Organic matter characterization and decomposition dynamics in sub Antarctic streams impacted by invasive beavers

Abstract

ABSTRACT. Despite being a relatively remote and well conserved area, the sub-Antarctic ecoregion faces pressing global threats from climate change, the ozone hole and introduced species. Its freshwater ecosystems are one of the least studied components of this biome, but they are known to confront a host of invasive taxa including trout and beavers. We set out to understand the basic characterization and dynamics of organic matter processing and decomposition in sub-Antarctic streams under natural forest (NF) conditions and in ponds constructed by North American beavers (Castor canadensis) (BP). We found these streams have a naturally stable benthic organic matter regime throughout the year with a peak in leaf input from Nothofagus pumilio in autumn. Beaver ponds significantly increased the retention of organic matter and caused significantly higher decomposition rates, probably associated with increased density and biomass of Hyalella spp. As expected, leaf decay rates for N. pumilio, a deciduous species, were higher (NF: -0.0028 day -1 ± 0.0001 SE; BP: -0.0118 day -1 ± 0.0009 SE) than N. betuloides (a broad-leaf evergreen) (NF: -0.0018 day -1 ± 0.0005 SE; BP: -0.0040 day -1 ± 0.0003 SE). Overall these results indicate that the naturally low decomposition rates (slower than 89% of a global survey of decay rates) for these cold, oligotrophic streams are being modified by introduced beavers to resemble more temperate latitudes. Keywords: Castor canadensis, carbon cycle, decay rate, decomposition, leaf packs, Nothofagus.Caracterización de la materia orgánica y la dinámica de descomposición en arroyos subantárticos impactados por castores invasores RESUMEN. A pesar de ser una zona relativamente remota y bien conservado, la ecorregión subantártica se enfrenta a presiones por amenazas globales por el cambio climático, el agujero de ozono y las especies introducidas. Los ecosistemas de agua dulce son uno de los componentes menos estudiados de este bioma, pero se sabe que enfrentan una serie de taxones invasivos como la trucha y los castores. El propósito de este estudio es entender la caracterización básica y dinámica de descomposición de la materia orgánica en arroyos subantárticos de bosques naturales (NF) y condiciones de los estanques construidas por diques del castor norteamericano (Castor canadensis) (BP). Todos los arroyos estudiados presentaron un régimen naturalmente estable de materia orgánica bentónica, durante todo el año, con un mayor ingreso de hojas de Nothofagus pumilio en el otoño. La retención de materia orgánica aumentó significativamente en BP y causó una mayor tasa de descomposición, probablemente asociada a una mayor densidad y biomasa de Hyalella spp. Como era de esperar, las tasas de descomposición de hojas de N. pumilio, una especie de hoja caduca, fueron mayores (NF: -0,0028 día -1 ± 0,0001 SE; BP: -0,0118 día -1 ± 0,0009 SE) que las de N. betuloides, un árbol de hoja ancha perenne (NF: -0,0018 día -1 ± 0,0001 SE; BP: -0,004 d -1 ± 0,0003 SE). En general, estos resultados que las bajas tasas ...

Engineering by an invasive species alters landscape‐level ecosystem function, but does not affect biodiversity in freshwater systems

“…By engineering new lentic habitats with greater retention than previous lotic zones, the overall BOM carbon budget in these benthic ecosystems increased by about 70% at the stream network scale. Previous research has shown that beaver‐induced increases in BOM affect stream food webs, enhancing secondary production of benthos by an order of magnitude (Anderson & Rosemond, ) and transitioning benthic production and BOM decomposition rates in sub‐Antarctic streams to values more similar to temperate zones (Anderson & Rosemond, ; Ulloa et al ., ). It should be noted, however, that the effects of beavers on other ecosystem functions need to be more fully explored, as our analyses have only focused on benthic ecosystems.…”

“…Therefore, rather than concluding from these data that the invasive beaver is not harmful to TDF because it ‘merely’ engineers ponds whose benthic biodiversity is similar to natural lentic habitats, we can now more precisely determine the actual way it has transformed this landscape, which is via effects on ecosystem processes. Other studies have also shown that this invasive species not only changes standing crops, but increases decomposition rates of BOM (Ulloa et al ., ).…”

“…Full taxonomic information in Appendix S1. enhancing secondary production of benthos by an order of magnitude (Anderson & Rosemond, 2010) and transitioning benthic production and BOM decomposition rates in sub-Antarctic streams to values more similar to temperate zones (Anderson & Rosemond, 2007;Ulloa et al, 2012). It should be noted, however, that the effects of beavers on other ecosystem functions need to be more fully explored, as our analyses have only focused on benthic ecosystems.…”

“…These natural lentic landscape units may or may not be equivalent to beaver ponds, as it is unknown whether the key physical characteristics of natural impoundments are similar to those engineered by the beaver. On the other hand, regardless of whether beaver ponds are quantitatively and qualitatively similar to unmodified impoundments, their damming activities would be expected to increase the retention of materials and alter ecosystem processes in invaded watersheds (Anderson & Rosemond, ; Anderson & Rosemond, ; Ulloa et al ., ). Therefore, we predict that the beaver's influence on biodiversity will depend on the landscape's broader context, while the effect on ecosystem function is expected to be cumulative.…”

Fatty acids elucidate sub-Antarctic stream benthic food web dynamics invaded by the North American beaver (Castor canadensis)

“…Beaver effects on benthic food webs are relevant at the landscape-scale in Tierra del Fuego's sub-Antarctic streams because, on average, they increased the retention of organic matter in watersheds by 72% (Anderson et al 2014), and they also increase the decomposition rate of N. pumilio leaves (Ulloa et al 2012). By integrating the present study's FA results to previous knowledge of sub-Antarctic limnology, we now can add several new insights and re-affirm other aspects of aquatic ecology in southern Patagonia.…”