Plant-ant interaction in xerophilous ephemeral vegetation of S.E. Sardinia.

Frequency and abundance of annual plants have been measured in 40 square plots, 0.36 m2 each, settled in an area of approximately 5 ha at the foothill of the Sarrabus massif, in S.E. Sardinia. 20 plots were randomly distributed at less than 5 m from the ant nests, further 20 at more than 30 m from the ants nests. The results showed a significantly lower plant density next to the ant nests, together with an higher species richness. Far from the nests, plots resulted dominated by relatively few species with a patchy distribution, while next to the nests the distribution of frequencies resulted more homogeneous. It is suggested that harvesting ants may contribute to the small-scale regulation of frequency and abundance of annual species and therefore contrast the natural tendency towards increasing the spatial order (and reducing the plant diversity) by the formation of patches. Being ants nests more frequent on secondary vegetation linked to a slight grazing-pressure, such traditional land-use might benefit indirectly the spatial heterogeneity and diversity of plant communities dominated by annuals.