Introduced Bullfrogs (Rana catesbeiana) in Western Canada: Has Their Ecology Diverged?

Defenses of whirligig beetles against native and invasive frogs

Many native insects have evolved defenses against native predators. However, their defenses may not protect them from non-native predators due to a limited shared history. The American bullfrog, Aquarana catesbeiana (Anura: Ranidae), which has been intentionally introduced to many countries, is believed to impact native aquatic animals through direct predation. Adults of whirligig beetles (Coleoptera: Gyrinidae), known for swimming and foraging on the water surface of ponds and streams, reportedly possess chemical defenses against aquatic predators, such as fish. Although whirligig beetles potentially encounter both bullfrogs and other frogs in ponds and lakes, the effectiveness of their defenses against frogs has been rarely studied. To assess whether whirligig beetles can defend against native and non-native frogs, we observed the behavioral responses of the native pond frog, Pelophylax nigromaculatus (Anura: Ranidae), and the invasive non-native bullfrog, A. catesbeiana, to native whirligig beetles, Gyrinus japonicus and Dineutus orientalis, in Japan. Adults of whirligig beetles were provided to frogs under laboratory conditions. Forty percent of G. japonicus and D.orientalis were rejected by P. nigromaculatus, while all whirligig beetles were easily consumed by A. catesbeiana. Chemical and other secondary defenses of G. japonicus and D. orientalis were effective for some individuals of P. nigromaculatus but not for any individuals of A. catesbeiana. These results suggest that native whirligig beetles suffer predation by invasive non-native bullfrogs in local ponds and lakes in Japan.

Hypersensitive response to interferon-stimulated gene (ISG)-inducing double-stranded RNA in American bullfrog tadpole fibroblasts

American bullfrogs are thought to be carriers of ranaviruses and contribute to their global spread via trade. Bullfrog tadpoles succumb to ranaviral infection's more severe and deadly effects than bullfrog adults. Presently, little is known about bullfrog tadpoles' innate antiviral immunity, possible due to the lack of available bullfrog tadpole cell lines. In this study, we describe a novel bullfrog tadpole fibroblast cell line named BullTad-leg. Its general cellular attributes, gene expression and function of class-A scavenger receptors (SR-As), and responses to poly IC (a synthetic dsRNA mimicking viral dsRNAs and a potent inducer of the interferon (IFN)-mediated antiviral responses) are investigated. Its abundant expression of vimentin corroborated with the cells' fibroblast morphology. BullTad-leg cells expressed transcripts of four SR-A members: SR-AI, SCARA3, SCARA4, and SCARA5, but transcripts of MARCO, the fifth SR-A member, were not detected. BullTad-leg cells expressed functional SR-As and could bind AcLDL. BullTad-leg cells exhibited cytotoxicity in response to poly IC treatment via SR-As. Additionally, very low doses of poly IC were able to induce dose-dependent expressions of ISGs including Mx, PKR, ISG20, and IFI35. This research sheds new light on the innate immune response, particularly SR-A biology and dsRNA responsiveness, in bullfrog tadpoles.

Bombardier beetles repel invasive bullfrogs

Invasive non-native predators negatively affect native species; however, some native species can survive the predation pressures of invasive species by using pre-existing antipredator strategies or evolving defenses against invasive predators. The American bullfrog Lithobates catesbeianus (Anura: Ranidae) has been intentionally introduced to many countries and regions, and has impacted native animals through direct predation. Bombardier beetles (Coleoptera: Carabidae: Brachininae: Brachinini) discharge chemicals at a temperature of approximately 100 °C from the tip of the abdomen when they are attacked by predators. This "bombing" can successfully repel predators. However, adults of a native bombardier beetle Pheropsophus (Stenaptinus) occipitalis jessoensis have been reportedly found in the gut contents of the introduced bullfrog L. catesbeianus in Japan. These records suggest that the invasive bullfrog L. catesbeianus attacks the native bombardier beetle P. occipitalis jessoensis under field conditions in Japan; however, the effectiveness of the bombing defense against invasive bullfrogs is unclear. To test the effectiveness of the bombing defense against bullfrogs, we investigated the behavioral responses of L. catesbeianus juveniles to P. occipitalis jessoensis adults under laboratory conditions. Contrary to previous gut content results, almost all the bullfrogs (96.3%) rejected bombardier beetles before swallowing them; 88.9% rejected the beetles after being bombed, and 7.4% stopped attacking the beetles before being bombed. Only 3.7% successfully swallowed and digested the beetle. All of the beetles collected from non-bullfrog-invaded sites could deter bullfrogs, suggesting that the pre-existing defenses of bombardier beetles played an essential role in repelling bullfrogs. When treated beetles that were unable to discharge hot chemicals were provided, 77.8% of bullfrogs successfully swallowed and digested the treated beetles. These results indicate that bombing is important for the successful defense of P. occipitalis jessoensis against invasive bullfrogs. Although invasive bullfrogs have reportedly impacted native insect species, P. occipitalis jessoensis has an existing defense mechanism strong enough to repel the invasive predators.

