RELATIONSHIPS BETWEEN PLANT-DENSITY, OUTCROSSING RATES AND SEED SET IN NATURAL AND EXPERIMENTAL POPULATIONS OF SCABIOSA-COLUMBARIA

Outcrossing rates were estimated in both natural and experimental populations of Scabiosa columbaria, a self-compatible, entomophilous, gynodioecious, protandrous perennial. In natural populations, estimates of the outcrossing rate in hermaphrodites were near to one and ranged from 0.84 +/- 0.07 to 1.12 +/- 0.11. The effect of plant density on outcrossing rates was studied in two experimental populations of 27 individuals. Contrary to expectation the estimates of the out-crossing rate in hermaphrodites were about 100% for both densities. However, in the sparse population, the fraction of developed seeds of plants used to estimate outcrossing rates was significantly lower than of plants in the dense population (0.41 +/- 0.06 and 0.68 +/- 0.08, respectively). Artificial pollinations of these plants in the greenhouse showed that the fraction of developed seeds was 0.60 +/- 0.01 and 0.83 +/- 0.05 after self- and cross-pollination, respectively. The combined results suggested that the differential success of self- and cross-pollination might have caused equalization of the outcrossing rates in the experimental populations, despite different plant densities. The implications of the results for conservation biology are discussed.