The conservation value of freshwater habitats for frog communities of lowland fynbos

Amphibians are more threatened than any other vertebrate class, yet evidence for many threats is missing. The Cape lowland fynbos (endemic scrub biome) is threatened by habitat loss, and natural temporary freshwater habitats are removed in favour of permanent impoundments. In this study, we determine amphibian assemblages across different freshwater habitat types with special attention to the presence of invasive fish. We find that anuran communities differ primarily by habitat type, with permanent water habitats having more widespread taxa, while temporary water bodies have more range restricted taxa. Invasive fish are found to have a significant impact on frogs with toads most tolerant of their presence. Temporary freshwater habitats are a conservation priority in the area, and their amphibian assemblages represent endemic taxa that are intolerant of invasive fish. Conservation of a biodiverse amphibian assemblage in lowland fynbos areas will rely on the creation of temporary freshwater habitats, rather than a northern hemisphere pond based solution.

Range‐wide persistence of the endangered arroyo toad ( Anaxyrus californicus ) for 20+ years following a prolonged drought

Prolonged drought due to climate change has negatively impacted amphibians in southern California, U.S.A. Due to the severity and length of the current drought, agencies and researchers had growing concern for the persistence of the arroyo toad (Anaxyrus californicus), an endangered endemic amphibian in this region. Range‐wide surveys for this species had not been conducted for at least 20 years. In 2017–2020, we conducted collaborative surveys for arroyo toads at historical locations. We surveyed 88 of the 115 total sites having historical records and confirmed that the arroyo toad is currently extant in at least 61 of 88 sites and 20 of 25 historically occupied watersheds. We did not detect toads at almost a third of the surveyed sites but did detect toads at 18 of 19 specific sites delineated in the 1999 Recovery Plan to meet one of four downlisting criteria. Arroyo toads are estimated to live 7–8 years, making populations susceptible to prolonged drought. Drought is estimated to increase in frequency and duration with climate change. Mitigation strategies for drought impacts, invasive aquatic species, altered flow regimes, and other anthropogenic effects could be the most beneficial strategies for toad conservation and may also provide simultaneous benefits to several other native species that share the same habitat.

Differences in Corticosterone Release Rates of Larval Spring Salamanders (Gyrinophilus porphyriticus) in Response to Native Fish Presence

Simple Summary

In amphibians, glucocorticoid hormones play a key role in the response to predation stress. Predators can directly affect prey via injury and death, but they can also have indirect effects due to the activity of glucocorticoids. The regulation of glucocorticoids can differ between populations that have co-evolved with predators and those that have not. We measured glucocorticoids at baseline and in response to a novel stressor in free-living larval salamanders that either live with or without fish predators naturally. We found that salamanders living with fish predators had lower measures of glucocorticoids than those without fish predators. Our study indicates that predator presence alters glucocorticoid regulation, which may allow species to better cope with native and introduced predators.

Abstract

Invasive fish predators are an important factor causing amphibian declines and may have direct and indirect effects on amphibian survival. For example, early non-lethal exposure to these stressors may reduce survival in later life stages, especially in biphasic species. In amphibians, the glucocorticoid hormone corticosterone is released by the hypothalamo–pituitary–interrenal axis (HPI), as an adaptive physiological response to environmental stressors. The corticosterone response (baseline and response to acute stressors) is highly flexible and context dependent, and this variation can allow individuals to alter their phenotype and behavior with environmental changes, ultimately increasing survival. We sampled larvae of the spring salamander (Gyrinophilus porphyriticus) from two streams that each contained predatory brook trout (Slavelinus fontinalis) in the lower reaches and no predatory brook trout in the upper reaches. We measured baseline and stress-induced corticosterone release rates of larvae from the lower and upper reaches using a non-invasive water-borne hormone assay. We hypothesized that corticosterone release rates would differ between larvae from fish-present reaches and larvae from fish-free reaches. We found that baseline and stressor-induced corticosterone release rates were downregulated in larvae from reaches with fish predators. These results indicate that individuals from reaches with predatory trout are responding to fish predators by downregulating corticosterone while maintaining an active HPI axis. This may allow larvae more time to grow before metamorphosing, while also allowing them to physiologically respond to novel stressors. However, prolonged downregulation of corticosterone release rates can impact growth in post-metamorphic individuals.

