Phylogeography and Molecular Ecology of Selected Invasive Species in the Hawaiian Islands

Most alien (non-native) species which become naturalized are not disruptive to natural ecosystems. However, the small fraction that do spread and become invasive can have severe environmental and economic impacts. These impacts are often irreversible as control efforts normally only start after species are widespread. Molecular ecology renders a new approach to better understand invasions and can help in their management by resolving taxonomic issues, elucidating geographical source(s), detecting hybridization and introgression, and tracking dispersal and spread. Using a molecular ecological approach, some of these phenomena were investigated in this dissertation research for three different plant invaders in Hawaii. Molecular markers were developed for Pennisetum setaceum (fountaingrass), Miconia calvescens (velvet tree) and Senecio madagascariensis (fireweed) to answer ecological and management related questions. Molecular and quantitative genetic variation indicated that fountaingrass is monoclonal throughout Hawaii and furthermore, that this clone or “super-genotype” is shared globally among invasive and native ranges. This indicates phenotypic plasticity as the sole mechanism behind fountaingrass’ invasive success. Fountaingrass is unlikely to evolve resistance against successful control mechanisms. Subsequent herbicide trails indicated that grass-selective herbicides are ineffective against fountaingrass. Phylogenetic and population genetic structure showed that fireweed introduced to Hawaii originated from eastern South Africa. Effective and host-specific biological control agents against fireweed are most likely to be found in this region. The high genetic diversity found in fireweed is indicative of multiple introductions. Genetic spatial autocorrelation revealed a diffusive dispersal pattern in the Hawaiian Islands. Genetic structure indicated that bottlenecked populations of velvet tree are highly inbred in Hawaii and southern Pacific Islands. The data further suggested that invasive populations throughout northern and southern Pacific islands are genetically similar despite differential invasive success. These results indicate that invasions in both hemispheres are potentially fi-om similar geographic origin and/or that Hawaiian infestations are the result of a secondary introduction directly from Tahiti. The introduction of genotypes pre-adapted for various morphological, physiological and life history traits facilitate invasion success of velvet tree. Climatological similarities between the Society and the Hawaiian Islands indicate that Hawaiian infestations of velvet tree have not yet reached an optimum. Biological control would be the only effective control method against velvet tree with most productive control agents likely to be found in Mexico

The ecology, biogeography, history and future of two globally important weeds : Cardiospermum halicacabum Linn. and C. grandiflorum Sw.

Members of the balloon vine genus, Cardiospermum, have been extensively moved around the globe as medicinal and horticultural species, two of which are now widespread invasive species; C. grandiflorum and C. halicacabum. A third species, C. corindum, may also have significant invasion potential. However, in some regions the native status of these species is not clear, hampering management. For example, in South Africa it is unknown whether C. halicacabum and C. corindum are native, and this is a major constraint to on-going biological control programmes against invasive C. grandiflorum. We review the geography, biology and ecology of selected members of the genus with an emphasis on the two most widespread invaders, C. halicacabum and C. grandiflorum. Specifically, we use molecular data to reconstruct a phylogeny of the group in order to shed light on the native ranges of C. halicacabum and C. corindum in southern Africa. Phylogenetic analyses indicate that southern African accessions of these species are closely related to South American taxa indicating human-mediated introduction and/or natural long distance dispersal. Then, on a global scale we use species distribution modelling to predict potential suitable climate regions where these species are currently absent. Native range data were used to test the accuracy with which bioclimatic modelling can identify the known invasive ranges of these species. Results show that Cardiospermum species have potential to spread further in already invaded or introduced regions in Australia, Africa and Asia, underlining the importance of resolving taxonomic uncertainties for future management efforts. Bioclimatic modelling predicts Australia to have highly favourable environmental conditions for C. corindum and therefore vigilance against this species should be high. Species distribution modelling showed that native range data over fit predicted suitable ranges, and that factors other than climate influence establishment potential. This review opens the door to better understand the global biogeography of the genus Cardiospermum, with direct implications for management, while also highlighting gaps in current research

The status of alien bamboos in South Africa

CITATION: Canavan, S. et al. 2021. The status of alien bamboos in South Africa. South African Journal of Botany, 138:33-40. doi:10.1016/j.sajb.2020.11.027.The original publication is available at https://www.sciencedirect.com/journal/south-african-journal-of-botanyThe growing interest in commercial cultivation of bamboos (Poaceae subfamily Bambusoideae) has led to the introduction of new alien species into South Africa. The rate at which bamboos are being planted in South Africa is a cause for concern because of the impacts of bamboo invasions in other parts of the world. To understand the risks associated with new introductions and new plantings, we assess the outcomes of past introductions of bamboos into South Africa. To this end we: (1) produce an inventory of alien bamboo taxa; (2) assess the distribution of bamboos; (3) determine the rate of spread of bamboo at a site with a high density of naturalised stands; and (4) evaluate the current regulatory status of alien bamboos in South Africa. We used a combination of expert opinion, literature, historical records of populations, and public participation to produce a species list and locate populations of alien bamboos. We also attempted to confirm species identities using DNA barcoding. We found that 28 currently-accepted species of bamboo have been recorded in South Africa. However, we have little confidence in this estimate, as 20 of the species could not be confirmed or identified as present in the country. Bamboos are an inherently challenging group to identify using vegetative material, and DNA barcoding was inconclusive. The distribution of bamboos across the country varied with the type or lineage (e.g. herbaceous, tropical or temperate) and the source of information (e.g. herbarium records, in-field observation or public contribution). Although alien bamboos are naturalised at several sites, we found no large invasive stands nor evidence of widespread negative environmental impacts. Nonetheless, we recommend caution regarding future introductions of bamboos for commercial cultivation, as the nature of the plantings will likely differ from the historical situation in both the location, configuration, and the scale of cultivation, and as new species are likely to be introduced. We propose several changes to the current listing of bamboo taxa in national legislation pertaining to alien and invasive species.https://www.sciencedirect.com/science/article/pii/S0254629920311868?via%3DihubPublisher’s versio

