Our assessment showed that patch-scale impacts of exotic beavers were predictable based on studies in their native range with localized decreases in richness and increases in productivity. Beavers have invaded 30-50% of the archipelago’s stream networks and 2-15% of its area, which caused the retention of organic material and enhanced the magnitude of cross boundary organic matter flows from terrestrial to aquatic ecosystems. While at the habitat-level beavers significantly altered benthic community structure, at the landscape-scale their influence on diversity and assemblage was much less, since their engineered habitats were similar to natural landscape features (bogs and lakes). In contrast, the total effect on stream function was cumulative and significantly changed carbon flow dynamics. In conclusion, we found that beaver invasion in the world’s southernmost forested ecosystem provoked the largest land cover alteration since the last ice age. Nonetheless, the ecological effects of this invasive ecosystem engineering were largely predictable based on underlying ecological mechanisms; at the patch-scale engineering effects were related to habitat heterogeneity and trophic dynamics, while at the landscape-scale it was necessary to have an adequate comprehension of the context of surrounding native ecosystems and species-specific characteristics of native biotic communities. We conclude that beaver eradication strategies and ecosystem restoration efforts should focus on the ecology of native ecosystems rather than the biology of this invasive species per se.