Tora Finderup Nielsen:
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M.T. Lange wrote “Culture leads to a uniformity of the flora, which is not beautiful...” in his flora from 1857. More than one and a half century later, we may conclude that he was right.
Biodiversity is under pressure by human activity at global, regional and at local scale. Globally, species richness is declining; however, it is not evident that the same applies to regional and local scale. At the same time, structural changes at local or regional scale can lead to loss of species at global scale, without species declining in numbers. Time lags in biodiversity responses, the “shifting baseline syndrome” and shifts in intensity of drivers make temporal scale matter in the evaluation of biodiversity change over time. Hence, matching biotic responses with environmental forcing in time and space is of key importance to understand how human drivers shape biodiversity and thereby to gain knowledge on how to conserve biodiversity at a global scale.
In this thesis, I use historical plant surveys from mid-19th to early 20th century to quantify spatial and temporal changes in Danish plant species richness and composition from local to regional scale. Furthermore, I use species' properties and land-use data to identify the major anthropogenic drivers of change and I attempt to use herbarium specimens to disentangle these drivers.
I found that species richness increased in all regions, but that change varied at the local scale. The turnover of species was large and non-random, leading to homogenization of species composition at localities within a region and between regions. Species declining in abundance and occupancy were mainly low-growing, historically rare species adapted to high light and/or low nutrient levels. Increasing species were tall-growing, historically common species thriving at mesic and/or nutrient rich biotopes. The drivers of change were double sided as changes in both quantity and quality of habitats were found to play important roles. The main drivers of change in Danish plant species over more than a century were the drainage of forests and open land - potentially followed by conversion to arable land or built-up, overgrowth and afforestation as well as a general increase in nutrients and lack of disturbance in terrestrial and freshwater habitats. To be able to separate the strength of the identified human drivers of change, I attempted to use herbarium moss specimens as a direct indicator of historical nitrogen deposition. Unfortunately, the results of this analysis were not reliable, probably due to contamination of the moss while it was stored in the herbarium. In the last chapter of the thesis, I therefor argue for cautious use of this method.
The results of this thesis demonstrate that despite a general increase in richness, systematic change in species composition leads to homogenization of the flora. In this way, richness trends were detached from beta diversity change at local to regional scale over more than a century. The results presented here add to the understanding of how historical drivers have shaped the richness and distribution of the species we see today. The knowledge gained can be used to manage the future of the earth's biodiversity.