Corresponding author: Kevin Jancowski (
Academic editor: E. García-Berthou
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,
The American bullfrog,
Stomach contents analyses from both native and invasive alien populations.
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Argentina: Buenos Aires | Non-native | 35 | 3 |
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Brazil: Minas Gerais | Non-native | 113 | 1 |
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Canada: British Columbia | Non-native | 13 | 1 |
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Canada: British Columbia | Non-native | 150 | 4 |
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China: Daishan Island | Non-native | 121 | 1 |
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Germany: Baden Wuerttemberg | Non-native | 44 | 1 |
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Japan: Kyoto | Non-native | 128 | 1 |
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USA: California | Non-native | 5 | 1 |
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USA: California | Non-native | 30 | 1 |
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USA: California | Non-native | 107 | 2 |
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USA: Michigan | Native | 166 | 2 |
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USA: Missouri | Native | 455 | 1 |
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USA: Missouri | Native | 4 | 1 |
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USA: Nebraska | Non-native | 1 | 1 | Bomberger Brown and Brown 2009 |
USA: Nevada | Non-native | 28 | 2 |
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USA: New Mexico | Non-native | 138 | 1 |
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USA: New Mexico | Non-native | 85 | 1 |
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USA: Ohio | Native | 158 | 1 |
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USA: Ohio | Native | 1 | 1 |
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USA: Oklahoma | Native | 52 | 1 |
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Venezuela | Non-native | 338 | 1 |
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Total for all locations | 2172 | 29 |
From previous studies, a number of commonalities emerge. Bullfrogs consume a large number and variety of prey species (
Populations of alien, invasive bullfrogs, geographically isolated and arising independently, are scattered along the southeast coast of Vancouver Island–their origins are often obscure. However, in the mid-1980s, a population of American bullfrogs became established just north of the City of Victoria at the extreme southern end of Vancouver Island (
The term “site” is used here, as in
Latitudinal range of study sites on southeastern Vancouver Island, British Columbia, Canada.
All fieldwork was carried out by one 2-person team working full-time, approximately 125 nights per season (April-September). Adult and juvenile bullfrogs were captured live using a patented manual “electro-frogger” technique that stuns them momentarily in the water so that they can be netted. They were later euthanized in a two-stage process that cooled them to torpor below 2⁰ C before being quick frozen (
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Juveniles < 80 | 338 | 496 | 212 | 224 | 397 | 453 | 2120 | 46 |
Young males 80-120 | 70 | 113 | 182 | 214 | 313 | 102 | 994 | 22 |
Mature males > 120 | 7 | 74 | 95 | 41 | 53 | 31 | 301 | 6 |
Young females 80-120 | 110 | 111 | 111 | 139 | 242 | 212 | 925 | 20 |
Mature females > 120 | 3 | 60 | 61 | 35 | 67 | 36 | 262 | 6 |
Totals | 528 | 854 | 661 | 653 | 1072 | 834 | 4602 | 100 |
% of Total with contents | 12 | 19 | 14 | 14 | 23 | 18 | 100 | |
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Juveniles < 80 | 44 | 19 | 15 | 67 | 52 | 90 | 287 | 61 |
Young males 80-120 | 8 | 9 | 8 | 7 | 3 | 3 | 38 | 8 |
Mature males > 120 | 3 | 19 | 19 | 10 | 7 | 8 | 66 | 14 |
Young females 80-120 | 14 | 4 | 2 | 5 | 6 | 10 | 41 | 8.5 |
Mature females > 120 | 2 | 9 | 6 | 9 | 10 | 5 | 41 | 8.5 |
Totals | 71 | 60 | 50 | 98 | 78 | 116 | 473 | 100 |
% of Total | 15 | 13 | 11 | 21 | 16 | 24 | 100 | |
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Total sample (with contents + without) | 599 | 914 | 711 | 751 | 1150 | 950 | 5075 | |
% with contents | 88% | 93% | 93% | 87% | 93% | 88% | 91% |
Six calendar months were available for fieldwork (April to September, inclusive) but only one site was sampled in all six calendar months (Florence Lake,
The range of organisms found in the stomachs of adult and juvenile bullfrogs spans 15 taxonomic classes (
Prey remains identified to class.
