Spatial patterns of plant association in grazed and ungrazed shrublands in the semi-arid Karoo, South Africa

The investigation of vegetation pattern and plant association by spatial statistics has become increasingly popular among plant ecologists. Recently, Individual-centered analysis (ICA) has been introduced as a new tool for analysis of multi-species co-occurrence patterns. We tested this new technique by applying it to spatial data from grazed and ungrazed shrub communities in the semi-arid Great Karoo, South Africa. There were substantial but complex and scale-dependent differences in pattern between grazed and ungrazed vegetation. Unpalatable species that increase in abundance in grazed vegetation possibly play a key role in the change of vegetation pattern. At small scales we found indications of aggregation (< 30 cm) at the ungrazed, but of repulsion (30 - 40 cm) at the grazed site. An additional non-random pattern at 60 - 170 cm at the grazed site was probably due to the clumped distributions of some species on broader scales. We show that the interpretability of ICA results is improved when the actual observed and expected frequencies of species combinations are added to the program output. The main strength of ICA is that it has the potential to detect association patterns that involve more than two species.