BIODESERT Fertile Island

Laburpena

We examined the fertile island effect by comparing 24 soil physical, chemical and functional attributes beneath the canopy of perennial vegetation compared with their adjacent unvegetated interspaces across global drylands. The 24 attributes were assembled into three synthetic functions that represent the capacity of soils to mineralise organic matter (Decomposition), enhance fertility (Fertility), and conserve water and maintain stability (Conservation, see Methods). We gathered data from 288 dryland sites across 25 countries on six continents (Fig. 1) to test the following two contrasting hypotheses. First, we expected that the magnitude of the fertile island effect would increase with increasing levels of both recent (standardised dung mass) and long-term or historic (heuristic assessment; ungrazed to high) grazing pressure (Hypothesis 1a). This prediction is based on the understanding that greater grazing pressure will destabilise surface soils, mobilising sediment, seed, nutrients, and organic matter from unvegetated interspaces to plant patches, strengthening fertile islands. Additionally, livestock might be expected to have a greater effect than wild herbivores because they have not co-evolved with indigenous vegetation and therefore have more deleterious effects on both island plants and their soils, Hypothesis 1b). Alternatively, changes in climate and plant traits, factors that operate at much larger (regional and global) scales, could overwhelm the impacts of grazing, a factor that operates at the local scale, on fertile islands (Hypothesis 2a). More specifically, irrespective of grazing pressure, we would expect that plants would make a greater contribution to fertile islands in arid and hyper-arid ecosystems where soils are extremely bare and infertile compared with less arid ecosystems where the influence of plants would be relatively lower. For example, reduced rainfall and/or increased temperature would increase the harshness of the interspaces compared with the vegetated and more protected islands, thereby strengthening the fertile island effect. Plant effects might also be expected to vary among broad functional groups (tree vs shrub vs grass; Hypothesis 2b). These broad groups could have varying effects on soil biogeochemistry because of marked differences in shape, size, and structural complexity. Quantifying the contribution of grazing by different herbivores at different pressures, plant traits, climate, and soil properties on fertile islands allowed us to assess current and future impacts of grazing on ecosystem structure and functioning across global drylands, where woody vegetation is a predominant plant form.