To ease the accelerating species extinction caused by environmental destruction, it is necessary to identify the cause of extinction for each species and develop a conservation strategy which can be applied for a wider range of species. My research focuses on an endangered species Drosera anglica, which is an index species for vulnerable wetland ecosystem and rapidly decreasing in number in many regions around the world. The question of this research is why D. anglica is threatened compared to sympatric widespread congener D. rotundifolia. To answer this question, I addressed differences between D. anglica and D. rotundifolia in terms of environmental conditions and reproductive traits.
In natural habitats, D. anglica grows in lighter and high water level conditions compared to D. rotundifolia at a microhabitat scale. Based on an assumption that the narrow microhabitat preference of D. anglica may have developed with the failure of seedling establishment in low water level, the relationship between recruitment and survival rate and environmental factors were investigated in a three-year census in a natural habitat of Sarobetsu mire. The recruitment rate for D. anglica was high around their parents but the survival rate of the species was low in high water level and overstory vegetation. The effects of environment to recruitment and survival were not seen on D. rotundifolia. The seed sowing and seedling transplant experiments were also conducted with water-table, light and habitat type manipulation. The seedling growth was large in mediate water-table for both species, but it was significantly low for D. anglica with decreasing light. The seedling growth of D. anglica seedlings was also suppressed on Sphagnum-mat, however the seedlings of D. anglica grew large on bare ground with enough light and low water level. Based on these results, it was considered that the less tolerance to luck of light in early life stage of D. anglica narrowed its possible habitats compared to D. rotundifolia.
D. anglica and D. rotundifolia reproduce sexually by self- fertilization and propagate axillary buds as new ramets by clonal growth, but it is not clear how the two reproductive modes are used in combination and how significant each mode would be in an adaptive sense for each species. The mechanism and constraining factor of the two reproductive modes were compared by focusing on the meristem allocation between the number of flowers, scapes versus ramets. The sexual reproduction and clonal growth of both species were size-dependent and there was no trade-off between two reproductive modes. The ramet- producing individuals were significantly large in size throughout the growing season. Flower production negatively affected following growth and survival rate of mature individuals, but ramet production had no effects on growth and reproduction in the two species. From these results, it was considered that D. anglica and D. rotundifolia had the same reproductive mechanism in which newly produced meristems were allocated to reproductive meristems first and to growth meristems, i.e., new ramet the next. The high frequency of ramet production of D.anglica thanks for its large size and the large number of flowers of D. rotundifolia for the meristem activation are seemed to be suitable to maintain their population in each microhabitat.
In conclusion, the narrow microhabitat preference of D. anglica in the early life stage is the cause for the high extinction rate. Therefore, clonal growth may be more effective than seed reproduction for population maintenance under low overstory vegetation where the individuals can fully photosynthesize.