Andrew G. Young

The population genetic consequences of habitat fragmentation for plants

Habitat fragmentation reduces the size and increases the spatial isolation of plant populations. Initial predictions have been that such changes will be accompanied by an erosion of genetic variation and increased interpopulation genetic divergence due to increased random genetic drift, elevated inbreeding and reduced gene flow. Results of recent empirical studies suggest that while genetic variation may decrease with reduced remnant population size, not all fragmentation events lead to genetic losses and different types of genetic variation (e.g. allozyme and quantitative variation) may respond differently. In some circumstances, fragmentation actually appears to increase gene flow among remnant populations, breaking down local genetic structure.

Genetics, demography and viability of fragmented populations.

Foreword P. Brussard 1. Introduction: genetics, demography and the conservation of fragmented populations G. Clarke and A. Young Part I. Introductory Concepts: 2. Managing and monitoring genetic erosion W. Sherwin and C. Moritz 3. Inbreeding and outbreeding depression in fragmented populations M. Dudash and C. Fenster 4. Demography and extinction in small populations K. Holsinger 5. The metapopulation paradigm: a fragmented view of conservation biology P. Thrall, J. Burdon and B. Murray 6. Population viability analysis for conservation: the good, the bad and the undescribed M. Burgman and H. Possingham 7. Applications of population genetics and molecular techniques to conservation biology P. Hedrick Part II. Animal Case Studies: 8. Inbreeding in small populations of red-cockaded woodpeckers: insights from a spatially-explicit individual-based model S. Daniels, J. Priddy and J. Walters 9. Genetic erosion in isolated small mammal populations following rain forest fragmentation S. Srikwan and D. Woodruff 10. The Tumut experiment - integrating demographic and genetic studies to unravel fragmentation effects: a case study of the native bush rat D. Lindenmayer and R. Peakall 11. Demographic evidence of inbreeding depression in wild golden lion tamarins J. Dietz, A. Baker and J. Ballou 12. Inferring demography from genetics - a case study of the endangered golden sun moth, Synemon plana G. Clarke 13. Genetic population structure in desert bighorn sheep: implications for conservation in Arizona G. Gutierreez-Espelta, S. Kalinowski and P. Hedrick Part III. Plant Case Studies: 14. Limited forest fragmentation improves reproduction in the declining New Zealand mistletoe Peraxilla tetrapetala (Loranthaceae) D. Kelly, J. Ladley, A. Robertson and D. Norton 15. Ecology and genetics of Grevillea (Proteaceae) 16. Genetic and demographic influences on population persistence: gene flow and genetic rescue in Silene alba C. Richards 17. Fragmentation in central American dry forests - genetic impacts on Swietenia humulis (Meliaceae) G. White and D. Boshier 18. Population viability analysis of the rare Gentiana pneumonanthe: importance of genetics, demography and reproductive biology J. Oostermeijer 19. Genetic erosion, restricted mating and reduced viability in fragmented populations of the endangered grassland herb Rutidosis leptorrhynchoides A. Young, A. Brown, B. Murray, P. Thrall and C. Miller Part IV. Conclusions and Future Directions: 20. What do we know about the genetic and demographic effects of habitat fragmentation and where do we go from here? A. Young and G. Clarke Index.

Assessing the benefits and risks of translocations in changing environments: a genetic perspective

Translocations are being increasingly proposed as a way of conserving biodiversity, particularly in the management of threatened and keystone species, with the aims of maintaining biodiversity and ecosystem function under the combined pressures of habitat fragmentation and climate change. Evolutionary genetic considerations should be an important part of translocation strategies, but there is often confusion about concepts and goals. Here, we provide a classification of translocations based on specific genetic goals for both threatened species and ecological restoration, separating targets based on ‘genetic rescue’ of current population fitness from those focused on maintaining adaptive potential. We then provide a framework for assessing the genetic benefits and risks associated with translocations and provide guidelines for managers focused on conserving biodiversity and evolutionary processes. Case studies are developed to illustrate the framework.

