Seeing Weeds with ‘New Eyes’ — 2

Are weeds Alien?

Edward Salisbury, a Professor of Botany at University College, London, popularized the use of the term ‘alien’ in his book on “Weeds & Aliens”, published in 1961. He was also the Director of Kew Gardens in London during 1943–56 and someone who had considerable interest in weeds. The term, of course, had been used much earlier by renowned botanists in the mid-19th Century who dealt with extensive collections of plants sent to the Kew Herbarium from various parts of the British Empire.

The word alien (from Latin, “alienus”) means belonging to another, not one’s own, strange, or foreign. The term first appears among annotations and notes on the side panels of old herbarium specimens of some species that the 19th Century botanists were examining.

When botanists applied the term ‘alien’ to describe a group of plants, way back in the mid-19th Century, their intention was not to deride weeds. One of the earliest users was Hewett Cottrell Watson (1804–1881), an eminent British botanist and phyto-geographer, who defined ‘alien plants’ as those: ‘…now more or less established but either presumed or certainly known to have been originally introduced from other countries…”.

Watson’s objective was only to categorize plants found in Britain, those days, according to their known distributions from Europe and elsewhere. His book — Cybele Britannica (Watson, 1847) stands as the first, earnest attempt to put British geographic and distributional botany on an exact scientific basis. It was also the first publication to replace vague generalizations with concrete facts, and analyze its character and content of the British flora. Watson intended absolutely no disrespect for introduced plant species.

Fifty years after Hewett Watson, one of the earliest Kew botanists who used the term ‘alien’ was Stephen Troyte Dunn (1868–1938). In introducing his book — The Alien Flora of Britain (Dunn, 1905), Dunn’s words were: “…The term alien is used to designate any species which, though now spontaneous, originated in Britain through human agency…”.

The early writers and botanists recognized the role of humans in moving plants across biogeographical regions but also appreciated that natural agencies (such as cyclones, floods, and high winds) also cause plants to spread between and across continents. Those days, as the human population grew and interactions across continents increased through trade, conquests, and colonization, many plant species spread widely. It was important for botanists to understand the factors that caused the changes in the biogeographical distribution of species, the agencies (both human and natural) and causes of spread and the habitat preferred by the species, which successfully established themselves in the new environments.

The early writings, particularly of Hewett Watson and Stephen Dunn, indicate a great deal of caution in categorizing plant species in this way, as it was difficult to assign any species as a ‘native’ or ‘introduced alien’ plant without historical knowledge. Of course, those botanists knew they were studying common species and not aliens from another planet. Their purpose was not to slander plant species, but to draw attention of other botanists on the risks of introducing plants across the continents, particularly with the exchanges of live specimens among botanic gardens. Likely, they were also aware of spreading plant species along with movements of livestock, fodder, people, and military equipment, at that time. It is most likely that Salisbury followed this practice and used the term ‘alien’ interchangeably with the term ‘introduced’. Some authors use the term to refer to plants becoming weedy when transferred from their native to an alien environment, meaning a new environment. Here, while the emphasis is on the new environment, the organism is also regrettably branded as an alien foreigner.

This term ‘alien’ is now often directly attributed to Edward Salisbury’s book as if it is original. Inadvertently, he has indeed, given those who dislike weeds and want 100% control of colonizing species the perfect weapon! Taking the cue from him, other senior botanists also used the term, as Hiram Wild, a renowned botanist from South Africa did in discussing Weeds and Aliens in Africa and their origin, as potentially ‘American Immigrants’ (Wild, 1967). Peter Michael (1981) and Peter Kloot (1983), Australian botanists, also borrowed the term for discussing naturalized plants that had been introduced to Australia from overseas. The term ‘alien’ was advisedly used by these well-known taxonomists, and Peter Michael, in particular, clarified that he was referring to Hewett Watson’s categorization. I might still offer the view that the term ‘alien’ was, perhaps, superfluous in these historical publications for their key botanical messages because, a decade or two later, the term began to be mis-used to deride introduced species.

‘Invasive Aliens’ — a misleading narrative

The concept of ‘invasive species’ was first raised by the British Ecologist Charles Elton (Elton, 1958). His landmark treatise prophetically suggested that some animal and plant species may spread widely across continents, and potentially “invade” (he really meant, ‘colonize’) other bio-geographical regions, which are non-native to the original populations.

