Line Transects by Design: The Influence of Study Design, Spatial Distribution and Density of Objects on Estimates of Abundance

Abstract

Abstract:The line transect distance sampling method provides unbiased estimates of abundance when organisms are distributed randomly or line transects are laid out randomly, sample sizes are large and other assumptions of the method are met; such, however, is rarely the case in real life. We conducted a simulation study to investigate how spatial distribution and density of objects, and total length, layout and number of transects influence bias, precision, and accuracy of estimates of abundance obtained by distance sampling along line transects. Overall, density estimated using the distance sampling method was within 4.9% of the true density, but it varied substantially depending upon spatial distribution of objects. Of the three spatial distribution patterns considered, estimates of density were least biased, and most precise and accurate when objects were distributed randomly; they were most biased, and least precise and accurate when objects followed a clumped distribution. The estimated bias (% difference between true density and estimated density) for clumped, random and uniform distribution was 13.1%, -0.4%, and 2.1%, respectively; precision (% coefficient of variation, CV( D )) was 13.7%, 9.1%, and 9.2%; and accuracy (root mean-squared error, RMSE) was 27.9%, 7.4%, and 11.7% for clumped, random, and uniform distribution, respectively. Increasing total transect length and using several short transects (as opposed to few long transects) generally reduced bias, and increased accuracy and precision of estimates of abundance. A systematic layout of transects worked as well as, or better than, random layout, except when objects were distributed uniformly in space. This study advances the utility of the line transect method by providing information both on how study design affects accuracy and precision of abundance estimates, and how it can be improved when assumptions of the method are not strictly met based on a priori knowledge of the spatial distribution and presumed density of the target organism through appropriate changes in the study design.

Validating a common tick survey method: cloth-dragging and line transects

“…In areas with generally higher tick abundance as Bogesund, there is a higher chance to collect larvae. Consequently, tick abundance and distribution in combination will strongly influence repeatability (Medlock et al 2013 ; Nomani et al 2012 ).…”

“…Cloth-dragging for ticks is usually performed as a line transect survey method (LTSM), i.e., with predetermined lines in the study area used for repeated observations or collections to estimate the abundance of a species (Rulison et al 2013 ; Schultze et al 1997 ). The LTSM is a widely used method in ecological studies for estimating species diversity and abundance of different kinds of organisms (Nomani et al 2012 ), including mammals (Plumptre 2000 ; Romero et al 2016 ), birds (Sutherland 2006 ), marine species (Colin et al 2013 ), plants (Kenny et al 2018 ) and insects (Kral et al 2018 ). Although recommended, validation of the precision and repeatability of LTSM is rarely done (Plumptre 2000 ) and may be challenging, especially when applied to organisms that move or are displaced after sampling (Colin et al 2013 ).…”

Response of a globally endangered canopy insectivore to habitat degradation in an East African tropical rainforest: The role of differential forest protection levels

“…The transects were selected in a stratified fashion to reflect the degree of protection and relative disturbance levels: lowest disturbance and highest active protection level with no utilization in the northern block; moderate disturbance with regulated access in the southern block; high disturbance with moderate to low protection level in Kisere block ( Table 2). The stratification was aimed at obtaining representative samples as well as facilitating spatial comparison of results (Nomani et al, 2012). In selecting and laying the transects, the main challenge was logistical especially as much of the closed-canopy TE habitat both in the north and the south was characterized by thick under-storey, thus precluding straight transects.…”

“…Further, it was ensured that transects were located at a separation distance of not less than 200 m from the adjacent ones, to avoid double counting (Nomani et al, 2012). Such a mix of systematic and random transect layout has been found to improve precision in density estimates using distance (Nomani et al, 2012). Sampling points for measuring percent forest canopy cover on quadrats.…”

Line Transect and Triangulation Surveys Provide Reliable Estimates of the Density of Kloss’ Gibbons (Hylobates klossii) on Siberut Island, Indonesia

“…However, few studies compare different survey methods and designs for large mammals under the same conditions, for instance, the same time of year and same location (Nijman and Menken 2005; Nomani et al . 2012; Viquerat et al . 2012).…”