Frog Density Research Articles

Impact of an environment-friendly rice-production method on the population density of Pelophylax nigromaculatus frogs

Impact of an environment-friendly rice-production method on the population density of Pelophylax nigromaculatus frogs

Does agricultural intensification impact pest regulation service by frogs in a natural multi-trophic system?

Agricultural intensification is a major driver of biodiversity loss. However, the exact impacts of such loss of key ecosystem service (ES) provisions in agriculture require more scrutiny. We assessed the population loss impacts of a key ES-providing species in an agricultural landscape. We tested the hypothesis that intensive agriculture causes density reduction of frogs known as pest regulators and that negatively affects pest regulation. Different frog densities (high and low) observed in a previous study in low and high agricultural intensification areas were used as treatments in a semi-controlled field experiment. Functional response of the frog species was also studied. Neither high nor low frog density had any significant effect on respective pest populations. Limited feeding rate of the frog species might have contributed to reduced predation pressure on pests. Surprisingly, at low density, frogs significantly reduced the arthropod natural enemies, probably their preferred prey due to the latter’s agility. Unlike in low frog density treatment, increased intraspecific competition at high frog density made them seek out pests as alternative prey to the preferred arthropod pest enemies, whose populations were not affected by frog density. This study for the first time links the population loss of frogs, a potential bio-control agent to ES provision in a multi-trophic system.

In a rough spot: Declines in Arthroleptella rugosa calling densities are explained by invasive pine trees

AbstractBasing conservation interventions on evidence is important for justifying their associated cost and gauging their effectiveness. For amphibians, the number of studies available to support conservation action plans is limited. Here, we sought to determine the effect of invasive pine trees on the calling densities of a Critically Endangered frog species, endemic to the Western Cape province in South Africa. The Rough Moss frog, Arthroleptella rugosa, is an anuran restricted to a small patch of fire‐driven fynbos habitat prone to invasion by Cluster Pines, Pinus pinaster. We use acoustic spatially explicit capture–recapture methods to estimate frog densities at multiple sites (n = 12) over ten years. Sites were classified as invaded or uninvaded by P. pinaster, and this information, along with the time since the last fire, were used as explanatory variables for frog density in a generalized linear mixed model. Frog densities were found to be significantly affected by P. pinaster invasion status. At invaded sites, there was a negative relationship between call densities and time since fire, while at uninvaded sites the same relationship was positive. These results confirm previous suggestions that invasive pine trees cause population declines in A. rugosa. Our findings can be used to support conservation interventions for A. rugosa, specifically the use of fire in an adaptive management context to control pine invasion across its range.

Warming increases activity in the common tropical frog Eleutherodactylus coqui

Tropical ecosystems are expected to experience climate warming, with predicted increases in drying and heat extremes in the coming years. Understanding how these changes will affect terrestrial vertebrates such as amphibians is limited. The Tropical Responses to Altered Climate Experiment in the Luquillo Experimental Forest in northeastern Puerto Rico allows us to study how the tropical forest responds to warming within a replicated plot design. From September 2018 to August 2019, we used mark-recapture sampling to investigate how the spatial population ecology of the common coqui frog (Eleutherodactylus coqui) is impacted by experimentally increasing surface temperatures by 4 °C above ambient. We compared estimates of baseline detection, space use, and the density of frogs in control and warmed plots. Coqui space use and population density did not differ between control and warmed plots. However, coqui detection probabilities were higher in warmed plots, suggesting an increased level of activity relative to individuals in the control (unwarmed) plots. Frog detection increased in all plots with increased precipitation. Our results suggest that, at least in the short-term, the density of an ecological generalist frog like E. coqui does not change as a response to increased surface temperatures. However, short-term responses to warming such as changes in behavior may lead to changes in population dynamics in the long-term. Our research highlights the need to consider mutiple repsonses in order to understand the effects of climate warming on tropical vertebrates.

