Abstract
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 2009–2010 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.
Highlights
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]
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, 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
Surveys consisted of recording the number of detections of each species using two methods: (i) a search under natural and artificial cover objects and (ii) visual searches to find basking or active animals along directed transect routes
Summary
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. 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, 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
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