Inbreeding Depression, Environmental Stress, and Population Size Variation in Scarlet Gilia (Ipomopsis aggregata)

Abstract

Despite a large body of theory, few studies have directly assessed the effects of variation in population size on fitness components in natural populations of plants. We conducted studies on 10 populations of scarlet gilia, Ipomopsis aggregata, to assess the effects of population size and year‐to‐year variation in size on the relative fitness of plants. We showed that seed size and germination success are significantly reduced in small populations (those 100 flowering plants) of scarlet gilia. Plants from small populations are also more susceptible to environmental stress. When plants from small and large populations were subjected to an imposed stress (combined effects of transplanting and experimental clipping, simulating ungulate herbivory) in a common garden experiment, plants from small populations suffered higher mortality and were ultimately of smaller size than plants from large populations. In addition, experimental evidence indicates that observed fitness reductions are genetic, due to the effects of genetic drift and/or inbreeding depression. When pollen was introduced from distant populations into two small populations, seed mass and percentage of germination were bolstered, while pollen transferred into a large population had no significant effect. Year‐to‐year variation in population size and its effects on plant fitness are also discussed. In one small population, for example, a substantial increase in size from within did not introduce sufficient new (archived) genetic material to fully overcome the effects of inbreeding depression.

Effect of population size on the prospect of species survival

“…MENGES 1991, RAIJMANN et al 1994, HESCHEL & PAIGE 1995, KÉRY et al 2000, FISCHER et al 2000. The absence of this relationship in the data on seed production per plant shows that the conclusion of the no relationship studies (e.g.…”

Genetic rescue in interconnected populations of small and large size of the self-incompatible Ranunculus reptans

“…Numerous studies of plants (and animals) have recorded reduced fitness in small populations in nature and when individuals from small populations are raised in a common environment (Menges, 1991;Oostermeijer et al, 1994;Heschel and Paige, 1995;Fischer and Matthies, 1998;Morgan, 1999;Fischer et al, 2000a;Kéry et al, 2000;Luijten et al, 2000;Schmidt and Jensen, 2000;Mavraganis and Eckert, 2001;Jacquemyn et al, 2002;Paschke et al, 2002;Severns, 2003;Vergeer et al, 2003;Brys et al, 2004). There are three likely genetic causes for reduced fitness of plants from smaller populations.…”

“…A few plant studies have measured the fitness consequences of intra-and interpopulation crosses in the context of population size (Van Treuren et al, 1993;Hauser and Loeschcke, 1994;Ouborg and Van Treuren, 1994;Heschel and Paige, 1995;Paland and Schmid, 2003). Only two of these included a sufficient number of populations (45) to estimate effects of population size (Van Treuren et al, 1993;Paland and Schmid, 2003).…”

Genetic and demographic dynamics of small populations of Silene latifolia

“…There have been several studies of how population size and degrees of isolation have negative affects on survival and fecundity in many plant taxa, but few longitudinal studies have tracked population size and measures of genetic variation in natural populations through time (Heschel and Paige, 1995). For this reason, the correlation between measures of genetic diversity and population fitness, as measured through survival and fecundity, has been complex.…”

“…While the relation between the loss of genetic variation and population decline has been studied in experimental and natural populations (Menges, 1991;van Treuren et al, 1991;Oostermeijer et al, 1994;Heschel and Paige, 1995;Newman and Pilson, 1997;Fischer and Matthies, 1998;Hanski and Ovaskainen, 2000), the dynamical relation between genetic diversity and demography is less well-documented (but see Westemeier et al, 1998). Studies of genetic erosion and demographic decline in natural populations often compare classes of populations that differ in size, age, or degree of isolation (Menges, 1991;van Treuren et al, 1991;Oostermeijer et al, 1994;Ouborg and van Treuren, 1995).…”