Ecological disequilibrium drives insect pest and pathogen accumulation in non-native trees

CITATION: Crous, C. J., et al. 2017. Ecological disequilibrium drives insect pest and pathogen accumulation in non-native trees. AoB PLANTS, 9(1):1-16, doi:10.1093/aobpla/plw081.The original publication is available at https://academic.oup.com/aobplaNon-native trees have become dominant components of many landscapes, including urban ecosystems, commercial forestry plantations, fruit orchards and as invasives in natural ecosystems. Often, these trees have been separated from their natural enemies (i.e. insects and pathogens) leading to ecological disequilibrium, that is, the immediate breakdown of historically co-evolved interactions once introduced into novel environments. Long-established, non-native tree plantations provide useful experiments to explore the dimensions of such ecological disequilibria. We quantify the status quo of non-native insect pests and pathogens catching up with their tree hosts (planted Acacia, Eucalyptus and Pinus species) in South Africa, and examine which native South African enemy species utilize these trees as hosts. Interestingly, pines, with no confamilial relatives in South Africa and the longest residence time (almost two centuries), have acquired only one highly polyphagous native pathogen. This is in contrast to acacias and eucalypts, both with many native and confamilial relatives in South Africa that have acquired more native pathogens. These patterns support the known role of phylogenetic relatedness of non-native and native floras in influencing the likelihood of pathogen shifts between them. This relationship, however, does not seem to hold for native insects. Native insects appear far more likely to expand their feeding habits onto non-native tree hosts than are native pathogens, although they are generally less damaging. The ecological disequilibrium conditions of non-native trees are deeply rooted in the eco-evolutionary experience of the host plant, co-evolved natural enemies and native organisms from the introduced range. We should expect considerable spatial and temporal variation in ecological disequilibrium conditions among non-native taxa, which can be significantly influenced by biosecurity and management practices.https://academic.oup.com/aobpla/article/9/1/plw081/2737455Publisher's versio

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

CITATION: Hirsch, H. et al. 2017. Unresolved native range taxonomy complicates inferences in invasion ecology : acacia dealbata Link as an example. Biological Invasions, 19(6):1715-1722. doi:10.1007/s10530-017-1381-9The original publication is available at https://www.springer.com/journal/10530Elaborate 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.Publisher’s versio

The more the better? the role of polyploidy in facilitating plant invasions

CITATION: Te Beest, M., et al. 2019. The more the better? the role of polyploidy in facilitating plant invasions. Annals of Botany, 109(1):19-45, doi:10.1093/aob/mcr277.The original publication is available at https://academic.oup.com/aobBackground: Biological invasions are a major ecological and socio-economic problem in many parts of the world. Despite an explosion of research in recent decades, much remains to be understood about why some species become invasive whereas others do not. Recently, polyploidy (whole genome duplication) has been proposed as an important determinant of invasiveness in plants. Genome duplication has played a major role in plant evolution and can drastically alter a plant's genetic make-up, morphology, physiology and ecology within only one or a few generations. This may allow some polyploids to succeed in strongly fluctuating environments and/or effectively colonize new habitats and, thus, increase their potential to be invasive. Scope: We synthesize current knowledge on the importance of polyploidy for the invasion (i.e. spread) of introduced plants. We first aim to elucidate general mechanisms that are involved in the success of polyploid plants and translate this to that of plant invaders. Secondly, we provide an overview of ploidal levels in selected invasive alien plants and explain how ploidy might have contributed to their success. Conclusions: Polyploidy can be an important factor in species invasion success through a combination of (1) ‘pre-adaptation’, whereby polyploid lineages are predisposed to conditions in the new range and, therefore, have higher survival rates and fitness in the earliest establishment phase; and (2) the possibility for subsequent adaptation due to a larger genetic diversity that may assist the ‘evolution of invasiveness’. Alternatively, polyploidization may play an important role by (3) restoring sexual reproduction following hybridization or, conversely, (4) asexual reproduction in the absence of suitable mates. We, therefore, encourage invasion biologists to incorporate assessments of ploidy in their studies of invasive alien species.https://academic.oup.com/aob/article/109/1/19/154024Publisher's versio

Evolutionary dynamics of tree invasions: complementing the unified framework for biological invasions