Effects of elevated temperature, reduced hydroperiod, and invasive bullfrog larvae on pacific chorus frog larvae

Climate change and invasive species threaten many ecosystems, including surface freshwater systems. Increasing temperatures and reduced hydroperiod due to climate change may promote the persistence of invasive species and facilitate new invasions due to potentially higher tolerance to environmental stress in successful invaders. Amphibians demonstrate high levels of plasticity in life history characteristics, particularly those species which inhabit both ephemeral and permanent water bodies. We tested the influence of two projected effects of climate change (increased temperature and reduced hydroperiod) on Pacific chorus frog (Pseudacris regilla) tadpoles alone and in combination with the presence of tadpoles of a wide-spread invasive amphibian, the American bullfrog (Lithobates catesbeianus). Specifically, we explored the effects of projected climate change and invasion on survival, growth, mass at stage 42, and development rate of Pacific chorus frogs. Direct and indirect interactions between the invasive tadpole and the native tadpole were controlled via a cage treatment and were included to account for differences in presence of the bullfrog compared to competition for food resources and other direct effects. Overall, bullfrogs had larger negative effects on Pacific chorus frogs than climate conditions. Under future climate conditions, Pacific chorus frogs developed faster and emerged heavier. Pacific chorus frog tadpoles developing in the presence of American bullfrogs, regardless of cage treatment, emerged lighter. When future climate conditions and presence of invasive American bullfrog tadpoles were combined, tadpoles grew less. However, no interaction was detected between climate conditions and bullfrog presence for mass, suggesting that tadpoles allocated energy towards mass rather than length under the combined stress treatment. The maintenance of overall body condition (smaller but heavier metamorphs) when future climate conditions overlap with bullfrog presence suggests that Pacific chorus frogs may be partially compensating for the negative effects of bullfrogs via increased allocation of energy towards mass. Strong plasticity, as demonstrated by Pacific chorus frog larvae in our study, may allow species to match the demands of new environments, including under future climate change.

Profiling insular vertebrates prone to biological invasions: What makes them vulnerable?

Invasive alien species (IAS) are a major threat to insular vertebrates, although the ecological characteristics that make insular communities vulnerable to IAS are poorly understood. After describing the ecological strategies of 6015 insular amphibians, birds, lizards, and mammals, we assessed the functional and ecological features of vertebrates exposed to IAS. We found that at least 50% of insular amphibian functional richness was hosted by IAS-threatened amphibians and up to 29% for birds. Moreover, all IAS-threatened groups except birds harbored a higher functional richness than species groups threatened by other threats. Disentangling the ecological strategies threatened by IAS, compared to those associated with other threats, we showed that birds, lizards, and mammals were more likely to be terrestrial foragers and amphibians to have larval development. By contrast, large-bodied species and habitat specialists were universally threatened. By considering the functional aspect of threatened insular diversity, our work improves our understanding of global IAS impacts. This new dimension proves essential for undertaking relevant and effective conservation actions.

Predation Pressure of Invasive Marsh Frogs: A Threat to Native Amphibians?

Anurans have been introduced in many parts of the world and have often become invasive over large geographic areas. Although predation is involved in the declines of invaded amphibian populations, there is a lack of quantitative assessments evaluating the potential risk posed to native species. This is particularly true for Pelophylax water frogs, which have invaded large parts of western Europe, but no studies to date have examined their predation on other amphibians in their invaded range. Predation of native amphibians by marsh frogs (Pelophylax ridibundus) was assessed by stomach flushing once a month over four months in 21 ponds in southern France. Nine percent of stomachs contained amphibians. Seasonality was a major determinant of amphibian consumption. This effect was mediated by body size, with the largest invaders ingesting bigger natives, such as tree frogs. These results show that invasive marsh frogs represent a threat through their ability to forage on natives, particularly at the adult stage. The results also indicate that large numbers of native amphibians are predated. More broadly, the fact that predation was site- and time-specific highlights the need for repeated samplings across habitats and key periods for a clear understanding of the impact of invaders.

