Stomach contents from invasive American bullfrogs Rana catesbeiana (= Lithobates catesbeianus) on southern Vancouver Island, British Columbia, Canada

Kevin Jancowski,Stan Orchard

doi:10.3897/neobiota.16.3806

Abstract

Invasive alien American bullfrog populations are commonly identified as a pernicious influence on the survival of native species due to their adaptability, proliferation and consequent ecological impacts through competition and predation. However, it has been difficult to determine conclusively their destructive influence due to the fragmentary and geographically dispersed nature of the historical database. An expanding meta-population of invasive American bullfrogs, Rana catesbeiana (= Lithobates catesbeianus), became established on southern Vancouver Island, British Columbia, Canada in the mid- to late 1980s. An on-going bullfrog control program begun in 2006 offered a unique opportunity to examine the stomach contents removed from 5,075 adult and juvenile bullfrogs collected from 60 sites throughout the active season (April to October). Of 15 classes of organisms identified in the diet, insects were numerically dominant, particularly social wasps and odonates (damselflies and dragonflies). Seasonality and site-specific habitat characteristics influenced prey occurrence and abundance. Native vertebrates in the diet included fish, frogs, salamanders, snakes, lizards, turtles, birds, and mammals, including some of conservation concern. Certain predators of bullfrog tadpoles and juveniles are commonly preyed upon by adult bullfrogs, thereby suppressing their effectiveness as biological checks to bullfrog population growth. Prey species with anti-predator defences, such as wasps and sticklebacks, were sometimes eaten in abundance. Many prey species have some type of anti-predator defence, such as wasp stingers or stickleback spines, but there was no indication of conditioned avoidance to any of these. Results from this study reinforce the conclusion that, as an invasive alien, the American bullfrog is an opportunistic and seemingly unspecialized predator that has a uniquely large and complex ecological footprint both above and below the water surface.

Highlights

The American bullfrog, Rana catesbeiana (= Lithobates catesbeianus), is widely considered one of the most ecologically destructive of invasive alien vertebrate species (Lowe et al 2000, Kraus 2009, CABI 2011)
The range of organisms found in the stomachs of adult and juvenile bullfrogs spans 15 taxonomic classes (Table 3)
This study found that early in the bullfrog active season, Odonata were a consistently important prey for all size-classes of bullfrogs, and this has been reported by Werner et al (1995)

Summary

Introduction

The American bullfrog, Rana catesbeiana (= Lithobates catesbeianus), is widely considered one of the most ecologically destructive of invasive alien vertebrate species (Lowe et al 2000, Kraus 2009, CABI 2011). Bullfrogs consume a large number and variety of prey species (Bury and Whelan 1984) with insects usually numerically dominant (Korschgen and Moyle 1955, Cohen and Howard 1958, McCoy 1967, Bruggers 1973, Werner et al 1995, Hirai 2004, Laufer 2004, Barrasso et al 2009, Hothem et al 2009, Silva et al 2009). Adult bullfrogs are known to eat larger prey (Bruneau and Magnin 1980), and this is often vertebrates–frequently frogs (Korschgen and Moyle 1955, Stuart and Painter 1993, Werner et al 1995, Govindarajulu et al 2006, Diaz De Pascual and Guerrero 2008)

Methods

Results

Discussion

Conclusion