Landscape planners in semi-arid and arid regions often plant native shrubs to reduce water and fertilizer use; however, landscape management practices might alter shrub resource use and make them more dependent on irrigation and fertilization. Our objective was to assess whether water and nutrient subsidies in managed landscapes affected plant resource use. We measured carbon (delta 13C) and nitrogen (delta 15N) stable isotopes and tissue N concentrations in leaves of native Mediterranean-type vegetation growing in adjacent urban and natural areas at six different universities throughout Southern California. These data are used to test the hypothesis that native shrubs growing in natural areas with no or infrequent irrigation would have higher delta 13C but lower delta 15N values than conspecifics growing in managed landscapes. Leaf delta 13C declined as aridity increased across our six campuses, but only for plants growing in natural areas, suggesting that irrigation decoupled water use efficiency (WUE) from natural climate conditions. Plants growing in urban landscapes had lower delta 13C values than conspecifics growing in natural areas, suggesting that irrigated plants had lower WUE. Plants also had higher leaf delta 15N in managed settings, suggesting that fertilization and irrigation increased N availability and mobility. Our results indicate that semi-arid plants that normally exhibit stomatal control over water loss in natural areas might be more sensitive to drought if irrigation systems fail or are shut off due to high water costs. Using native plants in urban landscapes may not result in water savings because native plants are less efficient in their water use.