Ongoing harlequin toad declines suggest the amphibian extinction crisis is still an emergency

The study addresses whether the amphibian extinction crisis remains in an emergency phase by using harlequin toads (Atelopus) as a worst-case model. Amphibians are among the most threatened vertebrates, with high levels of global biodiversity loss and documented rapid declines since the late 20th century. Although more than 30 Atelopus species have been rediscovered after long absences, raising hopes of recovery, it remains unclear if these reflect genuine population improvements. The authors analyze standardized assessments of Atelopus population status from 2004 and 2022, and evaluate current and emerging threats, to determine if conservation actions have translated into improved status. The purpose is to quantify change in population status across species and to assess the persistence of threats, thereby informing conservation priorities and gauging progress toward reversing declines.

Previous global assessments revealed severe amphibian declines, with the 2004 IUCN Global Amphibian Assessment estimating ~32% of species threatened and numerous extinctions. Updated 2022 IUCN evaluations indicate nearly 35% of assessed species threatened. Traditional threats (habitat destruction/degradation) and novel/synergistic stressors, particularly infectious diseases like chytridiomycosis caused by Batrachochytrium dendrobatidis (Bd), and climate change, are implicated in declines. The Amphibian Conservation Action Plan outlines strategies including research, monitoring, habitat protection, disease mitigation, capacity building, community engagement, and ex situ conservation. Harlequin toads (Atelopus), with >100 species across Central and South America, experienced dramatic declines since the 1980s–2000s, with many species listed as Critically Endangered and some possibly extinct. Despite fears of genus-wide collapse, many Atelopus species have been rediscovered, prompting investigation into whether these represent real recoveries.

Study design and data sources: The authors updated and expanded the La Marca et al. (2005) expert-compiled database on Atelopus population trends. The study period for data gathering was October 2019 to December 2022. They contacted and interviewed 105 experts (scientists and conservationists with specialized knowledge of Atelopus), often in multiple rounds. Of contributors to the original database, 13 provided updated information; an additional 92 researchers contributed taxonomy and species observations. Initial expert estimates on distribution, population sizes, and trends were consolidated species-wise, followed by a second round requesting corrections. Taxonomic scope: The analysis considers 131 species (including undescribed taxa and tentatively elevated subspecies), of which 100 are formally described. The 2004 database included 94 taxa. Recognizing ongoing taxonomic issues, the team used the most current taxonomy and included nine identified but undescribed species. Population status coding: Following La Marca et al. with revised terminology, each species was categorized as: steady (one or more populations persist; no population declined >50%); shrinking (at least one population declined >50%); or data pending (insufficient trend data). Species considered by experts as possibly extinct or extinct in the wild were flagged as “probably vanished” but not treated as a separate status category due to uncertainty and frequent rediscoveries. For each species, they also recorded distribution, number of known populations (1, 2, >>2), year last seen (YLS), presence of Bd, and occurrence in protected areas. Statistical analysis: Differences in population status distribution between the 2004 and 2022 datasets were tested using the Freeman-Halton extension of Fisher’s exact test (one-sided) with exponential Bonferroni-Holm correction. Analyses used an online Fisher test calculator (accessed January 2023). Bd presence assessment: Bd presence was compiled from direct testing (PCR or histology) and inferred presence based on proximity to Bd records. The team used records from the Aquatic Parasite Observatory and the Amphibian Disease Portal. They georeferenced 776 Atelopus records for 40 species; if a Bd record occurred within a defined spatial buffer of an Atelopus record, the species was considered potentially Bd-exposed/present. Climate change vulnerability: The Climate Stability Index (CSI) was used to assess potential future climate impacts at Atelopus sites. CSI provides a high-resolution measure of climate stability from the Pliocene (~3.6 Ma) to 2100. For future projections, the authors used the SSP5-8.5 scenario from preindustrial baselines (1970–2000) to 2100, downloaded March 2, 2022. CSI values range from −1 (most stable) to 1 (least stable). CSI was summarized across species to evaluate climate vulnerability and potential elevational range shifts. Ex situ conservation data: All known institutions holding or having held captive assurance colonies of Atelopus were contacted (seven institutions within range countries plus numerous North American and European institutions, including coordination via Project Golden Frog). Data on species held, breeding attempts, and breeding success (including multi-institution and multigenerational successes) were compiled (Supplementary Table 4).

