Manipulating water for amphibian conservation

Hydroperiod Influences Tadpole Growth and Development in the Endangered Littlejohn's Tree Frog (Litoria littlejohni)

Amphibians are among the most threatened vertebrate taxa globally. Their global decline necessitates effective conservation actions to bolster populations across both the larval and adult stages. Constructing man-made ponds is one action proven to enhance reproduction in pond-breeding amphibians. However, to achieve successful conservation outcomes, extensive knowledge about the ecology and behavior of the target species is required. In this study, we investigated how different hydroperiod regimes impacted the growth and development of Litoria littlejohni tadpoles. Over a 28-week period, tadpoles were exposed to three hydroperiod treatments: constant high, declining, and constant low water levels. Weekly measurements of snout-vent length, body mass, and Gosner stage were taken to assess treatment-related changes. To determine whether different treatments affected locomotor performance, jump tests were conducted 3 weeks post-metamorphosis. Individuals exhibited limited developmental plasticity in response to declining water, with a mean time to metamorphosis of 85.1 days ± 12.1. Comparatively, when L. littlejohni tadpoles were exposed to low water volumes, they were able to speed up development and reduce time to metamorphosis, with a mean time of 63.7 days ± 10.3. The speeding up of development had an apparent consequence for L. littlejohni. We found support for trade-offs between rapid development and reduced morphometric measurements postmetamorphosis which resulted in reduced locomotive ability. Individuals from constant low water treatments exhibited an average total jumping distance of 171 cm ± 13.6 over 10 consecutive jumps, compared with 236 cm ± 17.3 in constant high and 210 cm ± 14.8 in declining treatments. Rapid larval development aids tadpoles in escaping suboptimal aquatic conditions, but its effects on locomotion may impact foraging efficiency and predator escape ability. Understanding developmental plasticity in threatened amphibians, especially in response to hydroperiod variations, is crucial for conservation programs, particularly under future climate change scenarios predicting increased drought and reduced hydroperiods in aquatic environments.

Managing invasive hybrids with pond hydroperiod manipulation in an endangered salamander system

When invasive and endangered native taxa hybridize, the resulting admixture introduces novel conservation challenges. Across a large region of central California, a hybrid swarm consisting of admixed endangered California tiger salamanders (CTS) (Ambystoma californiense) and introduced barred tiger salamanders (BTS) (Ambystoma mavortium) has replaced native populations, threatening the genetic integrity of CTS and the vernal pool systems they inhabit. We employed a large-scale, genomically informed field experiment to test whether shortening breeding pond hydroperiod would favor native CTS genotypes. We constructed 14 large, seminatural ponds to evaluate the effect of hydroperiod duration on larval survival and mass at metamorphosis. We tracked changes in non-native allele frequencies with a 5237-gene exon capture array and employed a combination of custom Bayesian and generalized linear models to quantify the effect of pond duration on salamander fitness. Earlier work on this system showed hybrid superiority under many conditions and suggested that hybrids are favored in human-modified ponds with artificially long hydroperiods. Consistent with these earlier studies, we found overwhelming evidence for hybrid superiority. Very short hydroperiods substantially reduced the mass (1.1-1.5 fold) and survival probability (10-13 fold) of both native and hybrid larvae, confirming that hydroperiod likely exerts a strong selective pressure in the wild. We identified 86 genes, representing 1.8% of 4723 screened loci, that significantly responded to this hydroperiod-driven selection. In contrast to earlier work, under our more natural experimental conditions, native CTS survival and size at metamorphosis were always less than hybrids, suggesting that hydroperiod management alone will not shift selection to favor native larval genotypes. However, shortening pond hydroperiod may limit productivity of hybrid ponds, complementing other strategies to remove hybrids while maintaining vernal pool ecosystems. This study confirms and expands on previous work that highlights the importance of hydroperiod management to control invasive aquatic species.

Decline and Extinction of the Italian Agile Frog Rana latastei from Core Areas of Its Range

Detecting the trends of species and populations is fundamental to identifying taxa with high conservation priority. Unfortunately, long-term monitoring programs are challenging and often lacking. The Italian agile frog Rana latastei is endemic to Northern Italy and adjacent countries, is considered vulnerable by the IUCN, and is protected at the European level. However, quantitative estimates of its decline are extremely scarce. In this study, we document the trends in abundance and distribution of Rana latastei within Monza Park, which currently represents the area closer to the type locality of the species and holds unique genetic features. Wetlands within the park were monitored from 2000 to 2023; counts of egg clutches were taken as a measure of reproductive output and the abundance of breeding females. In 2000, the species occurred over a significant proportion of the park. Total abundance showed strong yearly variation but remained rather constant from 2000 to 2019. However, Rana latastei disappeared from the park around 2021 and was never detected in 2022-2023. The decline is probably related to the joint effect of multiple factors, including the conversion of breeding sites for farming, inappropriate water management, invasive alien species, and severe drought. The local extinction of Rana latastei occurred despite legal protection, highlighting the need for more effective and stringent tools for the conservation of European biodiversity.

Complex Organisms Must Deal with Complex Threats: How Does Amphibian Conservation Deal with Biphasic Life Cycles?

