When Is a Species Declining? Optimizing Survey Effort to Detect Population Changes in Reptiles

Beebee TJC (2012) When Is a Species Declining? Optimizing Survey Effort to Detect Population Changes in Reptiles. PLoS ONE 7(8): e43387. doi:10.1371/journal.pone.0043387 When Is a Species Declining? Optimizing Survey Effort to Detect Population Changes in Reptiles David Sewell 0 Gurutzeta Guillera-Arroita 0 Richard A. Griffiths 0 Trevor J. C. Beebee 0 Brock Fenton, University of Western Ontario, Canada 0 1 Durrell Institute of Conservation and Ecology, School of Anthropology and Conservation, University of Kent , Canterbury, Kent , United Kingdom , 2 School of Life Sciences, University of Sussex , Falmer, Brighton , United Kingdom , 3 National Centre for Statistical Ecology, School of Mathematics, Statistics and Actuarial Science, University of Kent , Canterbury, Kent , United Kingdom Biodiversity monitoring programs need to be designed so that population changes can be detected reliably. This can be problematical for species that are cryptic and have imperfect detection. We used occupancy modeling and power analysis to optimize the survey design for reptile monitoring programs in the UK. Surveys were carried out six times a year in 20092010 at multiple sites. Four out of the six species - grass snake, adder, common lizard, slow-worm -were encountered during every survey from March-September. The exceptions were the two rarest species -- sand lizard and smooth snake - which were not encountered in July 2009 and March 2010 respectively. The most frequently encountered and most easily detected species was the slow-worm. For the four widespread reptile species in the UK, three to four survey visits that used a combination of directed transect walks and artificial cover objects resulted in 95% certainty that a species would be detected if present. Using artificial cover objects was an effective detection method for most species, considerably increased the detection rate of some, and reduced misidentifications. To achieve an 85% power to detect a decline in any of the four widespread species when the true decline is 15%, three surveys at a total of 886 sampling sites, or four surveys at a total of 688 sites would be required. The sampling effort needed reduces to 212 sites surveyed three times, or 167 sites surveyed four times, if the target is to detect a true decline of 30% with the same power. The results obtained can be used to refine reptile survey protocols in the UK and elsewhere. On a wider scale, the occupancy study design approach can be used to optimize survey effort and help set targets for conservation outcomes for regional or national biodiversity assessments. - Funding: The authors thank the Esmee Fairburn Foundation (http://www.esmeefairbairn.org.uk/) for financial support. The work of G.G.-A. was supported by an EPSRC/NCSE (Engineering and Physical Sciences Research Council/National Centre for Statistical Ecology; http://gow.epsrc.ac.uk/) grant. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. There is widespread evidence of worldwide declines in populations of vertebrates [1] including fish [2,3], amphibians [4,5,6], reptiles [7,8], birds [9,10,11] and mammals [12]. Declines have been attributed to a number of causes including habitat loss or change [7]; disease [13]; pollution [2] and climate change [14,15,16]. Action to address these declines requires sound scientific data on population trends at different scales. At the individual population level, long-term monitoring can provide useful data on the nature of population fluctuations and drivers of population change [17]. However, what may be more useful for conservation purposes are data on changes in the number of populations at the wider landscape level [18]. Such approaches present challenges in terms of logistics and expertise, particularly for cryptic species that are difficult to detect and identify. This raises the issue of how much survey effort is required to reliably identify population changes. A National Amphibian and Reptile Recording Scheme (NARRS) was instituted in Britain in 2007, with a view to assessing future status changes of the herpetofauna of the UK (United Kingdom), including the six native reptile species, slowworm Anguis fragilis Linnaeus 1758, common or viviparous lizard Zootoca vivipera Jacquin 1787, sand lizard Lacerta agilis Linnaeus, 1758, adder Vipera berus Linnaeus 1758, grass snake Natrix natrix Linnaeus 1758 and smooth snake Coronella austriaca Laurenti 1768. NARRS primarily uses trained volunteers who carry out presenceabsence surveys using a standard protocol. In this study, we applied an occupancy modeling technique that accounts for imperfect detection to the current NARRS survey protocol for reptiles. This modeling framework is based on the patterns of detection and non-detection of species at a range of sites over multiple visits, and estimates both site occupancy and detection probability simultaneously [19]. Once the detection probability of any particular species is estimated it is possible to determine the number of survey visits required at an occupied site for the species to be detected to a given level of certainty [2023]; note that a similar assessment can be performed without conditioning on species presence [24]. Survey effort can be allocated into number of sites and number of survey visits in different ways. For a given combination of occupancy and detection probabilities, there is an optimal level of replication in terms of estimator precision [25,26,27]. Thus, if sufficient surveyors are available to carry out 1000 surveys, is it better to survey 250 sites four times or 100 sites ten times? Such decisions are important for optimizing the deployment of survey effort. Equally, it is essential to design the survey so that biologically significant changes in species occupancy over time can be detected reliably. In this study we used an occupancy modeling approach on all six native reptile species in the UK to determine (1) occupancy and detectability of all the species across a range of sites; (2) the optimal number of survey visits to carry out per site; and (3) the required sampling effort to detect population declines at different power levels for the four commoner species. Materials and Methods Study Areas and Field Work In 2009 29 sites were surveyed for reptiles on six occasions, once each in March, April, May, June, July and September. All chosen sites were in southeast England as this was the only part of the United Kingdom where all six native reptile species occurred. In 2010 the same sites were again surveyed six times and additional sites were incorporated to bring the total sample size to 45 (see Fig. S1). These were from a wider geographical area, embracing Wales to the west, East Anglia to the east and Yorkshire to the north. The 45 site data set was thus more repres (...truncated)