Landscape-level effects of North American beavers in the Fuegian Archipelago: Is the introduction of beaver the largest threat to South America’s sub-Antarctic forest in the Holocene?

Background/Question/Methods

South America’s sub-Antarctic ecoregion not only hosts the world’s southernmost forests, but also the largest extent of temperate wetlands, forests and ice fields south of the equator. Additionally, its remoteness, with no latitudinal replica in the Southern Hemisphere, has kept it isolated from most modern environmental threats including nutrient deposition and habitat fragmentation, leading it to be declared one of the world’s last wilderness areas. Nonetheless, the globalized phenomenon of introduced species does affect this region, and today at the extreme southern tip – in the Fuegian Archipelago – the terrestrial mammalian assemblage is dominated by exotics 2:1. Among the introduced fauna, the North American beaver is believed to cause the largest impact to this landscape in the Holocene. To assess this assertion, we compared beaver’s effects on stream benthic taxa richness, assemblage and production in natural lotic (forested and grassland streams) and lentic (bogs, lakes) habitats and compared them to disturbed habitats (streams affected by forestry management, beaver ponds and beaver meadows). We evaluated this invasion at the habitat and landscape-scales by a) calculating the extent of invasion (% impacted stream length) and b) modeling natural and engineered landscapes with regards to diversity and ecosystem function.

Results/Conclusions

Beavers have invaded 30-50% of the archipelago’s streams, causing the retention of organic material (+40-115%). At the patch-scale, impacts of exotic beavers were predictable based on studies in their native range with localized decreases in richness and increases in productivity. Forest harvesting with a riparian buffer had no detectable impact on benthic communities, while beaver engineering (ponds and meadows) significantly altered assemblages, compared to streams, but were not significantly different than those in natural lentic habitats. Therefore, influence on benthic assemblages at the landscape-scale was much less, but in contrast, the total effect on stream function significantly changed carbon dynamics. In conclusion, since engineered patches were similar to natural habitats (bogs and lakes), beavers did not significantly alter landscape-level community patterns, but significantly affected ecosystem function. At the same time, beaver introduction did constitute the largest modern impact to sub-Antarctic forests, but this affect is attenuated by the natural lentic features found in the Fuegian landscape. Understanding the underlying ecological mechanisms operating differentially at patch versus landscape scales and between community and ecosystem levels of organization allowed us to predict the effects of this species introduction, which demonstrates the power of ecology for management and conservation decision-making as well.