136
I. Washitani
insect visitation during a continuous monitoring period for a total of sixteen hours
with 68 flowers (Washitani et al. 1991). The entire insect fauna of the reserve
appears impoverished, but the absence of long-tongued bumble bees may be the
main reason for the fruitless fall of the species, since the queens of long-tongued
bumblebees, of which tongue length is nearly equal to corolla tube length of P.
sieboldii, are known to be especially effective pollinators (Washitani et al. 1994b).
The strong fertility selection for the rare homostyle morph suggests the possibility of a large loss of genetic variation within a few generations. Simulation with
a genetic population model predicts either a strong genetic bottleneck due to fertility selection or an overall recruitment failure in the case of the presence of very
strong inbreeding depression (Washitani 1996).
3 Reproductive Failure Due to Solitude:
Low Quantity and Quality of Progeny
Pollinator loss is not the sole reason for seed set failure of plant populations in
fragmented landscapes. Reduced opportunity for mating due to isolation, i.e., a
typical form of the Allee effects (Allee 1951), may be another major cause of fruitless falls for small populations or isolated genets of declining populations. Evidence for a case of the Allee effect was obtained from our study, in which seed set
and various biological and other environmental factors potentially affecting reproductive success were compared among 20 P. sieboldii local populations in a fragmented landscape in the Hidaka region, southern Hokkaido (Matsumura and
Washitani, unpublished). Although various biological agents including herbivores
and pathogens affected the fertility of the plants (Washitani et al. 1996), seed set of
the populations was strongly dependent on the population size and pollinator availability, which can be assessed by craw marks left on the flower petals by longtongued bumblebee queens (Washitani et al. 1994b). Among the 20 populations
investigated, negligible numbers of seeds were set in smaller populations consisting of less than four genets, while in larger populations, population mean seed set
strongly depended on pollinator availability especially in years with generally low
pollinator activities.
Results of our study in another deciduous forest habitat suggested that not only
the quantity of the seeds, but also their quality, i.e., the fitness of the few seeds
which were produced on isolated mother plants of P. sieboldii was also significantly lower than those of the seeds from less isolated mother plants having potential mating partner(s) in their vicinity (Otsuka, Goka and Washitani, unpublished).
The study was performed in a typical habitat of the species in central Honshu, i.e.,
a forest dominated by Quercus mongolica Fischer ex Ledeb. ssp. crispula (Blume)
Menitsky (35
0
57' N, 138
0
28' E, 1350 m a.s.l.), and seed set, germinability of the
seeds and survival and growth of the germinated seedlings were examined for many
mother plants growing along a stream. Seed set as well as germination of the seeds
and early survival of the geminated seedlings were significantly lower and more
I. Washitani
insect visitation during a continuous monitoring period for a total of sixteen hours
with 68 flowers (Washitani et al. 1991). The entire insect fauna of the reserve
appears impoverished, but the absence of long-tongued bumble bees may be the
main reason for the fruitless fall of the species, since the queens of long-tongued
bumblebees, of which tongue length is nearly equal to corolla tube length of P.
sieboldii, are known to be especially effective pollinators (Washitani et al. 1994b).
The strong fertility selection for the rare homostyle morph suggests the possibility of a large loss of genetic variation within a few generations. Simulation with
a genetic population model predicts either a strong genetic bottleneck due to fertility selection or an overall recruitment failure in the case of the presence of very
strong inbreeding depression (Washitani 1996).
3 Reproductive Failure Due to Solitude:
Low Quantity and Quality of Progeny
Pollinator loss is not the sole reason for seed set failure of plant populations in
fragmented landscapes. Reduced opportunity for mating due to isolation, i.e., a
typical form of the Allee effects (Allee 1951), may be another major cause of fruitless falls for small populations or isolated genets of declining populations. Evidence for a case of the Allee effect was obtained from our study, in which seed set
and various biological and other environmental factors potentially affecting reproductive success were compared among 20 P. sieboldii local populations in a fragmented landscape in the Hidaka region, southern Hokkaido (Matsumura and
Washitani, unpublished). Although various biological agents including herbivores
and pathogens affected the fertility of the plants (Washitani et al. 1996), seed set of
the populations was strongly dependent on the population size and pollinator availability, which can be assessed by craw marks left on the flower petals by longtongued bumblebee queens (Washitani et al. 1994b). Among the 20 populations
investigated, negligible numbers of seeds were set in smaller populations consisting of less than four genets, while in larger populations, population mean seed set
strongly depended on pollinator availability especially in years with generally low
pollinator activities.
Results of our study in another deciduous forest habitat suggested that not only
the quantity of the seeds, but also their quality, i.e., the fitness of the few seeds
which were produced on isolated mother plants of P. sieboldii was also significantly lower than those of the seeds from less isolated mother plants having potential mating partner(s) in their vicinity (Otsuka, Goka and Washitani, unpublished).
The study was performed in a typical habitat of the species in central Honshu, i.e.,
a forest dominated by Quercus mongolica Fischer ex Ledeb. ssp. crispula (Blume)
Menitsky (35
0
57' N, 138
0
28' E, 1350 m a.s.l.), and seed set, germinability of the
seeds and survival and growth of the germinated seedlings were examined for many
mother plants growing along a stream. Seed set as well as germination of the seeds
and early survival of the geminated seedlings were significantly lower and more