BIOLOGICAL CONSERVATION 128 (2006) 467– 474 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon The effects of invasive North American beavers on riparian plant communities in Cape Horn, Chile Do exotic beavers engineer differently in sub-Antarctic ecosystems? Christopher B. Andersona,b,*, Clayton R. Griffitha, Amy D. Rosemonda, Ricardo Rozzib,c,d, Orlando Dollenzc aInstitute of Ecology, University of Georgia, Athens, GA 30602-2202, USA bParque Etnobota´ nico Omora, Puerto Williams, Cape Horn County, Chile cDepartamento de Ciencias y Recursos Naturales, Universidad de Magallanes, Casilla 113-D, Punta Arenas, XII Regio´n de Magallanes, Chile dDepartment of Philosophy and Religion Studies, University of North Texas, 225 EESAT, P.O. Box 310920, Denton, TX 76203-0920, USA ARTICLE INFO ABSTRACT Article history: North American beavers (Castor canadensis) were introduced into southern South America Received 20 July 2005 in 1946. Since that time, their populations have greatly expanded. In their native range, Received in revised form beavers shape riparian ecosystems by selectively feeding on particular plant species, 25 September 2005 increasing herbaceous richness and creating a distinct plant community. To test their Accepted 3 October 2005 effects as exotic engineers on sub-Antarctic vegetation, we quantified beaver impacts on Available online 15 November 2005 tree canopy cover and seedling abundance and composition, as well as their impacts on herbaceous species richness, abundance and composition on Navarino Island, Cape Horn Keywords: County, Chile (55°S). Beavers significantly reduced forest canopy up to 30 m away from Cape Horn streams, essentially eliminating riparian forests. The tree seedling bank was greatly Castor canadensis reduced and seedling species composition was changed by suppressing Nothofagus betulo- Diversity ides and Nothofagus pumilio, but allowing Nothofagus antarctica. Herbaceous richness and Introduced abundance almost doubled in meadows. However, unlike beaver effects on North American Global change herbaceous plant communities, much of this richness was due to invasion by exotic plants, Nothofagus and beaver modifications of the meadow vegetation assemblage did not result in a signif- Sub-Antarctic icantly different community, compared to forests. Overall, 42% of plant species were shared Wilderness between both habitat types. Our results indicate that, as predicted from North American studies, beaver-engineering increased local herbaceous richness. Unlike in their native range, though, they did not create a unique plant community in sub-Antarctic landscapes. Plus, the elimination of Nothofagus forests and their seedling bank and the creation of invasion pathways for exotic plants together threaten one of the world’s most pristine temperate forest ecosystems. Ó 2005 Elsevier Ltd. All rights reserved. * Corresponding author: Tel.: +1 706 542 4366; fax: +1 706 542 4819. E-mail address: [email protected] (C.B. Anderson). 0006-3207/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2005.10.011 468 BIOLOGICAL CONSERVATION 128 (2006) 467– 474 1. Introduction rapidly increased to very high levels (0.2–5.8 colonies kmÀ1; Lizarralde, 1993; Skewes et al., 1999; Briones et al., 2001), com- Exotic species are a primary cause of global ecological change pared to North American densities (0.08–1.4 colonies kmÀ1; (Vitousek et al., 1996). Sub-Antarctic islands are particularly Gurnell, 1998). Today, beavers inhabit nearly every available vulnerable to species invasions and are also the site of some watershed on most of these islands (Sielfeld and Venegas, of the most significant effects of global climate change, such 1980; Lizarralde, 1993; Skewes et al., 1999), except a few uncol- as warming and UV–B exposure (Chapuis et al., 2004). Predict- onized refuges in the far southern and western portions of ing the impacts of particular introduced species, therefore, on the archipelago (Anderson et al., in press). ecosystem level processes requires understanding the mech- The ecological context and natural history of sub-Antarc- anisms of invasion and linking species with their ecosystem- tic broad-leafed Nothofagus (Fagaceae) forests are important level consequences (Jones and Lawton, 1994; Courchamp to understand the effects of beavers in this non-native eco- et al., 2003). system. Nothofagus forests on the islands near Cape Horn In southern Chile’s Cape Horn County, which includes the are embedded is a complex mosaic of habitats that includes archipelago south of Tierra del Fuego Island, terrestrial mam- several bog types, such as Sphagnum peatlands, cushion bogs mals and freshwater fish assemblages are dominated by exo- and rush wetlands (Rozzi et al., 2004a). In addition, the for- tic species. Greater than 50% of taxa in these groups are ested portions have relatively low native flora and fauna introduced (Anderson et al., in press). An important conse- diversity, compared to similar latitudes in the Northern Hemi- quence is that intact, pristine ecosystems are embedded in sphere. Many islands completely lack terrestrial