Estimation of vascular plant occupancy and its change using kriging

Abstract

The grid square occupancies of vascular plants are estimated for the periods 1978–1994 and 1995–2011 across Great Britain. This was achieved by selecting well-surveyed 4 km2 grid squares from the pool of all records and deriving detection/non-detection data from them. From these, it was possible to extrapolate occupancy probabilities to the whole area. Grid squares were selected by setting a minimum threshold for the recording effort for each grid square based upon the number of species recorded and the number of surveys conducted. The spatial distribution of plants was modelled for these selected sites using variography, and kriging was used to interpolate across the whole area. This was conducted for the two time periods and differences in the occupancy probability were compared. This approach has several advantages over other methods. Recording effort is assessed and can be compared between time periods and regions. Absolute occupancy change can be estimated, rather than just relative change. The differences in the spatial distribution pattern of each species are accounted for and the predicted occupancy probabilities and their variance can be mapped. The change in occupancy probabilities between time periods is discussed with reference to known environmental drivers of change and the spread of neophytes. An important result was that in England each 4 km2 gained on average an additional neophyte every three years.