ReviewHow successful are plant species reintroductions?
Introduction
Wild plants are under increasing threat throughout the world. Humans may have accelerated the rate of extinction by 100- to 1000-times the natural rate (Ricketts et al., 2005, Thuiller, 2007). The best place to conserve plant biodiversity is in the wild, where a large number of species present in viable populations can persist in their natural habitats with their associated ecological interactions (Mc Naughton, 1989). However, degraded and altered habitats have become a major portion of the landscape mosaic (Vitt and Havens, 2004). For the foreseeable future, accelerating demands for natural resources will continue to degrade habitat and push an increasing number of plants towards extinction (Havens et al., 2006). Habitat restoration is a good conservation approach that may allow many plant populations to recover without the use of introduced propagules (Menges, 2008). However, as many plants have transient seed banks (Thompson et al., 1997), and many are dispersal-limited (Clark et al., 2007), spontaneous recovery of rare plant populations in restored sites may be constrained by the absence of naturally occurring propagules. In this case, the reintroduction of individual plants in the wild is an essential measure to conserve threatened species (Akeroyd and Wyse Jackson, 1995). The basic biological purpose of reintroductions is establishing new or augmenting existing populations in order to increase a species’ survival prospects (Pavlik, 1996, Van Groenendael et al., 1998, Luijten et al., 2002).
Reintroduction of native species has become increasingly important in conservation worldwide (e.g., Maunder, 1992, Hodder and Bullock, 1997, Rout et al., 2009). The value of species reintroduction has been increasingly acknowledged in international treaties and legislation, including the Convention on Biological Diversity, the Bern Convention, the Global Strategy for Plant Conservation, the European Strategy for Plant Conservation, the Gran Canaria Declaration on Climate Change and Plant Conservation, and the European “Habitat” Directive 92/43/EEC. These agreements increase public acceptance of reintroduction efforts as an integral component of biodiversity conservation. As a result, many reintroduction efforts have been initiated. In the last 10 years the European Union consistently supported reintroductions through specific projects approved under the LIFE programme (http://ec.europa.eu/environment/life/index.htm). However, with few exceptions, there has been little effort to report reintroduction protocols and outcome. Only a few studies have reported reintroduction trials in plant species (Bottin et al., 2007). Furthermore, case studies, best practices and experiences of reintroduction most often remain in the grey literature, rather than published in the scientific literature (Hodder and Bullock, 1997, Fischer and Lindenmayer, 2000). Moreover, published literature suffers from a bias towards positive results (Deredec and Courchamp, 2007). The lack of adequate documentation may therefore be especially acute for reintroductions that are regarded as failures (Griffith et al., 1989). In reviewing the grey literature we found that reintroduction projects exist for at least 234 plant species in Europe. This information was compiled from various sources, e.g. the IUCN Re-introduction Practitioners Directory (Soorae and Seddon, 1998), the ENSCONET Database (enscobase.maich.gr) and the Italian Botanical Society Database (www.societabotanicaitaliana.it). However, the existence of different databases does not facilitate information retrieval. Furthermore, no information is available on the techniques used or the results obtained.
Recently, Menges (2008) reviewed factors that contribute to the success of reintroduction, examining the ways reintroductions have been evaluated at various stages during the process of restoration. By answering the question “when is a reintroduction successful?” this paper is a major contribution in restoration ecology. However, Menges (2008) did not answer the question “how successful are reintroductions?” Such work has been carried out for animal species by various authors (e.g., Griffith et al., 1989, Cade and Temple, 1995, Wolf et al., 1996, Cade, 2000, Fischer and Lindenmayer, 2000). Except for Guerrant and Kaye’s (2007) review of 10 plant reintroduction projects, we are unaware of any comprehensive reviews despite an earlier call for a synthesis of data on specific reintroductions to improve overall success of these efforts (Hodder and Bullock, 1997). Indeed, many projects illustrate that various plant species seem to be particularly difficult to reintroduce (e.g. Allen, 1994, Parsons and Zedler, 1997, Helenurm, 1998, Morgan, 1999, Krauss et al., 2002). Consequently, reintroduction trials have sometimes been criticised (Fahselt, 2007). However, negative results must be viewed as an advancement of knowledge (MacNab, 1983). As reintroduction is recognised as a relatively high-risk, high-cost activity (Maunder, 1992, Gorbunov et al., 2008), disseminating the results of successful – and unsuccessful – experiments is important to provide examples and case studies that will allow development of common standards and methodologies.
This study analyzes plant species reintroduction trials worldwide by focusing on the methods used and the results obtained. Three main goals were identified. The first is to examine how successful plant reintroductions have been in establishing or significantly augmenting viable, self-sustaining populations in nature. The second is to determine the conditions required for plant reintroductions to be successful. Specifically, we test whether project type, material type, number of founder individuals, number of individuals introduced, provenance of material introduced, demographic status of source population, introduction method and field manipulations play a major role in the success of reintroductions. Our third objective is to use our results to inform future plant reintroduction efforts.
Section snippets
Definitions
In this study, we use the terms reintroduction, reinforcement and translocation according to Akeroyd and Wyse Jackson, 1995, IUCN, 1998. Reintroduction is a general term that describes the controlled placement of plant material into a natural or managed ecological area. It has also a stricter definition, i.e., the release and management of a plant into an area in which it formerly occurred, but in which it is now extinct or believed to be extinct (also called reinstatement or reestablishment).
Success of plant reintroductions
Overall, both the literature (Kruskal–Wallis test H = 22.1, n = 80, P < 0.001) and the survey data (Kruskal–Wallis test H = 10.8, n = 104, P = 0.013) show a significant downward trend over time in the survival of reintroduced plants (Fig. 1A). However, survival rates reported in the literature are remarkably higher than those mentioned by survey participants (Mann Whitney test: U = 6854.0, n = 184, P < 0.001). The percentages of flowering (Kruskal–Wallis test H = 13.0, n = 122, P = 0.005) and fruiting (Kruskal–Wallis
Success of plant reintroductions
A key qualitative measure of the ultimate fate of reintroductions is the ability of transplants to flower and set fruit (Morgan, 2000, Tyndall and Groller, 2006, Menges, 2008). Our study reveals that survival, flowering and fruiting rates in reintroduction projects are generally quite low. Furthermore, the data indicate a downward trend with time (e.g., after 4 years, flowering percentage was only 6% on average). Few studies included data on seed production and recruitment, which would allow
Acknowledgements
The authors wish to thank Christophe Lavergne, Ian Taylor, Joanna Markiewicz, Catherine Gaultier, Mari Miranto, Frédéric Blanchard, Andreas Groeger, Bert van den Wollenberg, Peter Enz, Hector Correa Cepeda, and the Italian Botanical Society for providing information on reintroduction programmes in their country. Maïté Delmas, Frédéric Hendoux and Daniel Malengreau have strongly advocated this study by encouraging participation from members of their networks. Richard Primack and three anonymous
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