- Biological invasions are the second largest threat to biodiversity following habitat loss, and studying invasions has been a focus of ecological the past two decades. Despite the intense research, many basic questions remain empirically unanswered, particularly in the realm of behavior. The purpose of this research was to explore the interaction between native and invasive competitors at multiple scales in experimental and field settings. Using the native signal crayfish (Pacifastacus leniusculus) and invasive red swamp crayfish (Procambarus clarkii) in Oregon's Southern Willamette Valley, we experimentally explored one-on-one aggression and dominance relationships and group shelter occupancy in mesocosms, while we performed a survey to determine field microhabitat associations. In our one-on-one aggression and dominance experiment, we tested agonistic relationships based on differences in species, size and sex. We found that aggression is affected more by similarity or dissimilarity in sex between opponents than by any other factor, and that while species or size difference did not affect aggression, they did affect the degree of dominance between opponents. In our mesocosm experiment, we observed patterns of shelter occupancy and microhabitat use based on species composition and shelter density. When alone, invasive red swamp crayfish used shelter infrequently, regardless of shelter density, while native signal crayfish used shelter in a density-dependent pattern. In mixed species treatments, signal crayfish did not alter their shelter use patterns compared to single species treatments, while red swamp crayfish patterns of shelter use were identical to those of the signal crayfish: they changed shelter occupancy from a density-independent response when alone to a density-dependent response in the presence of signal crayfish. Furthermore, patterns of microhabitat use paralleled the shelter occupancy patterns, yielding a narrow suite of microhabitats for native signal crayfish and a broad suite of microhabitats for invasive red swamp crayfish. In our field survey, we employed an epicenter-based design at two regional field sites and performed visual snorkel surveys to detect crayfish. Both field sites were in systems impacted by urban development and agriculture. Red swamp crayfish were more abundant, found at more field sites, and used a broader range of microhabitats than signal crayfish.
Cohesively, our results indicate that red swamp crayfish and signal crayfish do not differ in aggression or dominance, that behavior mediates changes in microhabitat use across environmental and community contexts, that factors other than behavior are important in determining the composition of crayfish assemblages, and that changes in habitat are likely to synergistically aid the spread of invasive red swamp crayfish at the expense of native signal crayfish.