Spatio-temporal niche plasticity of a freshwater invader as a harbinger of impact variability

Invasive alien fishes have detrimental ecological effects on aquatic ecosystems and the services they provide. Impacts from an invasion in a single ecosystem may differ across space and time due to variability in prey availability and environmental conditions. We hypothesize that such variability can be profound, even within a single ecosystem. Stable isotopes analysis (SIA) is commonly used to quantitatively describe the trophic niche of a species. However, spatial and temporal variability in occupied niches are often not incorporated into management strategies and policy options. Here, we used long-term monitoring data to investigate the invasion stage as well as SIA to analyse the trophic niche of the invasive channel catfish Ictalurus punctatus in Lake Kasumigaura (Japan), a long-term ecological research site (LTER), across distant sampling sites and years. We found a significant spatio-temporal variability in relative growth and isotopic niche occupation. Moreover, we defined a new index, the Isotopic Plasticity Index (IPI), which is the ratio between core and total home range of an occupied isotopic niche, to be used as a proxy for the trophic niche stretch or density. We found that this IPI varied considerably, confirming the spatio-temporal variability in trophic niches, suggesting the IPI to be an adequate new isotopic metric. Our results further provide evidence for the existence of variation across invaded landscapes, implying heterogeneous impacts on recipient native communities. Therefore, our work emphasizes the importance of exploring trophic plasticity in feeding ecology and growth as such information enables a better understanding of impacts and can inform the design and implementation of effective management responses.

MYCOBACTERIA IN SKIN LESIONS AND THE HABITAT OF THE ENDANGERED HOUSTON TOAD (ANAXYRUS HOUSTONENSIS)

Head-starting of the federally endangered Houston toad (Anaxyrus houstonensis), that is, the release of egg strands, tadpoles, and metamorphic juveniles produced in captivity into the original breeding ponds, requires assessment of potential threats for the transmission of pathogens from captive to free-ranging toads. We used Illumina-based 16S rRNA V3 amplicon sequencing to investigate the community structure of bacteria from skin lesions of captive Houston toad and habitat (pond) samples. Proteobacteria, alone or together with Actinobacteria and, in some samples, Cyanobacteria represented virtually all reads in tissue lesion samples, whereas pond samples were much more diverse, with Acidobacteria, Actinobacteria, Bacteriodetes, Chloroflexi, Cyanobacteria, Firmicutes, Planctomycetes, Proteobacteria, and Verrucomicrobia present with little variation between samples. If present in lesions, Actinobacteria were largely represented by Mycobacteriaceae, and here mainly by one sequence identical to sequences of members of the Mycobacterium chelonae-abscessus complex. In pond samples, mycobacteria represented only a small portion of the actinobacteria, although at higher diversity with six distinct reads. Sequences for reads obtained from pond samples were identical to those representing the M. chelonae-abscessus complex, a group with Mycobacterium marinum, Mycobacterium kansasii, Mycobacterium avium, a group with Mycobacterium vaccae, Mycobacterium fortuitum, Mycobacterium poriferae, and a group with Mycobacterium elephantis and Mycobacterium celeriflavum, whereas sequences of high similarity were detected for reads related to those of Mycobacterium holsaticum, Mycobacterium pallens, and Mycobacterium obuense, and Mycobacterium goodii. Our results indicated that lesions observed on the Houston toad in captivity are not the result of mycobacteria in every case, and that the presence of mycobacteria in the captive colony does not represent a novel pathogen threat to the wild populations because such bacteria are also seen in the natural pond habitats for the Houston toad.

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.

