Population enumeration and assessing conservation status in a widespread amphibian: a case study of Rana temporaria in Ireland

Authors


  • Editor: Trent Garner

    Associate Editor: Benedikt Schmidt

Abstract

Global amphibian declines are a major element of the current biodiversity crisis. Monitoring changes in the distribution and abundance of target species is a basic component in conservation decision making and requires robust and repeatable sampling. For EU member states, surveillance of designated species, including the common frog Rana temporaria, is a formal requirement of the ‘EC Habitats & Species Directive’. We deployed established methods for estimating frog population density at local water bodies and extrapolated these to the national and ecoregion scale. Spawn occurred at 49.4% of water bodies and 70.1% of independent 500-m survey squares. Using spawn mat area, we estimated the number of adult breeding females and subsequently the total population assuming a sex ratio of 1:1. A negative binomial model suggested that mean frog density was 23.5 frogs ha−1 [95% confidence interval (CI) 14.9–44.0] equating to 196 M frogs (95%CI 124 M–367 M) throughout Ireland. A total of 86% of frogs bred in drainage ditches, which were a notably common feature of the landscape. The recorded distribution of the species did not change significantly between the last Article 17 reporting period (1993–2006) and the current period (2007–2011) throughout the Republic of Ireland. Recording effort was markedly lower in Northern Ireland, which led to an apparent decline in the recorded distribution. We highlight the need to coordinate biological surveys between adjacent political jurisdictions that share a common ecoregion to avoid apparent disparities in the quality of distributional information. Power analysis suggested that a reduced sample of 40–50 survey squares is sufficient to detect a 30% decline (consistent with the International Union for Conservation of Nature Category of ‘Vulnerable’) at 80% power providing guidance for minimizing future survey effort. Our results provide a test case for other EU member states to follow when conducting future conservation assessments for R. temporaria and other clump-spawning amphibians.

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