Effects of population size on performance in the rare plant Gentianella germanica

1 We studied the relationships between population size, reproduction and population growth rate in 23 populations of the rare, short-lived plant Gentianella germanica. We also investigated a possible correlation of population size effects with environmental variation in climate, topography, soil, vegetation, and management. To ascertain whether the differences between populations have a genetic basis, we grew 20 seed families from each of the populations in a common garden experiment. 2 In 1993, population sizes ranged from 40 to 5000 flowering plants in the field populations (geometric mean 386). Plants in small populations had fewer seeds per fruit and fewer seeds per plant than plants in larger populations, whereas seed mass was independent of population size. From 1993 to 1995 population size decreased in most populations (mean annual population growth rate 0.876), and this decrease was larger in small populations than in large populations. 3 In the common garden the positive correlation between the number of surviving plants per planted seed and original population size increased steadily over time. After 17 months there were significantly more flowering plants and more flowers per planted seed for seeds from large populations than for those from small populations. 4 Although environmental variables accounted for significant variation in population growth rate and plant performance both in the field and in the common garden, effects of population size detected in stepwise multiple regressions were not confounded with environmental effects. 5 We conclude that the reduced performance observed in plants from smaller populations was explained by genetic effects rather than by habitat quality, although pollinator limitation may have contributed to fitness reductions in the held. The observed patterns suggest further reductions of population sizes in Gentianella germanica in the future, especially in already small populations. Our study lends support to the view that genetic problems are a major concern in plant conservation.