Orchids in Scotland: changes in their occurrence since 1950

Abstract

The numbers of tetrad records (plus hectads without any tetrad records) in the BSBI Distribution Database collected annually for twenty species of orchids in Scotland increased 5.9-fold between the periods 1950-69 and 2010-2019. Similarly, the total numbers of records for all plant taxa (duplicates removed) also increased, but even more so (13.6-fold between 1950-69 and 2010-19). These increases were progressive and are thought to reflect an increase in the intensity of recording. Support for this suggestion was provided by regressing mean annual numbers of orchid tetrad records for five time periods since 1950 against the corresponding records for all plant taxa. This revealed that the increases in both were highly correlated (R² = 0.99). To nullify the effect of the increase in recording effort on numbers of orchid tetrad records I used the corresponding total number of records for all plant taxa as a proxy for changes in amounts of recording. When this was done, compared to 1970-86, two-thirds of the orchid species had declined by more than 50% and two by 80% or more. One species had apparently increased.