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15,827 | 84.1 | 93.0 | 95 |
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874 | 4.6 | 12.4 | 51 |
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770 | 4.1 | 10.9 | 50 |
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644 | 3.4 | 10.3 | 62 |
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247 | 1.3 | 4.2 | 72 |
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166 | 0.9 | 2.8 | 32 |
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107 | 0.6 | 1.4 | 25 |
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59 | 0.3 | 0.9 | 20 |
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40 | 0.2 | 0.9 | 32 |
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25 | 0.1 | 0.6 | 27 |
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20 | 0.1 | 0.3 | 17 |
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12 | 0.06 | 0.2 | 15 |
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12 | 0.06 | 0.2 | 2 |
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8 | 0.04 | 0.1 | 5 |
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3 | 0.02 | 0.06 | 2 |
Totals | 18,814 | 100 |
Out of 18,814 instances of identifiable remains, 84% were insects. Insects were also found in 93% of bullfrog stomachs and were consumed at 95% of the 60 sites sampled. The range in types of insects consumed is highly variable. Most insect parts were not identifiable to species but were at least attributable to one of 47 broader categories of variable taxonomic resolution (
Occurrence of individual prey remains identifiable as insect. The 21 most abundant insect prey categories are shown (See Table S3 for other insects identified).
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Apr | May | June | July | Aug | Sept | |||||
Social Wasp | 2,674 | 14.0 | 16.0 | 50 | < 1 | < 1 | 1 | 13 | 64 | 22 |
Aphid | 1,982 | 10.0 | 4.9 | 20 | 1 | 2 | 1 | 2 | 71 | 24 |
Damselfly | 1,947 (17% nymph) | 10.0 | 23.0 | 68 | 2 | 18 | 35 | 13 | 25 | 7 |
Dragonfly | 1,415 (27% nymph) | 7.5 | 22.0 | 87 | 1 | 21 | 25 | 17 | 23 | 13 |
Water Strider | 1,259 | 6.7 | 12.0 | 41 | 42 | 13 | 17 | 12 | 11 | 5 |
Unidentified Beetle | 1,157 | 6.1 | 18.0 | 67 | 13 | 27 | 16 | 13 | 15 | 16 |
Brachycera fly | 726 (61% larvae) | 3.8 | 8.9 | 42 | 4 | 3 | 7 | 10 | 21 | 55 |
Ground Beetle | 675 | 3.6 | 9.6 | 67 | 20 | 26 | 15 | 7 | 19 | 13 |
Nematocera fly (not crane flies) | 472 | 2.5 | 6.9 | 30 | 8 | 34 | 7 | 14 | 24 | 13 |
Ant | 415 | 2.2 | 6.3 | 42 | 7 | 16 | 11 | 21 | 39 | 6 |
Predaceous diving beetle | 399 | 2.1 | 6.8 | 67 | 12 | 31 | 18 | 9 | 23 | 7 |
Butterfly/Moth | 365 (55% larvae) | 1.9 | 5.4 | 55 | 5 | 14 | 36 | 12 | 28 | 5 |
Weevil | 324 | 1.7 | 4.6 | 28 | 6 | 12 | 4 | 13 | 18 | 47 |
Other bee | 257 | 1.4 | 3.4 | 18 | 4 | 2 | 7 | 50 | 18 | 19 |
Honey bee | 254 | 1.4 | 2.5 | 11 | 1 | < 1 | 8 | 70 | 16 | 5 |
Unidentified insect | 234 | 1.2 | 4.6 | 47 | 13 | 19 | 16 | 11 | 20 | 21 |
Back-swimmer | 225 | 1.2 | 3.4 | 50 | 2 | 30 | 25 | 9 | 8 | 26 |
Caddisfly | 206 (10% larvae) | 1.1 | 2.8 | 28 | 38 | 45 | 6 | 1 | 5 | 5 |
Non-social wasp | 124 | 0.7 | 2.4 | 31 | 3 | 6 | 13 | 22 | 41 | 15 |
Click beetle | 108 | 0.6 | 2.0 | 27 | 23 | 52 | 19 | 3 | 3 | 0 |
Giant water bug | 96 | 0.5 | 1.9 | 37 | 1 | 43 | 24 | 9 | 14 | 9 |
Ladybird beetle | 87 | 0.5 | 1.6 | 18 | 3 | 5 | 3 | 12 | 33 | 44 |
At least 87% of adult and juvenile bullfrogs had food in their stomachs irrespective of month (
Collectively, non-insect invertebrates made up just over 13% of prey remains with spiders and mites (
Non-insect invertebrate prey remains.
