9. Creeping Fruitless Falls
141
Table 1. Seed set (mean±SD) and established seedling density in the remaining populations of Crepidiastrum ameristophyllum on Hahajima island of the
Bonin islands (GOlO and Washitani, unpublished)
Seed set per flower (%)
------------------Large population
Shoot no.=1200
Small population
Shoot no.=31
Isolated plant
Shoot no.=1
27.0±18.2
6.7±7.4
o
Seedling density (m2)
7
o
o
7 How to Strive Against Creeping 'Fruitless Fall'
On the present-day globe, habitat fragmentation ranks among the most serious causes
of biodiversity degradation (Wilcox and Murphy 1985; McNeely et al. 1990). Habitat fragmentation as well as other threats to biodiversity, such as over exploitation
and biological invasion, results in small population size and also in isolated populations or genets that elevate the probability of extinction (Lande 1987; Menges
1991a; Holsinger and Gottlieb 1991) through various mechanisms represented schematically in Fig 3. Impairment of species interaction essential to reproduction may
have a detrimental impact on an isolated plant population (Janzen 1974; Howe
1984). Especially, reduced pollinator services due to fauna degradation occur ubiquitously in present day landscapes throughout the world (Buchmann and Nabhan
1996). Absence of pollinators and solitude of isolated plants may impose several
types of drawbacks on a plant population depending on the species' reproductive
biology and population history: reproductive failure (Jennersten 1988; Bawa 1990;
Washitani et al. 1994a; Groom 1998), decrease in effective population size through
reducing gene flow, and breeding shifting to more sclfing (Bawa 1990; Menges
1991b; Aizen and Feinsinger 1994; Washitani et al. 1994a). Thus an altered reproductive pattern in turn may cause loss of genetic diversity and/or decreasing progeny fitness due to inbreeding depression (Karron 1989; Barrett and Kohn 1991;
Menges 1991b; Washitani 1996).
The results of our studies showing that seed production of both heterostylous
and monomorphic plant species is limited by compatible pollen supply through
reduced availability of either pollinators or mating partners suggest rather ubiquitous occurrence of 'fruitless falls' for wild plants with mating systems facilitating
outbreeding. Plant species depending on specific pollinators, i.e., pollinator specialists, such as Primula, have a higher risk of pollination limitation due to pollinator loss compared to a pollinator generalist such as Persicaria. However, solitude is
141
Table 1. Seed set (mean±SD) and established seedling density in the remaining populations of Crepidiastrum ameristophyllum on Hahajima island of the
Bonin islands (GOlO and Washitani, unpublished)
Seed set per flower (%)
------------------Large population
Shoot no.=1200
Small population
Shoot no.=31
Isolated plant
Shoot no.=1
27.0±18.2
6.7±7.4
o
Seedling density (m2)
7
o
o
7 How to Strive Against Creeping 'Fruitless Fall'
On the present-day globe, habitat fragmentation ranks among the most serious causes
of biodiversity degradation (Wilcox and Murphy 1985; McNeely et al. 1990). Habitat fragmentation as well as other threats to biodiversity, such as over exploitation
and biological invasion, results in small population size and also in isolated populations or genets that elevate the probability of extinction (Lande 1987; Menges
1991a; Holsinger and Gottlieb 1991) through various mechanisms represented schematically in Fig 3. Impairment of species interaction essential to reproduction may
have a detrimental impact on an isolated plant population (Janzen 1974; Howe
1984). Especially, reduced pollinator services due to fauna degradation occur ubiquitously in present day landscapes throughout the world (Buchmann and Nabhan
1996). Absence of pollinators and solitude of isolated plants may impose several
types of drawbacks on a plant population depending on the species' reproductive
biology and population history: reproductive failure (Jennersten 1988; Bawa 1990;
Washitani et al. 1994a; Groom 1998), decrease in effective population size through
reducing gene flow, and breeding shifting to more sclfing (Bawa 1990; Menges
1991b; Aizen and Feinsinger 1994; Washitani et al. 1994a). Thus an altered reproductive pattern in turn may cause loss of genetic diversity and/or decreasing progeny fitness due to inbreeding depression (Karron 1989; Barrett and Kohn 1991;
Menges 1991b; Washitani 1996).
The results of our studies showing that seed production of both heterostylous
and monomorphic plant species is limited by compatible pollen supply through
reduced availability of either pollinators or mating partners suggest rather ubiquitous occurrence of 'fruitless falls' for wild plants with mating systems facilitating
outbreeding. Plant species depending on specific pollinators, i.e., pollinator specialists, such as Primula, have a higher risk of pollination limitation due to pollinator loss compared to a pollinator generalist such as Persicaria. However, solitude is