Efficiency of snake sampling methods in the Brazilian semiarid region

Abstract

The choice of sampling methods is a crucial step in every field survey in herpetology. In countries where time and financial support are limited, the choice of the methods is critical. The methods used to sample snakes often lack objective criteria, and the traditional methods have apparently been more important when making the choice. Consequently researches using not-standardized methods are frequently found in the literature. We have compared four commonly used methods for sampling snake assemblages in a semiarid area in Brazil. We compared the efficacy of each method based on the cost-benefit regarding the number of individuals and species captured, time, and financial investment. We found that pitfall traps were the less effective method in all aspects that were evaluated and it was not complementary to the other methods in terms of abundance of species and assemblage structure. We conclude that methods can only be considered complementary if they are standardized to the objectives of the study. The use of pitfall traps in short-term surveys of the snake fauna in areas with shrubby vegetation and stony soil is not recommended.

The Costs of Evaluating Species Densities and Composition of Snakes to Assess Development Impacts in Amazonia

“…We calculated the cost to find a snake as $ 120 USD. Mesquita et al [45] spent $ 0.49 USD per snake found by visual search in a semiarid region of Brazil, more than 230 times cheaper than this study. Differences in costs should be more evident among regions with different climate, terrain, vegetation and logistics, but those authors did not present a detailed description of the spatial distribution of the sites observed, and therefore the independence of sampling units was not clear.…”

Snakes in a seasonally dry tropical forest in northeastern Brazil

“…Time-constrained search and the use of local collectors are efficient methods for determining the richness and abundance of species in a given area, as opposed to pitfall traps with drift fences. Although snakes are often rather difficult to encounter (as compared to lizards and anurans) as many species demonstrate cryptic or secretive behaviors (Sazima & Haddad 1992, Sawaya et al 2008, time-constrained search and local collectors have proven to be adequate methods for sampling snakes in many other herpetofaunistic inventories undertaken in the Caatinga and other Brazilian biomes (e.g., Sazima & Haddad 1992, Sawaya et al 2008, Santana et al 2008, Freire et al 2009, Pontes et al 2008, Mesquita et al 2013b, Caldas et al 2016, Pereira-Filho et al 2017, Sampaio et al 2018. While visual search represents a low-cost technique, their strength will depend on the sampling effort and experience of the researchers involved (Mesquita et al 2013b).…”

“…Although the use of pitfalls traps is quite common, this method should only be considered as a complementary sampling technique associated with other sampling methods in long-duration studies of snake assemblages (Cunha & Nascimento 1993, Greenberg et al 1994, Cechin & Martins 2000. As such, comparing the deficiencies of each sampling method and considering financial and time investments in relation to the numbers of species and individuals recorded, the use of pitfall traps in short-term studies of snakes in Caatinga areas with shallow rocky soils is not indicated (Cechin & Martins 2000, Mesquita et al 2013b.…”

Differences in Movement Pattern and Detectability between Males and Females Influence How Common Sampling Methods Estimate Sex Ratio

“…Passive capture methods, such as pitfall traps and mist nets, are commonly used in biodiversity studies of vertebrates, and many studies support the efficiency of these methods in correctly sampling the diversity [ 3 , 4 , 7 – 9 ]. However, our results highlight that passive methods may not provide good estimates of a population’s sex ratio when sampling effort (number of sampling days and/or number of passive traps) is low.…”

“…Evaluating the efficiencies of these methods is an essential step in improving our ability to sample biodiversity. Many field studies have evaluated the efficiency of these methods regarding their ability to sample a region’s species composition and richness [ 6 – 8 ], and how these methods influence the estimates from species abundance distributions [ 3 , 4 ]. Other field studies have also evaluated how changes to the arrangement and characteristics of these methods, for example trap size and design, may influence their outcomes [ 9 ].…”