Climate change might drive the invasive tree Robinia pseudacacia into nature reserves and endangered habitats
Introduction
The designation and maintenance of protected areas is the single most important conservation strategy world wide (WCPA, 2008). Protected areas regulate human land uses in order to safeguard endangered ecosystems and habitats of endangered species. One of the problems associated with the establishment of static networks of nature reserves is that they neglect spatial and temporal variation in ecological processes and ecosystem properties (Pressey et al., 2007) and disregard the dynamic nature of species ranges. The latter aspect is especially relevant in the face of large-scale environmental changes like global warming which will potentially shift the distribution of suitable habitats for many species along latitudinal and altitudinal gradients (Grabherr et al., 1994, Parmesan and Yohe, 2003, Parmesan, 2006). Concerns that current reserve networks might hence not be able to guarantee the long-term persistence of the species they should protect have already been expressed (Peters and Darling, 1995, Scott et al., 2002, Araújo et al., 2004, Hole et al., 2009). However, comparatively little attention has been paid to the related problem that climate warming may not only drive endangered species out of reserves but might also foster the invasion of alien species into reserve networks. This might especially be the case in temperate regions like non-Mediterranean Europe which are characterized by a pronounced latitudinal and altitudinal gradient in invasion risk (Pyšek et al., 2002c, Pyšek and Richardson, 2006, Chytry et al., 2009) and where the current distribution of many alien plants, in particular, is thought to be mainly limited by climatic constraints (Pyšek et al., 2002a, Walther, 2003). If these aliens compete with the species to be protected, or alter habitat quality and ecosystem functioning, a climate warming driven range expansion into yet un-invaded reserves might hence further devaluate the long-term conservation value of such areas (Gurevitch and Padilla, 2004, Yurkonis et al., 2005).
Competing land use interests usually constrain the area of reserve networks and occurrence of endangered species and habitats is hence hardly restricted to protected areas. Ideally, any assessment of climate change driven invasion risk into habitats of conservation value (e.g. endangered habitats or habitats which are important for endangered taxa) should also focus on relevant sites currently not under legal protection. This is the more important as such an evaluation may guide the designation of future reserves protecting similar habitats less sensitive to invasions.
Robinia pseudacacia L. (black locust), a pioneer tree native to North America, is one of the most problematic alien plants in Central Europe (Kowarik, 2003a, Daisie, 2008). Black locust can colonize a wide array of different habitats in its secondary range. However, throughout central and western Europe it particularly invades nutrient poor dry and semi-dry environments which comprise some of the most species rich and most endangered habitat types of these regions (Holzner, 1986, Fischer and Stöcklin, 1997). Due to its symbiotic nitrogen fixing nodule bacteria, its encroachment into nutrient poor stands severely increases productivity and modifies nutrient cycles (Kowarik, 2003a). In grassland vegetation, light regimes and microclimate are changed by the formation of a tree-layer (Kowarik, 1996, Kowarik, 2003a). In combination, these alterations of ecosystem properties and functions usually result in a near-total loss of rare species adapted to nutrient poor conditions and in a considerable decrease of plant species diversity (Kowarik, 1996, Rice et al., 2004).
Niche-based species distribution models (Guisan and Zimmermann, 2000, Guisan and Thuiller, 2005) have become an important tool for assessing the potential range of species under current as well as under future environmental conditions (e.g. Thuiller et al., 2005). Basically, such models use sample data to relate the occurrence of species to a suite of mostly abiotic site conditions by means of a variety of alternative statistical tools (e.g. Elith et al., 2006). The calibrated statistical functions are then applied for spatial or temporal extrapolation, i.e. to predict occurrence probabilities at sites which have not been surveyed, or under altered environmental conditions.
In this paper we use niche-based predictive modelling to assess the potential future distribution of R. pseudacacia in Austria under different climate change scenarios. We expect that a warmer climate will enlarge the area potentially invasible for R. pseudacacia and ask if and to what extent such a potential range expansion increases the infestation risk for an existing reserve network and a variety of habitat types of conservation value outside this network. In general, we intend to highlight that triggering range expansions of invasive species is a potentially underestimated way of how climate warming may affect biodiversity conservation.
Section snippets
Study area
Austria is a land-locked country in Central Europe, covering an area of 83,858 km2, with a population of slightly more than 8 million inhabitants mainly living in the lowlands and in the major valleys of the Alps. Two thirds of Austria are dominated by mountainous regions. The landscape at low to medium altitudes is shaped by a long tradition of human land use.
Study species
R. pseudacacia (Fabaceae) is an up to 25 m tall tree native to southeastern North America. In its native range, it is a typical element
Current pattern of R. pseudacacia distribution and SDM evaluation
Currently R. pseudacacia is recorded as naturalized in 737 raster cells of the FMA, an occupancy rate of 28.2% (Fig. 1A). These 737 raster cells are clustered in the warm and dry Pannonic region of eastern Austria and the adjacent foothills of the Alps, where the species is widespread and often occurs in large stands. Isolated populations of black locust are scattered in other low-lying regions, e.g. in the lowest valley bottoms of the Alps.
With AUCs between 0.896 and 0.914 and kappa between
Climate warming and R. pseudacacia invasion
Our niche-based models clearly indicate that temperature constraints strongly control the current distribution of R. pseudacacia in Austria. Under a warmer climate the species is hence predicted to expand its distribution across the country considerably. In fact, mean annual temperatures in Austria have risen significantly recently (2000–2008: >0.7 °C) compared to the mean of the years 1961–1990, which we have used as standard period (cf. Thuiller et al., 2005). Along with an overall range
Conclusions
Our results demonstrate that climate warming is indeed a factor which might drive aliens into nature reserves and endangered habitats. Such climate change driven invasion into areas of conservation value might particularly affect montane regions, where alien species are currently still less frequent and problematic than in the lowlands, at least in Europe (Pyšek et al., 2002c, Pyšek and Richardson, 2006, Chytry et al., 2009). Detrimental effects of at least parts of these aliens, like black
Acknowledgements
The study was partly financed by the Austrian Academy of Sciences. S.D. additionally received funds from the EU-project ECOCHANGE (066866GOCE). We are obliged to H. Niklfeld, L. Schratt-Ehrendorfer and T. Englisch for access to the distribution data of the project “Mapping the Flora of Austria”. Valuable unpublished distribution data have been contributed by numerous colleagues. We are grateful for the comments of two anonymous reviewers and of the editor.
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These authors have contributed equally to this paper.