Seasonal Spatial Patterns of Two Sympatric Frogs: California Red-legged Frog and American Bullfrog

—The introduced American Bullfrog (Lithobates catesbeianus) has been implicated in the decline of native amphibians. Bullfrogs have become widespread in California and are a threat to the native California Red-legged Frog (Rana draytonii). The two species are ecologically similar, and the bullfrog is a predator to the red-legged frog that may influence its habitat use patterns. We analyzed the spatial and temporal locations and body sizes of both frog species within a large, fishless seasonal marsh during 1996. Overall, California Red-legged frogs and American Bullfrogs showed similar spatial distributions; however, seasonal changes were observed. California Red-legged Frogs increased their mean distances between conspecifics in the marsh seasonally from winter through summer while frog numbers decreased. In contrast, bullfrogs showed the opposite pattern where mean distances between conspecifics decreased over time, but the abundance of bullfrogs increased from winter through summer. During each season the mean distances between red-legged frogs and bullfrogs was greater compared to distances to their respective conspecifics, except for spring when bullfrog conspecifics were slightly more distant than red-legged to bullfrog distances. American Bullfrogs were significantly larger than California Red-legged Frogs at our site. Our findings suggest that frog abundance, which is strongly influenced by breeding behavior, changing habitats, and possibly predation risk of red-legged frogs by bullfrogs are the dominant factors driving the spatial patterns observation at Ledson Marsh. Also, once winter breeding was completed, California Red-legged Frogs seemed to avoid predation by American Bullfrogs by spatial separation within the marsh or by leaving the marsh altogether.