Changes in the flora of the Netherlands in the 20th century Throughout the world, biodiversity is under major threat from human activity. As the gravity of the situation unfolds, critical analysis shows that our knowledge and understanding of these developments is still very incomplete, both globally and in individual countries. This study is concerned specifically with changes in abundance of the approximately 1,500 species of vascular plant growing wild in the Netherlands. Using species checklists, in the course of the 20th century no less than 10 million individual distribution records have been obtained for these plants at a scale level of about 1 km². These data are stored in two large databases: pre-1950 data in FLORIVON (in ‘quarter cells’), and post-1975 data in Florbase (in ‘kilometre cells’). The principal aim of this study is to provide a quantitative description and interpretation of the changes in the Dutch flora during the 20th century. This aim has been elaborated in four chapters with the following main themes: floristic survey bias and correction thereof (chapter 2); overall ecological changes in the Dutch flora, and the impact of ‘traditional’ environmental factors such as eutrophication, acidification and groundwater depletion (chapter 3); changes in the Dutch flora associated with climate change (chapter 4); and changes in the Dutch flora due to influx of alien species (chapter 5). Appended to the main text is the 2003 edition of the ‘Standard List of the Flora of the Netherlands’, in which use has been made of some of the ecological and methodological insights to have emerged from this study. Chapter 2 opens by describing the various types of floristic survey bias and goes on to develop several new methods to analyse and correct for them. For five kinds of survey bias full or partial solutions are presented. In the first place a new method has been developed that makes due allowance for (topographical) differences in scale and cartographic projection pre- and post-1950 and for differences in species distribution patterns. By implementing this method a corrected data set was obtained that is far more consistent than the one available to date. A second key issue is crosscountry variability in the amount of data collected, and a method was also developed to address this problem as satisfactorily as possible. The other categories of survey bias considered are incomplete species lists, differences in survey intensity and duration, and problems with species recognition. For these, too, solutions have been articulated and practically implemented. Chapter 3 describes the changes in the Dutch flora as these relate to the ‘traditional’ factors cited above. The extent to which Dutch conservation efforts and environmental policies of recent decades have already had a tangible effect is also examined. To this end the approximately 10 million, now-corrected distribution data were broken down into three periods: pre-1950 and two roughly equal periods post-1975. Each plant species was then assigned to one of 83 ‘ecological groups’ indexed to soil salinity, vegetation structure, moisture regime, nutrient availability and pH. The changes in national distribution of these ecological groups were then described and statistically tested. Of the factors examined in this study, eutrophication proved by far the most important cause of floristic change. Species of oligotrophic, pHneutral habitats have undergone serious decline, with those of eutrophic habitats exhibiting a marked increase. The second main cause is loss of saline habitats, particularly during the first half of the 20th century. A third important change relates to vegetation structure, viz. a decline in grassland species with an attendant increase in species of woodland and ruderal herbaceous vegetations. In terms of conservation and environmental policy, it was found, based partly on the results of the next chapter (see below), that despite emission reduction measures, for the century as a whole eutrophication still ranks as the single most important cause of floristic decline in the Netherlands. Acidification, by comparison, was found to constitute far less of a problem and there are also signs that it declined in importance during the last decade of the century. Finally, it was found that restoration projects have achieved a measure of success, especially for certain kinds of wet, oligotrophic ecosystems like bogs and dune slacks. Chapter 4 examines the possible effects of climate change on the Netherlands’ flora. To this end it was investigated whether the floristic changes could be explained by observed increases in average temperature, precipitation or CO2 levels. Again, use was made of the corrected data set, as described above. In this part of the analysis, changes in individual plant species presence was the issue of interest. Prior to about 1980 there was a demonstrable but small increase in both ‘warmth-loving’ (thermophilic) and ‘cold-loving’ species. The increase in the latter group can be explained by the historic growth in the area and quality of Dutch woodland. After about 1980 the only trend to be observed is a pronounced increase in thermophilic species, coinciding with a marked rise in temperature during the same period. More specifically, there has been a definite increase in species of drier habitats. No clearcut effect of increased precipitation on the Dutch flora could be established. As a separate issue, it could be demonstrated that increased urbanisation — and the warmer climate of urban environments — has also been a key factor contributing to the increased abundance of thermophiles. In the last two decades of the 20th century, climate change was the second most important cause of changes in the Dutch flora, behind urbanisation but before eutrophication. As yet, the effects of climate change are asymmetrical: an increase in thermophiles, but as yet no decline in ‘cold-loving’ species, contributing to what is probably only a temporary rise in the total number of species. In the literature, non-native (‘alien’ or ‘exotic’) species are cited as a major threat to biodiversity worldwide. To investigate the role of the approximately 360 exotic species now naturalised in the Netherlands, reported in chapter 5, a distinction was made between archeophytes and neophytes: species naturalised before and after 1500, respectively. One-quarter of the Netherlands’ wild flora consists of non-native species, about one-third of them archeophytes and two-thirds neophytes. The pattern of change among archeophytes is virtually identical to that among indigenous species, with a minor increase in common species and a decline in rarer species. In contrast, all groups of neophytes show an increase that is inversely proportional to the period of naturalisation. Of the hypotheses tested to explain the success of common or rapidly expanding exotics, two are best able to explain the facts: escape from the ‘predator-pathogen complex’ and increased availability of disturbed, eutrophic sites. As regards the relative number of species on the Red List, no difference was found between indigenous species and archeophytes; there are even several neophytes on the List. Neither was any significant difference found between indigenous species, archeophytes and neophytes with respect to the percentage now extinct. Contra the claims of exotic plant species having a major negative impact on indigenous floras, there are as yet no indications of these posing a threat to the Dutch flora at the national level. Finally, a number of important issues emerging from the study are discussed and the overall conclusions and recommendations presented in the final chapter. The main focus of the methodological discussion are the corrections made for survey bias. Despite the various options developed for correcting the raw floristic data, there is still a major need for a new and more coherent procedure for use in future surveys. A proposal to that end is made, proceeding from the same overall effort as at present. In the discussion on substantive issues a wide range of topics is considered in more detail, two of which are cited here by way of illustration. In evaluating biodiversity impacts as a function of scale level, it should be noted that despite the sensitivity of km-cell data to changes, on a smaller, micro-scale changes may be greater still. At the micro-scale, exotic species may indeed be having an impact on the Dutch indigenous flora. The effects of climate change also need to be further disentangled from those of urbanisation and exotics, to gain a better idea of climate impacts.