Species turnover at small scales in dune stack plant communities

Patterns of both species accumulation with increasing area and of individual species occurrences depend on the scale level considered. This study investigated community diversity and individual species turnover patterns between four scale levels within 2 x 2 m(2) nested plots situated in a dune stack plant community. The number of species increased with plot area following a log-log function, with a slope of 0.23. However, species turnover was higher between the lowest scale levels, indicating limitations on species occurrences at the 25 x 25 cm(2) scale level. Alpha diversity in rectangular plots was significantly higher than in square plots of the same area. There were strong differences between individual species turnover patterns. Most species occurrence patterns had a box-counting fractal dimension value between 0.8 and 1.6, which is rather tow compared with other studies on larger scale levels. Analyses of occurrence probabilities and scale area plots showed that there is a systematic deviation from self-similarity at the smallest scale level. Species had a lower frequency than expected from a fractal distribution, suggesting a higher level. of species aggregation. The higher species diversity turnover at the smallest scale level can be linked to a higher spatial aggregation of individual species, due to biotic or abiotic limitations on their occurrence. These results confirm the general nature of the pattern of breakdown of self-similarity at the smallest scale level considered. (C) 2004 Elsevier GrnbH. All rights reserved.