9
Creeping 'Fruitless Falls':
Reproductive Failure in
Heterostylous Plants in
Fragmented Landscapes
IZUMI W ASHIT ANI
Institute of Biological Sciences, University of Tsukuba, Tsukuba 305-8572, Japan
Abstract
In recent years, increasing instances of 'fruitless falls', i.e., seed set failure of flowering plants, have been appreciated for both cultivated and wild plants. Species
having a sophisticated entomophilous breeding system such as heterostyly are likely
to be most vulnerable to the detrimental effects of pollinator loss, resulting in fruitless falls through compatible pollen limitation. Our studies on pollination and seed
set in wild populations of heterostylous species, Primula sieboldii, P. kisoana and
Persicaria japonica, and a few remaining populations of a highly endangered monomorphic species Crepidiastrum ameristophyllum suggested that fruitless falls have
been already rather common among wild plants in present-day fragmented landscapes. Serious effects of pollinator loss have been recognized for a PrimuLa sieboldii
population in a small nature reserve in the floodplain of the Arakawa River, which
is an 'insular habitat' surround.ed by the urbanized area of Greater Tokyo. Fertility
of the rare short-homostyle far surpasses those of normal heterostylous morphs, of
which fertility is severely limited by pollinator availability. Model simulation predicts the possibility of a large loss of genetic variation within a few generations
under the present strong fertility selection for the homostyle. For small populations
or isolated individuals in fragmented habitats, reduced opportunity for mating, i.e.,
one typical form of the Allee effects, is another major cause of seed set failure.
Among 20 populations of P. sieboldii investigated in southern Hokkaido, only a
negligible number of seeds were set in smaller populations consisting of less than
four genets, while in larger populations, population mean seed set depended strongly
on pollinator availability which can be assessed by craw marks left on the flower
petals by effective pollinator queen bumblebees. Seed set failure due to reduced
Key words. Allee effect, biodiversity, biological interaction, Crepidiastrum ameristophyllum,
genetic variation, habitat fragmentation, heterostyly, isolation, pollination, seed set, small
population, Persicaria japonica, Primula kisoana, Primula sieboldii, reproductive failure
133
Creeping 'Fruitless Falls':
Reproductive Failure in
Heterostylous Plants in
Fragmented Landscapes
IZUMI W ASHIT ANI
Institute of Biological Sciences, University of Tsukuba, Tsukuba 305-8572, Japan
Abstract
In recent years, increasing instances of 'fruitless falls', i.e., seed set failure of flowering plants, have been appreciated for both cultivated and wild plants. Species
having a sophisticated entomophilous breeding system such as heterostyly are likely
to be most vulnerable to the detrimental effects of pollinator loss, resulting in fruitless falls through compatible pollen limitation. Our studies on pollination and seed
set in wild populations of heterostylous species, Primula sieboldii, P. kisoana and
Persicaria japonica, and a few remaining populations of a highly endangered monomorphic species Crepidiastrum ameristophyllum suggested that fruitless falls have
been already rather common among wild plants in present-day fragmented landscapes. Serious effects of pollinator loss have been recognized for a PrimuLa sieboldii
population in a small nature reserve in the floodplain of the Arakawa River, which
is an 'insular habitat' surround.ed by the urbanized area of Greater Tokyo. Fertility
of the rare short-homostyle far surpasses those of normal heterostylous morphs, of
which fertility is severely limited by pollinator availability. Model simulation predicts the possibility of a large loss of genetic variation within a few generations
under the present strong fertility selection for the homostyle. For small populations
or isolated individuals in fragmented habitats, reduced opportunity for mating, i.e.,
one typical form of the Allee effects, is another major cause of seed set failure.
Among 20 populations of P. sieboldii investigated in southern Hokkaido, only a
negligible number of seeds were set in smaller populations consisting of less than
four genets, while in larger populations, population mean seed set depended strongly
on pollinator availability which can be assessed by craw marks left on the flower
petals by effective pollinator queen bumblebees. Seed set failure due to reduced
Key words. Allee effect, biodiversity, biological interaction, Crepidiastrum ameristophyllum,
genetic variation, habitat fragmentation, heterostyly, isolation, pollination, seed set, small
population, Persicaria japonica, Primula kisoana, Primula sieboldii, reproductive failure
133