Forest conservation genetics: principles and practice

Forest management must be sustainable not only in ecological, economic and social, but also genetic terms. Many forest managers are advocating and developing management strategies that give priority to conserving genetic diversity within production systems, or that recognise the importance of genetic considerations in achieving sustainable management. Forest Conservation Genetics draws together much previously uncollected information relevant to managing and conserving forests. The content emphasises the importance of conserving genetic diversity in achieving sustainable management.nnEach chapter is written by a leading expert and has been peer reviewed. Readers without a background in genetics will find the logical sequence of topics allows easy understanding of the principles involved and how those principles may impact on day-to-day forest planning and management decisions. The book is primarily aimed at undergraduate students of biology, ecology, forestry, and graduate students of forest genetics, resource management policy and/or conservation biology. It will prove useful for those teaching courses in these fields and as such help to increase the awareness of genetic factors in conservation and sustainable management, in both temperate and tropical regions.

Genetic erosion, inbreeding and reduced fitness in fragmented populations of the endangered tetraploid pea Swainsona recta.

Genetic variation and fixation coefficients were measured for 17 fragmented populations of the endangered tetraploid pea Swainsona recta ranging in size from 1 to 430 flowering plants. Allelic richness and fixation coefficient were correlated with the log population size, suggesting that reduced population size is accompanied by genetic erosion, primarily due to a loss of rare (q<0.1) alleles, and increased inbreeding. Comparative germination and growth studies of seed from five populations representing three different levels of inbreeding (low F=0.34, medium F=0.43, high F=0.57) showed a significant reduction in percentage seed germination at 2 weeks in the single high F treatment population. There were no effects on survivorship and growth beyond this up until 141 days. Results suggest that polyploidy has not prevented erosion of genetic variation at the population level, as has previously been suggested. However, the production of partial heterozygotes, e.g. AABC and AAAB, under inbreeding may be mitigating inbreeding depression assuming a partial dominance model of gene expression. Conservation effort should concentrate on populations larger than 50 sexually reproductive plants, as these appear capable of maintaining high genetic diversity and exhibit no immediate evidence of inbreeding depression, despite some elevation of the fixation coefficient.

Population size, self-incompatibility and genetic rescue in diploid and tetraploid races of Rutidosis leptorrhynchoides (Asteraceae)

Self-incompatibility systems function to prevent inbreeding, and work effectively in large, genetically diverse populations. However, a decrease in population size can reduce genetic diversity at the self-incompatibility locus, which leads to a reduction in mate availability and has important demographic implications for small populations. Currently, little is known about the response of self-incompatible polyploid species to a reduction in population size. In Rutidosis leptorrhynchoides there was a significant decrease in the within-population probability of fertilization with a decline in population size for diploid populations and a marginally significant relationship for tetraploid populations, suggesting that in small populations of both chromosome races fertilization success is reduced due to a decrease in self-incompatibility allele (S-allele) diversity. There was no significant difference between the slopes of the fertility–population size relationship for diploid and tetraploid populations which indicates a similar rate of decline in fertilization success with population size for both chromosome races. Fertilization success increased when crosses were undertaken between populations and this was significantly related to population size for diploid and tetraploid populations, indicating that small populations gain the greatest benefit to fertilization success from crossing between populations. For tetraploid populations the benefits of crossing between populations tended to decline more rapidly with increasing population size. These results suggest that for small populations that have reduced fertilization success, genetic rescue by introducing new genetic material from other populations is an important means of ameliorating mate limitation issues associated with reduced S-allele diversity in both diploid and tetraploid races.

Reproductive success does not decline in fragmented populations of Leucochrysum albicans subsp. albicans var. tricolor (Asteraceae).

