When no color pattern is available: Application of double observer methods to estimate population size of the Alpine salamander

Romano, Antonio; Roner, Luca; Costa, Andrea; Salvidio, Sebastiano; Trenti, Matteo; Pedrini, Paolo

doi:10.1080/15230430.2021.1994103

Cited by 6 publications

(5 citation statements)

References 49 publications

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“…For this reason, we relied on the assumption that clutch count matches the total number of females breeding in a pond (Lodé et al, 2005). In future studies, alternative approaches such as the double observer approach could provide more accurate estimates of abundance (Nichols et al, 2000; Romano et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

Similar species, different fates: Abundance dynamics in spatially structured populations of common and threatened frogs

Dalpasso

Ficetola

Giachello

et al. 2022

Diversity and Distributions

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Aim Understanding which factors determine the variation in population size across space and time is crucial to plan sound conservation interventions. Amphibians are often characterized by large demographic changes, therefore a better understanding of factors driving these changes could help mitigate their global crisis. We investigated drivers of abundance dynamics of two similar frog species to understand why they met different fates in the same study area. Location Northern Italy. Methods In seven different years between 2004 and 2020, we performed repeated counts of egg masses of two similar frog species, the agile frog (Rana dalmatina) and the Italian agile frog (Rana latastei), in 31 wetlands, and used Bayesian models to estimate the relationships between frog abundance and candidate drivers acting at the scale of (i) site: wetland surface, shading percentage and the presence of the invasive red swamp crayfish (Procambarus clarkii); (ii) landscape: forest cover around wetlands; (iii) climate: yearly precipitation and (iv) spatially structured population (SSP): clutch incidence of the focal species and crayfish incidence in the surrounding landscape. Results The two species showed sharp differences in population size and trends: R. dalmatina was abundant and showed a stable trend throughout the entire study period; R. latastei showed low abundance in the first years and then almost disappeared. The abundance of R. dalmatina was positively related to forest cover, shading, wetland area and precipitation, while negatively related to the occurrence of invasive crayfish at both local and SSP level. R. latastei abundance increased with wetland area and precipitation, while models were not able to detect relationships with other factors. Main conclusions The high sensitivity to drought and demographic stochasticity could have contributed to the quasi‐extinction of R. latastei in the study area, highlighting that similar species can meet different fates even under the same environmental conditions.

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Section: Discussionmentioning

confidence: 99%

Similar species, different fates: Abundance dynamics in spatially structured populations of common and threatened frogs

Dalpasso

Ficetola

Giachello

et al. 2022

Diversity and Distributions

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“…For this reason, although considering that very rare species have a limited ecological relevance on the biotic interaction across the realized community, finding new tools to allow analysis of rare species is of paramount importance. In this respect, multinomial N-mixture models, relying on different sampling protocols, such as multiple observers (Costa et al 2020;Romano et al 2021) or mark-recapture distance sampling (Rosa et al 2022b), have proved to be a reliable and cost-effective tool to estimate abundance and other demographic parameters of amphibians and reptile populations, even when density and detection probability are low (Costa et al 2020). Therefore these model may represent a promising pathway for inferring species co-occurrence and interaction also for rare reptile species.…”

