(Un)remarkable floras
Regional floras are isolated from each other by physical or climatic barriers that prevent plant species from migrating without limits around the world. The uniqueness of floras is defined by taxonomic uniqueness – the species a flora shares or does not share with other regions, and phylogenetic uniqueness – distinctiveness of the evolutionary history captured by its species.
Two types of species can be compared: native species grow in the area naturally and belong there, while naturalised species have overcome environmental and reproductive barriers and established self-sustaining populations in the new region. Due to these species, regions around the world are becoming more similar and a trend towards homogenised floras can be seen. The extent of this homogenisation depends on various factors.
One such factor is geographic distance. The further floras are from each other, the less similar they are. The primary climatic factors are precipitation and temperature. However, distant regions have become less distinct due to naturalised species. This trend is most prominent in distant regions with similar climates. Initially, a plant grew only in one region that was bordered by a barrier it could not overcome. It was then artificially spread to other regions, where it naturalised. This process was most successful in climatically similar regions. The explanation for this is that even though climatically similar regions are geographically distant, the plants evolved similar adaptations for the same conditions. As a result, plants appeared in similar conditions to the ones they came from and spread easily.
Taxonomic and phylogenetic similarities have also increased thanks to naturalised plants. Because substantial numbers of these plants grow in new regions, they are often considered invasive. However, naturalised plants do not always cause homogenisation of the floras. The similarity between regions can be both increased and decreased by naturalised plants. A species induces floristic similarity when it is native in one region and spreads to others. It also occurs when it is brought to two regions, but is native in neither. Widely spread species are usually responsible for homogenisation. By contrast, floristic differentiation occurs when a species naturalises in one region but not another.
And what enables the plants to travel and naturalise? The easiest mode of transmission is probably through humans. People travel around the world, but plants are spreading the most between regions that have a current or past administrative link within a country, in colonies, overseas territories, and so on. These regions exhibit a stronger taxonomic homogenisation than those with no administrative relationship.
Overall, both taxonomic and phylogenetic floristic homogenisation are strong. Naturalised species mean that even with increasing geographic distance, floristic similarity is not decreasing as it would naturally. This suggests that the uniqueness of floras is endangered around the world.
Eliška Leštinová
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