Reliable effective number of breeders/adult census size ratios in seasonal-breeding species: Opportunity for integrative demographic inferences based on capture-mark-recapture data and multilocus genotypes

Fine-scale functional connectivity of two syntopic pond-breeding amphibians with contrasting life-history traits: an integrative assessment of direct and indirect estimates of dispersal

Assessing patterns of functional connectivity among amphibian demes is crucial to unravel their population dynamics and prevent their isolation and eventual extinction. Integrative studies based on direct (capture-mark-recapture) and indirect (genetic) estimates of dispersal provide robust, biologically realistic inferences on population structure and connectivity, with applications for conservation efforts. We focused on two pond-breeding amphibians with contrasting life-history traits: the short-lived, semi-arboreal Hyla molleri and the long-lived, fossorial Pelobates cultripes. We PIT-tagged 2150 individuals of both species in two ponds (Laguna and Gravera, separated by 700 m) and monitored them from 2009 to 2021 to document the frequency and spatial extent of dispersal events. In addition, we genotyped individuals from these and two additional breeding populations at a maximum distance of 5 km with 15–16 microsatellites to characterize fine-scale patterns of genetic structure. We detected dispersal events connecting Laguna and Gravera in both species, albeit at low frequencies (4.8% and 7.7% of recaptured individuals of H. molleri and P. cultripes, respectively). However, both species were capable of covering long distances, with individual accumulated displacements up to 3.5 km (Hyla) and 1.8 km (Pelobates). Breeding populations > 2 km apart were genetically differentiated, indicating lower connectivity at this spatial scale. Estimates of pairwise migration rates differed between species and were asymmetrical, with different ponds representing “source” populations contributing more migrants to other populations in each species. We discuss the role of differences in life history traits and ecological preferences in shaping population dynamics in the two species and highlight management implications of our results.

Artificialwater bodies as amphibian breeding sites: the case of the common midwife toad (Alytes obstetricans) in central Spain

Natural breeding sites for amphibians are decreasing in quantity and quality in temperate regions, resulting in local extinctions and increasing population fragmentation. Artificial water bodies (e.g., water tanks or cattle troughs) can represent suitable reproductive habitats for some amphibians, but demographic data are required to assess this assumption. We evaluated the role of artificial water bodies in the persistence of a species of population concern, the common midwife toad, Alytes obstetricans (Laurenti, 1768), at local and regional scales. We surveyed 275 water bodies to characterize the distribution of the species and detected 63 breeding populations of A. obstetricans where we estimated larval abundance. In addition, we monitored two populations for three consecutive breeding seasons using capture-mark-recapture methods based on photo-identification, assessing abundance, breeding success and the use of space of adult individuals captured on multiple occasions. Our results show that artificial sites are preferentially used as breeding sites in the region compared to natural aquatic habitats, providing key habitat for the species and hosting much larger numbers and densities of larvae than natural sites. At local scale, populations of A. obstetricans in artificial sites were abundant and characterized by high male breeding success. However, adults are spatially aggregated around breeding sites, with small home ranges, implying high vulnerability to population fragmentation. Our results suggest artificial breeding sites can sustain viable populations of A. obstetricans, provided measures promoting connectivity among breeding nuclei are considered.

Telomere attrition with age in a wild amphibian population

Telomere shortening with age has been documented in many organisms, but few studies have reported telomere length measurements in amphibians, and no information is available for growth after metamorphosis, nor in wild populations. We provide both cross-sectional and longitudinal evidence of net telomere attrition with age in a wild amphibian population of natterjack toads (Epidalea calamita). Based on age-estimation by skeletochronology and qPCR telomere length measurements in the framework of an individual-based monitoring programme, we confirmed telomere attrition in recaptured males. Our results support that toads experience telomere attrition throughout their ontogeny, and that most attrition occurs during the first 1-2 years. We did not find associations between telomere length and inbreeding or body condition. Our results on telomere length dynamics under natural conditions confirm telomere shortening with age in amphibians and provide quantification of wide telomere length variation within and among age-classes in a wild breeding population.

