This book is about plants that have been introduced to new areas, usually new continents. It is about the attempts that have been made to characterize which introduced species will become serious weeds, about their impacts on native communities, and on how introduced weeds might be controlled. Studies of invasive plants have provided a rich literature in applied plant ecology. The invasion of plants into a new environment is an example of succession in action and an experiment on the role of species in communities. In the following chapters we bring together theory and application to focus on both what the study of introduced plants reveals about ecological processes and what ecological processes might be applied to management programs. We consider how community and population ecology can be brought to bear on the topic of invasive plants. Because many introduced plants become invasive and are considered to be weeds, we start here by defining weeds. Next, in this introductory chapter, we describe the socio-economic context surrounding introduced plants and introduce topics to be discussed in more detail in following chapters.

Weeds and the value of native species

Weeds are plants growing out of place or plants whose value has not yet been discovered. They have demanded the time, attention and resources of farmers, gardeners and proud homeowners for centuries. Despite being pulled, sprayed, cursed and competed with, in the long run the weeds always seem to come back.

Why study plant populations?

In this book we have thus far focussed on introduced plant species and their impacts on and relationships to native plant communities. We are interested in the patterns of spread, the stability of established populations, and the impact of control procedures particularly biological control. For all of these considerations environmental and biological heterogeneity are major perturbing factors. Land managers are confronted with serious questions about interactions among populations of plant species, particularly those between native and introduced species. Therefore it becomes imperative to measure, predict and interpret changes in populations.

When plant species move into new communities their dynamics create challenges for population ecologists. That introduced species are able to establish and invade a new plant community indicates either: 1) the presence of empty niches in the native community, 2) that the introduced species itself creates a new niche, or 3) that the introduced species is a superior competitor able to respond to disturbance or utilize resources better than existing species. The introduced plant species will probably also differ from species in the native community by having less herbivore and disease pressure (Chapter 3). During establishment and spread, the introduced species will increase in density and distribution. Following the invasion, the dynamics of the invader may not be very different from those of the native species other than that densities will be higher and therefore intraspecific competition will be strong.

What are life history traits?

Traits that affect the fitness of individuals directly are ‘life history traits’ (Stearns 1992). For plants, these include size and number of seeds, seed dormancy, growth pattern and the size or age at first reproduction. Plant species differ in how and how often they reproduce, how rapidly they grow, and their physical structure. These characteristics can influence the evolution, adaptation, population growth and the population dynamics of species. Some characteristics associated with variation of life history patterns are given in Table 4.1. These same characteristics could contribute to whether an introduced plant species might become established in a new area and then whether it might invade and outcompete the native plants. We are interested here to know if invasive plants have life history characteristics that contribute to their success. We will first consider some of the variation in life history characteristics that might be important.

The growth rate and time to first reproduction of plant species determine how quickly a population can grow. Characteristics that influence plant survival will determine the persistence of a species in a habitat. The growth form and root structure of a species will influence its ability to compete. Characteristics that influence whether plants outbreed, inbreed or reproduce asexually will determine the levels of genetic variation that they maintain, and thus how rapidly they might respond to changing selection and varying environmental conditions. The persistence of seeds in the soil can buffer plants against poor conditions.

In this chapter, we consider the magnitude and scope of the invasive species problem, the language that is used when discussing introduced species, historical patterns of introductions, and the dynamics of colonization and spread. We also examine interactions between introduced species, habitat fragmentation and disturbance, and briefly describe some ecological theory relating to invasive species. We begin with a consideration of the role of invasive plants in the global environmental changes that are currently occurring.

The biota of the earth is dynamic both in the short-term and long-term. As students of evolutionary biology are quick to learn, most species that have ever existed on the earth have become extinct. For example, it is estimated that 96% of all species extant or living 245 million years ago may have become extinct (Raup 1979). Speciation and extinction are fundamental outcomes of evolutionary processes. However, rates of extinction vary over time and there have been periods of ‘mass extinction’, which can be seen in the fossil record as the disappearance of large numbers of species (Benton 1995, Eldredge 1997).

