A comparison of impacts from silviculture and North American beaver invasion on sub-Antarctic stream benthic macroinvertebrate community structure and function

Background/Question/Methods

The sub-Antarctic biome of South America is the world’s southernmost forested ecosystem and is considered one of the few remaining wilderness areas on the planet. Nonetheless, the region confronts various anthropogenic environmental impacts, such as the invasive North American beaver (Castor canadensis) and timber harvesting, particularly in Nothofagus pumilio stands. To understand the influence of these disturbances on sub-Antarctic watersheds, we characterized physical habitat conditions (pH, dissolved oxygen, conductivity, temperature, stream size, distance to riparian forest, bank slope, substrate heterogeneity, benthic organic matter) and sampled invertebrate community structure (density, richness, diversity, evenness) and function (biomass, functional feeding group percent) in 19 streams located in the Argentine portion of Tierra del Fuego Island. To explain the effects of beaver invasion and harvesting, we compared the physical and biotic variables among four habitat types: a) beaver meadows, b) shelterwood cut harvested areas without riparian buffers, c) variable retention harvested areas with riparian buffers, and d) unmanaged old-growth primary forests. 

Results/Conclusions

Most habitat variables were similar from all sites, except for dissolved oxygen (significantly higher in pristine streams of unmanaged old-growth primary forests compared to the other habitat types) and distance to riparian forest (significantly greater in shelterwood cut areas without riparian buffers and beaver meadows). Benthic communities in beaver meadows had significantly lower diversity, compared to streams of unmanaged old-growth primary forests and variable retention harvested areas with riparian buffers, while shelterwood cut harvested areas without riparian buffers presented intermediate values. Functionally, the benthic community in beaver meadows displayed a reduction in all functional feeding groups except collector-gatherers; again variable retention harvested areas with riparian buffers were similar to unmanaged old-growth primary forest streams, while  shelterwood cut harvested areas without riparian buffers occupied an intermediate position. These results indicated that i) current forestry practices including both variable retention and legally mandated riparian buffers may be effective in mitigating impacts on stream benthic communities and ii) provide further evidence of the large impact of beavers in the sub-Antarctic.