The Distance Decay of Similarity in Boreal Stream Communities

Janne Soininen Anna Astorga Riku Paavola Risto Virtanen Timo Muotka

University of Helsinki, Finnish Environment Institute and University of Oulu

Compositional similarity between two communities typically decreases with increasing distance. Such decay of community similarity with distance is caused by spatially limited dispersal, being a basic premise of the neutral theory of biodiversity. By contrast, the concept of environmental control of species distributions, niche theory, predicts that community similarity should decrease with environmental distance, irrespective of geographical proximity. The recent studies of distance decay have highlighted the predictable differences in rate of decay across communities. We examined the relationship between community similarity and environmental and geographical distance in three freshwater taxa (bryophytes, insects and diatoms) across Finland using Mantel tests and partial Mantel tests. Our results showed that community composition was more strongly related to environmental than to geographical distance. Differences in decay rates with environmental distance were observed between taxa, implying that these groups are responding differently to stream environmental variation. Moreover, different dispersal capacity among the organism groups was reflected as the different rate of decay in community similarity with geographical distance when controlling for the variation in environmental factors.