Beyond the Indicators concept - introducing a new method for evaluating population changes in groups of bird species
Abstrakt
When trying to identify causes of decline or increase of a bird population, the species are often grouped by certain criteria (e.g. by habitat, diet or migratory strategy) and their trends or indices are evaluated in groups. Currently, indicators (geometric mean of species' indices) are widely used, but such indicator itself doesn't provide any means to assess the trend of the whole group statistically or to perform comparison among several groups.
For this purpose, the common practice is to process the group trends using statistical tests like t-test or ANOVA. This approach has several issues.
We discuss these issues and propose a new method which further develops the concept of indicators so that we can directly evaluate the trends of the the species' groups (with their confidence intervals) and perform comparisons among them. We also propose a new method for controlling for the effect of one indicator on another.
In this method, all indicators are computed at once using a single linear model. Application of these methods is presented on Czech common birds, grouping them by several ecological traits.