Epsilon Open Archive

Abstract

In pre-industrial times beavers, ecosystem engineers, inhabited most of the Holarctic. Intensive exploitation over the last millennium, particularly in Europe during the 18th and 19th centuries, led to a dramatic decline in beaver populations. By the end of the 19th century the Eurasian beaver (Castor fiber) was at the verge of extirpation in many regions of Eurasia. However, during the 20th century, changes in management policy and reintroductions have resulted in a fast recovery of the species. Presently, Eurasian beavers are colonizing large parts of their former distribution range, and their population is still growing. Once again, beaver engineering and its outcomes are becoming prominent features of many streams in the boreal landscapes.

The goal of this thesis was to investigate how the reintroduction and population recovery of Eurasian beavers might affect ecosystem functioning at local and landscape scales. The work was based on measurements from 12 beaver systems of various colonization histories (pioneer and recolonized) in Swedish boreal forests. Based on water chemistry measurements together with field and lab experiments, I evaluated the effect of beavers on processes of mercury methylation, litter decomposition, and the growth and respiration of benthic biofilms. I also investigated how beavers alter the nutrient limitation of biofilm activity. To assess the environmental effects of the reintroduction of beavers at the landscape scale, this empirical approach was complemented by a meta-analysis of 76 published studies on a total of 16 environmental factors. The results showed that beavers increased MeHg concentrations, decreased algal biomass accrual, and sometimes increased litter decomposition rates downstream compared to upstream beaver systems. However, distinct contrasting patterns were observed in pioneer and recolonized systems: While all the above effects were prominent in pioneer systems, they were moderate or absent in recolonized systems. Albeit algal accrual and community respiration were nutrient limited, beaver systems had no effect on the degree of limitation. Based on the meta-analysis, this thesis demonstrates that, when compared to a reference site, beavers can potentially affect all studied factors. The meta-analysis also identified gaps in knowledge regarding the dependency of beaver effects on different stages of age and colonization history.

In conclusion, this thesis highlights: a) the importance of incorporating the successional stage and the colonization history of beaver systems when considering the effects of reintroducing beavers into stream ecosystems; and b) the lack of knowledge regarding these issues.