Toward the restoration of a sustainable population of a threatened aquatic plant, Nymphoides peltata: Integrated genetic/demographic studies and practices

Abstract

We provide a brief review of practices and relevant studies for restoration of Nymphoides peltata (yellow floating heart, Asaza), a distylous floating-leaf clonal plant, in Lake Kasumigaura, Japan to develop a model of integrated genetic and demographic conservation for threatened plants. Several theoretically expected issues, such as sudden extinction of small populations with low genetic diversity, limited seed production in small-sized local populations, and significantly high heterozygosity in adults that have survived environmental change, were ascertained through integrated studies on demography with discrimination of genets and genetics using highly polymorphic genetic markers. Investigation of genetic properties of the remnant soil seed bank suggested that the seed bank could potentially restore genetic diversity, although the fitness reduction of seed banks caused by inbreeding could affect the success of restoration. As a result of restoration efforts, increases in the number of local populations and genets in the Lake Kasumigaura metapopulation have led to population recovery.

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.