This term ‘invasive’ only became common in Weed Science in the late-1980s and it was primarily in the USA (Davis, 2011). I can safely vouch that in the early-1980s, in the UK, it was sufficient to refer to the plants with colonizing abilities just as ‘weeds’, until the narrative changed. The proceedings of two of the most influential milestone events in the evolution of Weed Science as a discipline, put more emphasis on understanding the global spread of weeds and other animals as part of ecological phenomena of plant succession, adaptations and colonization. These books rarely mention ‘invasions’ in the sense that the term is used nowadays (see the edited books — Harper, 1960 and Baker and Stebbins, 1965).

Following everything American as good is a well-known populist trend, partly due to America’s overwhelming economic success and its flow-on effects on the rest of the world. It is undeniable that other countries try to emulate the economic success of the USA and, at the same time, follow American trends without too much thought on their potential socio-cultural effects impacts. Samuel Huntington (1996, p. 310) questioned the potential negative impacts of following everything American, as below:

“…Awareness of cultural diversity will lead to understanding and perhaps to challenging the Western, particularly American, belief in the universal relevance of Western culture. This belief holds that all societies want to adopt Western values, institutions, and practices. If they seem not to have the desire and are committed to their own traditional cultures, they are, in the view of many, victims of a false consciousness.

Normatively, the Western belief posits that people throughout the world should embrace Western values and culture because they embody the highest, most enlightened, liberal, rational, modern, and civilized thinking of humankind. The Western belief in the universality of Western culture suffers three problems: it is false, it is immoral, and it is dangerous to agricultural progress…”

Some colonizing plants and animals are now permanently branded as ‘invasive species’ because they are capable of successful colonization of new environments. Absurd parallels are drawn with military invasions. In the 1990s, another adjective was added to brand the successful colonizers as ‘Invasive Alien Species’ (IAS). The combination of the two terms has been a real game-changer, the second adjective adding a potent but distasteful dimension to an already highly-charged term. With this acronym, there are significant amounts of funds doled out to various bodies to manage the alien invader armies, which are rapidly moving across the globe, threatening our existence. Exaggeration is a true reflection of the times we live in, to which this narrative fits well. Nowadays, most issues are prosecuted with hyperbole, instead of thoughtful reflections on the effects emotive words would have on the public.

The term IAS spread fast in English-speaking, ‘Westernized’ countries, including New Zealand, Australia, the USA and Europe. Regrettably, it is also commonly used in the largely non-English-speaking Asian-Pacific countries, which chose to follow the ‘trend’ rather than question its scientific basis. The flippant way in which the term is thrown around at weed conferences and also by the media indicates that now we really have a problem on our hands!

Even words and concepts evolve with time. Perhaps, an improved understanding of how some highly successful weeds and animal species can spread rapidly across the globe, crossing borders with or without assistance from humans, may have led some genuine researchers to call them ‘invasives’, invoking Charles Elton’s thesis. But much more likely, it is an artifact of the fierce competition for limited funds, globally, for research. To get a piece of this funding, the narrative must change to fit the prevalent thinking of the time, or a new narrative must be devised, and overstatement helps! Nonetheless, some credit must go to the proponents for placing the human agency at the centre of the argument. The IAS narrative (Convention of Biological Diversity, CBD, 2001) recognizes that disturbed habitats, colonized by these ‘alien’ invaders have often been wholly or partially created by man, whose activities are also largely responsible for their global spread.

Who are these alien intruders? Why do we have to use such dramatic words, which have potency to create fear and apprehension? How unfortunate is it that these terms have not been challenged enough by weed scientists? Is it because we fear of retribution and castigation by our scientific peers? Imagine the confusion on the minds of undergraduate biology students if the Ecology teacher does not correctly explain how these terms came about? I know of many weed scientists who are awestruck by these terms, and just go with the flow. Presently, I can only direct them not to be captivated by these powerful words but get more acquainted with the evolution of the terms (see discussions in Colautti and MacIssac, 2004; Shackleton, et al., 2019), the context of their use, and more broadly, on the history of Weed Science, well covered elsewhere (Timmons, 1970; Wyse, 1992; Hamill et al., 2004).

