engleski

Seasonal patterns and species associations of earthworms in grassland habitat

The spatial and temporal variability of earthworm populations is very high, both within and between differently managed types of land. Earthworms show clumped, patchy distribution (Rossi and Lavelle, 1998, Jiménez et al., 2001, Decaëns et al., 2003). This horizontal distribution is mainly a response to a spatial heterogeneity primarily determined with physico- chemical parameters of soil and food availability on various scales. In a given plot, the earthworm density depends on a group of different factors and which factor is the most limiting varies between ecosystems and earthworm species (Decaëns et al., 1997). One approach in the description and explanation of variation in their distribution is a geostatistical analysis. The aim of this study was to characterize the spatial variability of earthworm communities in grassland habitat in two consecutive seasons in relation to the habitat microtopography in order to facilitate earthworm sampling design. The variability was analyzed according to the earthworm species, ecological category (epigeic, endogeic or anecic), age (adult or juvenile) and habitat microtopography. Earthworms were sampled along a vertically stratified transect at 7 locations, at each location a 49 sampling points were arranged in a regular grid. To obtain high resolution microtopographic profile, elevation on each site was measured with the optical leveling instrument on multiple points. Data were used to create a digital elevation model (DEM) of each site revealing very subtle depressions and mounds. To test the spatial distribution of a single species a Morisita index of intraspecific aggregation (Iδ) was calculated, spatial autocorellation was tested with Geary's C and Moran's I index and semivariograms were calculated for each species, season and location.The results of this research show a patchy distribution of majority of adult individuals, and almost all juveniles. The Morosita index values from the literature are mostly higher, i.e indicate a stronger aggregation from the values obtained within this research. However, the differences in index values are probably the consequence of earthworm population seasonal dynamics. Namely, as it is evident from our results that the earthworm patches change their size and position over time, i.e. the strength of aggregation varies. The index values for a particular species changed independently of season and location. In this research the range of semivariograms for all species and life stages was 8.67 m. For adult earthworms a range was somewhat smaller (8.37 m), while for juveniles it was bigger (9.2 m). The semivariogram range for a particular species in this research where lower than the ones in the literature. With this research a structure on small distances were detected which confirmes that earthworm populations are structured, including autocorrelation on spatial distance of <5-10 m (Rossi, 2003). From the kriged sampling areas it is visible that the earthworm populations are not static and that the position of ceratin species, as well as a total area, that they covered on the sampled location, were changing. These comparisons could be many for several more species and the conclusion is tha the spatial distribution of adults and juveniles of the same sepcies is very similar. Sometimes the spatial distribution of juveniles and adults correspond within season, but sometimes a temporal shift is noticeable and the spatial distribution of juveniles in the summer corresponds to a spatial distribution of adults of the same species in the spring.

spatial distribution ; geostatistics ; microtopography ; species assemblages

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