Water is essential for life on Earth, yet little is known about how water acts as a trophic currency, a unit of value in determining species interactions in terrestrial food webs. We tested the relative importance of groundwater and surface water in riparian food webs by manipulating their availability in dryland floodplains. Primary consumers (crickets) increased in abundance in response to added surface water and groundwater (contained in moist leaves), and predators (spiders and lizards) increased in abundance in response to added surface water, in spite of the presence of a river, an abundant water source. Moreover, the relative magnitude of organism responses to added water was greatest at the most arid site and lowest at the least arid site, mirroring cricket recruitment, which was greatest at the least arid site and lowest at the most arid site. These results suggest that water may be a key currency in terrestrial dryland food webs, which has important implications for predicting ecosystem responses to human‐ and climate‐related changes in hydrology and precipitation.

The limiting effects of food and water on juvenile growth rates in the lizard Anolis aeneus were investigated in the field (Grenada, West Indies) and laboratory. Growth rates of lizards in the field were unrelated to their snout—vent lengths, but both prey biomass and rainfall had significant effects on juvenile growth rates. Laboratory experiments indicated that water had a primary limiting effect on growth; even when food supplies were superabundant, growth rates were low when drinking water was curtailed. Laboratory and field experiments suggest that limited water availability reduces growth rates for most (67%) of the dry season, whereas food levels are sufficiently low to limit growth during the weeks of the dry season when rainfall is sufficient for growth. During the wet season there is no evidence of water scarcity, food levels are high and average growth rates are 85% of the maximal rates observed under optimal conditions in the laboratory.