It is quite clear that ‘invasion ecology’ has enjoyed a rapid ascension in the public domain, owing in part to the extensive use of powerful adjectives like ‘invasive’, ‘alien’, ‘noxious’ and ‘exotic’ (Colautti and MacIssac, 2004). A species is considered ‘native’ if it has existed in a given biogeographical area for an extended period of time, and/or if it has undergone significant evolutionary changes in this area, over a long period of time. ‘Exotic’, ‘non-native’ and non-indigenous species (NIS) are simply the opposites of ‘native’. However, it is not easy to determine which plant species is ‘native’ to a region, or ‘naturalized’, and to differentiate native from non-native species.

The confusions and loose terminology lead to the unscientific branding of potentially useful taxa as some sort of villains. Besides, not everyone is convinced that the maligned ‘invasive’ plant species are harmful to the environment all the time (see discussions on Davis and Thomson, 2000; 2001). Many of the so-called ‘invasive’ species are highly beneficial to not just humans and animals, but also to the environment, under certain situations.

Mark Sagoff, an environmental philosopher, challenged the idea that ‘non-native’, ‘exotic’, or introduced species cause widescale ecological harm in the new environments to which they have been either deliberately or accidentally introduced. He also decried the use of pejorative terms in this discourse, which go against scientific norms, as follows:

“…Are non-native species harmful? That depends on your perspective. That non-native species harm the natural environment is a dictum so often repeated that one may assume it rests on evidence. It does not. Biologists often use pejorative terms such as “pollute,” “meltdown,” “harm,” “destroy,” “disrupt,” and “degrade” when speaking about non-native species. These words, along with metaphors borrowed from war and from cancer pack political punch.

“…Insofar as they convey aesthetic, moral, or spiritual judgments, they have a place in political debates and policy discussions. What troubles me as a philosopher is that these value-laden terms and their underlying concepts pervade the scientific literature of conservation biology and invasion ecology. These concepts are not well defined; generalizations based on them are not tested. Indeed, if you try to prove that invasive species harm natural environments, you’ll find your-self in a scientific maze of dead ends and circular logic…” Mark Sagoff (2005)

My view is that the term ‘invasive’ has been used within the ‘invasion’ biology theme as a descriptor of a specific capacity that an organism has (i.e., capability to colonize and establish), rather than to describe an ecological phenomenon. Objecting the overreach of the ‘invasion’ biology theme, Mark Davis (Davis, 2011), also strongly expressed his view, with which I agree:

“…Focused and persistent research will always be able to document some adverse effects of any species, native or non-native, on at least some other species. However, even if negative effects on other species are documented, ecologists should not feel empowered to declare a species to be “invasive” (harmful). Declaring harm is a value-based social decision, one that needs to be made through collaboration with the larger citizenry. This is not a scientific decision, even if scientists are making it…”

“…But for 30 years, it has been primarily invasion biologists, not their critics, who have been telling just half the story. Only recently has a more balanced perspective begun to emerge, a perspective the public needs to hear, since it is usually the public’s resources that are used to manage these species…”

Regrettably, there is still much confusion about the terminology in the IAS narrative. Despite objections, the provocative metaphors are still widely used in the discussions on weeds, misleading the public. The dominant discourse may also confuse young weed scientists. Therefore, it is time for Weed Science, as a mature discipline, to make a change in the use of the term ‘invasion’ to the more ecologically correct term ‘colonization’, which is a component of plant succession. Revisiting the attributes of successful colonizers (Baker, 1965) would make people understand weeds better. Attention should then focus on the processes by which weedy taxa ‘colonize’ new habitat. If one understood the factors that determine the outcome — success or failure of those colonization attempts — that would undoubtedly be helpful in how we may respond to an undesirable colonization event.

Can we change attitudes?

The hardened attitude towards colonizing plants (weeds) in many countries is due to the profits that can be made by landholders through farming. Despite agricultural production representing only a declining percentage of gross domestic product (GDP) in most countries, farmers, particularly in the developed countries form powerful political constituencies and lobby groups. Many growers and farmers who are wary of weeds have deeply entrenched opinions. They often mistrust alternatives and resist change because of personal experiences and biases, as well as property-related and economic factors. Pure and simple, it is a question of money.

