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Of Weeds and Men: Some Perspectives Nimal Chandrasena Principal Ecologist, GHD Pty Ltd., Parramatta, NSW, Australia Email:
[email protected]
Abstract The relationship between weeds and men is an old one; however, it is changing fast. Increasing public concerns about the impacts of overuse of herbicides and other destructive farming practices, under the guise of increasing food security for humans, have prompted a re-think on whether we need to take reckless action against plant taxa that only pose problems under certain sets of conditions. A critical issue for Weed Science to deal with is the persistent and uninformed maligning of colonizing plants (weeds) by some, which inhibits others from admiring them and appreciating their redeeming values. Common definitions of a weed: “…a plant, which has a detrimental effect on economic, conservation, or social values...” or ‘”…undesirable plants, which are out of place…” tend to create an flawed, negative perception that weeds are an ‘environmental problem’ and all colonizing plants are bad, under all circumstances. Redressing this requires recognition of the beneficial impacts of weeds, as part of the Earth’s rich bio-diversity. Often introduced to different continents deliberately or accidentally by men, weeds are trekking the globe as the shadows of men’. Humans are clearly “weeds par excellence”. thereby threatening the sustainability of natural ecosystems and primary production. As human enterprises expand, population increases and pursuit of material wealth continues, the mode of existence of some colonizing plant taxa will clash with our existence. It is through no intrinsic fault of these plants. The same attributes that make a plant ‘invasive’ will be sought after under a different set of circumstances. The way forward is to broaden our understanding of plant resources and their crucial and integral role in biological communities. Weeds In the case of taxa with strong colonizing abilities, their resilience, tenacity, and capacity to adapt to environmental disturbances need to be recognised. The success of these plants in the environment is also their weakness! It has brought them into an unnecessary conflict with Homo sapiens. Perhaps this understanding would help modify our attitudes, or allow us to avoid creating conflicts with plant taxa, and getting into situations from which we cannot win. As in many other fields, it is necessary from time to time, to realign the focus of a scientific discipline, and Weed Science may have reached that stage. Whilst there is a vast amount of disparate literature, the future requires a ‘body of knowledge’ of utilization of colonizing species to be established, so that present and future generations could benefit from that knowledge. Keywords: Definitions of weeds; Values of weeds; Utilization of weeds 1
Introduction “The word weed is taken to mean a species or race, which is adapted to conditions of human disturbance. By this definition weeds are not confined to plants. Animals such as the English sparrow, the starling, the "statuary" pigeon, the house mouse, Drosophila melanogaster, and others are especially fitted to environments provided by human disturbance. Indeed, perhaps no species thrives under human disturbance more than Homo sapiens himself. In this ecological sense, man is a weed…” Halan and De Wet, 1965. “Homo sapiens is perhaps the weediest of all species, and the more he dominates the landscape, the more he seems to thrive. If we confine the concept of weeds to species adapted to human disturbance, then man is by definition the first and primary weed under whose influence all other weeds have evolved...” Harlan, 1975. “One might argue that man is a domesticated animal rather than a weed. But man existed a very long time before he domesticated any other species; he has never seriously or consistently attempted to improve the race by selection or breeding as he has with other domesticates; and if we apply the test of unwantedness, the current alarm over the population explosion would appear to place man more in the category of weeds than domesticated animals. If man does succeed in controlling his own population size, we shall have an example of a weed becoming domesticated...” Harlan, 1975.
As far back as in 1908, Roland M. Harper, an American Botanist provided an illuminating view on weeds and their probable origin through introgression, hybridization and deliberate introductions of European imports to the new American Colonies. Since then, many others, notably, Baker (1965), De Wet (1966); Harlan and De Wet (1965) and Harlan (1975) have pointed out that, in many ways, weeds and man have co-evolved, each shadowing the other. They have not just evolved alongside humans; they have also been introduced to different environments and globally, across continents often deliberately, or sometimes, accidentally by men. As a result, it would be fair to say that weeds are trekking the globe in the ‘shadows of men’. In sharing the habitat, disturbed and occupied by men, weeds, by their very nature, entered into a ‘conflict’ presenting significant challenges to productive activities of humans. These conflicts arise from weeds being one cause of reduced agricultural production (both quantity and quality); causing poisonings, allergies and physical discomforts to humans and animals. Weeds also reduce recreational opportunities and extract significant energy inputs from both humans through the efforts to control them. Their excessive growth may also displace some native species under certain circumstances in different habitats; impede the flows in irrigation canals; reduce the uses of waterways, and potentially, harbour disease-causing organisms.