The effect of integrated rice–frog ecosystem on rice morphological traits and methane emission from paddy fields

Integrated Rice–frog Ecosystem (IRFE) has the potential to reduce methane (CH4) emission and maintain yields from paddy fields. However, the quantitative relationship between rice morphological traits and CH4 emission remains to be explored. In this study, a 2-year field experiment was conducted to evaluate the effect of IRFE on rice morphological traits and CH4 emission from paddy fields and the ecological mechanisms. This study was conducted to analyze twelve aboveground and eight underground rice morphological traits, rice yields, and CH4 flux and emission from the paddy fields with six frog densities (0, 3750, 7500, 15,000, 30,000, and 60,000 frogs ha−1). The results showed that IRFE reduced CH4 emission by 24.70%–41.75% and 21.68%–51.21% in the 2018 and 2019 rice growth seasons, respectively. Moreover, CH4 emission decreased with the increase of frogs. Frogs also increased the diameter, biomass, and volume of rice roots, thus promoting rice growth. Root biomass, thousand-grain weight, and harvest index were also closely related to the yield. Root porosity and oxygen secretion capacity were negatively correlated with CH4 flux. Frogs increased root porosity and oxygen secretion, thereby reducing CH4 emission. The present study demonstrated that reducing CH4 emission and improving rice yields could be simultaneously achieved by altering rice morphological traits in IRFE.

Temporary Water Holes May Benefit the Breeding of the Common Skipper Frog Euphlyctis cyanophlyctis (Anura: Dicroglossidae)

Seasonal fluctuations in adult Euphlyctis cyanophlyctis density were recorded in two permanent ponds in Southeastern Bangladesh. From a two-year dataset of seven habitat parameters, the seasonal suitability of two different pond habitats was examined to understand these fluctuations of frog density. The combined influence of the parameters on the density was significant in all three seasons. One combination was found to be optimal: water temperature of 24 °C, a pH of approximately 6, and a water depth of 2.29 m. These conditions supported more than 24 frogs per acre in the round pond. In the forestry pond, the optimal parameters were a water temperature of 27 °C and a water depth of less than 1 m, which accommodated 27 frogs per acre. The result indicates that a slight increase in any of these parameters was found to sharply lower the accommodation capacity of the studied ponds to less than half of the aforesaid densities, especially in the rainy season. During this season, the adults were found to breed in the small waterholes adjacent to the ponds. This may be the response of anuran to temporal unsuitability, for example, the depth of water and the pH of the permanent ponds.

HIGHER INFECTION PREVALENCE IN AMPHIBIANS INHABITING HUMAN-MADE COMPARED TO NATURAL WETLANDS.

It is unclear how suitable human-made wetlands are for supporting wildlife and how they impact wildlife disease risk. Natural wetlands (those that were created without human actions) can support more diverse and resilient communities that are at lower risk of disease outbreaks. We compared frog community composition and infection with the pathogenic fungus Batrachochytrium dendrobatidis (Bd) between human-made and natural wetlands in Tippecanoe County, Indiana, US. We conducted visual encounter surveys of frog communities and quantified Bd infection prevalence at four natural and five human-made wetlands. Water parameters associated with human practices (e.g., pH, salinity) and surrounding land use were also compared across sites. We found higher Bd infection prevalence at human-made sites than at natural sites, with monthly differences showing highest infection in spring and fall, and decreasing infection with increasing water temperature. However, we found no differences between human-made and natural sites regarding amphibian community composition, water quality, or surrounding land use. Further, we found frog density increased with distance to nearest roads among both human-made and natural sites. These findings might suggest that human-made wetlands can support frog communities similar to natural wetlands, but pose a greater risk of Bd infection.

