Fiona A. C. Impson

Biological Control of Australian Acacia Species and Paraserianthes lophantha (Willd.) Nielsen (Mimosaceae) in South Africa

In total, ten agent species have been released in South Africa for the biological control of ten invasive Australian Acacia species and Paraserianthes lophantha (Willd.) Nielsen (Mimosaceae). Besides a single fungal pathogen species which affects both reproductive and vegetative growth of its host plant, Acacia saligna (Labill.) H.L.Wendl., there are nine herbivorous insect species which predominantly suppress the reproductive output of their host plants. These include five seed-feeding weevil species, two flower-galling fly species and two bud-galling wasp species. An indigenous basidiomycete fungus, which causes die-back disease of Acacia cyclops A. Cunn. ex G. Don, has also been investigated. During the last ten years, considerable effort has been directed at searching for new agents in Australia and in collecting additional material to bolster populations of recently-established agents in South Africa. Concurrently, ongoing evaluation studies in South Africa have measured the dynamics of the introduced agents as well as their impact on the vigour and fecundity of their host plants and the extent to which their damage is reducing the density, distribution and invasiveness of the Acacia species. Progress with all of these projects is reviewed.

Unresolved native range taxonomy complicates inferences in invasion ecology: Acacia dealbata Link as an example

Elaborate and expensive endeavours are underway worldwide to understand and manage biological invasions. However, the success of such efforts can be jeopardised due to taxonomic uncertainty. We highlight how unresolved native range taxonomy can complicate inferences in invasion ecology using the invasive tree Acacia dealbata in South Africa as an example. Acacia dealbata is thought to comprise two subspecies based on morphological characteristics and environmental requirements within its native range in Australia: ssp. dealbata and spp. subalpina. Biological control is the most promising option for managing invasive A. dealbata populations in South Africa, but it remains unknown which genetic/taxonomic entities are present in the country. Resolving this question is crucial for selecting appropriate biological control agents and for identifying areas with the highest invasion risk. We used species distribution models (SDMs) and phylogeographic approaches to address this issue. The ability of subspecies-specific and overall species SDMs to predict occurrences in South Africa was also explored. Furthermore, as non-overlapping bioclimatic niches between the two taxonomic entities may translate into evolutionary distinctiveness, we also tested genetic distances between the entities using DNA sequencing data and network analysis. Both approaches were unable to differentiate the two putative subspecies of A. dealbata. However, the SDM approach revealed a potential niche shift in the non-native range, and DNA sequencing results suggested repeated introductions of different native provenances into South Africa. Our findings provide important information for ongoing biological control attempts and highlight the importance of resolving taxonomic uncertainties in invasion ecology.

Historical range contraction, and not taxonomy, explains the contemporary genetic structure of the Australian tree Acacia dealbata Link

Irrespective of its causes, strong population genetic structure indicates a lack of gene flow. Understanding the processes that underlie such structure, and the spatial patterns it causes, is valuable for conservation efforts such as restoration. On the other hand, when a species is invasive outside its native range, such information can aid management in the non-native range. Here we explored the genetic characteristics of the Australian tree Acacia dealbata in its native range. Two subspecies of A. dealbata have previously been described based on morphology and environmental requirements, but recent phylogeographic data raised questions regarding the validity of this taxonomic subdivision. The species has been widely planted within and outside its native Australian range and is also a highly successful invasive species in many parts of the world. We employed microsatellite markers to investigate the population genetic diversity and structure among 42 A. dealbata populations from across the species’ native range. We also tested whether environmental variables purportedly relevant for the putative separation of subspecies are linked with population genetic differentiation. We found no relationship between population genetic structure of A. dealbata in Australia and these environmental features. Rather, we identified two geographically distinct genetic clusters that corresponded with populations in the northeastern part of mainland Australia, and the southern mainland and Tasmanian range of the species. Our results do not support the taxonomic subdivision of the species into two distinct subspecies based on environmental features. We therefore assume that the observed morphological differences between the putative subspecies are plastic phenotypic responses. This study provides population genetic information that will be useful for the conservation of the species within Australia as well as to better understand the invasion dynamics of A. dealbata.