Shifting the emphasis of weeds from ‘foe’ to friend’ requires vigorous campaigning by enlightened scientists, working within or outside governments. Presently, this view is championed mainly by popular websites and patrons of sustainable lifestyles who have not much to do with governments. However, recognition of the potential for utilization of weeds as bioresources by governments in different countries is necessary to have a broad societal effect. Relaxing the attitude towards colonizing species will come with time, but this can be hastened by economic incentives to manage weeds as part of the biodiversity within individual farmlands and vast farming landscapes, rural areas, or countryside.

The collective wisdom of all weed scientists and weed managers across continents may be required to bring about a change in farmers’ mind-set, as well as an attitude change among landholders and governments. The recognition of biodiversity values of weeds and the tolerance of beneficial weeds in arable weeds has been recommended in European countries (see Marshall, 2002; Marshall et al., 2003; Storkey, 2006; Storkey and Westbury, 2007; and discussions in Chandrasena, 2007; 2014).

As far back as in 1980s, agro-ecologists Miguel Altieri and Matt Leibman built the case to argue that eliminating all weeds from the farming ecosystems can destroy valuable habitat for natural enemies of insect pests, and thereby increase costs for insect pest control (Altieri and Leibman, 1988). Stamping out weeds may even contribute to human malnutrition. In developing countries, replacing traditional poly-cultures that tolerate or even encourage some weed growth with large scale monocultures and near-100% weed control has undermined food security in rural communities (Altieri, 1999). In addition to posing threats to local food production, industrial-scale farming eliminates palatable, nutritious weeds from farmers’ fields, robbing low-income communities of important sources of dietary vitamins and minerals. Many rural societies depend on edible weeds for food before their traditional crops mature, and especially in the event of crop failure. Such food systems are not served by an ‘all-out war’ against weeds.

There is a great deal of evidence of colonizing plants as some of the most useful medicinal plants in traditional medicine, as well as the sources of many modern pharmaceuticals. Although there is a general belief that the primary tropical forests, undisturbed and mystical, are the most likely habitat to discover new pharmaceuticals, perhaps because of their high biodiversity and endemism. However, the evidence from many traditional cultures is that this may not be true as they predominantly rely on non-forested, disturbed habitat for their medicinal plants (Voeks, 1996; Stepp, 2004; Stepp and Moerman, 2001). Stepp’s (2004) analysis of 101 plant species from which 119 modern pharmaceuticals are derived, showed that at least 36 species are widely regarded as weeds. The results were an order of magnitude higher than would be predicted by random occurrence of weeds in the modern pharmacopeia.

There is mounting evidence that weeds are relatively high in bioactive secondary compounds and are, thus, likely to hold promise for future drug discovery. Secondary compounds in weeds perform a variety of ecological functions. Chief among these is allelopathy, where such compounds may inhibit the germination and growth of neighbouring plants and also act as chemical defences against herbivory. Many weed species interfere with crops through the release of allelopathic secondary metabolites. However, because allelopathy usually occurs through the complex chemical matrix of the soil, it has been hard to show a causal relationship (Zimdahl, 1999) conclusively. Thus, disturbed environments, even within forests, which are the province of colonizing species, appear to be areas most likely to harbour novel compounds that may become future medicines.

Colonizing species will always be the ultimate survivors in the conflict with man. Rather than a zero-tolerance towards particular taxa, it would be prudent and responsible to ecologically manage problematic weeds, on a ‘case-by-case’ basis, with an eye on their potential benefits. This requires moving away from autecological, ‘species-led’ approaches that are reactions to problems posed by single species. The agroecology practices promoted by Altieri (1999) are invaluable ecological risk management models, in the sense that they have long-proven benefits in ecosystems. Agroecology also encourages people to integrate closely with all components of biological diversity, including colonizing species.

An Ethno-biological perspective- Linking Plants and Humans

In discussing the relative variety and intensity of uses of common reed (Phragmites australis) by human groups, Kiviat and Hamilton (2001) suggested that the utility of a plant is related to several factors. These include (1) abundance and distribution of the plant; (2) length of time the plant and a human group have been in contact; (3) invention or transmission of traditional ecological knowledge of the plant; (4) ease of managing, acquiring, and processing the plant; (5) physical and chemical qualities of the plant (e.g., pharmaceutical or toxicological properties, fibre characteristics, nutritional composition); and (6) availability and variety of alternate taxa. These ideas reveal why some taxa are much valued, and others much disliked. Discussions of such ethnobiological perspectives would help in building better relationships between weeds and humans, particularly in developed countries where the conflicts between the two are most profound.

