ABSTRACT

The endangered herb Rutidosis leptorrhynchoides from south-eastern Australia has had its population numbers and sizes severely reduced by habitat loss and fragmentation. Assessment of allozyme variation in 18 diploid and five tetraploid remnant populations shows a strong positive relationship between the logarithm of population size and allelic richness for diploid populations, but not for tetraploids, which generally maintain higher diversity. Diploid populations smaller than 200 plants also show reduced S allele richness at the self-incompatibility locus, which constrains mate availability. Reduced mate numbers are associated with increased variance in male fitness in small isolated populations as reflected by high correlated paternity and increased deviation in allele frequencies between adult and pollen gene pools. Such changes reduce effective population sizes in small populations below their census size, further exposing them to effects of genetic drift.

Stochastic matrix projection models based on two years of demographic data from five populations show that population persistence is related in a general way with both higher genetic diversity and lower correlated paternity. Variation in population performance is primarily due to differences in the magnitude of the variance in transition probabilities rather than their mean values, which are generally not significantly different.

Measurement of genetic variation and fixation indices suggests that small re-established populations suffer the same constraints as small remnant populations. Conservation strategies for R. leptorrhynchoides should focus on maintaining population sizes above 200 plants to avoid demographic effects of S allele erosion and future re-establishment efforts should source seed broadly for the same reasons.