Genetic structure of American bullfrog populations in Brazil

Non-native species are a major problem affecting numerous biomes around the globe. Information on their population genetics is crucial for understanding their invasion history and dynamics. We evaluated the population structure of the non-native American bullfrog, Aquarana catesbeiana, in Brazil on the basis of 324 samples collected from feral and captive groups at 38 sites in seven of the nine states where feral populations occur. We genotyped all samples using previously developed, highly polymorphic microsatellite loci and performed a discriminant analysis of principal components together with Jost’s D index to quantify pairwise differentiation between populations. We then amplified 1,047 base pairs of the mitochondrial cytochrome b (cytb) gene from the most divergent samples from each genetic population and calculated their pairwise differences. Both the microsatellite and cytb data indicated that bullfrogs comprise two populations. Population grouping 1 is widespread and possesses two cytb haplotypes. Population grouping 2 is restricted to only one state and possesses only one of the haplotypes from Population grouping 1. We show that there were two imports of bullfrogs to Brazil and that there is low genetic exchange between population groupings. Also, we find that there is no genetic divergence among feral and captive populations suggesting continuous releases. The limited genetic variability present in the country is associated to the small number of introductions and founders. Feral bullfrogs are highly associated to leaks from farms, and control measures should focus on preventing escapes using other resources than genetics, as feral and captive populations do not differ.

Reproductive characteristics of American bullfrogs (Lithobates catesbeianus) in their invasive range of the Pacific Northwest, USA

Invasive species pose a major threat to global biodiversity. The effects of invasive species can be strongly influenced and potentially mediated by their reproductive characteristics, such as fecundity, egg production, and duration and number of reproductive events. Selection for smaller body size at first reproduction can also play a role in their establishment, facilitating colonization and spread. The American bullfrog, native to the eastern U.S. (Lithobates catesbeianus), is a species that has invaded more than 40 countries across 4 continents. This species has become especially prevalent in the western United States since its introduction in the early 1900s. This study characterized reproductive characteristics of bullfrogs with emphasis on the minimum size at which males and females reach sexual maturity in the Willamette Valley, Oregon, USA invasion range. We collected and dissected 121 individuals in 2013 and 2017, quantifying characteristics of sexual maturity including snout-vent length, total length, sex, tympanum diameter, presence of distended oviducts or eggs for females, and testes length and sperm activity in males. Our results showed that the minimum reproductive size of both males and females was smaller relative to bullfrogs in their native range as well as in populations across their invasive range. Reduction in size at reproductive maturity is likely impacting the invasive success of American bullfrogs and this study gives us insight on management actions to control the invasion. Applying this insight, managers can adjust their definition of reproductively active adults, increasing the target population of culling and other control methods.

Historical museum collections and contemporary population studies implicate roads and introduced predatory bullfrogs in the decline of western pond turtles

The western pond turtle (WPT), recently separated into two paripatrically distributed species (Emys pallida and Emys marmorata), is experiencing significant reductions in its range and population size. In addition to habitat loss, two potential causes of decline are female-biased road mortality and high juvenile mortality from non-native predatory bullfrogs (Rana catesbeiana). However, quantitative analyses of these threats have never been conducted for either species of WPT. We used a combination of historical museum samples and published and unpublished field studies shared with us through personal communications with WPT field researchers (B. Shaffer, P. Scott, R. Fisher, C. Brown, R. Dagit, L. Patterson, T. Engstrom, 2019, personal communications) to quantify the effect of roads and bullfrogs on WPT populations along the west coast of the United States. Both species of WPT shift toward increasingly male biased museum collections over the last century, a trend consistent with increasing, female-biased road mortality. Recent WPT population studies revealed that road density and proximity were significantly associated with increasingly male-biased sex ratios, further suggesting female-biased road mortality. The mean body size of museum collections of E. marmorata, but not E. pallida, has increased over the last 100 years, consistent with reduced recruitment and aging populations that could be driven by invasive predators. Contemporary WPT population sites that co-occur with bullfrogs had significantly greater average body sizes than population sites without bullfrogs, suggesting strong bullfrog predation on small WPT hatchlings and juveniles. Overall, our findings indicate that both species of WPT face demographic challenges which would have been difficult to document without the use of both historical data from natural history collections and contemporary demographic field data. Although correlational, our analyses suggest that female-biased road mortality and predation on small turtles by non-native bullfrogs are occurring, and that conservation strategies reducing both may be important for WPT recovery.

