Danish islands help to disentangle how plant dispersal characteristics shape species richness patterns.
Above: The Danish coastline with the island Hjelm in the background. © Anders Sanchez Barfod.
Suppose you hear the names Galapagos, Hawaiian or Canary Islands. In that case, I am sure you have a picture in mind right away. These islands are well known in general and very popular in island biogeography. We have seen pictures and documentaries or read evolutionary and ecological studies about these islands. But have you every head of islands like Anholt, Egholm, Fur or Hjelm? These are islands scattered along the Danish coast, and I would like to convince you that such islands have great potential to help us solve pending questions in island biogeography.
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Walentowitz, A., Troiano, C., Christiansen, J. B., Steinbauer, M. J., & Barfod, A. S. (2022). Plant dispersal characteristics shape the relationship of diversity with area and isolation. Journal of Biogeography, 49, 1599–1608. https://doi.org/10.1111/jbi.14454
Denmark has more than 1400 immensely diverse islands. Some are home to Danes while others are uninhabited; they are located in the North Sea, Baltic Sea or in bights and fjords and differ in size. My co-authors Prof. Dr Manuel Steinbauer (University of Bayreuth) and Assoc. Prof. Dr Anders Sanchez Barfod (Aarhus University) were intrigued by these islands and their flora. They raised the question: can the way plants disperse to islands help explain the species richness patterns on these islands?
Indeed, they can. For example, a plant like the common oak tree (Quercus robur) has heavy acorns dispersed by mammals or birds (zoochore dispersal). The chances are high for an acorn to reach and get established on a larger island. The transporting animal was looking for a spot with enough resources for survival and therefore decided to head for a larger island. For plants dispersed by water (hydrochore dispersal) like the common cordgrass (Spartina anglica), chances are high that these are adapted to habitats associated with coastal areas. Now simple mathematics can help to explain why larger islands have proportionally fewer hydrochore plant species than smaller islands: If the coastline doubles, island area roughly quadruples.
It was most astonishing to unveil such relationships between plant dispersal and island characteristics despite the human impact on plant communities on Danish islands for centuries. These patterns seem so fundamental and robust that anthropogenic encroachment could not overwrite them. However, if we additionally include human impact in our models, we can explain plant species diversity on islands even better.
While working with Danish Islands, I was highly impressed by two former botanists and scientists, who massively shaped this study, although I never had the chance to meet them. Eric Wessberg was a Danish botanist who inventoried numerous islands jointly with his team. His plant records built the baseline of our study. Such data are essential for the studies we conduct as island biogeographers. I was also profoundly impressed by the theories and thoughts developed by Alvar Palmgren, a Finnish botanist who lived from the late 19th until the mid-20th century. Back in his day, he developed first thoughts on how plant dispersal characteristics shape richness patterns on islands long before the discipline of island biogeography was established. Alvar Palmgren seems to suffer a bit from what I call the “Alfred Russel Wallace syndrome”, as his genius work is little remembered. Alfred Russel Wallace developed theories about evolution at the same time as Charles Darwin. Still, his counterpart is better known to be the founder of evolutionary biology. With our study, we hope to not only put Danish islands onto the map of island biogeographical research but also value the work of Eric Wessberg and Alvar Palmgren.
I learned from this study that numerous islands are out there whose potential for island biogeography has not been explored yet. We should continue to be on the lookout for such islands that help answer questions on how island biodiversity is being shaped by natural and anthropogenic forces.
Anna Walentowitz, M.Sc., PhD candidate
University of Bayreuth
Department of Biogeography
Universitaetsstrasse 30 95447 Bayreuth