Thomas C. Giarla

CONTAGION OR COMPRESSION? PREDATOR EFFECTS ON COLONIZATION IN ADJACENT AQUATIC HABITATS. Thomas C. Giarla¹, James R. Vonesh¹, Biology Department, Washington University, St. Louis, MO¹

Historically, ecologists have focused on the lethal effects of predators, but recent studies have highlighted the importance of “fear” responses, which may dramatically alter patterns of habitat selection. In aquatic communities, many taxa avoid breeding in habitats that contain fish, and prey perception of risk may have consequences for colonization in adjacent, predator-free habitats. Habitat compression (when avoidance of predator habitats increases colonization in remaining predator-free habitats), and habitat “contagion” (when predator cues are associated with adjacent predator-free habitats, reducing colonization), are two possible landscape-level patterns of fear response. A contagion effect of predation was recently shown in the gray treefrog (Hyla chrysoscelis). Predator-free habitats adjacent to fish pools had fewer colonists compared to habitats adjacent to fishless pools. In this study we reexamine the effect of fish predators on gray treefrog colonization in adjacent habitats, focusing on how far the effect extends into the landscape and how it shapes the pattern of abundance over longer timescales. To accomplish this, we conducted an experiment in which we manipulated the presence/absence of green sunfish (Lepomis cyanellus) in 24 300-gallon cattle tanks widely spaced throughout the Tyson Research Center and then monitored treefrog colonization in 100-L focal pools placed at two distances from the larger tank. Results show that the presence of fish strongly altered patterns of treefrog colonization in adjacent fishless habitats, but focal pools adjacent to fish pools had significantly more frog colonization—not less. Furthermore, while colonization decreased with distance from the central tanks, the effect of predators on colonization did not. These results suggest habitat compression rather than contagion. Differences in experiment scale and landscape quality may explain the differences among studies. Finally, initial differences in colonization did not alter final tadpole abundances, suggesting post-colonization processes (e.g., intraspecific competition) may have overwhelmed initial differences due to habitat selection.