A new method for the estimation of minimum adult frog density from a large-scale audial survey

Audial surveys of anuran amphibians (frogs, toads and similar) are cost-effective and allow for the coverage of large areas, but they are usually regarded as unsuitable for population size estimations due to imperfect detection. Our study demonstrated a method for obtaining minimum adult population size estimates from vocalising anuran counts by using sex ratios, life history and vocalising behaviour parameters from other studies. We collected data from 2016 to 2018 for seven taxa on 65 plots (each 25 km2) representing the entirety of Latvia. Among taxa, average breeding waterbody audible detection probabilities ranged from 0.56 to 0.88 per plot, minimum adult frog density (MAFD) estimates were from 12.0 to 51.7 individuals per km2, but the estimated fraction of population covered by MAFD varied from 57 to 86%. The least accurate density estimates were in taxa with brief calling activity and quiet mating calls (Rana temporaria), and in taxa with a calling activity dependent on the numbers of males in a pond (Bufo bufo). Our study suggests that lek-breeders would be more suitable than explosive-breeding taxa for minimum population size estimates from audial data. The use of MAFD allowed for coarse minimum population size estimates for the entire country from the audial monitoring data, these ranged from 3.7 ± 0.5 thousand (Bombina bombina) to 1.64 ± 0.47 million (B.bufo) adults.

Informing Amphibian Conservation Efforts with Abundance-Based Metapopulation Models

Science-based management strategies are needed to halt or reverse the global decline of amphibians. In many cases, sound management requires reliable models built using monitoring data. Historically, monitoring and statistical modeling efforts have focused on estimating occupancy using detection–nondetection data. Spatial occupancy models are useful for studying colonization–extinction dynamics, but richer insights can be gained from estimating abundance and density-dependent demographic rates. We developed an integrated abundance-based metapopulation model of the processes contributing to spatiotemporal variation in patch population density. We fit our model to a combination of detection–nondetection and count data from a 14-yr study of a reintroduced metapopulation of federally threatened Chiricahua Leopard Frogs (Lithobates chiricahuensis). Pond-specific population growth rate was influenced by pond hydroperiod and frog density, such that permanent and semipermanent ponds with low densities of adult frogs experienced the highest annual population growth rates. Immigration rate declined as the distance among ponds increased. After reintroduction in 2003, metapopulation-level abundance increased and appeared to stabilize around 1300 adult frogs (95% CI = 1192–1471) by year 2015. Further, changes in metapopulation abundance were driven mostly by changes in abundance at a few ponds. These high-density populations, which would not have been identifiable with traditional occupancy-based metapopulation models, are likely especially important for species recovery in the area. Abundance-based metapopulation models can be widely applied to inform conservation efforts, by providing higher quality information needed to prioritize habitat patches for management and can be used to make more accurate predictions of metapopulation extinction risk.

Native and non-native frogs responded differently to modernization at Japanese paddy fields

Rice farming modernizations, especially changes in the irrigation system, have been accused as one of the reasons of frog population decline in Japan. Here, we examined responses of native frog, Daruma pond frog (Pelophylax porosus porosus) and non-native frog, Rice frog (Fejervarya kawamurai) population to the modernization in Japanese paddy field. We tested the following two hypotheses: 1) native frog’s density will be lower in the modernized paddy field, and 2) non-native frog density will not be affected by modernization. After two periods of field studies (2017 and 2018), we found that native frog’s density is indeed lower in modernized paddy fields, and the non-native frog’s density is lower in the non-modernized sites. Presumably, changes of shallow soil ditches to concrete ditches in modernized paddy fields act as a death trap for Daruma pond frog. Ditch depth and the distance from forest also significantly affecting the density of Daruma pond frog. On the other hand, the population of Rice frog did not show any negative response to the modernizations. The non-native frog population remained stable because they may have high fecundity.

