Estimating plant population density: Time costs and sampling efficiencies for different sized and shaped quadrats

Given the increasing number of rare and listed plant species, and decreasing resources dedicated to monitoring such species, the need to implement powerful and efficient sampling designs has never been greater. Previous studies have examined statistical efficiencies of sampling designs; however, few studies have considered associated field efficiencies. We attempted to assess fairly the relative field-based time costs of sampling programs designed to estimate plant density. We applied a standardized field sampling protocol to quadrats that comprised 25 different combinations of size and shape in which the density and spatial pattern of ‘plants’ was known. Time costs were estimated using two techniques – one presuming pre-fabricated quadrats were used, and the other using string and stakes to construct quadrats. Estimates of density were recorded to enable calculation of bias for each quadrat size and shape. We found prefabricated quadrats much more efficient, taking approximately 50% less time to sample. When using string-and-stake quadrats, set-up and take-down time increased both as a function of increasing size and rectangularity. Using pre-fabricated quadrats, set-up and take-down times were roughly the same across all quadrat sizes and shapes. For pre-fabricated quadrats, there is no appreciable difference in processing, set-up and take-down times for quadrats 4m or less in area. However, processing time does significantly increase for larger quadrat sizes. Large quadrats also appear to generate measures of population density that are underestimates. Integrating these results with estimates of sampling variance and transit time will enable the development of comprehensive recommendations for optimal design of sampling programs.