We further expected that the quality (precision) of the estimated parameters deteriorates when surveys are carried out along roads rather than on true transects ( Varman and Sukumar 1995, Tomas et al. We expected, therefore, that key population parameters (abundance and density) estimated from roads are different from randomly positioned transects. Edge effect itself might cause problems, as some species are attracted to these disturbed places while others are repelled. Vegetation along their margins might be disturbed by edge effects, impairing the detectability of animals by the observer. If behaviors affecting detectability are not random in relation to roads, estimates will be compromised, despite any random geographic distribution of animals.īesides the behavioral issue, sampling along roads and trails has other problems. This argument further implies that the behavioral repertoire of animals sampled along roads is not different from that of animals observed along true transects, an important factor as behavior can potentially affect detectability ( Ramsey and Harrison 2004, Hounsome et al. This argument implies that animal distributions in relation to roads fulfill the randomness assumption of distance sampling, something that has been confirmed for a rather small number of species ( Venturato et al. Some researchers may choose to sample animals using pre-existing trails or unpaved roads, due to lack of time, access restrictions or logistical limitations ( Varman and Sukumar 1995, Ruette et al. 2012).Ī recurrent violation of the aforementioned assumptions involves the use of roads as surrogates of true line transects. However, the quality of estimates will depend on the ability of the researcher to model the errors and/or bias introduced by the violation of those assumptions ( Marques and Buckland 2003, Buckland et al.
Currently, some degree of relaxation of these assumptions is possible. Some key classical assumptions of line transect distance sampling are (1) distance between animal and line transect is measured accurately and precisely (2) animals are spotted on their original position when first detected by the observer (3) all animals on the line transect are detected with certainty and (4) transects are randomly positioned in relation to animals’ distribution ( Buckland et al. As every other method, the distance sampling has its own assumptions, which should be validated by the researcher if he/she intends to make valid inferences about density or abundance of the sampled population ( Buckland et al. Among the methods available for estimating abundance of tropical forest mammals of medium to large body sizes, line transect distance sampling is one of the most popular methods ( Buckland et al. Keywords: behavior caviomorphs distance sampling population estimation urban matrix IntroductionĮstimates of population abundance are a basic step toward the establishment of a successful management and conservation strategy. We conclude that while avoiding roads is unfeasible given time and logistical limitations, researchers should be aware of the potential bias, making this clear when discussing and comparing their results. Together, these findings indicated that (1) we detect agoutis differently on these two transect types and (2) road sampling more seriously violates the assumption of randomness of transects in relation to animals. We also found that agoutis move/interact and forage/feed differently on roads in comparison with transects and were more vigilant in transects than in roads during the first season we sampled (wet). However, road data had a different variance contribution, lower detection probability and poorer model fitting. Road densities (347–432 agoutis/km 2 for dry and wet seasons, respectively) were statistically similar to those from transects (373–322 agoutis/km 2, respectively). We sampled an isolated agouti population during two contrasting seasons. Here, we used distance sampling to compare abundance and behavior of agoutis between transects and unpaved roads.
However, relatively few studies have attempted to address this, simultaneously comparing behavior between roads and transects, a key issue as behavior can potentially affect detection. Managers/researchers sometimes sample unpaved roads assuming that these do not violate important assumptions. The distance sampling is one of the most widely used techniques for estimating abundance and density, particularly for mammals.
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