Invasive species shifts ontogenetic resource partitioning and microhabitat use of a threatened native amphibian

1. Invasive species may have direct effects on native species, such as predation or competition, as well as indirect effects such as altering habitat or the behaviour of a native species. Indirect effects can be difficult to detect, but whole-system removal of invasive species presents ail excellent opportunity for revealing these more subtle effects. 2. American bullfrogs were introduced to California in the late 1800s and have been implicated in the declines of native ranid species. Invasive American bullfrogs were removed from a series of 12 ponds to test for changes in microhabitat use of federally threatened California red-legged frogs, using a neighbouring field site with six ponds as a control. 3. There was a marked increase in the total number of adult California red-legged frogs seen in all of the ponds after the first year of bullfrog removal, suggesting that these adults were in the ponds, but hiding when invaders were present. Ontogenetic partitioning of habitat in this species was documented, as well as a shift in that partitioning and increased hiding behaviour with adult bullfrog presence. California red-legged frogs used willows significantly more as cover, and were found on bare shores half as often when adult bullfrogs were present. 4. These results support recent Suggestions that behavioural responses of prey to predators can play a Substantial role in total predator effects and suggest that careful management of invasive species may be necessary to accurately evaluate population sizes of this threatened amphibian species. Copyright (C) 2009 John Wiley & Sons, Ltd.