Metrics of disturbance in a redwood forest ecosystem: responses of stream amphibians to repeated sediment infusions

AbstractDisturbances are part of the natural dynamics of Earth's ecosystems, with these events more common now in the Anthropocene. Yet metrics for calibrating these impacts and measuring an ecosystem's capacity to recover are lacking. Highway construction in 1989 to bypass Prairie Creek Redwoods State Park in California resulted in storm‐driven infusion of exposed sediments into five streams; five nearby streams that were not intersected by the bypass construction were not affected by this event and served as controls for a natural experiment. A second large storm event in 1995 contributed sediment loads into all ten streams resulting in a disturbance gradient that allowed us to examine the effects of repeated sediment disturbances. We evaluated the impacts of these stresses on three resident stream amphibian species in 1990 and again in 1996. In 1990, the impacted streams had sixfold higher pool sediment loads and significantly lower larval tailed frog (Ascaphus truei) densities and lower densities of coastal giant salamanders (Dicamptodon tenebrosus) and southern torrent salamanders (Rhyacotriton variegatus) compared with the un‐impacted streams. During the six years between these storm events, pool bowl sediment loads increased 14‐fold in the previously un‐impacted streams and threefold in the previously impacted streams. Larval tailed frogs and torrent salamanders further declined in both sets of streams in 1996 although non‐significantly. In contrast, giant salamander densities increased in both stream sets. Of the three species, giant salamanders appeared the most resistant to the depositional events, while the other two species appeared to decline relative to the intensity of the sediment disturbances but still persist. We believe these results demonstrate the usefulness of these three amphibians as metrics for measuring the effects of this common disturbance type on the ecological resilience of stream networks in this and other temperate northwest forest ecosystems.

Invasive coqui frogs are associated with differences in mongoose and rat abundances and diets in Hawaii

With the increasing rate of species being introduced to areas outside of their native ranges, non-natives are likely to interact in ways that influence each other’s populations. The high densities of invasive coqui frogs (Eleutherodactylus coqui) in Hawaii have been hypothesized to increase non-native mongoose (Herpestes auropunctatus) and rat (Rattus spp.) abundances, and in turn increase bird nest depredation rates. We compared the relative abundances of rats and mongooses and artificial bird nest predation rates at 12 sites that had plots with similar habitat invaded and not invaded by coqui frogs across the island of Hawaii. We interpret our results considering mongoose and rat stomach analyses and camera trap data collected to monitor coqui scavengers. We found that coqui presence was associated with 30% greater mongoose abundance and 17% lower Pacific rat (R. exulans) abundance. Based on our diet analyses and scavenging data, both mongooses and rats consume coquis, but mongooses were the most important consumers of coquis, which may have contributed to their increase in coqui plots. We speculate that coquis are competing with rats for invertebrate prey due to reduced Pacific rat abundance and greater amounts of fruit in rat stomachs collected in coqui-invaded compared to uninvaded plots. We did not observe any difference in bird nest predation rates in coqui-invaded and uninvaded plots. Our results suggest that the coqui invasion may increase or decrease non-native mammal populations, and non-native amphibians may serve as both novel prey and competitors to non-native mammals.

Survival analysis of brown plant hoppers (Nilaparvata lugens) in rice using video recordings of predation events

The brown plant hopper, Nilaparvata lugens Stål, is a major rice pest in South-East Asia. While brown plant hopper (BPH) populations can be regulated by natural enemies, there is limited quantitative information available about the contribution of different predator species to BPH mortality. Our study has three aims: (i) assess the relative contribution of different predator species to BPH mortality in rice fields, (ii) assess diurnal patterns in BPH predation, and (iii) assess the seasonal variation in BPH predation. We quantified predation of live mobile BPH in three rice fields using video recording and assessed densities frogs, a major predator group, by direct counts. In 864 h of video recording, 102 mortality events were observed. Frogs (Ranidae), wolf spiders (Lycosidae) and jumping spiders (Salticidae) were the main predators, accounting for 76%, 13% and 9% of the BPH predation events, respectively. There were large differences in frog density across fields, and there was more predation during the evening (63% predation events) than during the day (37%). Survival analysis indicated that predation risk quickly decreased with time after the onset of recording sessions and that most predation happened within the first 10 min. The results confirm the often overlooked contribution of frogs to BPH predation, but also highlight the substantial variation in predator pressure and frog abundance across farmers’ fields. While camera observations provide compelling information on the identity and relative importance of natural enemies in predation of pests, further development of methods is needed to minimize possible biases resulting from disturbance when making camera observations to quantify predation risk.

