volumes estimated. The same area was sampled once more in 2015, when each individual ant-hill mapped in 2007 was measured again, without reference to the 2007 data.  

On each ant-hill, two measurements were recorded of its horizontal diameter and four of its height according to established methodology (King, 1977b, 1981a).  To calculate volumes and surface areas, ant-hills were regarded as spherical caps.  If the average radius of the base of the cap is a cm, and its average height is h cm, then the volume of the ant-hill is πh (3a2 + h2)/6000 Litres, and its surface area is π(a2 + h2)/10000 square metres. The percentage cover of bare soil was estimated by eye. The presence or otherwise of a Lasius flavus colony within the mound was recorded on each occasion. In fewer than 3% of cases some individuals of Lasius niger or Myrmica spp. were present. The percentage cover of Thymus drucei and Helianthemum chamaecistus was recorded by eye and the numbers of flowers of Arenaria serpyllifolia and Cerastium fontanum were counted. The calculated surface areas of each mound allowed the areas of bare soil, T. drucei and H. chaecistus to be estimated. The non-parametric Wilcoxon signed-ranks test for matched pairs (Sokal & Rohlf, 2012) was used to compare the 2007 and 2015 data for the same ant-hills because the underlying distribution pattern of the data was sometimes difficult to transform to normality.  Plant nomenclature follows Stace (2019).