Evidence of niche shift and invasion potential of Lithobates catesbeianus in the habitat of Mexican endemic frogs

Invasive alien species are one of most severe threats to biodiversity and natural resources. These biological invasions have been studied from the niche conservatism and niche shifts perspective. Niche differentiation may result from changes in fundamental niche or realized niche or both; in biological invasions, niche differences between native and non-native ranges can appear through niche expansion, niche unfilling and niche stability. The American bullfrog Lithobates catesbeianus is an invasive species that can have negative impacts on native amphibian populations. This research examines the climate niche shifts of this frog, its potential range of expansion in Mexico and the risk of invasion by bullfrog in the habitats of 82 frog species endemic to Mexico, that based on their climatic niche similarity were divided in four ecological groups. The results indicate that species in two ecological groups were the most vulnerable to invasion by bullfrog. However, the climate niche shifts of L. catesbeianus may allow it to adapt to new environmental conditions, so species from the two remaining groups cannot be dismissed as not vulnerable. This information is valuable for decision making in prioritizing areas for conservation of Mexican endemic frogs.

Reproductive phenology of the American Bullfrog in subtropical Brazil: photoperiod as a main determinant of seasonal activity

The North American bullfrog Lithobates catesbeianus continues to invade ecosystems worldwide, potentially causing population declines and even extinctions. Within its native distribution, bullfrogs show prolonged reproductive seasons and high fertility. However, data on breeding biology of bullfrogs ex-situ in invaded localities mainly comes from anecdotal reports. Understanding how invasive species are adjusting their life histories to new colonized environments is important for conservation purposes. Here we describe temporal and spatial abundance, calling activity, spawning and tadpole distribution of bullfrogs in southern Brazil. Eighteen samplings occurred during one year. The abundance of individuals was positively related to longer photoperiods and higher temperatures. Reproductive activity was also positively associated with longer photoperiods. Calling sites, spawning and tadpoles were associated with microhabitats presenting hydrophytes, which may provide shelter and thermal stability to bullfrogs. The reproductive seasonal activity of bullfrogs can be highly variable across its growing geographical range, but in subtropical Brazil it is associated with photoperiod, a highly predictable abiotic determinant. In our study area, bullfrogs presented a breeding season twice as long as that observed in some native localities. We suggest that management strategies directed to bullfrog populations must consider the habitat structures and seasonal regimes determined by each invaded environment.

Introduced bullfrogs and their parasites: Haematoloechus longiplexus (Trematoda) exploits diverse damselfly intermediate hosts on Vancouver Island

The lung fluke, Haematoloechus longiplexus, is the most prevalent and abundant parasite of introduced bullfrogs on Vancouver Island, British Columbia, Canada. The ecological success of this trematode in invasive bullfrogs is related to the fluke's ability to utilize native intermediate hosts for transmission. The purpose of this study was to identify the odonate (dragonfly/damselfly) species involved in the transmission of H. longiplexus to the introduced bullfrog. The prevalences and mean intensities of 21 species of odonates (nymphs and adults) were examined for metacercariae infections. Haematoloechus longiplexus is a second intermediate host specialist, being found only in damselflies. Six damselfly species exhibiting the "climber" ecological habit were identified as second intermediate hosts of H. longiplexus. Enallagma carunculatum (prevalence = 75.0%, mean intensity = 17.2 ± 10.8), Ischnura cervula (65.2%, 8.9 ± 4.3), Ischnura perparva (45.5%, 15.4 ± 10.3), and Enallagma boreale (40.7%, 4.8 ± 7.8) were the most commonly infected damselfly species. Metacercariae were absent in damselflies collected from sites lacking bullfrogs. Haematoloechus longiplexus was likely introduced along with the bullfrog, and subsequently adapted to the physid snail and diverse damselfly intermediate hosts present in ponds on Vancouver Island.

