Introduced deer reduce native plant cover and facilitate invasion of non-native tree species: evidence for invasional meltdown

Abstract

Invasive species are a major threat to native communities and ecosystems worldwide. One factor frequently invoked to explain the invasiveness of exotic species is their release in the new habitat from control by natural enemies (enemy-release hypothesis). More recently, interactions between exotic species have been proposed as a potential mechanism to facilitate invasions (invasional meltdown hypothesis). We studied the effects of introduced deer on native plant communities and exotic plant species on an island in Patagonia, Argentina using five 400 m 2 exclosures paired with control areas in an Austrocedrus chilensis native forest stand. We hypothesized that introduced deer modify native understory composition and abundance and facilitate invasion of introduced tree species that have been widely planted in the region. After 4 years of deer exclusion, native Austrocedrus and exotic Pseudotsuga menziesii tree sapling abundances are not different inside and outside exclosures. However, deer browsing has strongly inhibited growth of native tree saplings (relative height growth is 77% lower with deer present), while exotic tree sapling growth is less affected (relative height growth is 3.3% lower). Deer significantly change abundance and composition of native understory plants. Cover of native plants in exclosures increased while cover in controls remained constant. Understory composition in exclosures after only 4 years differs greatly from that in controls, mainly owing to the abundance of highly-browsed native species. This study shows that introduced deer can aid the invasion of non-native tree species through negatively affecting native plant species.

The role of plant resistance and tolerance to herbivory in mediating the effects of introduced herbivores

“…However, when herbivores are introduced, outcomes associated with category 4 are likely. For example, beaver in Chile, deer in Argentina, rabbits in Australia, possums in New Zealand, deer in Canada all represent category 4 introductions with strong negative effects on associated biodiversity and ecosystem function largely due to the lack of resistance and tolerance traits in the population (see other papers in this issue -Martin et al 2009;Oduor 2009;O'Reilley-Wapstra and Cowan 2009;Relva 2009;Wallem 2009). …”

Distance to flood meadows as a predictor of use of Nothofagus pumilio forest by livestock and resulting impact, in Patagonia, Argentina

“…Herbivory by large exotic ungulates is one of the main anthropic disturbances in the Patagonian Andes forests (Kitzberger et al 2005;Vázquez 2002; Relva et al 2010;Relva and Veblen 1998). In 1880 began a period of massive European settlement in northern Patagonia and the establishment of livestock occurred during the coming decades (Veblen and Lorenz 1988).…”

Bases ecológicas para implementar un sistema de uso silvopastoral en la región patagónica de Aysén, Chile

“…El pastoreo con ganado doméstico es una de las principales actividades humanas en los bosques templados del sur del continente americano (Relva y Veblen 1998, Vázquez 2002, Kitzberger et al 2005, Relva et al 2010, Quinteros et al 2012. En la Patagonia chilena, los grandes fuegos de principios del siglo XX y la introducción del ganado condujeron a la deforestación de grandes superficies de bosque nativo (Veblen y Lorenz 1988), y al desplazamiento de grandes herbívoros silvestres como el huemul (Hippocamelus bisulcus) y el guanaco (Lama guanicoe) (Jaksic et al 2002).…”

“…Varios trabajos han destacado los efectos del pastoreo sobre la dinámica de regeneración natural de los bosques patagónicos (Relva y Veblen 1998, Pastur et al 2002, Vázquez 2002, Cavieres y Fajardo 2005, Kitzberger et al 2005, Blackhall et al 2008, Vila y Borrelli 2011) y sobre la estructura de los pastizales bajo el dosel arbóreo, si bien estos últimos han sido objeto de una menor atención (Relva et al 2010, Quinteros et al 2012. Esto contrasta con el amplio conocimiento existente sobre la estructura y la dinámica sucesional de las comunidades de pastizal de otras regiones en el mundo (Pineda et al 1981a, Gómez Sal 1986, Milchunas et al 1988, Noy-Meir et al 1989, McNaughton 1985, González Bernáldez 1991, Hobbs y Mooney 1995, Knapp et al 1999, Meiners et al 2002, Eriksson et al 2002, Kahmen y Poschlod 2004, Peco et al 2006, Moola y Vasseur 2008, García-Ruiz y Lana-Renault 2011, Pakeman 2011.…”