INBREEDING DEPRESSION, ENVIRONMENTAL-STRESS, AND POPULATION-SIZE VARIATION IN SCARLET-GILIA (IPOMOPSIS-AGGREGATA)

Despite a large body of theory, few studies have directly assessed the effects of variation in population size on fitness components in natu ral populations of plants We conducted studies on 10 populations of sc arlet gilia, Ipomopsis aggregata, to assess the effects of population size and year-to-year variation in size on the relative fitness of pla nts. We showed that seed size and germination success ave significantl y reduced in small populations (those less than or equal to 100 flower ing plants) of scarlet gilia Plants from small populations are also mo re susceptible to environmental stress. When plants from small and lar ge populations were subjected to an imposed stress (combined effects o f transplanting and experimental clipping, simulating ungulate herbivo ry) in a common garden experiment plants from small populations suffer ed higher mortality and were ultimately of smaller size than plants fr om large populations. In addition experimental evidence indicates that observed fitness reductions are genetic, arte to the effects of genet ic drift and/or inbreeding depression When pollen was introduced from distant populations into two small populations, seed mass and percenta ge of germination were bolstered while pollen transferred into a large population had no significant effect. Year-to-year variation in popul ation size and its effects on plant fitness are also discussed In one small population, for example, a substantial increase in size from wit hin did not introduce sufficient new (archived) genetic material to fu lly overcome the effects of inbreeding depression.