The importance of traditional cultures, their wisdom and sustainable interactions with plants and animals are routine subjects in Anthropology, and Social Science. Interactions between the humanities and Weed Science are almost non-existent and hence, both sides may gain from a closer exchange of views. Journals dedicated to Ethnobotany and Economic Botany often carry articles relating to human uses of colonizing plants. Increased appreciation of plant taxa can be achieved by studying these ethnobiological appraisals, as well as by exercising more common sense. Improved ecological knowledge and an understanding of a broader range of cultures, societies, and plants of value to humanity may assist those who apply ‘weed risk assessments’ when deciding whether or not to list particularly resourceful taxa as ‘invasives’ that should be controlled at any cost. I object to the presumptive ‘branding’ of taxa, carried out by bureaucrats, which tends to stick in the minds of the public. Applying ‘a guilty until proven innocent’ approach to taxa with colonizing abilities, as practiced in some countries, belies common sense. It is also disrespectful to Nature and may not be tenable for long.

In a study in semi-arid areas of Brazil, Dos Santos et al. (2013) posed a series of questions: “Are invasive species considered useful by traditional societies? How are they useful? Are they more or less useful than non–invasive species? Is there a relationship between the use categories and taxonomic groups (families, genera, and species)? What plant parts are preferentially used and how are they distributed by categories of usage? Are there differences in the perceived usefulness of native vs. exotic invasive plants?”

In their study, a total of 56 invasive species were recorded, of which 55 were considered useful, and invasive species were considered useful more often than non–invasive species. The predominant use was as animal fodder, followed by medicine, food, and raw materials for industries. Nearly half (44%) of the animal fodder species also served as medicine for people. Herbaceous plants were the most common. Uses varied significantly within taxonomic ranks (species, genus, and family). The most recognized plants were also those that were most used locally. This study, just one of many from different countries, underscores the value of invasive species (weeds) in semi–arid Brazil, as well as the need to include local people in regional and national strategies to address invasive species management.

Weeds and Humans– the future

There is no simple remedy for the weed problem in its many manifestations. Therefore, we need to continue our studies on the best management strategies and control tactics to manage their negative impacts. As a discipline, Weed Science does understand quite well the reasons why colonizing species come to dominate landscapes. Weed management approaches need to be designed to prevent the introduction of potentially problematic taxa to new habitats and to provide rapid responses to minimize undesirable impacts where conflicts arise between man and colonizing species. I believe that this will be done best with a proper ecological understanding, and with a balanced view of economic implications, but without dramatizing weed issues, and certainly avoiding messages that create a visceral dislike for the colonizing plant taxa.

Evidence-based policy making is a sound goal in any country. However, only a small proportion of agricultural or environmental research has had the desired policy impacts. Most researchers in science are not trained to create policy effects from their work. Engagement with policymakers is not always encouraged, nor is it rewarded in most settings. Communication of scientific findings occurs mostly within the academic community; rarely outside it. There are exceptions, but across the various fields of human endeavour and mainly in science, little is done to link scholarship to policy systematically.

To exemplify, utilization opportunities for weeds is a topic not readily discussed at weed conferences. Is this because of some fear? Is it because weeds are so problematic that looking at them with a fresh set of eyes goes against the grain of Weed Science? I tend to agree with others (R. Zimdahl, 2019, personal communications, 28 December) who believe that it is mainly an educational problem. Nevertheless, there is a strong case building for investments in the utilization of weeds not least because it is a sensible weed management practice, but also because it provides a positive message for the public on the values of these plants, so severely mismanaged across the globe. Making a case for the utilization of weeds as bioresources is not difficult (see Kim and Shin, 2007; Chandrasena, 2007; 2014). The compilation of existing knowledge from different cultures should assist this task and, there is much to learn from the existing Economic Botany and Ethnobotany knowledge. A renewed attempt to explore weeds as bioresources requires efforts to highlight how traditional societies use all available knowledge of colonizing plants wisely and ‘co-exist’ with them.

