BSBINews103

NotesReporting on Local Change17• The pattern of change for taxa whosedistribution appeared not to have changedmuch between the two surveys can be usedas a guide to the 'real' change in distrib~tionof the other taxa. The authors make Itclear, in Appendix 2 and in their conch~sionsand recommendations, that thISassumption may not be true and cannot beverified. Unless I have misunderstoodsomething, therefore, the numericalindices of change presented throughout thebook are representing data which do notexist and which we have no objectivereason for supposing would be obtained inany survey.• The indices of change were expressed asproportions or percentages. A change in,say, 4 tetrads, could translate into apercentage change of some size for a raretaxon but make no significant difference inthe case of a widespread taxon. Theauthors therefore used another formula tochange the raw data for rare taxa to givecomparable measures for these two categories.This appears to be an extension oftheprocess of deriving change indices whichrepresent non-existent data.Using the report's dataOne result of the approaches described abovecould be a devaluation of the importance oftheraw data actually obtained. Fortunately, however,the authors of the report do make use ofthem. They record the tetrad gains and lossesfor individual taxa, and refer to them in theirtaxon summaries. It is also possible to discoverhow many tetrads were recorded for a particulartaxon in Local Change - you just add the' gains'to the 'refmds'. Likewise, for the MonitoringScheme, you add the 'losses' to the 'refinds':There are a number of ways of approachmgthe raw data which allow us to be pleased thatthe level of recording in Local Change was somuch higher than previously, rather than dis~ppointed,as the authors apparently were. WhIleon the surface these approaches appear to beless precise than the numerical indices in thereport, they may be closer to reality. All arecovered in the report despite the authors' reservationsabout validity. They are:• There is a great deal of information on whathas been found and where during a surveyof commendable thoroughness.• The raw losses and gains can be consideredas they stand, bearing in mind that, becauseof improved recording: the losses are morelikely to be real than the gains; the greaterthe size of a gain, the greater are its chancesof being a real one; care should be taken ininterpreting the results for rarer taxa.• The two sets of data combined give a usefulpicture ofthe total recorded range of occurrencefor each taxon from 1997-2003.• While there are deficiencies in the data,these can be viewed positively as raisingquestions which need further investigation.• The data provide a helpful supplement tothose for hectads in the New Atlas (Prestonet al. 2002).• The raw data provide information on somerare and scarce taxa.Suggestions for further analysis .Though it seems unlikely that valid mathematIcalindices of absolute change can be calculatedfrom surveys of this kind, it is possible to calculatethem for change in individual or groups oftaxa relative to other taxa or to the whole bodyof taxa. Indeed, it could be argued that this waswhat the authors ofthe report were really doing.Below is a method which I suggest producesvalid mathematical indices of such changedirectly from the raw data without any adjustmentsand with simple mathematics which doesnot cut off most readers from understandingwhat is happening. I offer it step by step forcomment.1. Take the raw number of tetrads recordedfor each individual taxon in the MonitoringScheme. Call this the Monitoring SchemeTaxon Tetrad Score.2. Add together the individual MonitoringScheme Taxon Tetrad Scores. Call thesummed scores the Monitoring SchemeTaxon Tetrad Scores Total.3. For each individual taxon, express theMonitoring Scheme Taxon Tetrad Score asa percentage of the Monitoring SchemeTetrad Scores Total. Call this the MonitoringScheme Taxon Tetrad ScorePercentage.4. Repeat steps 1-3 with the Local Changeraw scores to obtain a Local ChangeTaxon Tetrad Score for each individualtaxon, a Local Change Taxon TetradScores Total, and, for each individualtaxon, a Local Change Taxon TetradScore Percentage.5. For each individual taxon, subtract theMonitoring Scheme Taxon Tetrad ScorePercentage from the Local Change TaxonTetrad Score Percentage. Call the result theRelative Taxon Percentage Change. Itwill be positive for an increase andnegative for a decrease.