Toward the restoration of a sustainable population of a threatened aquatic plant, Nymphoides peltata: Integrated genetic/demographic studies and practices
Introduction
The ultimate goal for conservation of threatened plants is to establish or maintain self-sustaining natural populations that are genetically diverse and demographically stable. The processes required to achieve this goal include several steps: assessing the status of each population to set restoration goals, hypothesizing mechanisms of the decline, and implementing and managing adaptively. To assess the status of a population appropriately, integrated demographic and genetic studies are indispensable (Lande, 1988, Schemske et al., 1994, Oostermeijer et al., 2003). Because the degradation of a population is generally recognized by a decrease in its size, a primary plan for restoration usually includes measures to recover the number of individuals. However, even if population size can be recovered, negative genetic effects, such as loss of original genetic variation, may potentially cause reductions in the ability of a population to adapt to a changing environment (Barret and Kohn, 1991, Lande and Shannon, 1996). Furthermore, a reduction in the immediate fitness of progeny due to inbreeding depression (Charlesworth and Charlesworth, 1987, Keller and Waller, 2002) may persist for several or even hundreds of generations (Nei et al., 1975, Newman and Pilson, 1997; Kirkpatrick and Jarne, 2000). Hence, genetic assessment using appropriate genetic markers and indices can help to assess the risk of such impacts (Frankham et al., 2002).
In the case of demographic assessment of a clonal plant, investigations should be based on the genet rather than the ramet. The genet number fundamentally affects viability (Charlesworth and Charlesworth, 1987, Lande and Shannon, 1996) and evolvability (Frankel, 1974, Avise, 1994) of a population, while the ramet number contributes to population persistence via the longevity of a genet (Honney and Bossuyt, 2005). Hence, highly polymorphic genetic markers, which allow discrimination of genets, are useful for precise assessment of the status of clonal plant populations, especially in species without apparent morphological variations among genets.
Plans designed to recover degraded plant populations should cover all key life-history stages of the target plant, not only ‘above-ground stages’ from germination to reproduction, but also ‘below-ground stages’ or the soil seed bank, which is an important component of population dynamics and persistence (Silvertown and Charlesworth, 2001). Persistent soil seed banks seem to be an ideal source for recovering a plant population because they preserve some of the genetic diversity that the aboveground population had before its degradation (Levin, 1990, McCue and Holtsford, 1998). However, the effectiveness of seed banks for genetic restoration varies, depending on species traits and population histories. If a seed bank is largely the result of inbreeding caused by population size reduction, seedlings from the seed bank and the immediately following generations may have low fitness caused by inbreeding depression (Apparicio and Guisande, 1997). Therefore, genetic properties of a seed bank population, as well as seed bank size, should be incorporated in plans for restoring a sustainable population.
Restoration programs should be conducted using an adaptive management approach, in which experimental restoration measures are conducted to test hypotheses on the mechanisms of the decline, because such programs are usually planned under conditions of uncertainty (Washitani, 2001a, Heywood and Iriondo, 2003). In the adaptive management cycle, citizen involvement in the implementation of management measures, as well as in goal setting, planning, and monitoring is recognized as an important component (Ludwig et al., 2001, Stringer et al., 2006).
Although a large number of studies on the demographic and genetic studies concerning conservation of threatened plants, case studies on population restoration fulfilling all above mentioned criteria are rare. Here, we provide a review of practices and relevant studies for the restoration of a threatened aquatic plant, Nymphoides peltata (yellow floating heart; Japanese common name, Asaza), in Lake Kasumigaura, Japan, to contribute to develop an integrated genetic/demographic study model. This species has advantages as a model species. First, the highly polymorphic genetic markers are available. Second, the demographic and genetic status of the population is well studied. Thus, hypotheses on the mechanisms of the population decline can be developed. Third, a comparatively large scaled restoration practice using soil seed banks has been implemented by The Japan Ministry of Land, Infrastructure, and Transport. Finally, this species has been used as a flagship species in an ecosystem restoration program coordinated by a NGO and many citizens, including school children, have participated in the program.
Section snippets
Study species
N. peltata (Gmel.) O. Kuntze, Menyanthaceae, a floating-leaf perennial plant, is widely distributed in temperate and subtropical zones of Eurasia including Japan. This species has a heterostylous (distylous) reproductive system: seed production therefore usually requires cross-pollination between long- and short-styled floral morphs (Ornduff, 1966). Although an unusual homostylous morph, occurring at a low frequency, can self-fertilize, the fitness of the offspring from self-pollination is
Importance of the Lake Kasumigaura population for conservation of N. peltata in Japan
In Japan, N. peltata was once common in freshwater habitats such as lakes, ponds, and brooks. However, most populations have disappeared as a result of anthropogenic habitat loss, so that the species is now listed as vulnerable in the Japanese Red Data Book (Environmental Agency of Japan, 2000). A nationwide population survey in 2001–2003 revealed that 27 metapopulations consisting of 64 local populations remained in the Japanese archipelago (Uesugi et al., 2009). In the survey, local
Recent status of the Lake Kasumigaura population revealed by ecological and genetic assessment
An assessment of all important life-history stages of the Lake Kasumigaura population of N. peltata was conducted using demographic and genetic approaches (Table 1).
Actual status of the Lake Kasumigaura population – the effect of restoration management
The assessment of demographic and genetic status of the Lake Kasumigaura population of N. peltata suggested a need for conservation of remnant adult clones to ensure continuity of the Lake Kasumigaura strain and achievement of regeneration from the soil seed bank to recover population size with genetic diversity (Table 1). Careful monitoring of the negative genetic impacts is also indispensable. The Japan Ministry of Land, Infrastructure, and Transport (MLIT) initiated a participatory program
Conclusions
As a result of restoration efforts, the number of local populations and genets and the area occupied by the Lake Kasumigaura metapopulation of N. peltata, which had been rapidly declining, recovered (Fig. 2). However, the ultimate goal, restoration of a self-sustaining population, has not yet been achieved, given that all the fully-grown and flowering genets from the soil seed bank were artificially planted after cultivation in the school pond. Adaptive management including careful monitoring
Acknowledgments
We thank Dr. Y. Tsumura, H. Kawaguchi, H. Marui, and A. Goto for advice and help in laboratory and fieldwork. We also thank Dr. S.R. Primack, H. Higuchi, A. Shimono, T. Kadoya, H. Kobori, and E. Platt for helpful comments on a preliminary manuscript. We acknowledge the Kasumigaura River Office of the Japanese Ministry of Land, Infrastructure, and Transport for providing opportunities for study, and the Asaza Fund and participants in the Asaza Project for joining in collaborative programs for
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