134
I. Washitani
opportunity for mating because of solitude was also demonstrated for isolated genets or smaller populations of other heterostylous species, Primula kisoana and
Persicaria japonica in highly fragmented deciduous forests and moist tall grasslands, respectively. We also demonstrated the detrimental effects of isolation, i.e.,
absence of mating partners, in a highly endangered homomorphic species,
Crepidiastrum ameristophyllum endemic to the Bonin Islands. Fruitless falls ascribed to the Allee effects are likely to be already ubiquitous among wild plants
subjected to habitat fragmentation or other threats to biodiversity. Habitat and/or
population restoration and 'pollinator therapy' management based on sound population and reproductive ecology are urgently required to strive against fruitless falls.
1 'Fruitless Falls': Signs of Biodiversity
Impairment Appearing in Plant Reproduction
Maintaining the earth's biodiversity and integrated ecosystems is among the most
important social aims of present-day human beings to guarantee intergenerational
long-term sustainability (Cristensen et al. 1996). We should have keen powers to
appreciate any signs or symptoms of impairment of biodiversity or ecosystem integrity. More than thirty years ago, Carson (1962) predicted a 'silent spring' devoid of
bird chorus and buzzing bees. The prediction invoked great social impacts and has
continuously drawn wide public attention to the problems of environmental pollution by synthetic chemicals until today. Another prediction she made in her famous
book entitled "silent spring" was a 'fruitless fall' in which flowers fail in producing
fruits and seeds because of pollinator absence (Carson 1962).
Fruitless fall has been creeping up more secretly than the silent spring, but
recently increasing instances of 'fruitless falls' have been appreciated for both cultivated and wild plants (Buchmann and Nabhan 1996). According to Buchmann
and Nabhan (1996), among 258 species in which limiting factors for fruit set under
natural conditions were investigated in detail, 62 percent were shown to suffer
limited fruit set from insufficient pollinator services.
As sessile organisms, plants have evolved a tremendous variety of mechanisms
assuring reproduction in case of isolation or pollinator absence, which include various
self-pollination and self-fertility mechanisms. Simultaneously, various physiological, morphological and phenological mechanisms to facilitate outbreeding and thus
to prevent selfing have been evolved (Silvertown and Lovett Doust 1993), probably
under strong selective pressures imposed by inbreeding depression (Barrett and
Kohn 1991). Contradictory demands for reproductive assurance vs. those for outbreeding and their temporal and spatial variations as selective forces would be
largely responsible for the highly diversified plant breeding systems we ean see in
the present day world. Reproductive resistance to isolation or pollinator absence
may vary according to the breeding system indigenous to the species or population.
Plants in which breeding systems tilt more to outbreeding may be less resistant to
habitat fragmentation or sudden population decline.
I. Washitani
opportunity for mating because of solitude was also demonstrated for isolated genets or smaller populations of other heterostylous species, Primula kisoana and
Persicaria japonica in highly fragmented deciduous forests and moist tall grasslands, respectively. We also demonstrated the detrimental effects of isolation, i.e.,
absence of mating partners, in a highly endangered homomorphic species,
Crepidiastrum ameristophyllum endemic to the Bonin Islands. Fruitless falls ascribed to the Allee effects are likely to be already ubiquitous among wild plants
subjected to habitat fragmentation or other threats to biodiversity. Habitat and/or
population restoration and 'pollinator therapy' management based on sound population and reproductive ecology are urgently required to strive against fruitless falls.
1 'Fruitless Falls': Signs of Biodiversity
Impairment Appearing in Plant Reproduction
Maintaining the earth's biodiversity and integrated ecosystems is among the most
important social aims of present-day human beings to guarantee intergenerational
long-term sustainability (Cristensen et al. 1996). We should have keen powers to
appreciate any signs or symptoms of impairment of biodiversity or ecosystem integrity. More than thirty years ago, Carson (1962) predicted a 'silent spring' devoid of
bird chorus and buzzing bees. The prediction invoked great social impacts and has
continuously drawn wide public attention to the problems of environmental pollution by synthetic chemicals until today. Another prediction she made in her famous
book entitled "silent spring" was a 'fruitless fall' in which flowers fail in producing
fruits and seeds because of pollinator absence (Carson 1962).
Fruitless fall has been creeping up more secretly than the silent spring, but
recently increasing instances of 'fruitless falls' have been appreciated for both cultivated and wild plants (Buchmann and Nabhan 1996). According to Buchmann
and Nabhan (1996), among 258 species in which limiting factors for fruit set under
natural conditions were investigated in detail, 62 percent were shown to suffer
limited fruit set from insufficient pollinator services.
As sessile organisms, plants have evolved a tremendous variety of mechanisms
assuring reproduction in case of isolation or pollinator absence, which include various
self-pollination and self-fertility mechanisms. Simultaneously, various physiological, morphological and phenological mechanisms to facilitate outbreeding and thus
to prevent selfing have been evolved (Silvertown and Lovett Doust 1993), probably
under strong selective pressures imposed by inbreeding depression (Barrett and
Kohn 1991). Contradictory demands for reproductive assurance vs. those for outbreeding and their temporal and spatial variations as selective forces would be
largely responsible for the highly diversified plant breeding systems we ean see in
the present day world. Reproductive resistance to isolation or pollinator absence
may vary according to the breeding system indigenous to the species or population.
Plants in which breeding systems tilt more to outbreeding may be less resistant to
habitat fragmentation or sudden population decline.