Integrative demographic study of the Iberian painted frog (Discoglossus galganoi): inter-annual variation in the effective to census population size ratio, with insights on mating system and breeding success

In the face of worldwide amphibian declines, integrative studies combining individual-based information and genetic data represent a powerful approach to produce robust, reliable, and comparable assessments of demographic dynamics. The Iberian painted frog (Discoglossus galganoi) is endemic to Spain and Portugal and shows decreasing population trends across its range, but few studies have attempted to estimate census sizes or assess genetic diversity in wild populations, and little is known about their reproductive biology. We applied an integrative approach based on the combination of capture-mark-recapture data and multilocus genotypes to monitor a breeding population of D. galganoi in central Spain during two consecutive breeding seasons, focusing on the estimation of demographic parameters and their temporal variation. Specifically, we estimated the number of adults (N_a ), the effective population size (N_e ), and the effective number of breeders (N_b ), as well as survival and migration rates. We documented a >50% decrease in the estimated number of adults of both sexes between the breeding seasons of 2018 and 2019, probably associated with reduced rainfall in the latter. Estimates of N_b and the N_b /N_a ratio were low in both seasons, with a 20-30% decrease in N_b and a 47% increase in the N_b /N_a ratio in 2019. Based on the reconstruction of pedigrees from larval and adult genotypes, we provide the first genetic evidence of polygamy in males and females of D. galganoi and the first estimates of breeding success in the species.

Contrasting demographic trends and asymmetric migration rates in a spatially structured amphibian population

Natural populations often persist at the landscape scale as metapopulations, with breeding units (subpopulations) experiencing temporal extinction and recolonization events. Important parameters to forecast population viability in these systems include the ratio of the effective number of breeders (N_b ) to the total number of adults (N_a ) and migration rates among subpopulations. Here, we present the results of a 10-year integrative monitoring program of a metapopulation of the Iberian green frog (Pelophylax perezi) in central Spain. We characterized population dynamics at two main breeding ponds (Gravera and Laguna) using capture-mark-recapture data to estimate N_a in each breeding season, and multilocus genotypes to estimate the effective population size (N_e ), N_b , individual breeding success, and migration rates. Both ponds experienced population decline after a dry season, with Gravera subsequently recovering and Laguna suffering a bottleneck associated with genetic impoverishment. In this subpopulation, average allelic richness and private alleles decreased from 2010 (10.87 and 1.67, respectively) to 2018 (8.0 and 0.20). The N_b /N_a ratio in Laguna in 2018 was twice as high (0.95) than in Gravera (0.41) or in pre-bottleneck Laguna (0.50), suggesting plasticity or genetic compensation through increased individual breeding success. Migration rates were asymmetric between ponds, with a stronger contribution from Gravera to Laguna (29.9% vs. 16.2% in the opposite direction) that may result in a rescue effect. This study emphasizes the importance of integrative demographic approaches for the monitoring of natural populations based on a better understanding of their spatio-temporal dynamics, which provides valuable information for conservation actions.

Reliable effective number of breeders/adult census size ratios in seasonal-breeding species: Opportunity for integrative demographic inferences based on capture-mark-recapture data and multilocus genotypes

The ratio of the effective number of breeders (Nb) to the adult census size (Na), Nb/Na, approximates the departure from the standard capacity of a population to maintain genetic diversity in one reproductive season. This information is relevant for assessing population status, understanding evolutionary processes operating at local scales, and unraveling how life-history traits affect these processes. However, our knowledge on Nb/Na ratios in nature is limited because estimation of both parameters is challenging. The sibship frequency (SF) method is adequate for reliable Nb estimation because it is based on sibship and parentage reconstruction from genetic marker data, thereby providing demographic inferences that can be compared with field-based information. In addition, capture-mark-recapture (CMR) robust design methods are well suited for Na estimation in seasonal-breeding species. We used tadpole genotypes of three pond-breeding amphibian species (Epidalea calamita, Hyla molleri, and Pelophylax perezi, n = 73-96 single-cohort tadpoles/species genotyped at 15-17 microsatellite loci) and candidate parental genotypes (n = 94-300 adults/species) to estimate Nb by the SF method. To assess the reliability of Nb estimates, we compared sibship and parentage inferences with field-based information and checked for the convergence of results in replicated subsampled analyses. Finally, we used CMR data from a 6-year monitoring program to estimate annual Na in the three species and calculate the Nb/Na ratio. Reliable ratios were obtained for E. calamita (Nb/Na = 0.18-0.28) and P. perezi (0.5), but in H. molleri, Na could not be estimated and genetic information proved insufficient for reliable Nb estimation. Integrative demographic studies taking full advantage of SF and CMR methods can provide accurate estimates of the Nb/Na ratio in seasonal-breeding species. Importantly, the SF method provides results that can be readily evaluated for reliability. This represents a good opportunity for obtaining robust demographic inferences with wide applications for evolutionary and conservation research.