A general method for measuring relative change in range size from biological atlas data
Introduction
Species threat statuses have played a crucial role in conservation. The publication of threat statuses in Red Data Books allows conservationists to set priorities, and target limited resources with the greatest effect in the battle against extinction.
Rarity was once the sole criterion used to allocate species status, but increasingly there is a focus on decline. This trend can be seen in successive editions of the British Red Data Book for vascular plants. In the first edition (Perring and Farrell, 1977), species were selected for inclusion if they had been recorded from 15 or fewer ‘10-km squares’ (10×10 km cells of the British national grid). The latest edition (Wigginton, 1999) uses the revised threat categories of the IUCN (1994), which have since been further revised (IUCN, 2001). The IUCN threat categories are based on five quantitative criteria, three of which include an assessment of decline in either range or population size.
For most of the biota of even the best-recorded countries, monitoring data do not exist with which to assess declines. However, biological atlas data sets do exist for many groups, and in Britain these provide the main source of data on the conservation status of the majority of the biota (Harding and Sheail, 1992). These data sets generally include the results of at least one national survey, plus collated historical records, and are used to produce distribution maps (e.g. Fig. 1). A general method for quantifying changes in range size from such data sets is needed.
Various ‘change statistics’ have been calculated from biological atlas data. The biases in biological atlas data are widely recognised (Rich, 1998, Dennis et al., 1999), typically including an increase in survey effort over time. However, existing methods to quantify changes in range size usually fail to compensate either for change in the geographical coverage of recording, or for change in recording intensity. Here we present a new method that we have developed to minimise such biases. Although the method proposed here can be used to compare the results of a single survey against accumulated historical data, it is particularly suited to the analysis of distributional data for groups which have been surveyed by comparable methods in two periods, for example the successive surveys of butterflies (Heath et al., 1984, Asher et al., 2001) and vascular plants (Perring & Walters, 1962, Preston et al., in press). After describing the new method and outlining the assumptions it makes, we then present results for three representative distributional data sets.
Section snippets
The general method for calculating relative change in range size
In outline, the method is first to define the set of grid cells that have been surveyed in both survey periods; subsequent calculations are based solely on these cells. For each species, the number of recorded grid cells is counted for each period. These counts of grid cells are expressed as proportions of the total survey area, and then logit-transformed. A linear regression model is then fitted to the relationship between counts of grid cells in earlier and later survey periods. A weighted
Application of the method to three representative datasets
Note that the botanical nomenclature used here follows Stace (1997), and that the nomenclature of other taxonomic groups follows the publications from which examples are drawn.
Calculating the index
The index of relative change in range size was calculated using data sets detailed in Table 1. All calculations were performed using the statistical package Minitab (release 13). Range sizes (numbers of occupied 10-km squares) were counted for each species in each time period. Scatter plots of range sizes in the early and later periods (Fig. 2 a–c) reveal approximately linear relationships between range sizes for all three taxonomic groups. This linear relationship implies that range sizes are
Correlates of change
To demonstrate the potential uses of the change index in detecting correlates of change, three examples are taken from the data sets analysed earlier.
Comparison with other methods of assessing change from atlas data
Previous numerical assessments of the change in the distribution of British species have usually been derived from atlas data by a direct comparison of the numbers of 10-km squares occupied in different recording periods. Gibbons et al. (1993) tabulated the number of 10-km squares in which breeding birds had been found in two recording periods (1968–1972 and 1988–1991), expressing the more recent value as a percentage of the earlier value to obtain a ‘% change’. Of species which were recorded
Acknowledgements
We thank Henry Arnold for preparation of Fig. 1; David Roy and Mark Hill for commenting on drafts of this paper; David Pearman, Martin Luff, Mark Williamson, Petr Pyšek, Vojtı̀ch Jarošik and Phil Shaw for useful discussions. The data sets used are the result of many years of work by volunteer experts to whom we are indebted. We have been encouraged by the interest shown by the Department for Environment, Food and Rural Affairs and the BSBI in the application of this method to the analysis of
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