In the last 25 years, beginning with Norman Myers (1976), many biologists and ecologists have adopted and echoed the refrain that we are presently in such a period of accelerated extinction. But, unlike past episodes of mass extinction, this one is directly caused by human activities and has been termed the ‘Biodiversity Crisis’ (see definitions in Box 2.1) (Wilson 1985, Wilson and Peters 1988).

In the first part of this chapter, we consider what makes an ecosystem or community prone to invasion by an introduced species. In the second part we look at the impacts of invasive plant species on communities and ecosystems. These two issues are critical to the interpretation that non-indigenous species are large contributors to the current biodiversity crisis (D'Antonio and Vitousek 1992, Chapin et al. 1998). To begin we briefly review relevant theory on the role of plants in communities in a broader ecological framework. In particular we begin by discussing the invasion of plants into communities as a dimension of the succession process.

Part 1 – Characteristics of native plant communities that influence plant invasions

The invasion of an introduced species into communities is a special case of plant succession. Succession, which was a major focus in early studies of plant ecology, determines the diversity of plant species in communities and this process continues to fascinate ecologists. In the late 1800s, Eugene Warming wrote the first book on plant ecology and in it he recognized that abiotic conditions such as soil type and soil moisture influence plant distributions (Sheail 1987).

Frederick Clements (1916) developed the first theory of plant succession and described the stages of vegetation development (Bradshaw 1993, Miles and Walton 1993). Clements viewed plant community development as a process that begins with bare substrate and progresses through a series of ‘seres’ or stages. Each stage gives way to the next until the ‘climax’ community is reached.

Introduction

We have previously discussed the historical, socio-economic, and ecological aspects of introduced plants. However, land managers and environmental consultants are faced with developing plans and procedures for responding to invasive plants. For them conservation and restoration are the goals. Some researchers would, no doubt, argue that a clear economic cost–benefit analysis of different strategies is essential prior to embarking on a weed control program (e.g. McNeely 2000, Naylor 2000). However, even with such an analysis, success cannot be guaranteed (Rea and Storrs 1999). Science can help to inform management decisions, but ultimately all such decisions about our environment are driven by political imperatives, social concerns and economic constraints. Although the latter factors often dominate, management decisions should still be based on good ecological principles when these are well established and requires, at the very least: (1) sound, defensible science, and (2) education about the issues.

In this chapter we summarize the techniques and programs that have been used to deal with introduced species, and the processes by which habitat managers and researchers may evaluate these programs. We also look at how management strategies have been developed or are developing at international, national, regional, and local scales. In this chapter we broaden our outlook to include considerations relating to non-indigenous species that are not purely ecological. We consider the perceived benefits of introduced plant species, as well as the costs, benefits and outcomes of large-scale and small-scale removal programs for invasive species.

Genetically modified plants: another time bomb?

In Chapter 1 we described what Naylor (2000) calls a time bomb. This is the prolonged period of low density observed for many species before they begin to rapidly increase (Figures 1.2 and 2.8). The situation with genetically modified plants may represent another invasion time bomb. The production and release into uncontrolled environments of genetically modified crop plants (GMOs) is another frontier for invasion ecology. It might be that modification of plants could increase their invasiveness and therefore produce super weeds. More likely is the invasion of genetic material into new continents or into natural ecosystems. The fundamental assumption underlying the technology of genetic modification, is that genes from other organisms, introduced by bacteria to target plant species, will direct the production of (useful) proteins that are not normally synthesized by that plant. Currently little peer-reviewed research exists about the impacts of releasing these essentially non-native organisms into the environment. One reason for this is that it is difficult to obtain modified plants from their industrial creators, prior to their release onto the agricultural market. Once the GMOs have been released, research is more feasible, but it may then be too late to act if problems are revealed.

