Productivity and function of arid and semiarid ecosystems (drylands) are optimised when critical resources such as water, organic matter and nutrients are assembled into discrete patches known as fertile patches or fertile islands. The strength of these islands is greater when the focal island plants are larger, but it is unclear whether this size effect holds true within similar-sized island species or across gradients in productivity. We examined the strength of the fertile island effect beneath three shrub species of similar structure but varying size across a gradient in productivity from ranges (low productivity), to footslopes (intermediate) and plains (high productivity). We predicted that fertility effects would be greater for larger plants and under lower productivity. There was no evidence of a productivity effect, but the fertility island effect increased with shrub size, irrespective of species. Furthermore, the fertile island effect was stronger for plant-related attributes (plants and litter) than functions associated with either soil fertility or soil stability. Our results reinforce the notion that plant size is an important determinant of the extent to which perennial plants enhance their understorey environment, irrespective of productivity or species. This has important implications for restoration programmes involving shrub plantings where the aim is to enhance the functionality of degraded dryland systems.