Conclusions

A vast amount of global scientific literature indicates that man has not looked after the Earth’s natural resources well. Most findings are that depletion of natural resources (soil, water, and vegetation) is almost unstoppable, and many resources, including tropical rain forests, are being depleted at an alarming rate and will soon reach unrecoverable levels. Continuing population growth in many parts of the world and the quest for profits from growing crops or over-exploiting natural resources (such as minerals, oil and gas and timber) remain the root causes of the high rate of biodiversity losses and depletion of those natural resources — not weeds! It is men — and not weeds — who face a profound dilemma.

Kenneth Bolding, an economist at University of Colorado said: “Anyone who believes exponential growth can go on forever in a finite world is either a madman or an economist” (see quote in NEF, 2010).

Agreeing with those sentiments, Tim Jackson (2012), a Professor of Sustainable Development at the University of Surrey, argued that the human society is faced with a profound dilemma: ‘to refrain from growth is to risk economic and societal collapse; to pursue it relentlessly is to endanger Mother Earth’s ecosystems on which our survival depends.

Science tells us that weeds are only ‘colonizing plants’, and their management will be best undertaken within an ecological framework. Wherever or whenever man disturbs a habitat, they will be among the first pioneers to make use of the opportunity of space (sensu lato, Bunting, 1960). Downplaying this ecological emphasis, because of a focus on weed control, is disingenuous. In natural or man-made ecosystems, many weeds serve valuable ecological functions that need more recognition. Examples of their complex biological role, such as providing resources for wildlife, pollinating insects, slowing erosion, building soil, and generally enriching biological diversity, are abundant in global literature; these need to be studied more and given more extensive publicity. In a strategic approach to managing weeds, more people — weed scientists and students — should explore different ways of using these taxa for improving the human condition.

The summary condemnation of plant taxa, because we dislike them in particular situations is not a sensible way to approach a complex man-made problem. The genetic attributes of weeds that confer superior colonizing ability, competitiveness, and survival could be beneficial, not just in repairing damaged ecosystems, but also in sustainably providing food and fibre for both humans and animals. A key to sustainable living is to learn from weeds to be more resourceful and not ask for more. If all men become thrifty, and asked for less, we could reduce our environmental impacts, both as individuals and as societies. Such a change would make our Earth a much safer place for all species.

To end this essay, I would pose the following questions to all weed scientists: ‘Would you live in a world free of weeds? Or, would you cherish understanding how our complex interactions with weeds will enrich our lives? In an environmental ethic that all life is sacred, weeds are no more villainous than man himself!

Literature Cited

Altieri, M. A. (1999). The ecological role of biodiversity in agro-ecosystems. Agriculture Ecosystems and Environment, 74: 19–31.

Altieri, M. A. and Liebman, M. (Eds.). (1988). Weed Management in Agroecosystems : Ecological Approaches. Vol. I. CRC Press, Boca Raton, Fla, USA, p. 354.

Baker, H. G. (1965). Characteristics and Modes of Origin of Weeds. In: Baker, H. G. and Stebbins, G. L. (Eds.) The Genetics of Colonizing Species. Proceedings 1st International Union of Biological Sciences Symposia (General Biology). Academic Press. New York. 147–169.

Baker, H. G. and Stebbins, G. L. (Eds.) (1965), The Genetics of Colonizing Species. pp. 147–172, Academic Press, New York.

Chandrasena, N. (2007). Liabilities or assets? Some Australian perspectives on weeds. In: Kim, K.U. et al., (Eds.) Utility of Weeds and their Relatives as Resources, pp. 9–56. Kyungpook National University, Daegu, Korea.

Chandrasena, N. (2014). Living with weeds — a new paradigm? Indian Journal of Weed Science, 46(1): 96–110.

Colautti, R. I. and MacIssac, H. J. (2004). A neutral terminology to define ‘invasive’ species. Diversity and Distribution, 10: 135–141.

Convention on Biological Diversity, CBD (2001). Assessment and management of alien species that threaten ecosystems, habitats and species. Proceedings of the 6th Meeting of the Subsidiary Body on Scientific, Technical and Technological Advice, Montreal, Canada, p. 123. (CBD Technical Paper №1).

