Central-Place Forager Effects on Food Web Dynamics and Spatial Pattern in Northern California Meadows

In this study, I examined the impacts of a presumed central-place forager on its prey and on the prey's resource. Western fence lizards, Sceloporus occidentalis, limit their activity and feeding in meadows to a variety of structures (e.g., stumps, logs, brush, and rock piles). The impact of lizards on their primary prey, grasshoppers, decreased at a decreasing rate with distance from structures that housed lizards; thus, grasshopper densities increased with distance. Consequently, the abundance of plants (grasshoppers' food) decreased with distance from these structures. However, the relative abundance of plants varied with distance. The proportion of herbaceous forbs (the grasshoppers' preferred resource) was greatest near lizard structures, whereas the proportion of less edible grasses was greater far from lizard structures. There were no such relationships between grasshoppers or plants and distance from similar structures that did not house lizards. Near the structure, an exclosure experiment showed a very strong impact of lizards on grasshopper populations. Plant biomass and the relative proportion of forbs were higher with lizards present than in lizard exclosures, suggesting a trophic cascade. An identical exclosure experiment far (15 m) from the structure, where lizard activity was greatly reduced, showed no such cascades. Thus, this study showed evidence for strong consumer effects in a grassland system. However, small-scale differences in foraging behaviors (possibly due to predation risk) may importantly affect the outcomes of these interactions.