Reproduction and genetics in fragmented plant populations
: Habitat fragmentation and smal population size can strongl y effect the reproductive succes inn species depending on pollinators or cross-pollination. Most Asteracea e have a generalist pollination syndromee and are self-incompatible. Members of this family may suffer less from pollination limitation andd more from limited cross-compatibl e mates resulting from drift. However, pollinator limitation may increasee the number of self-fertile plants, but increased homozygosit y associate d with the higher selfing ratess might lower plant performance . Inn the rare, self-incompatibl e Arnica montana (Asteraceae) , flower visitors and visitation rates were investigatedd in a large and smal population. In the large population, we observed a high and a low plot densit yy of flowering stems. In the smal population one plot was studied, resembling the low densit y plot off the large population. In the same two populations and a third large one, outcrossing rates, and the relationshi pp between individual heterozygosit y and plant size were analyzed. In eleven populations of variouss sizes, cultivated from seed samples , seed set after manual self-pollination was tested in the greenhouse . . ArnicaArnica montana was visited by hoverflies, 'other flies', butterflies, bees, beetles and bugs. In the large population,, hoverflies and butterflies were the main visitors, while 'other flies' was the dominating group inn the smal population. All visitors carried heterospecifi c pollen. Hoverflies and bees bore significantly moree pollen and conspecifi c pollen than beetles and butterflies. We assume that hoverflies and honeybee ss were the most efficient pollinators. Thee mean number of approache s to a genet and the number of flowerheads visited per genet were positivelyy correlated with the number of heads per genet , and they did not significantly differ among the plots.. The mean number of visits per flowerhead was independen t from the number of flowerheads per genet .. However, the visitation rate per flowerhead was significantly lower in the high densit y plot than inn the low densit y plot and the smal population. Populationn size was positively correlated with the median seed set after hand selfing, which disagrees withh the hypothesize d breakdown of the Si-system in smal populations. In all three populations, outcrossingg rates were high, suggesting that self-incompatibilit y still functions. In the smal population, fruitss within a capitulum had on average been sired by a higher number of pollen donors. This was in agreemen tt with the higher visitation rate in that population,, but additionall y suggest s that mate availabilityy is still sufficient. Offspring performance was positively related with individual heterozygosity ,, indicating that outcrossing is very important to maintai n fitness. INTRODUCTIO N N Inn Europe, nutrient-poor ecosystems are characterize d by relatively high plant species diversity.. Habitat fragmentation, eutrophication, and changes in land use have changed the speciess composition of these specific plant communities. The last decennia, many populations havee disappeare d or declined in number and size. This makes the species vulnerabl e to local extinctionn because of genetic, demographi c and environmental stochasticit y (Ellstrand & Elamm 1993, Menges 1991a, Menges 1992). A A