John W. Wilkinson

Science, statistics and surveys: a herpetological perspective

Bridging the gap between conservation science and conservation practice is a widely acknowledged issue in applied ecology (Hulme 2011). Nowhere is the gap greater than in the area of data collection, analysis and interpretation. Population assessments for conservation are frequently based on traditional practices that use rules of thumb and quasi-quantitative methods. This means that important decisions that have far-reaching conservation, and commercial and financial implications are often based on sketchy population assessments. This is particularly problematic for small-bodied, cryptic animals that have highly seasonal patterns of behaviour tied to prevailing weather conditions. Amphibians and reptiles are a case in point and illustrate many issues that have wider implications for biodiversity assessment. Despite a resurgence of interest in the conservation of these animals over the past two decades (Gibbons et al. 2000), there remain significant challenges in obtaining population data for amphibians and reptiles that are reliable enough to inform conservation decisions. Even in the UK, which has an impoverished herpetofauna that is relatively well studied, surveys are usually based on methods that have changed little over a quarter of a century. For example, great crested newt Triturus cristatus populations can be scored by systems based on simple counts (Table 1). When such counts are carried out as part of a licensed survey to inform recommendations for development impact mitigation, there is a requirement to standardize such counts and record such variables as torch power and water turbidity. However, there remains a multitude of site-specific and survey-specific variables that can affect such counts, particularly in wider surveys where standardization may not be required (Schmidt 2003). Consequently, many population assessments may more reliably reflect species detectability than actual population status. Numerous statistical tools are now available that take account of detectability, and can provide estimates of population size, population density or population presence–absence (Table 2). Despite the fact that some of these tools have been around for many years, their use remains largely confined to academia. But are rigorous, statistically defensible population assessments really helpful when it comes to conservation decision-making? Could their application actually divert resources away from more pressing issues? Exactly what type of evidence is needed for population assessment? In 2011–2012, we held five knowledge exchange workshops in England, Wales and Scotland to explore these issues with professional conservation practitioners. Participants included ecological consultants, planners, local authority ecologists, reserve managers, and government and non-government agency employees. To ensure discussions remained focussed, habitat assessment and spatial modelling were specifically excluded from the workshops, although nearly all participants considered these to be areas that offered good potential to guide future surveys. Participants were asked to brainstorm what types of survey (i.e. presence–absence, population indices, population densities, population estimates) were needed to guide conservation at different scales. They were then asked to assign priorities for improved design and analysis, and identify possible barriers to implementing changes to current practice. This article explores the main themes that emerged from these workshops, particularly with regard to surveys carried out within professional practice.

Assessing Historical and Current Threats to Common Frog (Rana Temporaria) Populations in Ireland

Abstract Ponds are an ephemeral feature of the landscape but their large-scale loss can have profound implications for biodiversity and the persistence of amphibian populations. We quantified rates of pond loss throughout Ireland over a period of approximately 125 yr. Environmental parameters and perceived risk factors associated with the current occurrence and density of the Common Frog, Rana temporaria, were also analyzed. The numbers of farmland ponds declined by 54% between 1887–1913 and 2005–11, with most ponds and the greatest losses in the East, coincident with agricultural intensification and human habitation. The decline of pond numbers was significant but, at approximately 0.5% per annum, was substantially less than losses recorded in other European countries. Losses were coincident with major changes to the agricultural landscape including extensive land drainage. However, losses of pond and natural wetland habitats throughout Ireland may have been partially or wholly mitigated by a synchronous expansion of artificial field margin ditches associated with drainage projects during the mid-20th Century. The ecology of the Common Frog in Ireland was similar to its ecology elsewhere and it appears largely unaffected by pollution and disturbance. Consequently, the conservation status of the frog in Ireland was judged “favorable” and should remain so for the foreseeable future.

Optimising monitoring efforts for secretive snakes: a comparison of occupancy and N-mixture models for assessment of population status

A fifth of reptiles are Data Deficient; many due to unknown population status. Monitoring snake populations can be demanding due to crypsis and low population densities, with insufficient recaptures for abundance estimation via Capture-Mark-Recapture. Alternatively, binomial N-mixture models enable abundance estimation from count data without individual identification, but have rarely been successfully applied to snake populations. We evaluated the suitability of occupancy and N-mixture methods for monitoring an insular population of grass snakes (Natrix helvetica) and considered covariates influencing detection, occupancy and abundance within remaining habitat. Snakes were elusive, with detectability increasing with survey effort (mean: 0.33 ± 0.06 s.e.m.). The probability of a transect being occupied was moderate (mean per kilometre: 0.44 ± 0.19 s.e.m.) and increased with transect length. Abundance estimates indicate a small threatened population associated to our transects (mean: 39, 95% CI: 20–169). Power analysis indicated that the survey effort required to detect occupancy declines would be prohibitive. Occupancy models fitted well, whereas N-mixture models showed poor fit, provided little extra information over occupancy models and were at greater risk of closure violation. Therefore we suggest occupancy models are more appropriate for monitoring snakes and other elusive species, but that population trends may go undetected.