Estimating density of calling male Eleutherodactylus coqui in Hawaii from audio recordings of the nighttime frog chorus

Analysis of sound pressure data obtained from audio recordings of night-time Eleutherodactylus coqui chorusing was used to estimate population density of calling male coqui frogs in the major Hawaiian island populations. Study plots 400 m2 in size (20 by 20 m; n = 32) were established on the islands of Hawaii and Maui to provide venues for field counting calling male frogs to investigate the ground truth relationship with the recorded sound pressure data. Male coqui frogs emit a two-note ‘co-qui’ advertisement call at regular intervals. Exploiting the temperature-related and relatively consistent male coqui call repetition rate and the average emitted ‘co’ call sound pressure level (70.4 dB at 1 m), cue counts obtained from a correlation process available in the program Raven were also used to estimate the density of calling male coqui. The study methodology included comparing linear regression results between the field-counted frogs and the number of frogs obtained from the cue count method. Both methods revealed a linear relationship between calling male population density and associated average sound pressure of the resulting chorus.

The Interactive Effects of Fish Predation and Conspecific Density on Survival and Growth of Tadpoles ofRana sylvaticain a Subarctic Wetland

In nature, prey are often simultaneously exposed to predators and competitors, leading to conflicting demands between survival and growth. Larval amphibians are model organisms to investigate the balancing of life history trade-offs because individuals must metamorphose before a wetland dries. Therefore, we deployed enclosures in a natural wetland to investigate the interactive effects of Wood Frog (Rana sylvatica) density (50 or 100 tadpoles) and Brook Stickleback (Culaea inconstans) presence on survival and growth of tadpoles. We hypothesized that the effects of conspecific density on tadpole survival and growth would be dependent on the presence of fish predators. Stickleback predation significantly decreased Wood Frog survival; however, a trend of higher tadpole survival with fish was detected at high conspecific densities. Wood Frog time to and mass at metamorphosis was not significantly affected by either conspecific density or stickleback presence. Our results demonstrate that Brook Sticklebacks ca...

Dietary overlap between the invasive coquí frog (Eleutherodactylus coqui) and the Hawaiian hoary bat (Lasiurus cinereus semotus) on the Island of Hawai’i

The invasive coqui frog is a likely insectivorous competitor to the native Hawaiian hoary bat. The frog is a sit-and-wait predator native to Puerto Rico, but it has the capacity for producing dense populations in its invasive range and the potential to reduce arthropod populations, including aerial arthropods in the orders Coleoptera, Lepidoptera and Isoptera, which are primary food sources of the Hawaiian hoary bat. Dietary analysis of the coqui frog showed that aerial insects made up 33.8 % of the diet. The dietary similarity of the bat and frog was relatively low (Pianka index = 0.25), however, coqui frogs consumed a wide range of juvenile flying insects. Population densities of coqui frogs at two low elevation sites were projected to range from 750 to 14,000 individuals/hectare, and could consume an estimated 1500–19,000 aerial insects/hectare/night, or nearly 90.6 % of all available aerial insects/hectare. Although direct competition between coqui frogs and Hawaiian hoary bats was not confirmed, the coqui frogs could reduce available prey for bats in newly invaded upland sites where bats appear to be more selective of prey species among available aerial insects.

