Combining pitfall traps and soil samples to collect Collembola for site scale biodiversity assessments
Introduction
Landscape-level biodiversity assessments, large-scale surveys and monitoring projects involve the processing of large numbers of animal or plant specimens. While arthropods particularly are quickly collected in large numbers, the identification of most taxa is very time consuming and therefore costly. As budgets are always limited, sampling and identification need to be optimized, in order to collect a sufficient number of sample units with the lowest possible effort. Collembola have rarely been considered in large-scale projects (Black et al., 2003, Sousa et al., 2006, Vanbergen et al., 2007), as they are found in high abundance in most terrestrial habitats, and their identification is especially intricate and labour and cost intensive.
As Collembola live in the soil pores, on the soil surface, in the litter layer and the vegetation, no single sampling method collects all species appropriately. The eu- and hemiedaphic species are normally extracted from soil and litter samples using Berlese-Tullgren or Macfadyen extractors. Between one to ten soil samples are usually collected per site and processed individually (see for example Deharveng, 1996, Black et al., 2003, Palacios-Vargas et al., 2007, Cole et al., 2008, Salmon et al., 2008). Alternatively, Bruckner et al. (2000) recommended collecting a large number of soil samples in the field and identifying subsamples (aliquots) after pooling all extracted animals. This reduces the large variation in abundance and species richness found in most sample units which is due to their aggregated spatial distribution in soil (Hopkin, 1997; p. 163).
The epedaphic or surface active species are best collected in pitfall traps; suction sampling (Stewart and Wright, 1995) is less frequently applied. Up to ten pitfall traps are placed per site and exposed for a few days to a few months (see for example Durbešić et al., 2006, Fountain et al., 2007). Comparing results from pitfall catches from different places or periods is difficult, because trap number, size, material, and conservation fluid all influence the catch considerably (Adis, 1979), and sampling is not yet standardized.
Few ecological studies combine coring and trapping (Jakel and Roth, 1998, Fountain and Hopkin, 2004, Bitzer et al., 2005) to adequately represent all life forms. However, combining the methods with a lower total number of sampling units may yield a better estimate of community composition of sites than one method alone. To test this hypothesis, we sampled the Collembola assemblages from 30 agricultural fields and compared the (i) pitfall trap, (ii) soil sample and (iii) pooled species richness. Additionally, we quantified the man hours spent for collecting, extracting, sorting and identifying the catch of each method to identify the most efficient sampling strategy. The results presented here are especially relevant for large scale biodiversity assessments, where the collembolan assemblages of a great number of sites are to be characterized as efficiently as possible.
Section snippets
Site description
As part of a study of on the landscape ecology of plant parasites and their predators (Drapela et al., 2008, Zaller et al., 2008a, Zaller et al., 2008b, Zaller et al., 2009), we designated an agricultural study region of 240 km2 size approximately 40 km east of Vienna, Austria (central coordinates: 16°57′E, 48°04′N). The main soil type of the region is chernozem; the climate is pannonian (continental). Within this region, 30 winter oilseed rape fields were selected, embedded in differently
Results
We collected 35,981 Collembola in total, 8042 from soil subsamples and 27,939 from pitfall traps. The activity density in the pitfalls varied between 82 and 3048 individuals and the abundance in the soil subsamples between 55 and 1200 individuals per site (five subsamples pooled). 70 species were recorded in total; 34 in the traps and 53 in the subsamples (Table 1). The site frequency of most species varied greatly between the two methods. Eighteen species were collected combining the methods;
Discussion
In this study, most epedaphic species were confined to the pitfall traps and euedaphic species to the soil subsamples from the pooled soil samples. The epedaphic species occurring in the subsamples were mostly juveniles or adults, probably migrating into deeper soil layers during periods of unfavorable conditions at the surface (for example during drought; Hopkin, 1997; p. 168). Similarly, euedaphic species coincidentally appeared in pitfall traps. These were possibly attracted by the
Acknowledgements
We thank Alex Bandion and Bettina Ibera for their help in the field. This project was funded by a scholarship of the University of Natural Resources and Applied Life Sciences, Vienna and by the Austrian Science Fund (grant no. P16972).
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