Abstract Fragmentation and isolation of plant populations can affect demographic processes such as seed production and cause reductions in fitness, but their relative effects are likely to depend on the life history of the species concerned (i.e. breeding system, dispersal syndrome and longevity). In 14 isolated, remnant populations of the short-lived Leucochrysum albicans subsp. albicans var. tricolor (Asteraceae), where reproductive population size differed from 74 to over 50,000 flowering plants, seed set and germinability was determined for one flowering season. The breeding system was determined in a hand-cross versus self-pollination experiment in the glasshouse, whilst mating system parameters were determined by molecular methods for four sites spanning the range of reproductive population sizes. Leucochrysum albicans subsp. albicans var. tricolor is a self-incompatible species and outcrossing rates were uniformly high (>90%) in the populations observed. Seed production was not linearly associated with log (population size), although there was substantial within and between-population variability. Seed germinability was rapid and substantial (>80%) in all populations and not linearly associated with log (population size). This study suggests that (previously identified) short-term factors, such as the maintenance of habitat and safe sites for regeneration, are of immediate importance to the persistence of all L. albicans subsp. albicans var. tricolor populations. To uncouple the effects of population size and within-site factors on plant demography in fragmented plant populations, more research effort should be directed at quantifying the significance of the local-scale interactions that occur between individual plants, pollinators and site environmental factors.

Low outcrossing rates and shift in pollinators in New Zealand pohutukawa (Metrosideros excelsa; Myrtaceae).

New Zealand pohutukawa (Metrosideros excelsa), a member of the Myrtaceae, is a large, mass-flowering tree endemic to northern New Zealand coastlines. Mainland populations have been reduced to fragmented stands, and the original suite of bird pollinators has been largely replaced by introduced species. The native pollinator fauna on several offshore islands is largely intact and includes three species of the New Zealand honeyeaters (Meliphagidae) and native, solitary bees. We estimated multilocus outcrossing rates for three mainland and two island populations and found that they were among the lowest in the Myrtaceae (t(m) = 0.22-0.53). The shift in pollinators had no measurable effect on the mating system. Mass-flowering facilitates geitonogamous selfing, and inbreeding depression in seedling height was detectable at 6 mo of growth. F(s) [Wrights (1965) Fixation Index] was consistently higher than F(m) in all populations, indicating that selection may eliminate selfed offspring from populations prior to achieving reproductive maturity. Results suggest that increased selfing in mainland populations due to pollinator changes is not responsible for current patterns of poor regeneration of this species.

Genetic diversity in tetraploid populations of the endangered daisy Rutidosis leptorrhynchoides and implications for its conservation

Polyploidy is an important variable in assessing the genetics of endangered plant species. Species consisting of populations with different chromosome numbers pose questions as to the mode of inheritance, relative variability status, population divergence and gene flow. The self-incompatible species Rutidosis leptorrhynchoides (Asteraceae) in south-eastern Australia is a good example. The remnant populations in the northern sector of the species range are diploid, whereas southern ones are either diploid or tetraploid. Allozyme analysis of the tetraploid populations showed tetrasomic inheritance confirming an autopolyploid genetic system, a modest increase in their allelic richness over diploid populations in the same region and a lack of genetic divergence. Conservation and replenishment strategies should take account of these genetic features of mixed ploidy.

Comparative analysis of the mating system of the rare woodland shrub Daviesia suaveolens and its common congener D. mimosoides

Controlled pollinations and allozyme markers were used to compare mating system parameters among three populations of the rare woodland shrub Daviesia suaveolens Crisp. and five populations of its common relative D. mimosoides R. Br. Pollination results show that both species require a vector to facilitate pollination and have strong self-incompatibility mechanisms. Multilocus estimates of outcrossing rates confirm this, with all populations being highly outcrossed (D. suaveolens tm=0.78−1.0; D. mimosoides tm=0.71−0.96). Smaller and less dense D. mimosoides populations had higher correlations of outcrossed paternity than larger ones. The divergence of pollen pool allele frequencies from population frequencies was greater in smaller populations than in larger ones, in keeping with a lower effective size of the male source. Regression analyses generally failed to show effects of either plant size or local flowering environment on estimates of single-plant outcrossing rates. Most variation in these rates probably reflects the combined effects of sampling error, correlated mating and the influence of marker diversity on outcross detectability.