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Spiders | 873 | 4.6 | 8.9 | 52 | 7 | 24 | 24 | 25 | 10 | 10 |
Snails | 533 | 2.8 | 8.1 | 58 | 12 | 12 | 12 | 11 | 15 | 20 |
Isopods | 481 | 2.6 | 5.3 | 40 | 22 | 17 | 17 | 6 | 9 | 26 |
Crayfish | 174 | 0.92 | 2.8 | 22 | 2 | 17 | 17 | 18 | 52 | 6 |
Amphipods | 115 | 0.61 | 0.24 | 2 | 0 | 62 | 62 | 9 | 1 | 0 |
Slugs | 108 | 0.57 | 1.60 | 38 | 22 | 10 | 10 | 20 | 23 | 3 |
Earthworms | 83 | 0.44 | 0.37 | 12 | 76 | 0 | 0 | 0 | 2 | 2 |
Millipedes | 59 | 0.31 | 0.91 | 20 | 22 | 5 | 5 | 5 | 10 | 12 |
Leeches | 24 | 0.13 | 0.48 | 20 | 17 | 8 | 8 | 0 | 13 | 37 |
Centipedes | 20 | 0.11 | 0.33 | 17 | 0 | 33 | 33 | 20 | 7 | 7 |
Clams | 8 | 0.04 | 0.11 | 5 | 0 | 25 | 25 | 0 | 0 | 12 |
Mites | 1 | 0.01 | 0.02 | 2 | 0 | 0 | 0 | 100 | 0 | 0 |
Spiders (
Fish (
The top 14 vertebrate prey categories in the bullfrog diet (See Table S4 for other vertebrates identified).
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Three-spined stickleback ( |
97 | 0.52 | 1.5 | 27 | 3 | 30 | 19 | 11 | 6 | 31 |
Pacific treefrog ( |
74 | 0.39 | 1.2 | 33 | 12 | 39 | 19 | 25 | 4 | 1 |
Bullfrog juveniles ( |
51 | 0.27 | 0.96 | 33 | 2 | 6 | 2 | 10 | 10 | 70 |
Rough-skinned newt ( |
50 | 0.26 | 0.87 | 21 | 0 | 36 | 18 | 8 | 26 | 12 |
Bullfrog tadpoles | 30 | 0.16 | 0.43 | 15 | 0 | 7 | 33 | 30 | 27 | 3 |
Sculpin ( |
25 | 0.13 | 0.46 | 3 | 20 | 8 | 8 | 0 | 40 | 24 |
Shrew ( |
24 | 0.13 | 0.48 | 18 | 4 | 17 | 17 | 4 | 54 | 4 |
Unidentified fish | 18 | 0.10 | 0.39 | 6 | 28 | 11 | 6 | 22 | 22 | 11 |
Townsend’s vole ( |
16 | 0.08 | 0.35 | 13 | 0 | 25 | 31 | 0 | 25 | 19 |
Pumpkinseed sunfish ( |
14 | 0.07 | 0.30 | 18 | 0 | 21 | 29 | 14 | 29 | 7 |
Western painted turtle ( |
12 | 0.06 | 0.17 | 2 | 81 | 19 | 0 | 0 | 0 | 0 |
Red-legged frog ( |
10 | 0.05 | 0.21 | 9 | 0 | 10 | 10 | 20 | 60 | 0 |
Northwestern salamander ( |
10 | 0.05 | 0.20 | 5 | 0 | 60 | 40 | 0 | 0 | 0 |
Coho salmon ( |
9 | 0.05 | 0.13 | 2 | 0 | 0 | 0 | 100 | 0 | 0 |
Cannibalism of bullfrog juveniles and tadpoles collectively made up only 0.43% of total prey remains (
The majority (60%) of the 40 individual mammals consumed were shrews, while the rest were all Townsend’s voles (
The approach used here is to focus primarily on instances of predation rather than on ingested volume or nutritional quality in the diet. We accept that one vertebrate is the nutritional equivalent of many insects or other invertebrates, but quantifying and analyzing the relative nutritional significance of each prey instance was beyond the scope of this study.