Chemical cues of an invasive turtle reduce development time and size at metamorphosis in the common frog

In aquatic systems, chemical cues are one of the major sources of information through which animals can assess local predation risk. Non-native red-eared sliders (Trachemys scripta elegans) have the potential to disrupt aquatic ecosystems in Central Europe because of their superior competitive abilities and omnivorous diets. In this study, we examined whether continuous predator-borne cues are tied to changes in the developmental rates, growth rates and sizes at metamorphosis of common frog tadpoles (Rana temporaria). Our results show rather rarely documented types of amphibian prey responses to caged predators. The presence of turtles shortened the time at metamorphosis of tadpoles from 110 ± 11.7 days to 93 ± 13.0 days (mean ± S.D.). The first metamorphosed individuals were recorded on the 65^th day and on the 80^th day from hatching in the predator treatment and in the control group, respectively. The froglets were significantly smaller (12.8 ± 0.99 mm) in the presence of the predator than in the control treatment (15.2 ± 1.27 mm). The growth rate trajectories were similar between the predator treatment and the control. Thus, predator-induced tadpole defences were evident in higher developmental rates and smaller sizes at metamorphosis without significant changes in growth.

Effects of invasive larval bullfrogs (Rana catesbeiana) on disease transmission, growth and survival in the larvae of native amphibians

The mechanisms by which invasive species negatively affect native species include competition, predation, and the introduction of novel pathogens. Moreover, if an invasive species is a competent disease reservoir, it may facilitate the long-term maintenance and spread of pathogens in ecological assemblages and drive the extinction of less tolerant or less resistant species. Disease-driven loss of biodiversity is exemplified by the amphibian–chytrid fungus system. The disease chytridiomycosis is caused by the aquatic chytrid fungus Batrachochytrium dendrobatidis (Bd) in anurans and is associated with worldwide amphibian population declines and extinctions. For amphibian species that metamorphose and leave infected aquatic habitats, the mechanisms by which Bd persists over winter in these habitats remains a critical open question. A leading hypothesis is that American bullfrogs (Rana catesbeiana), a worldwide invasive species, are tolerant to Bd and serve as a reservoir host for Bd during winter months and subsequently infect native species that return to breed in spring. Using outdoor mesocosms, we experimentally examined if two strains of Bd could overwinter in aquatic systems, in the presence or absence of bullfrog tadpoles, and if overwintered Bd could be transmitted to tadpoles of two spring-breeding species: Pacific treefrogs (Pseudacris regilla) and Cascades frogs (Rana cascadae). We found that only 4 of 448 total animals (one bullfrog and three spring breeders) tested positive for Bd after overwintering. Moreover, two of the three infected spring breeders emerged from tanks that contained overwintered Bd but in the absence of infected bullfrogs. This suggests that Bd can persist over winter without bullfrogs as a reservoir host. We found no effect of Bd strain on bullfrog survival after overwintering. For Pacific treefrogs, Bd exposure did not significantly affect mass at or time to metamorphosis while exposure to bullfrogs reduced survival. For Cascades frogs, we found an interactive effect of Bd strain and bullfrog presence on time to metamorphosis, but no main or interactive effects on their survival or mass at metamorphosis. In short, bullfrog tadpoles rarely retained and transmitted Bd infection in our experiment and we found limited evidence that Bd successfully overwinters in the absence of bullfrog tadpoles and infects spring-breeding amphibians.

Effective removal of the American bullfrog (Lithobates catesbeianus) on a landscape level: long term monitoring and removal efforts in Yosemite Valley, Yosemite National Park

Invasive alien species are a major threat to freshwater ecosystems, and American bullfrogs are among the world’s 100 most prominent aquatic invasive species causing negative direct and indirect effect on native aquatic fauna worldwide. Bullfrogs were intentionally introduced into Yosemite Valley, Yosemite National Park in the 1950s where they became well established in the subsequent years. Starting in 2005, the National Park Service (NPS) began bullfrog removal, targeting various life stages using hand, net, and spear techniques. Starting in 2015, the NPS conducted environmental DNA (eDNA) surveys and deployed audio recordings devices to ensure adequate detection of bullfrogs. During the first year of cencerted effort in the Valley in 2005, the NPS removed 86% of all recorded bullfrog. The subsequent decade was spent searching for individuals with lower return on effort. In 2012, the NPS removed the last observed signs of bullfrog breeding, and the last observed bullfrog in 2019. Following removal of the breeding bullfrog population, the NPS began restoration projects for species of special concern. The NPS introduced the federally threatened California red-legged frogs (Rana draytonii) into Yosemite Valley beginning in 2016. This is the first published successful eradication of bullfrogs on a landscape level. National Parks and Monuments often provide refuges for imperiled wildlife and should be managed to remove invasive species. Our work highlights effective bullfrog removal is obtainable and can lead to local recovery of endangered species.