Factors affecting frog density in the Solomon Islands

This paper identifies some important factors affecting the density of frogs in the Solomon Islands. Distance sampling was used to estimate frog density in all major frog habitats across 13 islands. A total of 109 transects, covering five forest types (coastal, freshwater marshes, lowland forest, hill or ridge, and montane forest) were used, with 16 species being sampled. Estimated densities ranged from 2 ha–1 to 675 ha–1. Akaike’s Information Criterion (AIC) was used to select the most parsimonious model of frog density. Factors identified in the selected model to predict density of Batrachylodes elegans, Batrachylodes vertebralis, Ceratobatrachus guentheri, Discodeles bufoniformis, Discodeles guppyi, Discodeles malukuna, Litoria thesaurensis, Palmatorappia solomonis, Platymantis guppyi, Platymantis neckeri, Platymantis solomonis, Platymantis sp., Platymantis weberi, and Rana kreffti were island, landform, and forest type. Additional factors such as disturbance, leaf litter, shrub, and understorey were also described by the regression model as predictors of density for B. vertebralis, C. guentheri, D. guppyi, D. malukuna, Pal. solomonis, P. guppyi, and R. kreffti. These findings have important management implications for the conservation of frogs in the Solomon Islands. High densities of most species were strongly related with forest type and low disturbance. Preservation of rainforest, in especially high conservation value old-growth forests, is imperative to protecting these species.

Immobile defence of a frog distracts attention of approaching predators to other prey

Escape theories predict that animals remaining motionless after being detected by a predator are less successful in predation avoidance because it simply allows the predator to come closer. Nonetheless, even in such a situation, some animals often remain motionless for a while before escaping. We hypothesized that this non-immediate escape response increases the survivorship of the prey by distracting the predator’s attention to another nearby prey that indifferently comes into the visual field of the predator that is carefully and inconspicuously approaching the former prey. Staged encounter experiments using frogs and snakes as prey and predator, respectively, yielded the results that support this hypothesis. Considering the density of frogs in the field, this phenomenon is feasible under natural conditions. Therefore, remaining motionless after being detected by predator could be an effective choice for prey to avoid predation under a certain condition.

Molecular phylogeny of stream treefrogs (Hylidae: Hyloscirtus bogotensis Group), with a new species from the Andes of Ecuador

We present a new molecular phylogeny of the stream treefrog genus Hyloscirtus, with an improved taxon sampling in the Hyloscirtus bogotensis group. The tree supports the existence of three clades within the genus (Hyloscirtus armatus group, H. bogotensis group and Hyloscirtus larinopygion group) in congruence with previous studies, and suggests the presence of at least three new species in the H. bogotensis group. Herein, we describe one of these species, Hyloscirtus mashpi n. sp. from the Pacific slope of the Ecuadorian Andes. The validity of the latter is supported by molecular, morphological and acoustic data. We also tested individuals of the new species for the chytrid fungus Batrachochytrium dendrobatidis, finding a prevalence of 17.6% (6 positives and 28 negatives). However, at sampled streams, frog densities were high, suggesting that H. mashpi n. sp. may be tolerant to the infection.

Living on the edge: Effects of body size, group density and microhabitat selection on escape behaviour of southern leopard frogs Lithobates sphenocephalus

Abstract Models of optimal escape strategy predict that animals should move away when the costs of fleeing (metabolic and opportunity costs) are outweighed by the costs of remaining. These theoretical models predict that more vulnerable individuals should be more reactive, moving away when an approaching threat is further away. We tested whether escape behaviour (including ‘escape calling’) of Lithobates sphenocephalus approached by a human was influenced by body size or the initial microhabitat that the individual was found in. Irrespective of their size, frogs in the open tended to remain immobile, enhancing their crypsis. Frogs in cover showed different responses according to their body size, but, contrary to our initial predictions, larger frogs showed greater responsiveness (longer flight initiation distance and distances fled) than small frogs. Small frogs tended to remain closer to water and escaped into water, while larger individuals were more likely to jump to terrestrial cover and call during escape. Density of frogs near the focal animal had no effect on escape behaviour. This study indicates a range of escape responses in this species and points to the importance of divergent escape choices for organisms which live on the edge of different environments.