Standardization and optimization of arthropod inventories—the case of Iberian spiders

Conservation of species requires accurate knowledge on their distribution. For most groups this can only be achieved through targeted biodiversity assessment programs that must explicitly incorporate comparability and efficiency in their definition. These require the standardization and optimization of sampling protocols, especially for mega-diverse arthropod taxa. This study had two objectives: (1) propose guidelines and statistical methods to improve the standardization and optimization of arthropod inventories, and (2) to propose a standardized and optimized protocol for Iberian spiders based on such guidelines and methods. Definition of the protocol has the following four steps. Firstly, the evaluation of the source data to ensure that the protocol is based on close to complete sampling of a number of sites. Secondly, optimizing the effort per collecting method, using an iterative algorithm that optimizes the combination of methods and samples per method tested in the different sites. Thirdly, defining the overall effort (stop-rules), considering not only desired sampling completeness levels, but also practical strategies during field work. Finally, standardizing the protocol, by finding common results between the optimal options for the different sites. The steps listed were successfully followed in the determination of a sampling protocol for Iberian spiders. A protocol with three sub-protocols of varying degrees of effort (24, 96 and 320 h of sampling) is proposed. I also present recommendations on how to apply the same principles to other regions and taxa.