Lobelia dortmanna is an iconic keystone species of northern softwater lakes in Europe as well as North America. It has suffered a dramatic decline in distribution in recent decades and the root causes are not well-known, although elements such as eutrophication, acidification and brownification have been suggested as underlying reasons for the decline.
In order to quantify the causes of change in the occurrence of L. dortmanna in Danish lakes, we firstly mapped the historical distribution of the species and secondly collected the available recent information on the presence of L. dortmanna along with data on environmental variables and plant community composition from the lakes. We identified 168 original Lobelia lakes of which L. dortmanna had, by now, gone extinct in 48%. A subset of 80 lakes formed the backbone of this study owing to the availability of data related to environmental conditions and aquatic plant communities.
Using piecewise structural equation models, we identified that only the mean height of the lake plant communities had a direct negative influence on the occurrence of L. dortmanna. The expected adverse effects of eutrophication and alkalinity were all indirect, and funnelled through the trophic affinity of the plant community, which in turn influenced the mean plant height. Direct effects of eutrophication via shading from phytoplankton, acidification or brownification were not observed.
We show that eutrophication, particularly in more alkaline water, is likely to have caused the dramatic decline in occurrence of L. dortmanna, but our models indicated that the effect of eutrophication acted via interspecific competition from other larger rooted aquatic plants.
Conservation efforts aimed at protecting small aquatic plant species should be prioritised in more alkaline lakes where such species are most at risk. Furthermore, managers should be aware of the threat of interspecific competition posed by larger aquatic plants on their smaller counterparts.