Arachnologische Mitteilungen 58

Suction sampling vs. pitfall trapping in gaps 25 In order to visualise and compare the distribution of our four main data (s, n, H’, [mm]) and their probability density, we used violin plots, which are basically box plots that also show the probability density of the data at different values, usually smoothed by a kernel density estimator (Hinze & Nelson 1998). We included every individual sample (30-30) for both sampling methods (Fig. 2.). During this analysis, we also used Student’s t-test to compare the datasets of the two sampling methods. We considered differences to be signifi- cant at p<0.05 values. To observe potential changes in the spider communities through the survey transect (i.e. between the gaps and forest stand) we organised the data by summarizing the samples lo- cated at the same relative positions in the transects for both methods. To make the results more comparable, we represen- ted S and n as percentages of the total catch results (Tab. 1.). We compared the family compositions of the two me- thods by species and specimen numbers, which were also re- presented as percentages of the total catch results (Tab. 2.). All these values were calculated by summarising the data from each sample in the same relative position.To classify the spider families into the two basic guild categories (web ma- kers and hunters), we used the work of Cardoso et al. (2011), and we represented the data in pie charts (Fig. 3.). Two different analyses were conducted to compare the similarities between the samples for the two methods. First, we computed the Renkonen similarity indices between the DV and PT samples (Tab. 3.). In addition, we also conducted an ordination analysis (Fig. 4.), where we applied non-metric multidimensional scaling (N-MDS). The similarity matrices were based on Bray-Curtis distance measures (Bray & Curtis 1975, Anderson &Willis 2003).The corresponding ST value was 0.13, which is within the preferred acceptance interval (Podani 1997).The data were analysed by collecting methods and by sampling position, and only mature specimens were included. Both analyses were computed using the PAST 3.2 program (Hammer et al. 2001). Finally, we used linear regression analysis to model the relationships between the distance from the centre of the gaps (d) and our measured data (n, S, [mm] and H’). We conside- red relationship to be significant at p<0.05 values (Tab. 4.). Results The pitfall traps collected 928 (463 juvenile) specimens, re- presenting 34 species. The suction sampling gathered 1254 (1087 juvenile) specimens, belonging to 41 species. This me- ans an average of 2 specimens/day/trap for pitfall trapping and an average of 21 specimens/sampling (equal to 0.1 m 2 ) for D-Vac sampling. Eleven species occurred only in pitfall traps, while nineteen species occurred only in D-Vac samples. The violin plots show that the mean and maximum values are higher in the pitfall samples in all four cases (S, n, H’, [mm]). The graph representing the distributions of the body sizes shows that data from the D-Vac samples are multimo- dal.The two peaks are in the ~4.5 and ~1.5 mm body ranges. This may indicate that the D-Vac sample collection method has assessed two different sized groups from the same com- munity. However, the samples of from pitfall trapping seem to be mostly be the ~4.5 mm body range, with many outlier data points in both the minimal and the maximal ranges. Ad- ditionally, the datasets of the two methods show significant differences in the case of all four variables (Fig. 1.). Both the S and n values are highest in the inner part of the transects (i.e. in the gaps) in the case of both methods. Addi- Fig. 2: Violin plots representing: S. the dis- tribution and probability density of species numbers; n. specimen numbers; H’. Shan- non diversities; [mm]. average body sizes [mm] of the ground-dwelling spider com- munities accessed by the different samp- ling methods; p values show the results of t-tests comparing the datasets; PT – pitfall trapping; DV – suction sampling 13.5 12.0 10.5 9.0 7.5 6.0 4.5 3.0 1.5 0.0 3.0 2.5 2.0 1.5 1.0 0.5 0.0 S n H' [mm] p: 0.041 p: 0.028 p: 0.003 p: 0.004 13.5 12.0 10.5 9.0 7.5 6.0 4.5 3.0 1.5 0.0 105 90 75 60 45 30 15 0 PT PT DV DV DV DV PT PT