Evolutionary processes greatly impact the outcomes of biological invasions. An extensive body of research suggests that invasive populations often undergo phenotypic and ecological divergence from their native sources. Evolution also operates at different and distinct stages during the invasion process. Thus, it is important to incorporate evolutionary change into frameworks of biological invasions because it allows us to conceptualize how these processes may facilitate or hinder invasion success. Here, we review such processes, with an emphasis on tree invasions, and place them in the context of the unified framework for biological invasions. The processes and mechanisms described are pre-introduction evolutionary history, sampling effect, founder effect, genotype-by-environment interactions, admixture, hybridization, polyploidization, rapid evolution, epigenetics and second-genomes. For the last, we propose that co-evolved symbionts, both beneficial and harmful, which are closely physiologically associated with invasive species, contain critical genetic traits that affect the evolutionary dynamics of biological invasions. By understanding the mechanisms underlying invasion success, researchers will be better equipped to predict, understand and manage biological invasions

Explaining the variation in impacts of non-native plants on local-scale species richness: the role of phylogenetic relatedness

ABSTRACT Aim To assess how the magnitude of impacts of non-native plants on species richness of resident plants and animals varies in relation to the traits and phylogenetic position of the non-native as well as characteristics of the invaded site. Location Global. Methods Meta-analysis and phylogenetic regressions based on 216 studies were used to examine the effects of 96 non-native plant species on species richness of resident plants and animals while considering differences in non-native species traits (life-form, clonality or vegetative reproduction, and nitrogen-fixing ability) and characteristics of the invaded site (ecosystem type, insularity and climatic region). Results Plots with non-native plants had lower resident plant (-20.5%) and animal species richness (-26.4%) than paired uninvaded control plots. Nitrogenfixing ability, followed by phylogeny and clonality were the best predictors of the magnitude of impacts of non-native plants on native plant species richness. Nonnitrogen-fixing and clonal non-native plants reduced species richness more than nitrogen-fixing and non-clonal invaders. However, life-form and characteristics of the invaded sites did not appear to be important. In the case of resident animal species richness, only the phylogenetic position of the non-native and whether invaded sites were islands or not influenced impacts, with a more pronounced decrease found on islands than mainlands. Main conclusions The presence of a phylogenetic signal on the magnitude of the impacts of non-native plants on resident plant and animal richness indicates that closely related non-native plants tend to have similar impacts. This suggests that the magnitude of the impact might depend on shared plant traits not explored in our study. Our results therefore support the need to include the phylogenetic similarity of non-native plants to known invaders in risk assessment analysis

Explaining the variation in impacts of non-native plants on local-scale species richness: the role of phylogenetic relatedness

ABSTRACT Aim To assess how the magnitude of impacts of non-native plants on species richness of resident plants and animals varies in relation to the traits and phylogenetic position of the non-native as well as characteristics of the invaded site. Location Global. Methods Meta-analysis and phylogenetic regressions based on 216 studies were used to examine the effects of 96 non-native plant species on species richness of resident plants and animals while considering differences in non-native species traits (life-form, clonality or vegetative reproduction, and nitrogen-fixing ability) and characteristics of the invaded site (ecosystem type, insularity and climatic region). Results Plots with non-native plants had lower resident plant (-20.5%) and animal species richness (-26.4%) than paired uninvaded control plots. Nitrogenfixing ability, followed by phylogeny and clonality were the best predictors of the magnitude of impacts of non-native plants on native plant species richness. Nonnitrogen-fixing and clonal non-native plants reduced species richness more than nitrogen-fixing and non-clonal invaders. However, life-form and characteristics of the invaded sites did not appear to be important. In the case of resident animal species richness, only the phylogenetic position of the non-native and whether invaded sites were islands or not influenced impacts, with a more pronounced decrease found on islands than mainlands. Main conclusions The presence of a phylogenetic signal on the magnitude of the impacts of non-native plants on resident plant and animal richness indicates that closely related non-native plants tend to have similar impacts. This suggests that the magnitude of the impact might depend on shared plant traits not explored in our study. Our results therefore support the need to include the phylogenetic similarity of non-native plants to known invaders in risk assessment analysis

Super-Genotype: Global Monoclonality Defies the Odds of Nature

The ability to respond to natural selection under novel conditions is critical for the establishment and persistence of introduced alien species and their ability to become invasive. Here we correlated neutral and quantitative genetic diversity of the weed Pennisetum setaceum Forsk. Chiov. (Poaceae) with differing global (North American and African) patterns of invasiveness and compared this diversity to native range populations. Numerous molecular markers indicate complete monoclonality within and among all of these areas (FST = 0.0) and is supported by extreme low quantitative trait variance (QST = 0.00065–0.00952). The results support the general-purpose-genotype hypothesis that can tolerate all environmental variation. However, a single global genotype and widespread invasiveness under numerous environmental conditions suggests a super-genotype. The super-genotype described here likely evolved high levels of plasticity in response to fluctuating environmental conditions during the Early to Mid Holocene. During the Late Holocene, when environmental conditions were predominantly constant but extremely inclement, strong selection resulted in only a few surviving genotypes