Distribution Analysis of Wolfspiders (Araneae, Lycosidae) in a Dune Area By Means of Principal Component Analysis

Abstract

1. The study was part of an extensive ecological program carried out in the dune are Meijendel of the Water Supply Company of The Hague. 2. The aim of this study was to analyse the spatial distribution of 12 wolfspider species occurring within one area, since it was expected that a competitive situation might exist. 3. The lycosid populations were sampled by 100 pitfalls grouped into four regions. The contents of these pitfalls were sampled weekly during a period of 7 years (1-3-1953 up till 24-2 1960 inclusive). 4. For a description of the relations between the distributions of the species the technique of Principal Component Analysis was used (R-technique) . 5. Principal Component Analysis was also used on the same data to obtain an ordination of the pitfalls (biotopes) (Q-technique). 6. From the R- and Q-technique, the following hypotheses suggested themselves: a. The degree of cover or a related factor is most important for the distribution of the species. b. A difference appears to exist between similar biotope types nearer to the sea and situated more landinward. 7. The optimum vegetation type appeared to be different for nearly each species. The optimum vegetation type for each species in my study area is in good agreement with the type of habitat stated in the literature for the species concerned. 8. A number of species showed a gradual shift in spatial distribution in the course of the seven years period of sampling. 9. Since it was shown that the spatial distribution of wolfspider species was linked to vegetation structure and since it was established that the vegetation itself had changed in the course of time, it was concluded that the chronological shift in the spatial distribution of the lycosid species was caused by changes in the vegetation. 10. The technique of Principal Component Analysis was amended at the following points: a. The linearity (additivity) of the principal components (factors) was changed in a more realistic multiplicative (fractional) model by taking the logarithms of the data. The additive model, however, still appeared to be a good approximation of the multiplicative one in this case. b. The proposition that each principal component, if necessary after rotation, can be interpreted as only one real environmental factor, does not hold for second and higher degree polynomial response curves. c. In the Q-technique, the mean and variances should be corrected, when the samples cannot be regarded as taken from one well-defined mathematical population.