• No Atelopus species improved in population status between 2004 and 2022. Statistical tests found no significant difference in status distributions between 2004 and 2022 (Fisher’s exact test: p = 0.152; N = 131). A reduced dataset limited to species assessed in 2004 also showed no change (p = 0.826; N = 94).
• Rediscoveries do not equate to recoveries: Of 29 species rediscovered since 2004, six have not been seen again for the past 10 years despite targeted searches, indicating extremely low and unstable populations.
• Spatial pattern of declines: Shrinking populations are concentrated in the Andes and Central America. No shrinking populations were noted among species from the lower Amazon basin, the Guianas, the Colombian Sierra Nevada de Santa Marta, and a subset of Chocó species (13 species total without shrinking status in these regions).
• Many species not observed recently: 61 species (46.6%) have a year last seen (YLS) of 2004 or earlier; 37 of these are considered probably vanished.
• Threats persist and are widespread: Habitat destruction/degradation threatens 93 species (71.0%). Bd presence was noted or inferred in 50 species (38.2%). Some species maintain steady populations despite Bd (five species), suggesting context-dependent persistence but fragile dynamics.
• Protected areas coverage increased but is insufficient: The number of Atelopus species occurring in protected areas increased from 84 (64.1%) in 2004 to 90 (73.3%) in 2022. However, populations of 43 species within protected areas are still shrinking, indicating protection alone does not mitigate key threats (e.g., Bd, climate change, ongoing habitat pressures).
• Climate change poses emerging risks: CSI analyses suggest increasing climatic instability and potential needs for rapid elevational shifts for many species, implying rising future vulnerability, including for high-elevation taxa that may face range constraints and increased solar exposure.
• Ex situ conservation: Captive assurance colonies exist for 26 species. Sixteen species (13 of them shrinking) have reproduced at least once in captivity with offspring reaching maturity, but only six species have achieved reproduction at two or more institutions and multigenerational success at least once. Attempts failed completely for three species (all individuals lost). Regional biases exist in ex situ efforts, with gaps in several range countries.
• Overall conclusion: The amphibian extinction crisis remains an emergency; conservation efforts have not reversed declines in this worst-case group.

Using harlequin toads as a worst-case indicator, the study demonstrates no improvement in population status over nearly two decades, directly addressing the research question and disproving the hypothesis of a post-decline recovery phase. Persistent, widespread threats—primarily habitat destruction/degradation and chytridiomycosis (Bd)—continue to drive declines. The increase in protected area coverage has not translated into population recovery for many species, indicating that protected status alone is inadequate when disease and other stressors are at play. The rediscoveries of multiple species, while encouraging, often represent small, fragile remnant populations and cannot be conflated with demographic recovery. Emerging climate instability, as indicated by CSI under SSP5-8.5, is projected to exacerbate risks by forcing elevational range shifts and potentially increasing exposure of high-elevation taxa, compounding existing threats. Ex situ conservation has provided critical insurance for select species and bought time for threatened taxa, but success remains uneven and limited; multi-institutional and multigenerational breeding successes are rare, and regional coverage is incomplete. Taken together, the results show that the amphibian extinction crisis is ongoing and acute, and that substantial scaling-up of both in situ threat mitigation (habitat protection/management, disease management) and ex situ capacity (breeding programs, genetic resource banking) is required to prevent further losses.

Comparisons between the 2004 and 2022 datasets reveal no species-level improvements in Atelopus population status, and rediscoveries have not generally yielded sustained recoveries. Persistent threats—habitat change, Bd—and emerging climate risks underscore that the amphibian extinction crisis remains an emergency. Meeting global goals to sharply reduce extinction rates over the next 20–30 years will require significantly expanded efforts and investments. Priorities include: scaling in situ mitigation such as habitat protection and management; developing and deploying innovative disease mitigation strategies; expanding ex situ conservation breeding capacity, including in-country technical and scientific capabilities; and enhancing genetic resource banking (gamete storage and gene banks) to support future reintroductions and genetic resilience. Continued monitoring, research, and community-based conservation will be essential. Future research should refine disease management in the wild, assess climate adaptation options (e.g., habitat corridors, assisted migration), improve demographic monitoring to detect genuine recoveries, and address taxonomic uncertainties that affect conservation planning.

The study relies heavily on expert elicitation, which introduces unavoidable uncertainty in estimates of distribution, population size, and trends. Population status categories are based on expert assessments and may be affected by detection biases, uneven survey effort, and varying data quality among species and regions. Some taxa remain taxonomically unresolved, and the inclusion of undescribed taxa and tentatively elevated subspecies may affect comparability with prior assessments. Not all species are assessed on the IUCN Red List (37 unassessed), limiting cross-referencing with standardized threat categories. Bd presence was inferred in part using spatial proximity to pathogen records, which may lead to over- or underestimation of true infection status. Climate risk assessments based on CSI use a worst-case SSP5-8.5 scenario and may not capture finer-scale microclimatic refugia or adaptive capacity.