Research

Woody plants were not widely considered to be important invasive alien species until fairly recently. Thousands of species of trees and shrubs have, however, been moved around the world. Many species have spread from planting sites, and some are now among the most widespread and damaging of invasive organisms [read a recent global review and the 2013 update of the database].

Pines (genus Pinus) and other conifers have featured prominently in my research on the dynamics of plant invasions. Because of the large number of species in the genus Pinus (about 111), the wide range of ecological adaptations in different species, and because many species have been so widely planted in many habitats outside their natural range, studies on pine invasions have been useful for shedding light on many aspects of plant invasion ecology. Recent research in my lab on pines as invasive species has focussed on understanding macroecological patterns, and providing guidelines for strategic management of pine invasions in the southern hemisphere.

Australian acacias or “wattles” (1012 recognized species native to Australia, previously grouped in Acacia subgenus Phyllodineae) are currently a strong focus of my research. Many species have been moved around the world by humans over the past 250 years. This has created the opportunity to explore how evolutionary, ecological, historical and sociological factors interact to affect the distribution, usage, invasiveness and perceptions of a globally important group of plants. Recent research on Australian acacias by my group has dealt with:

• Human perceptions of introduced species in different parts of the world;
• The evolution and phylogenetic placement of invasive species in the group;
• Phylogeographic consequences of different introduction histories;
• Exploring links between features of native range dimensions and invasiveness;
• A detailed study of the reproductive biology of the group;
• Seeking links between invasiveness and: species traits and genome size; and native climatic affinities, life history traits and human use;
• Examining biophysical impacts of invasive acacias and the development of a framework for improved restoration practices;
• Evaluation of global efforts to manage invasions of Australian acacias to develop guidelines for best-practice interventions;
• Detailed studies on selected emerging invaders to assess the feasibility of eradication and the development of management plans;
• The formulation of a national strategy for managing invasive Australian acacias in South Africa.

Other work on particular invasive tree species or genera has dealt with the invasion ecology of Brazilian peppertree (Schinus molle) and the ecology of Eucalyptus trees as introduced species around the world. My group is currently also working on aspects of Casuarina, Lantana camara and Prosopis invasions in South Africa.

South Africa’s fynbos biome has been severely invaded by non-native tree species. The management of invasive trees is one of the most taxing challenges facing conservation managers in this region. Substantial research effort has been and will continue to be devoted to exploring the complexity inherent in managing invasive woody plants in the fynbos. This work involves a range of modelling approaches, scenario planning, and other methods aimed at providing guidelines for more effective management.

Seed dispersal and pollination by animal agents and relationships between plants and micro organisms in the soil often determine whether a species will invade and/or the extent to which it will spread in a new environment. Many non-native plant species become invasive by infiltrating pollination or seed dispersal networks. Biological invasions also have a profound effect on naturally occurring mutualisms. Some such impacts are widely known, but the mechanisms that produce such impacts are poorly understood. Many types of invasive species have huge potential to alter evolutionary trajectories by causing many types of changes to prevailing mutualisms. This introduces a new layer of complexity to programmes aiming to reduce or mitigate the effects of biological invasions.

Riparian zones - the fringes of rivers or streams - throughout the world have been heavily affected by invasive alien plants. Sometimes invasions are a symptom of degradation caused by other factors, but plant invasions are very often the primary driver of degradation of riparian zones. This is the case in many parts of South Africa. Much recent work in my lab has dealt with the dynamics of riparian ecosystems in South Africa, in particular:

• Factors affecting the susceptibility of these ecosystems to invasion by alien plant species;
• Elements of resilience that determine how invasions impact on the structure and functioning of such systems;
• The capacity of riparian communities to recover following management intervention and the options for restoration.

The main aim of my work in this field is to contribute to frameworks for improved management of invaded riparian ecosystems.

The Eerste River which flows through Stellenbosch is the primary study site for work on this topic. A permanent transect along the entire length of the river (40km) was established for long-term research.

Invasion ecology is the study of the human-mediated introductions of organisms, especially to areas well outside their potential range as defined by their natural dispersal mechanisms and biogeographic barriers. The field covers all aspects relating to the introduction of organisms, their capacity to naturalize and invade in the target region, their interactions with resident biota and, increasingly, the consideration of costs and benefits of their presence and abundance with reference to human value systems. Interest in biological invasions has grown exponentially in the past two decades. I have a special interest in contributing to the development of clear and objective concepts in invasion ecology, particularly relating to plant invasions. Contributions in this area in recent years have included the following:

• Development of a lexicon and associated concepts for categorizing introduced plants; this was recently expanded to include concepts for all taxa and types of ecosystems;
• Conceptualization of the role of long-distance dispersal in biodiversity conservation and biological invasions;
• What’s hot in invasion ecology — an evaluation of insights from the most-highly cited papers in the field;
• Contributing to the conceptual unification of plant invasion ecology through the merging the concepts of species invasiveness and community invasibility;
• Assessing progress in invasion ecology since the publication of Charles Elton’s 1958 book The ecology of invasions by animals and plants: a 50-year-on review of Elton’s book; a detailed assessment of the book’s influence on the field; an edited book with contributions from >50 prominent researchers on progress in the field since Elton;
• Identifying geographical and taxonomical biases in invasion ecology;
• Conceptualization of global dispersal pathways and their role in invasion success; A proposed unified framework for biological invasions.

The increasing human domination of the world is forcing us to revisit key tenets of paradigms relating to conservation of biodiversity. Biological invasions are interacting with numerous other human-mediated drivers to bring about radical changes to ecosystems worldwide. Such changes provide numerous challenges for conservationists and resource managers.

I am very interested in contributing to the development of novel approaches for sustainable ecosystem management in our rapidly changing world. To this end, I have been involved in the following initiatives:

• Defining concepts and evaluating management implications of the notion of “novel ecosystems” — those that contain new combinations of species that arise through human action, environmental change, and the impacts of introduced organisms;
• Developing pragmatic and sustainable protocols for restoration of ecosystems degraded by invasive species. My recent focus has been on fynbos and riparian ecosystems in South Africa;
• Developing an objective approach to identify situations when the deliberate movement of species to new localities can be justified to enhance their chances of survival (“Managed relocation” or “assisted migration”);
• Identifying emerging pathways for biological invasions to allow for proactive intervention to reduce future impacts. The use of non-native plant species for biofuels has the potential to produce a new wave of invasions. Work in this area has involved assessing the potential impacts of South Africa’s Biofuels Industrial Strategy, and drawing lessons from the experience of commercial forestry to reduce risks of invasions from new plantings.
• Contributing to the formulation of a robust research agenda for conservation biogeography to include the consideration of biological invasions and many other factors.