Evaluating adaptive, carry-over, and plastic antipredator responses across a temporal gradient in Pacific chorus frogs

The development of antipredator traits is dependent on the frequency and intensity of predator exposure over evolutionary and ecological time. We hypothesized that prey species would respond with increasing accuracy when exposed to predators across generational, ontogenetic, and immediate time scales. We assessed larval Pacific chorus frog (PSRE; Pseudacris regilla) individuals that varied in population sympatry, embryonic conditioning, and immediate exposure to stocked populations of rainbow trout (Oncorhynchus mykiss). Using PSRE populations from sites with and without resident rainbow trout, we conditioned embryos to trout odor, PSRE alarm cues, trout odor in combination with alarm cues, or control water. After being hatched and reared in control water, individuals were exposed to the four predator cue treatments using a fully factorial design. Tadpoles from populations with resident rainbow trout did not behave or develop differently than tadpoles originating from fishless sites. However, we found evidence that PSRE reduced predation risk with a combination of carry-over effect (i.e., transfer of information across life history stages) and within-life stage phenotypically plastic mechanisms. We found both developmental and behavioral carry-over effects: tadpoles conditioned with trout odor as embryos grew more slowly and took refuge more often than control animals. Within-life-stage behavioral plasticity was observed in tadpoles from all treatment groups, responding to predator cues with increased refuge use. Potentially additive effects of predator exposure on prey response should be considered when predicting the ability of prey to recognize novel threats.

The role of predation risk in metamorphosis versus behavioural avoidance: a sex-specific study in a facultative paedomorphic amphibian

Evolutionary theory predicts the evolution of metamorphosis over paedomorphosis (the retention of larval traits at the adult stage) in response to life in unfavourable habitats and to the benefits of dispersal. Although many organisms are canalised into obligatory complex or simple life cycles, some species of newts and salamanders can express both processes (facultative paedomorphosis). Previous research highlighted the detrimental effect of fish on both metamorphic and paedomorphic phenotypes, but it remains unknown whether predation risk could induce shifts from paedomorphosis to metamorphosis, whether behavioural avoidance could be an alternative strategy to metamorphosis and whether these responses could be sex-biased. Testing these hypotheses is important because metamorphosed paedomorphs are dispersal individuals which could favour the long-term persistence of the process by breeding subsequently in more favourable waters. Therefore, we quantified the spatial behaviour and timing of the metamorphosis of facultative paedomorphic palmate newts Lissotriton helveticus in response to predation risk. We found that fish induced both male and female paedomorphs to hide more often, but behavioural avoidance was not predictive of metamorphosis. Paedomorphs did not metamorphose more in the presence of fish, yet there was an interaction between sex and predation risk in metamorphosis timing. These results improve our understanding of the lower prevalence of paedomorphs in fish environments and of the female-biased sex ratios in natural populations of paedomorphic newts. Integrating sex-dependent payoffs of polyphenisms and dispersal across habitats is therefore essential to understand the evolution of these processes in response to environmental change.

A global meta-analysis of the ecological impacts of alien species on native amphibians

The exponential increase in species introductions during the Anthropocene has brought about a major loss of biodiversity. Amphibians have suffered large declines, with more than 16% considered to be threatened by invasive species. We conducted a global meta-analysis of the impacts of alien species on native amphibians to determine which aspects of amphibian ecology are most affected by plant, invertebrate, fish, amphibian, reptile, or mammal introductions. Measures of fitness were most strongly affected; amphibian performance was consistently lower in the presence of alien species. While exposure to alien species caused a significant decrease in amphibian behavioural activity when compared with a no species control, this response was stronger towards a control of native impacting species. This indicates a high degree of prey naiveté towards alien species and highlights the importance of using different types of controls in empirical studies. Alien invertebrates had the greatest overall impact on amphibians. This study sets a new agenda for research on biological invasions, highlighting the lack of studies investigating the impacts of alien species on amphibian terrestrial life-history stages. It also emphasizes the strong ecological impacts that alien species have on amphibian fitness and suggests that future introductions or global spread of alien invertebrates could strongly exacerbate current amphibian declines.

