12 pages, 10 tables, 2 figures. We thank Georgina Álvarez for the organizational support in our fieldtrips to Chafarinas. T. Gómez, M. Igual, J. Charco, and P. Robles gave field assistance. R. López, J.M. Alegre, J. Cara and E. Gutiérrez carried out plant and soil chemical analyses. D. G. Sobey kindly provided some key references. J.M. Murillo and L. Sack made valuable comments on an earlier version of the manuscript.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 15N 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 era 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 15N, paralleled by a decrease in 13C 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.This research was supported by the Spanish Ministry of Environment, and T.M. by the DGES (grant PB97-1177).Peer reviewed