Water availability directly determines per capita consumption at two trophic levels

Abstract

Abstract. Community ecology has long focused on energy and nutrients as currencies of species interactions. Evidence from physiological ecology and recent studies suggest that in terrestrial systems, water may influence animal behavior and global patterns of species richness. Despite these observations, water has received little attention as a currency directly influencing animal species interactions. Here, we show that the per capita interaction strength between predatory wolf spiders and their primary prey, field crickets, is strong (À0.266) when predators and prey are maintained in ambient dry conditions, but is near zero (0.001) when water is provided ad libitum. Moreover, crickets consume 31-fold more moist leaf material in ambient dry conditions, switching from old litter to moist green leaves when free water is scarce. Under dry conditions, animals may make foraging decisions based first on water needs, not energy or nutrients, suggesting strong and predictable effects of alterations in aridity on species interactions.

Animal water balance drives top-down effects in a riparian forest—implications for terrestrial trophic cascades

“…Specifically, large spiders reduced cricket populations under dry ambient conditions, but had no effect with water supplementation (figure 1, B1 versus B2). This result followed expectations from previous research in cages [15], and thus supports the hypothesis that short-term, small-scale changes in per capita effects can roughly predict longer-term population-level dynamics [19]. Similarly, as predicted, large spiders had positive effects on moist leaf material (reduced herbivory) under dry conditions ( figure 1, B1), but in contrast to predictions (no effect), large spiders increased herbivory where direct free water was added (reverse of a trophic cascade; figure 1, B2).…”

“…Previous work has shown that water balance can influence consumption of moist food for particular consumers [9,15]. We hypothesized that this alteration of consumption would lead to changes in the strength of top-down effects in real food webs.…”

“…saturated soil or open water) are unavailable [9,10,15]. In these situations, terrestrial food webs can be viewed as water webs, with water driving trophic interactions [15]. Large-scale, open-air manipulative experiments examining the direct importance of animal water balance in structuring food webs are generally lacking (but see [16]).…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…In addition, environmental water conditions, usually categorized as wet or dry, have the potential to impact the consumption habits of riparian consumers [147,148]. Strikingly, the abundance of riparian organisms was greater where surface water and groundwater resources were added, regardless of the presence of a river, an abundant and natural water source [144].…”

“…Strikingly, the abundance of riparian organisms was greater where surface water and groundwater resources were added, regardless of the presence of a river, an abundant and natural water source [144]. When reviewing the interaction between two trophic levels, researchers found that predatory spiders altered their prey consumption; in dry conditions, crickets consumed more moist leaves than dry litter, and the spiders under the dry conditions consumed significantly more crickets under the same conditions [147]. These preferences indicate that food consumption can vary across multiple levels of the trophic system depending upon water availability [34,147].…”