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Concentration of seagulls in the Chafarinas Islands (three semi-arid North African islets) induces profound changes in soil properties including eutrophication, salinization, acidification and nutrient imbalances. Soils of heavily seabird-affected sites have significantly higher soluble K and NO3 levels ( > 20-fold), Zn and Fe availability ( > 5-fold), water-retention capacity ( > 40% increase) and N-15 enrichment, compared with control sites. These seabird-induced soil changes are paralleled by (i) different patterns of abundance of the two main chenopod shrubs: Suaeda vera abundance is higher ( > 30-fold) in seabird-affected sites while Salsola oppositifolia largely dominates in low-affected areas. No differences were found for Atriplex halimus. The abundance of the first two species may be largely explained by changes in soil soluble K-to-available Ca ratio whose value depends of the interaction between seabird products and soil constituents; (ii) significant increases in leaf N, P and Zn levels, and in all K ratios, paralelled by a decrease in Ca, in Salsola; but only by increased K/Na and P/Ca ratios in Suaeda leaves. These changes were significantly correlated to changes in species abundances; (iii) an increase of delta(15)N, paralleled by a decrease in delta(13)C values, in Salsola leaves but not in Suaeda.

Conjoint analysis of the seabird-related changes in soil properties, species abundance, leaf composition and differential response to the seabird-induced fertilization/severity gradient of the two main chenopod shrub species, has led us to propose an explanatory hypothesis of seagull-soil-shrub relationships. According to this, the seagull-induced soil changes, mediated by processes of (in)tolerance-facilitation, play a main role in determining the abundance and the leaf nutrient status of the studied chenopod shrubs, which have contrasting physiological adaptations.