Section: Discussionmentioning

confidence: 99%

How landscape and biotic interactions shape a Mediterranean reptile community

Romano¹,

Rosa

Salvidio

et al. 2022

Landsc Ecol

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Context Disentangling the effect of environment and biological interaction on community composition with observational data, within the environmental filtering framework, is challenging because the two processes produce non independent results. Objectives Adopting community N-mixture models with symmetric interactions, we aimed at estimating differential effects of landscape structure and biotic interactions on the local abundance of a Mediterranean reptile community including four lizards (Lacerta bilineata; Podarcis siculus; P. muralis; Chalcides chalcides) and two snakes (Hierophis viridiflavus; Natrix Helvetica). Methods We sampled reptiles for three consecutive years (2019–2021; 4 surveys/year) on 52 linear transects on a Mediterranean coastal landscape. We analyzed count data by means of a multi-species N-mixture model with symmetric interactions. Interactions within pair of species were estimated from the residual correlation of their realized abundances, after accounting for four landscape features: landscape heterogeneity calculated from land cover data, edge density of woody vegetation patches, tree cover density, net primary productivity. Results Most species displayed very low detection probability (p ~ 0.10). All species responded with different intensity and sensitivity to landscape predictors. Two biological interactions resulted significant: L. bilineata and P. siculus showed a positive interaction, while P. muralis and C. chalcides displayed a negative interaction. Conclusions Using community N-mixture models we demonstrated that, also with observational data obtained from a realized community, partitioning the filtering process of the landscape from the one of biotic interactions is possible.

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“…The surveys were timed to spot the peak of the breeding season of the two frogs and, during the sampling, the two observers walked across the entire wadable surface of the wetlands to count the clutches of the two species. We adopted a dependent double-observer approach, where a first observer indicates clutches to a second observer, who annotates the number of clutches not seen by the first observer [ 3 , 14 ]. Before beginning the fieldwork, we performed preliminary independent counts in a small subset of ponds and obtained identical counts from the different observers.…”

Section: Methodsmentioning

confidence: 99%

“…Every wildlife biologist has experienced the frustration of being unable to find any individual of his target species during a field survey, even in habitats where a colleague or a friend has reported several sightings of that species. This can occur because animals are not available in a given moment at a study site, or because the individuals are present but their detection probability is lower than one [ 1 , 2 ], which may depend on their behavior and characteristics, on the environmental conditions at the survey sites, or on the ability level of an observer to detect the individuals in a given area [ 3 , 4 , 5 ]. Overlooking individuals during counts at a site can result in biased estimation of species occurrence and abundance, as well as to misleading inferences on the key parameters of populations or of the relationships between species and habitats.…”

Section: Introductionmentioning

confidence: 99%

“…The dependent double-observer approach is a promising method to rapidly estimate the actual abundance of species and has been successfully applied to several amphibian species [ 3 , 14 ]. Previous studies on American amphibians ( Ambystoma maculatum , Rana sylvatica ) in the United States showed the high detectability of egg clutches [ 14 ].…”

Section: Introductionmentioning

confidence: 99%

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Assessing Population Trends of Species with Imperfect Detection: Double Count Analyses and Simulations Confirm Reliable Estimates in Brown Frogs

Falaschi

Gibertini

Parrino

et al. 2022

Animals

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Most animal species are detected imperfectly and overlooking individuals can result in a biased inference of the abundance patterns and underlying processes. Several techniques can incorporate the imperfect detection process for a more accurate estimation of abundance, but most of them require repeated surveys, i.e., more sampling effort compared to single counts. In this study, we used the dependent double-observer approach to estimate the detection probability of the egg clutches of two brown frog species, Rana dalmatina and R. latastei. We then simulated the data of a declining population at different levels of detection probability in order to assess under which conditions the double counts provided better estimates of population trends compared to naïve egg counts, given the detectability of frog clutches. Both species showed a very high detection probability, with average values of 93% for Rana dalmatina and 97% for R. latastei. Simulations showed that not considering imperfect detection reduces the power of detecting population trends if detection probability is low. However, at high detection probability (>80%), ignoring the imperfect detection does not bias the estimates of population trends. This suggests that, for species laying large and easily identifiable egg clutches, a single count can provide useful estimates if surveys are correctly timed.

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When no color pattern is available: Application of double observer methods to estimate population size of the Alpine salamander

Cited by 6 publications

References 49 publications

Similar species, different fates: Abundance dynamics in spatially structured populations of common and threatened frogs

Similar species, different fates: Abundance dynamics in spatially structured populations of common and threatened frogs

How landscape and biotic interactions shape a Mediterranean reptile community

Assessing Population Trends of Species with Imperfect Detection: Double Count Analyses and Simulations Confirm Reliable Estimates in Brown Frogs

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