Many agriculture crops have close relatives among native plants. For example squash, Cucurbita pepo, has been modified through transgenes to be resistant to viral disease. This species occurs outside cultivation, and interbreeds with native subspecies (Parker and Kareiva 1996).

Introduction

Densities of invasive, introduced plants are generally higher in the exotic habitat than in their native habitats. Possible reasons for this have been discussed in Chapter 3 and include a lack of specialist herbivores and new competitive interactions with other plant species in the exotic habitat. If the success of invasive weeds is due to a lack of specialized herbivores and diseases, the introduction of natural enemies from the native habitat should redress the problem. This rationale, however, produces an ethical dilemma. Should more foreign species be introduced to adjust the balance between native and introduced plant species?

A review of biological control in Canada showed that on average five to seven species of natural enemies were introduced for every exotic weed for which biological control was attempted. Of these, only 10% had any impact on host density. This ratio of introduced agents to targets is approximately 2.5 to 1 in other studies (McFadyen 2000). However, some biological control programs have involved a very large number of introductions of natural enemies. For example, over 20 species of natural enemies have been introduced in largely unsuccessful attempts to control Lantana (Broughton 2000) and 50 species of natural enemies were introduced early in the biological control programme against Opuntia cactus in Australia (Mann 1970). The practice of biological control increases the number of introduced species, and in this way has the potential to increase the ratio of exotic to native species.

Introduction

We would like to be able to predict the dynamics of introduced plant species in different situations, how they might respond to biological control, and how they might spread. Several different types of models have been used to integrate information on the populations of introduced species and their control. These models include (1) simulation models based on individual population units that can vary depending on survival and reproduction functions estimated from field studies and may involve stochasticity, (2) analytical models in which functions derived from simulation models or field measurements are used to describe the population processes, and (3) matrix models based on life table studies. In Chapter 5 we described the most basic aspects of population ecology – birth, immigration, death and emigration – and discuss how life tables could be used to summarize data on the transitions among different life stages. Also we described how the rate of growth, R0 or λ, of a population could be determined by relating the population density of one generation to that of the next. In this chapter we explore theoretical models of biological control, the use of models to study populations of introduced plant species, and then models of the spread of introduced species. The strengths and weaknesses of different models will be evaluated. A more extensive treatment of models of weed populations can be found in Cousens and Mortimer (1995).

Introduction

The future of invasion ecology will depend on the use of good quantitative techniques. A common theme throughout this book is that data should be collected to evaluate both the extent of problems and the success of control procedures. We have included in this section a brief overview of three areas that are particularly important in the ecology of invasions: population sampling methods, measuring species diversity, and a general description of global positioning systems GPS and geographic information systems GIS. This is meant only as an introduction and we refer to other sources of more detailed coverage.

Sampling methods

The plant population ecologist has a real advantage over the animal or microbial ecologist in that, to a large degree, the target organisms stand still and can be seen and counted and reproduction and survival estimated. Of course, there are challenges with plants such as purple loosestrife, Lythrum salicaria, for which an individual may produce over a million seeds. Counting exceeds the limits of even the most patient graduate student. Sampling procedures require careful consideration and must be related to the biological question being asked. Books on ecological methods, e.g. Krebs (1999), are a godsend to the applied ecologist and new packages of statistical programs greatly simplify analysis. We will provide only a general overview here of techniques and considerations that influence sampling designs for plant ecology.

First, sampling must be done efficiently and this puts constraints on what can be done.

The concept for this book goes back at least 15 years. In the meantime invasive plant species have become the ‘flavor of the month’ and the literature bursts with interesting new papers. Writing this book has been an exciting undertaking. We have written for a wide audience, and therefore take the chance that it falls between the interests of a variety of readers. Some may find sections to be too anecdotal. Others may find parts to be too technical. As we wrote we could not resist including some of the fascinating stories of the involvement of individuals in spreading plants. It is a scary thought that others may be introducing weeds of the future as we write. We hope that land managers who are charged with controlling invasive weeds and restoring habitats will find this book useful. We admire your efforts in tackling such complex problems. We value the great scientific and management contributions made by our colleagues in biological control, and are sorry we could not include all of the ideas and successes. For students, the experts of the future, we hope that invasion ecology and biological control stimulate your interest. There are many hypotheses to be tested and problems to be solved at this interface between basic and applied ecology. To all, we would be happy to get your feedback.