The unprecedented rate of global amphibian decline is attributed to The Anthropocene, with human actions triggering the Sixth Mass Extinction Event. Amphibians have suffered some of the most extreme declines, and their lack of response to conservation actions may reflect challenges faced by taxa that exhibit biphasic life histories. There is an urgent need to ensure that conservation measures are cost-effective and yield positive outcomes. Many conservation actions have failed to meet their intended goals of bolstering populations to ensure the persistence of species into the future. We suggest that past conservation efforts have not considered how different threats influence multiple life stages of amphibians, potentially leading to suboptimal outcomes for their conservation. Our review highlights the multitude of threats amphibians face at each life stage and the conservation actions used to mitigate these threats. We also draw attention to the paucity of studies that have employed multiple actions across more than one life stage. Conservation programs for biphasic amphibians, and the research that guides them, lack a multi-pronged approach to deal with multiple threats across the lifecycle. Conservation management programs must recognise the changing threat landscape for biphasic amphibians to reduce their notoriety as the most threatened vertebrate taxa globally.

Bending the curve: Simple but massive conservation action leads to landscape-scale recovery of amphibians

The global decline of amphibians is part of the global freshwater biodiversity crisis. In human-dominated landscapes, amphibian population declines are driven by multiple stressors. A better understanding of the benefits of conservation action can contribute to the halting and reversal of population declines. Our analysis of 20 y of monitoring data shows that the large-scale construction of hundreds of new ponds in northern Switzerland has halted or even reversed declining trends for the majority of amphibian species, including multiple Red-Listed species undergoing declines at the national level. This conservation success suggests that increasing habitat availability benefits threatened amphibian species despite the continued presence of stressors known to negatively affect populations.

Success stories are rare in conservation science, hindered also by the research-implementation gap, where scientific insights rarely inform practice and practical implementation is rarely evaluated scientifically. Amphibian population declines, driven by multiple stressors, are emblematic of the freshwater biodiversity crisis. Habitat creation is a straightforward conservation action that has been shown to locally benefit amphibians, as well as other taxa, but does it benefit entire amphibian communities at large spatial scales? Here, we evaluate a landscape-scale pond-construction program by fitting dynamic occupancy models to 20 y of monitoring data for 12 pond-breeding amphibian species in the Swiss state Aargau, a densely populated area of the Swiss lowlands with intensive land use. After decades of population declines, the number of occupied ponds increased statewide for 10 out of 12 species, while one species remained stable and one species further declined between 1999 and 2019. Despite regional differences, in 77% of all 43 regional metapopulations, the colonization and subsequent occupation of new ponds stabilized (14%) or increased (63%) metapopulation size. Likely mechanisms include increased habitat availability, restoration of habitat dynamics, and increased connectivity between ponds. Colonization probabilities reflected species-specific preferences for characteristics of ponds and their surroundings, which provides evidence-based information for future pond construction targeting specific species. The relatively simple but landscape-scale and persistent conservation action of constructing hundreds of new ponds halted declines and stabilized or increased the state-wide population size of all but one species, despite ongoing pressures from other stressors in a human-dominated landscape.

Age structure of amphibian populations with endemic chytridiomycosis, across climatic regions with markedly different infection risk

Threatening processes, such as disease, can drive major changes in population demographics of the host. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has led to the decline of at least 500 amphibian species across the globe and has been shown to truncate host age structure by lowering adult survival rates. This results in heavy reliance on annual recruitment and the inability to recover in the event of periodic recruitment failure. We used skeletochronology to determine the age structure, growth, and survival rates of populations of an endangered amphibian, Litoria raniformis, with endemic chytridiomycosis, across two climatically disparate regions in south-eastern Australia: semi-arid and temperate. Contrary to predictions, populations in the semi-arid region (in which chytrid prevalence is substantially lower due to high temperatures) displayed a more truncated age structure than populations in the temperate study regions. Maximum recorded age was only two years in the semi-arid region compared with up to four years in the temperate region. Wetland hydroperiod and average seasonal air temperature were correlated with age, and males had a slightly higher survival rate than females (0.31 for males and 0.27 for females). Despite the previously documented differences in chytrid prevalence between the two climatic regions, water availability and wetland hydroperiods appear the over-riding determinants of the age structure and survival rates of L. raniformis. Targeted management which ensures water availability and improves survival of 1-year-old frogs into their second and third breeding season would reduce the impact of stochastic events on L. raniformis, and this may be true for numerous frog species susceptible to chytridiomycosis.

Long-term drivers of persistence and colonization dynamics in spatially structured amphibian populations

Many organisms live in networks of local populations connected by dispersing individuals, called spatially structured populations (SSPs), where the long-term persistence of the entire network is determined by the balance between 2 processes acting at the scale of local populations: extinction and colonization. When multiple threats act on an SSP, a comparison of the different factors determining local extinctions and colonizations is essential to plan sound conservation actions. We assessed the drivers of long-term population dynamics of multiple amphibian species at the regional scale. We used dynamic occupancy models within a Bayesian framework to identify the factors determining persistence and colonization of local populations. Because connectivity among patches is fundamental to SSPs dynamics, we considered 2 measures of connectivity acting on each focal patch: incidence of the focal species and incidence of invasive crayfish. We used meta-analysis to summarize the effect of different drivers at the community level. Persistence and colonization of local populations were jointly determined by factors acting at different scales. Persistence probability was positively related to the area and the permanence of wetlands, whereas it was negatively related to occurrence of fish. Colonization probability was highest in semipermanent wetlands and in sites with a high incidence of the focal species in nearby sites, whereas it showed a negative relationship with the incidence of invasive crayfish in the landscape. By analyzing long-term data on amphibian population dynamics, we found a strong effect of some classic features commonly used in SSP studies, such as patch area and focal species incidence. The presence of an invasive non-native species at the landscape scale emerged as one of the strongest drivers of colonization dynamics, suggesting that studies on SSPs should consider different connectivity measures more frequently, such as the incidence of predators, especially when dealing with biological invasions.