Figure 1.

Top-down–trigger–transfer–reserve–pulse model (adapted from the trigger–transfer–reserve–pulse model of Ludwig et al. [24]), describes the movement of materials in semi-arid and arid lands. Solid arrows indicate flows of materials between elements (boxes) in ecosystems. Rainfall events function as a ‘trigger’ for ‘pulses’ of plant growth. Flows of wind and water ‘transfer’ materials and nutrients across the landscape. Transported materials are lost through ‘out-flow’ or captured by obstacles in the landscape which interrupt fluid flows, such as vegetation patches, where they become incorporated into the soil which functions as a ‘reserve’. Nutrients and seeds stored in the soil promote pulses of vegetation of growth. Vegetation growth and resources are recycled through the deposition of senescent plant material and seeds back into the ‘reserve’ or consumed by herbivores. Loss of vegetation cover due to drought or excessive grazing by herbivores can decouple feedback loops between vegetation growth, litter-fall and the local soil nutrient pool. Predators limit consumption of plant biomass through predation and the fear they instil (dashed arrow) and thus promote the positive feedback loop between vegetation production, litter deposition and the soil nutrient pool and the capture of transported nutrients by vegetation.