Many people helped in this project and two in particular deserve enormous thanks. First, Jamie Smith used up several red pens worth of ink editing the manuscript.

Originally entitled The naming of plants and the meanings of plant names, this book is in two parts. The first part has been written as an account of the way in which the naming of plants has changed with time and why the changes were necessary. It has not been the writer's intention to dwell upon the more fascinating aspects of common names but rather to progress from these to the situation which exists today; in which the botanical and horticultural names of plants must conform to internationally agreed standards. The aim has been to produce an interesting text which is equally as acceptable to the amateur gardener as to the botanist. The temptation to make this a definitive guide to the International Code of Botanical Nomenclature was resisted since others have done this already and with great clarity. A brief comment on synonymous and illegitimate botanical names and a reference to recent attempts to accommodate the various traits and interests in the naming of cultivated plants was added after the first edition.

The book had its origins in a collection of Latin plant names, and their meanings in English, which continued to grow by the year but which could never be complete. Not all plant names have meaningful translations. Some of the botanical literature gives full citation of plant names (and translations of the names, as well as common names). There are, however, many horticultural and botanical publications in which plant names are used in a casual manner, or are misspelled, or are given meanings or common names that are neither translations nor common (in the world-wide sense).

There can be no doubt that the diverse approaches to naming garden plants, by common names, by botanical names, by mixtures of botanical and common names, by group names and by fancy names, is no less complex than the former unregulated use of common or vernacular names. The psychology of advertising takes descriptive naming into yet new dimensions. It catches the eye with bargain offers of colourful, vigorous and hardy, large-headed, incurved Chrysanthemum cvs. by referring to them as HARDY FOOTBALL MUMS. However, we are not here concerned with such colloquial names or the ethics of mail-order selling techniques but with the regulation of meaningful names under the Code.

In 1952, the Committee for the Nomenclature of Cultivated Plants of the International Botanical Congress and the International Horticultural Congress in London adopted the International Code of Nomenclature for Cultivated Plants. Sometimes known as the Cultivated Code, it was first published in 1953 and has been revised several times at irregular intervals since then (Trehane, 1995). This Code formally introduced the term ‘cultivar’ to encompass all varieties or derivatives of wild plants which are raised under cultivation and its aim is to ‘promote uniformity and fixity in the naming of agricultural, sylvicultural and horticultural cultivars (varieties)’. The term culton (plural culta) is also mooted as an equivalent of the botanical term taxon.

The Cultivated Code governs the names of all plants which retain their distinctive characters, or combination of distinctive characters, when reproduced sexually (by seed), or vegetatively in cultivation.

Since making the assumption, in the second edition, that genetic manipulation of the properties of plants might require new consideration of the ways in which they are to be named, GM has proceeded apace. Not only can the innate genetic material be re-ordered – in ways that nature would have rejected through their exposure to natural selection by the environment – but alien genetic material, from other organisms, can be introduced to give bizarre results. Arabidopsis thaliana has only 10 chromosomes and has been the plant of choice for cytologists and nucleic acid workers because of this. The twenty-first century sees its genetic code mapped and its 25, 000 genes being examined individually to ascertain the ‘meaning of plant life’. From quite practical beginnings such as giving tomato fruits an extended keeping time, to esoteric developments such as building a luminescence gene from a jellyfish into a mouse, there is now a proposal to insert a gene from an electric eel into plants so that the plants can provide sources of electricity. This new ‘green revolution’ has an historical ring of familiarity about it!

The new century has not yet brought universal consistency in accepting the botanical and the horticultural codes. Yet science is already seeking to move towards an international biodiversity code for the naming of everything.