mammalian a landscape that is replete with potential invaders. One spe- predators (Venegas and Sielfeld, 1998), and the regional plant cies in particular has merited a great deal of attention from assemblage is relatively simple and homogeneous (Moore, scientists, wildlife managers and governmental authorities: 1983; Martı´nez Pastur et al., 2002). In the absence of a com- the North American beaver (Castor canadensis Kuhl). mon evolutionary history with beaver, Nothofagus trees lack Beavers were brought to the Argentine portion of Tierra del the types of defensive mechanisms and reproductive strate- Fuego Island near Fagnano Lake in 1946 as part of a govern- gies (Rebertus and Veblen, 1993; Cuevas and Arroyo, 1999) that ment effort to economically ‘‘enhance’’ the species-poor Fue- are found in North American forests to deal with the impacts gian landscape by introducing exotic furbearers, including caused by beaver foraging and flooding (Basey et al., 1988; beavers, muskrats and minks. While only 25 mating pairs Johnston and Naiman, 1990). Finally, a more diverse riparian were released initially, by the 1950s the expanding population assemblage in North America includes taxa such as conifers, had entered into the Chilean portion of the island, and by the which are not part of the sub-Antarctic assemblage, and that 1960s they had succeeded in crossing channels to the adja- are avoided by beaver simply due to their lower nutritional cent Chilean archipelago, including Navarino and Hoste quality and palatability (Jenkins, 1980; Johnston and Naiman, Islands (Sielfeld and Venegas, 1980); (Fig. 1). Beaver densities 1990). Fig. 1 – Map of southern South America. Dashed line delimits Cape Horn County, Chile, which also corresponds to the sub-Antarctic archipelago area of the Cape Horn Biosphere Reserve. The arrow indicates the study areas located on the north coast of Navarino Island. BIOLOGICAL CONSERVATION 128 (2006) 467– 474 469 As a consequence beaver herbivory and wetland creation and all were below 150 m elevation. In each watershed, in North America promotes the persistence of an unpalatable two study reaches of at least 100 m were chosen that repre- stand of riparian trees that remains and regenerates (Naiman sented: (1) a natural, mature riparian forest with no history et al., 1988; Mu¨ ller-Schwarze et al., 1994) and creates a more of beaver impact either in the stream or in the riparian complex and diverse habitat and herbaceous species assem- zone; and (2) a beaver-created meadow along a reach that blage (Pollock et al., 1995; Wright et al., 2002). In sharp con- was previously forested before the beaver’s arrival, which trast, the forests of the Cape Horn area have only three was determined by evidence of standing, dead trees up to major tree species, all in the genus Nothofagus, which are the stream edge. The study meadows were made by aban- broad-leafed trees that are all eaten by beavers (Sielfeld and doned beaver dams that were less than 5 years old, which Venegas, 1980). Furthermore, these forests naturally regener- was determined by personal observation. Both sites in each ate from sapling banks in a gap dynamic, since seed banks do catchment were within 1 km of one another and on the not persist for long periods in these soils and vegetative main stem of the catchment. Their slopes, measured with reproduction is rare (Cuevas and Arroyo, 1999). Also Nothofa- a stadia rod and surveyor’s level, were not significantly dif- gus forests in general have problems regenerating when ferent (forest stream slope = 0.03 ± 0.008, Meadow stream under a systematic or permanent disturbance, making them slope = 0.02 ± 0.001, P = 0.25). Field work was carried out extremely vulnerable to persistent disturbance (Rebertus from February to March 2003 and April 2004, the austral and Veblen, 1993). Furthermore, only one species (N. antarc- summer months. tica) is adapted for boggy soil conditions (Ramı´rez et al., 1985). Such profound differences in the natural history of 2.2. Field methods sub-Antarctic Nothofagus forests leads to the questions: as ecological engineers, how do introduced beavers affect sub- To quantify the effects of beaver on forest canopy cover as a Antarctic riparian forests? And, what are the implications function of distance from the stream, ten 1 · 50 m transects for the conservation of one of the world’s largest and most spaced 10 m apart were run perpendicularly to the stream pristine temperate forests (Mittermeier et al., 2001; Rozzi edge at each study reach. Shaded cover was recorded with a et al., 2004a)? spherical densitometer at 10 m intervals along each transect Therefore, we tested the hypothesis that beaver activities and averaged for each study stream. Additionally, twenty in this novel ecosystem would increase riparian plant species 0.5 m2 plots were placed in 10 m intervals along the stream richness and diversity and create beaver-specific community edge at each habitat type per stream to survey herbaceous assemblages, as expected from studies conducted in its na- layer (plants < 1 m in height) vegetation.