Do invasive bullfrogs in Victoria, British Columbia, Canada, show evidence of parasite release?

Few studies have examined vertebrate models of invasive species to explore parasite release as a proposed mechanism through which host species might become invasive. In this study, we examined evidence for parasite release in invasive American bullfrogs (Rana catesbeiana/Lithobates catesbeianus) from five sites in Victoria, British Columbia, Canada. We examined helminth species richness, as well as the prevalence, intensity and abundance of lung and kidney fluke infections. These flukes are expected to impose costs on host survival, growth and reproductive output. We compared measures of these parasite taxa with bullfrogs from Ontario and New Brunswick where they are endemic. Helminth species richness in bullfrogs from the Victoria sites was lower than in Ontario bullfrogs, but comparable to reported indices for other endemic populations. The prevalence of lung flukes (Haematoloechus spp.) in bullfrogs from Victoria was twice as high as was observed in the Ontario bullfrogs, and higher than has been reported from other endemic locations. In four of the five study sites in Victoria, numbers of Echinostoma spp. kidney cysts were lower than observed in endemic populations; however, the fifth site had uncharacteristically high numbers of cysts. In this study, there did not appear to be clear evidence to support parasite release using either parasite species numbers, or infection by specific parasite taxa. Instead, the invasive bullfrogs demonstrated high parasite species richness and high levels of infection for parasites known to be harmful to their hosts.

Bullfrog Lithobates catesbeianus (Amphibia: Ranidae) tadpole diet: description and analysis for three invasive populations in Uruguay

The North American bullfrog, Lithobates catesbeianus is a dangerous invasive species that has been introduced worldwide for commercial and ornamental purposes. Although studies of the bullfrog’s effects on invaded communities have been carried out, they have focused mostly on post-metamorphic stages. However, considering the experimental evidence and its ecological attributes, the L. catesbeianus tadpole could also have significant impact on invaded communities. The aim of this study was to perform a detailed qualitative and quantitative exploration of the L. catesbeianus larval diet. Gut content was studied and compared for three invaded locations in Uruguay (Departments of Canelones, Soriano and Cerro Largo). Although microscopic algae (Spirogyra spp., Euglena spp., Closterium spp., Volvox spp. and Scenedesmus spp.) were the most frequent prey, the diet was also made up of small invertebrates (especially rotifers) and eggs. Statistically significant differences in diet richness and prey identity were found among the three study sites, probably due to local conditions. One common feature among the three locations was the ingestion of prey with high protein levels, filamentous algae and animal prey. The high frequency of high-protein prey could explain the substantial growth rate of these larvae. Native tadpoles appear to have a much poorer diet than bullfrogs. We can thus assume that the L. catesbeianus larvae will be a major competitor and a potential predator for native communities. We propose that L. catesbeianus tadpoles may have significant effects on invaded communities, and should be considered in future research and managements plans.

Invasive bullfrogs as predators in a Neotropical assemblage: What frog species do they eat?

Anurans are important prey for the American bullfrog Lithobates catesbeianus, but field assessments of its diet in the context of a local prey assemblage are lacking. We aimed to identify the frog species consumed by an invasive bullfrog population in subtropical South America, and to assess their relative importance among other types of prey. Characterization of the frog assemblage in the study area also allowed us to calculate the degree of electivity of the recorded anuran prey, in order to gain insight regarding bullfrog feeding preferences and to test if the bullfrog prey composition differed from a random sample of the assemblage. A total of 32.6% of the bullfrogs had at least one anuran in the stomach contents, and post-metamorphic anurans represented 49.1% of the relative prey importance for adult bullfrogs. Anurans were preyed on by all size classes, and constituted the volumetrically most important prey category in the diet of individuals heavier than 100 g. Cycloramphidae, Hylidae and Leiuperidae were positively selected, and Hypsiboas pulchellus and Physalaemus cuvieri were the species most often taken. We found a low occurrence of cannibalism, despite the high density of bullfrogs at the study site. Our results showed that the degree of electivity differed among bullfrog prey types, suggesting that some frog species may be preyed on in a higher proportion than their relative abundance in the assemblage. Testing the clues provided by this assemblage-level approach may lead to a better assessment of the interactions between bullfrogs and the native frog fauna.