Population genetics and conservation biology of the endangered tree Aquilaria sinensis (Thymelaeaceae) in Hong Kong

Abstract

Poaching, habitat loss, and human-induced fragmentation are global issues that threaten species with extinction. Economically important species are especially susceptible. Aquilaria sinensis (Thymelaeaceae) is an evergreen tree endemic to South China that is intensively poached as a source of agarwood. Agarwood is the resinous heartwood of a small group of related trees that is highly prized in many Asian cultures for its fragrance and purported medicinal properties. As a result of agarwood’s popularity for use in a variety of products, all Aquilaria species have been heavily exploited in the wild, bringing about a drastic decrease in population sizes. Aquilaria sinensis is now assessed as Vulnerable in the IUCN global Red List. In this thesis, I use this species as an example to study the genetic legacy of a long history of anthropogenic disturbance on its conservation. This species has long been subject to rampant poaching because of rising demand for and soaring price of agarwood. Moreover, as urbanisation depletes South China’s natural areas, the countryside is subject to conversion, causing habitat loss and fragmentation, as well as poaching. In the present study, I sampled 1,464 individuals from 34 sites across the territory of Hong Kong. I genotyped the samples using 15 microsatellite markers to examine genetic diversity and population structure, test for bottlenecks and restricted gene flow, and identify priority populations for conservation. In Part One, life history traits and the demographic structure of A. sinensis in Hong Kong were examined. Genetic diversity was also compared between life history stages, as defined by size-class thresholds for onset of reproductive maturity and increased poaching risk, in order to understand whether life history traits confer tolerance to the genetic consequences of poaching. An array of life history traits associated with high fecundity and r-selection were confirmed in A. sinensis, and comparable levels of genetic diversity between life stages suggest that life history traits do grant a degree of genetic resilience. In Part Two, genetic diversity and population genetic structuring are placed in a spatial context. A pronounced genetic structure was revealed between sites on Lantau and Lamma Islands on the one hand, and in the New Territories and on Hong Kong Island on the other, with significant differences in genetic diversity indices detected between the two clusters: a lower heterozygosity and allelic richness, as well as a less diverse genetic composition, were observed among sites in the Lantau- Lamma cluster. The results imply that the two clusters have been subject to different demographic histories, genetic processes, and/or levels of anthropogenic disturbance, and thus require different conservation approaches. To inform management decision-making, Part Three sets out to identify conservation priorities among the 34 sites with respect to their heightened evolutionary distinctiveness and extirpation risk. It was revealed that different sites have varying levels of extirpation risk, and sites occupying longer branches in the Unweighted Pair Group Method with Arithmetic Mean (UPGMA) tree were consistently prioritised for conservation. This thesis presents a practical case study to bridge research output and management action.