Free water determines trophic structure in a desert riparian forest

Background/Question/Methods

Animal population density often reflects spatial and temporal variation in resource availability. In arid ecosystems, water is generally the most important limiting resource, but different sources of water can have varying effects on consumer abundance. Here, we use additions of water in two forms—free water (via water pillows) and water in moist food items (trophic water, here water laden leaves)—to understand the importance of water in determining the density of riparian animals at two trophic levels, detritivorous crickets (Gryllus alogus) and predatory spiders (Lycosidae). 
Results/Conclusions

Cricket density increased 4-5 fold within 5 days in plots supplied with additional water either as direct drinking water (water pillows) or trophic water (green leaves). The nearly identical response of crickets to free and trophic water suggests that crickets seek green leaves to harvest water. Predatory ground spiders also increased (by 1-2 fold), but only where water was supplied as a direct resource (water pillows). Thus, under our experimental conditions, spiders respond numerically to free water as a resource and this numerical response is equal or stronger to that of spiders to their primary prey species (G. allogus). These effects on community structure have clear implications for arid ecosystems. Changes in water availability (for instance, through pumping from aquifers, or removal of vegetation, or climate change) can have serious effects on species interactions and community structure.