The research team monitored three contrasting populations using capture–mark–recapture (CMR) protocols designed specifically for a species that is nearly invisible most of the time. Le Porge, on the Gironde coast, is an oceanic forest site with a permanent natural pond. Le Plan and Les Tours, both in the Mediterranean agricultural landscape of the Calavon watershed in southeastern France, host mainly man-made ponds of variable hydroperiod. Because spadefoot toads are nocturnal, fossorial and sensitive to temperature and humidity, the researchers conducted their surveys exclusively at night in terrestrial habitats, under light rainfall or relative humidity above 90 percent, using headlamps and random active searching within a 300-metre radius of the breeding ponds.

Every captured toad was sexed, measured and marked with a sterile PIT-tag implanted subcutaneously, a technique shown to have a 100 percent retention rate and no detectable effect on body mass in this species. The handheld RFID reader allowed individual identification without physically handling the animal during recapture. Monitoring spanned 22 field sessions at Le Porge in 2010–2011 and 30 and 26 sessions at Le Plan and Les Tours respectively between 2019 and 2021, conducted between March and November. The summer and winter months were deliberately excluded, as dry Mediterranean summers and cold winters render the species inactive. The resulting dataset comprised 726 captures of 328 marked toads at Le Porge, 1,496 captures of 741 toads at Le Plan and 233 captures of 111 toads at Les Tours.

The statistical backbone of the study was the Pradel Robust Design Survival and Seniority model, which allows simultaneous estimation of capture probability, apparent survival, recruitment and seasonal population size. Because no specific goodness-of-fit test exists for the Robust Design, the researchers used Cormack–Jolly–Seber tests in U-CARE, applying variance inflation factors where overdispersion was detected and comparing models with the quasi-likelihood Akaike Information Criterion corrected for small sample size. Model averaging was used to obtain final estimates, since several models received similar statistical support. Juveniles were excluded from the analyses to avoid transience effects, as too few had been tagged at two of the three sites.

The numbers that emerged reveal a striking demographic mosaic. Capture probabilities were low to moderate, ranging from 0.07 to 0.22, and varied mainly among seasons, with the highest mean detectability at Le Plan. Estimated seasonal population sizes were broadly comparable between Le Porge and Le Plan, typically 200 to 350 active adults per season, whereas Les Tours supported markedly smaller numbers of 50 to 100 adults, up to six times fewer. Across all three sites, abundance was generally higher in spring than in autumn, suggesting that the main surface activity period for this species occurs in spring at these latitudes, consistent with the biannual breeding phenology described for Mediterranean populations.

Apparent survival varied with season and sex, and was consistently higher over the autumn-to-spring interval than over the spring-to-autumn interval, a pattern that may reflect the energetic and physiological costs of reproduction. Mean survival estimates suggested lower survival at Le Plan (0.63) than at Le Porge (0.76) and Les Tours (0.83), with non-overlapping confidence intervals indicating strong differences among sites. Recruitment showed the opposite pattern, with markedly higher mean estimates at Le Plan (0.40) than at Le Porge and Les Tours (0.11–0.12), indicative of a system characterised by high adult turnover. These patterns should be interpreted cautiously, the authors stress, given the influence of sex, season and time-dependent effects, as well as the confounding effect of site and study period, since the sites were not monitored during the same years.

Nevertheless, the demographic contrast at Le Plan is difficult to ignore. This site is crossed by a secondary road with heavy traffic, averaging 2,028 vehicles per day and 25 vehicles per hour during the night, and its surroundings are subject to repeated mechanical soil tilling in a vineyard and lavender field. The lower apparent survival observed there is consistent with stronger anthropogenic pressures, in line with patterns documented in other amphibian populations inhabiting agricultural landscapes and areas close to road networks. Meanwhile, the higher recruitment at Le Plan may reflect compensatory mechanisms, similar to those described in the yellow-bellied toad (Bombina variegata), where reduced adult survival in human-altered environments has been associated with increased recruitment. If demographic compensation is operating, it could partly offset lower survival, whereas the combination of lower recruitment and higher survival at the other sites may reflect alternative, slower demographic strategies.

Sex ratios also proved strongly context-dependent. The oceanic population at Le Porge was male-biased (observed sex ratio 2.06), while the Mediterranean populations showed a weaker bias in favour of females, with some seasonal variation. This male bias at Le Porge is consistent with previous work showing that distance to the coastline is associated with variation in sex ratio, age ratio and morphology in this species, suggesting that salinity and sea-spray deposition may influence population structure. However, a comparable male bias has also been reported in a Mediterranean population in central Spain, indicating that this skew is not exclusive to oceanic environments. Differences likely reflect a combination of phenological variation, sex-specific site attendance and skipped breeding events among females.

The conservation implications are site-specific and urgent. At Le Plan, reducing road mortality through toad tunnels along major migration routes, combined with the creation of new breeding ponds encompassing diverse hydroperiods, represents a key priority. The authors recommend that any mitigation structure be evaluated using Before-After Control-Impact (BACI) designs to ensure evidence-based refinement of management actions. At Le Porge, regular monitoring of pond salinity and hydrological conditions is necessary to anticipate the effects of increasing salinity on reproduction and juvenile survival, and the creation of ponds further inland within the dune system could help buffer these changes. At Les Tours, the higher survival rates highlight the value of existing buffer zones, but the small population size and low connectivity warrant the creation or restoration of stepping-stone ponds within dispersal distance.

Connectivity measures may be rapidly effective when implemented within the species’ functional dispersal range. In the Calavon watershed, newly created or restored ponds were successfully colonised within two years of establishment, at distances of 300 and 700 metres from ponds where reproduction had been confirmed. Movement data revealed substantial individual variability, with maximum distances ranging from a few metres to 700 metres, and mean maximum distances of approximately 44 metres for males and 74 metres for females at Le Plan. A study of 12 amphibian species indicates that the probability of long-term pond occupancy more than triples when new ponds are created within 500 metres of two to four occupied ponds. The authors conclude that effective conservation of the Western spadefoot toad in France ultimately requires coordinated, long-term strategies underpinned by standardised monitoring protocols, and they call for the development of a dedicated National Action Plan to align stakeholder efforts and ensure species-level conservation at the national scale.

Subject of Research: A capture–mark–recapture study in France provides the first site-level demographic estimates for the Western spadefoot toad, revealing contrasting survival, recruitment and population sizes across climatic and agricultural gradients with implications for targeted conservation.

Article Title: Demography and terrestrial seasonal dynamics of a threatened fossorial toad ( Pelobates cultripes ) across climatic and landscape gradients: Implications for conservation

Article References: Priol, P., Renet, J., Brichard, J., Baudat-Franceschi, J., & Besnard, A. (2026). Demography and terrestrial seasonal dynamics of a threatened fossorial toad (Pelobates cultripes) across climatic and landscape gradients: Implications for conservation. Nature Conservation, 64, 135-157. https://doi.org/10.3897/natureconservation.64.202089

Image Credits: AI Generated

DOI: 10.3897/natureconservation.64.202089

Keywords: Pelobates cultripes, amphibian conservation, demography, capture-mark-recapture, fossorial toads, France, survival, recruitment, terrestrial, seasonal, dynamics, threatened