Insects were found in 93% of the 4,602 bullfrog stomachs with contents, which is consistent with
This study found that early in the bullfrog active season, Odonata (dragonflies and damselflies; May: 45% adults; June: 81% adults) were a consistently important prey for all size-classes of bullfrogs, and this has also been reported by
Bullfrogs are seemingly immune to many natural predator defenses. Previous studies have alluded to the toxic or potentially repellent effects of natural prey defense mechanisms on predatory bullfrogs. For example,
Sticklebacks were the most numerous vertebrate prey and were also one of the most defensively armed. Bullfrogs, however, were seemingly immune to the discomfort of stickleback spines, and we recovered as many as five of these fish from a single stomach. Bullfrogs are reported to have eaten both scorpions and rattlesnakes along the lower Colorado River (
Dragonfly nymphs are known to prey on bullfrog tadpoles (
In 2011, we observed an adult common garter snake (
A giant water bug (
Another organism found in the adult bullfrog diet, and also a predator of bullfrogs (
Bullfrogs routinely leave the water and migrate overland as adults and juveniles, presumably feeding as they travel. This may account for species turning up in the bullfrog diet that are strictly terrestrial, e.g. Townsend’s voles, terrestrial shrews, northern alligator lizards, western red-backed salamanders (
Aphids, because they are tiny, would seem to be an unlikely temptation to bullfrogs. However, aphids ranked second only to social wasps in number of instances of insect predation (
Cannibalism, though well known to occur in bullfrogs, has not been very comprehensively studied (
We sampled 448 bullfrogs that were greater than or equal to 130 mm in body length, or comparable in body size to the “large” category in
In the absence of alternative prey, cannibalism remains an option for this species that would be of variable importance from site to site, season to season, and year to year. In the long-term, unmanaged bullfrog populations might conceivably drive down native species numbers to the point where cannibalism becomes increasingly important to bullfrog population sustainability.
Of native amphibians, the Pacific treefrog was the most frequently eaten by bullfrogs (
Second to treefrogs are rough-skinned newts. Predation on newts peaked in May (36%) and then rose again in August (26%) (
Included in this study were the four sites sampled over 5 years by
The bullfrogs that figured in this study were not collected primarily for the purpose of examining their stomach contents. They were captured and euthanized as part of a research and development program exploring the feasibility and practicality of bullfrog eradication. This was carried out while testing and refining the electro-frogger technique on a regional scale. Most of the 60 sites included in this study (86%) were only visited in three or less of the six months available within the bullfrog’s active season, resulting in only 32% of the overall sample (Table S2). This is because, for the most part, they were smaller ponds where all of the adult and juvenile bullfrogs present could be removed in one or two evenings. In addition, there were a few single-evening reconnaissance missions to sites of interest. The remaining 14% of sites were the larger and more difficult ones where bullfrog densities, immigration rates, and problematical habitats required more effort to bring bullfrog numbers down. The most demanding (Florence Lake,
American bullfrogs have been identified as, or are suspected to be, a threat to the survival of various vertebrates world-wide, including native fish (
In British Columbia, three species of conservation concern relate to this study: the red-listed western painted turtle (
It is of economic interest that coho salmon (
An organism that lies dormant for almost six months of the year must replenish its fat reserves during the relatively brief six-month active season. Mature adults, in particular, should have the life experience to be proficient hunters. They also have energy demanding roles that include vocalizations, territorial defense, egg production, spawning, and may include overland migrations. Then they must end the season with sufficient reserves to overwinter for another six months. The percentage of mature adults of both genders with empty stomachs was, therefore, remarkably high (Table 2B).
1 As an “invasive alien” the American bullfrog is a highly adaptable, opportunistic, and seemingly unspecialized predator that has a uniquely large and complex ecological footprint both above and below the water surface.
2 Insects were the dominate prey group found in 84% of prey instances and 93% of stomachs with food, but seasonality influenced the relative importance of any one insect group over another at any given time period.
3 Cannibalism was found to be a minor component of the diet in terms of relative instances and accounted for approximately 34% of all instances of predation on amphibians.
4 Bullfrog control measures should be routinely factored into management plans for rare and endangered species, such as the western painted turtle on southern Vancouver Island, which are particularly vulnerable to bullfrog predation.
This program was made possible thanks to the consistent funding of Victoria’s Capital Regional District (CRD) departments of Water Services and Parks & Community Services. Funding has also been gratefully received from various municipalities throughout the region including: Langford, Saanich, Highlands, View Royal, Metchosin, and Sooke, and agencies such as the Hartland Landfill and the Swan Lake-Christmas Hill Nature Sanctuary. Many private citizens also made financial contributions to the program. The Veins of Life Watershed Society and the Highlands Stewardship Foundation were instrumental in getting this program off the ground. Finally, we must thank Mr. David Nagorsen for mammal identification, Dr. Alex Peden for equipment and the base map adapted for this report, and Dr. Sylwia Jancowski for laboratory assistance.
Supplementary tables. (doi: