The effect of forest fragmentation on genetic diversity, mating systems and effective population sizes of forest trees in Guanacaste, Costa Rica.

摘要

This thesis addresses the genetic impact of forest fragmentation on remnant tree populations in Guanacaste, Costa Rica. Chapter One justifies the research in terms of the importance of genetic diversity and possible fragmentation effects. Chapter Two reviews the latter. Chapter Three traces the deforestation history of the study zone and discusses its genetic implications, particularly those of increased forest linearity. Chapters Five and Seven report studies of two native species (Chapters Four and Six describe inheritance, neutrality, linkage of the allozytne markers used). Anacardium excelsum gene diversity (H_e) was similar to other tropical woody species. There was no apparent relationship between gene diversity and population size. There was moderate to large subpopulation differentiation overall, but less genetic differentiation within population groups, and there was an apparent relationship between genetic and geographical distances. F_st-based migration estimates were around 1 individual generation−1 overall, but 2–4 generation −1 within groups. Current gene flow to one isolated population (m) was 0.18. Outcrossing estimates (t_m) varied between fragments (mixed mating system), and were significantly positively related to neighbourhood density. Sheltered fragments on watercourses tended to higher flowering and flowering equitability. Growth rate in common garden experiments varied significantly between fragments; it was not significantly related to t_m, but tended to be lower in highly disturbed populations. Plumeria rubra H_e was similar to A. excelsum. Subpopulation differentiation was low to moderate. F_st-based migration estimates were 2–7 migrants generation−1, whilst m to one population was 0.13. Estimated population t_m were not significantly different from 100%. Capsule production was positively related to neighbourhood density. Results suggest differing susceptibilities to fragmentation. Disturbance may reduce A. excelsum effective population size due to inbreeding and high fertility variance. Despite smaller population sizes, anemochory, and aggregated populations, P. rubra retains high variation and little subpopulation differentiation, probably due to its highly mobile pollinator, whilst self-incompatibility precludes outcrossing effects. However, low genetic variation in one population, and density-related limitation of fruiting, suggest the species's resilience has limits. In Chapter Eight, general implications are discussed, with reference to species and forests of the zone, and to genetic and reproductive processes. The mitigation of fragmentation by improved husbandry of pastureland and riparian trees is considered.