Davis, M. A. (2011). Do native birds care whether their berries are native or exotic? BioScience 61: 501–502.

Davis, M. A. and Thompson, K. (2000). Eight ways to be a colonizer; two ways to be an invader: a proposed nomenclature scheme for invasion ecology. ESA Bulletin, 81: 226–230.

Davis, M. A. and Thompson, K. (2001) Invasion terminology: should ecologists define their terms differently than others? No, not if we want to be of any help. ESA Bulletin, 82: 206.

Dos Santos, L. L., et al. (2013). The Cultural Value of Invasive Species: A Case Study from Semi–Arid North-eastern Brazil. Economic Botany, 68 (3): 283–300.

Dunn, S.T (1905). The Alien Flora of Britain. London, West, Newman and Co. p. 236.

Elton, C. S. (1958). The Ecology of Invasions by Animals and Plants. Methuen, London. p. 181.

Hamill, A. S., Holt, J. S. and Mallory-Smith, C. A. (2004). Contributions of Weed Science to weed control and management. Weed Technology, 18: 1563–1565.

Harper, J. L (Ed.) (1960). The Biology of Weeds. A Symposium of The British Ecological Society. April, 1959. Blackwell Scientific Publications, Oxford. p 256.

Jackson, T. (2012). The Cinderella economy: an answer to unsustainable growth? The Ecologist, 27th July 2012.

Kim, K. U., Shin, D. H. and Lee, I. J. (Eds.) (2007). Utility of Weeds and Their Relatives as Bioresources. Kyungpook National University, Daegu, Korea. p. 222.

Kiviat, E. and Hamilton, E. (2001). Phragmites use by Native North Americans. Aquatic Botany, 69: 341–357.

Kloot, P. M. (1983). Early Records of Alien Plants Naturalised in South Australia. Journal of Adelaide Botanic Gardens, 6(2): 93–131.

Marshall, E. J. P. (2002). Weeds and Biodiversity. In: Naylor, R. (Ed.), Weed Management Handbook 9th Edition British Crop Protection Council. pp. 75–93. Blackwell Science, Oxford, U.K.

Marshall, E. J. P. Brown, V. K., Boatman, N. D., Lutman, P. J. W., Squire, G. R. et al. (2003). The role of weeds in supporting biological diversity within crop fields. Weed Research, 43:77–89.

NEF (2010). New Economics Foundation. Growth Isn’t Possible: Why we need a new Economic direction. Schumacher College.

Sagoff, M. (2005). Do non-native species threaten the natural environment? Journal of Agricultural and Environmental Ethics, 18: 215–236.

Salisbury, E. (1961). Weeds & Aliens. The MacMillan Co., New York. p. 381.

Shackleton, R. T., et al. (2019). Explaining people’s perceptions of invasive alien species: A conceptual framework. Journal of Environmental Management, 229: 10–26.

Storkey, J. (2006). A functional group approach to the management of UK arable weeds to support biological diversity. Weed Research, 46: 513–522.

Stepp, J. R. (2004). The role of weeds as sources of pharmaceuticals. Journal of Ethno-pharmacology, 92: 163–166.

Stepp, J. R. and Moerman, D. E. (2001). The importance of weeds in ethnopharmacology. Journal of Ethnopharmacology, 75: 25–31.

Timmons, F. L. (1970). A history of Weed Control in the United States and Canada. Weed Science, 1970. 18 (2): 294–307).

Voeks, R. A. (1996). Tropical forest healers and habitat preference. Economic Botany, 50: 381–400.

Storkey, J. and Westbury, D B. (2007). Managing arable weeds for biodiversity. Pest Management Science, 63: 517–523.

Watson, C. H. (1847). Cybele Britannica [Or, British Plants and Their Geographical Relations]. Vol. 1. London: Longman & Co (https://www.biodiversitylibrary.org/item/104172). The definition is on p. 63.

Wild, H. (1967). Weeds and Aliens in Africa: The American Immigrant. A Lecture given in the University College of Rhodesia. 10th Oct 1967 (https://core.ac.uk/download/pdf/43541105.pdf).

Wyse. D. L. (1992). Future of Weed Science Research. Weed Technology, 162–165.

Zimdahl, R. (1999). Fundamentals of Weed Science, 2nd Edition, Academic Press, New York, p. 556.