Multiple uprising invasions of Pelophylax water frogs, potentially inducing a new hybridogenetic complex

The genetic era has revolutionized our perception of biological invasions. Yet, it is usually too late to understand their genesis for efficient management. Here, we take the rare opportunity to reconstruct the scenario of an uprising invasion of the famous water frogs (Pelophylax) in southern France, through a fine-scale genetic survey. We identified three different taxa over less than 200 km²: the autochthonous P. perezi, along with the alien P. ridibundus and P. kurtmuelleri, which have suddenly become invasive. As a consequence, the latter hybridizes and may now form a novel hybridogenetic complex with P. perezi, which could actively promote its replacement. This exceptional situation makes a textbook application of genetics to early-detect, monitor and understand the onset of biological invasions before they pose a continental-wide threat. It further emphasizes the alarming rate of amphibian translocations, both at global and local scales, as well as the outstanding invasive potential of Pelophylax aliens.

The effects of sunfish on spotted salamander oviposition, hatching time, and larval survival

Introduced species have negative effects on amphibian populations by reducing local recruitment and consuming larvae before metamorphosis. Fish can naturally colonize ponds periodically because of flooding and connectivity with river floodplains. However, many fish introductions have been intentional for commercial, recreational, and biocontrol purposes. In southeast Missouri (USA), Ambystoma maculatum will attempt to breed in ponds even if introduced fish (Lepomis spp.) are present. We predicted that fish would have negative effects on salamander oviposition, hatching success and timing, and larval survival. In order to evaluate the effects of introduced fish on A. maculatum across life stages, we conducted pond surveys and a series of mesocosm experiments. Using field surveys, A. maculatum deposited significantly fewer eggs in ponds with fish. In short-term mesocosm experiments, we found that hatching time was not significantly affected by deposition site or fish cues, however, hatching success was lower for eggs deposited in fish ponds. No A. maculatum larvae survived when fish were present, regardless of egg deposition site. Our study is important because we found that, unless a female avoids depositing eggs with fish, one fish species can have profound effects on larval amphibian persistence. Therefore, small-scale fish introductions for recreation can act as a potential source for reduced recruitment and an increased risk of local extinction.

Frog eat frog: exploring variables influencing anurophagy

Background. Frogs are generalist predators of a wide range of typically small prey items. But descriptions of dietary items regularly include other anurans, such that frogs are considered to be among the most important of anuran predators. However, the only existing hypothesis for the inclusion of anurans in the diet of post-metamorphic frogs postulates that it happens more often in bigger frogs. Moreover, this hypothesis has yet to be tested.

Methods. We reviewed the literature on frog diet in order to test the size hypothesis and determine whether there are other putative explanations for anurans in the diet of post-metamorphic frogs. In addition to size, we recorded the habitat, the number of other sympatric anuran species, and whether or not the population was invasive. We controlled for taxonomic bias by including the superfamily in our analysis.

Results. Around one fifth of the 355 records included anurans as dietary items of populations studied, suggesting that frogs eating anurans is not unusual. Our data showed a clear taxonomic bias with ranids and pipids having a higher proportion of anuran prey than other superfamilies. Accounting for this taxonomic bias, we found that size in addition to being invasive, local anuran diversity, and habitat produced a model that best fitted our data. Large invasive frogs that live in forests with high anuran diversity are most likely to have a higher proportion of anurans in their diet.

Conclusions. We confirm the validity of the size hypothesis for anurophagy, but show that there are additional significant variables. The circumstances under which frogs eat frogs are likely to be complex, but our data may help to alert conservationists to the possible dangers of invading frogs entering areas with threatened anuran species.