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It is also important to note that weeds and men also share many characteristics; both are excellent opportunists, displaying extraordinary survival skills in varied environments. The capacity of humans to successfully adapt, colonize, reproduce, and populate new environments across varied climatic regions is undoubted. Humans compete for resources aggressively, both within our own species (intra-specific competition) and with other organisms (inter-specific competition), and often these confrontations are do-ordie battles. The victor then thrives in conquered environments (which are also highly disturbed), exploits resources as much as possible, and perpetuates its kind. Table 1 summarizes some similarities between weeds and men. Humankind, surely, must be regarded as one of the most successful weeds in the world!
Table 1
Some similarities and differences between man and weeds
Humans
Weeds
Relatively high fecundity (or potential); Maximum number of off-spring produced by an individual is determined by number of egg cells
High fecundity almost all the time; individual seed producers can produce very large numbers of offspring; Clonal species (vegetative reproduction) have almost unlimited capacity
Very low genetic variability; all humans 99.9% similar to other humans based on genes, nucleotide base pairs and allele frequencies (Jorde et al., 2000)
Very high genetic variability within species (in both widespread species and rare species (Stebbins, 1980; Gitzendanner, M.A. and Soltis, P.S. 2000; Rao and Hodgkin, 2002)
High adaptability to environmental conditions, mostly through innovations (e.g. warm clothes to live in cold climates) and inventions (e.g. building shelters)
Ability to withstand environmental extremes, such as droughts or cold - through heritable characteristics (e.g. expanded ecological amplitude) and adaptations
Ability for populations to withstand environmental extremes - through dispersal and movement (immigration and emigration into or out of unfavourable environments)
Ability to withstand environmental extremes, such as fire, typhoons, floods or earthquakes- through propagule dispersal and rapid re-colonization
Aggressive competitors; often unable to efficiently share resources, leading to violent conflicts
Aggressive competitors; but efficient in sharing resources through niche differentiation and other tactics (i.e. mimicry; plasticity and avoidance)
Most humans, lacking an in-depth understanding, deride and malign weeds, sometimes in ways that appear xenophobic (fear and dislike of anything foreign). However, ideas and attitudes towards weeds vary much across the globe. While weeds are much maligned and not tolerated in many countries, this dislike is not universal. In some cultures, weeds are accepted as part of nature, and are also highly valued. This sharply contrasting societal attitude towards weeds is a paradox that is worthwhile examining further. Weeds, taken individually by species, or as a class, are top-notch, skilled survivors, often thriving in inhospitable environments and extreme conditions. They appear to survive and make the best of any situation. As humans face significant uncertainty in a relatively unstable future climate, brought about by our own
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actions, the survival strategies of weeds would be great lessons to learn. These vagabond plants, derided as ‘fugitive plants’ - do not ask for much; instead they may take some of the earth’s resources for their growth and survival; they may also make humans toil a bit more, but they give back a lot more than we commonly realize. In a rapidly changing world, with limited resources, a burgeoning human population, weeds tell us how to share those limited resources, differentiate our ecological niches, and co-exist with nature. In this paper, I attempt to provide some social and ecological perspectives on weeds and men, reflecting on their often antagonistic relationship. Mistrust of weeds comes from mis-judgements and lack of understanding of what they really are. Primarily, the paper attempts to counter the human antagonism against weeds, and not the antagonists. It argues against the prevalent narrative that weeds - rather than man - are an ‘environmental problem’ causing unacceptable ecological harm in human-modified and natural landscapes.
Myths and Confusions Typically, some plants are designated as ‘weeds’ on the basis of a subjective assessment of their positive and negative characteristics by a few good men. Despite good intentions, often these assessments lack scientific credibility and are based on the relationship of the plant to the activities and experiences of those good men. To illustrate the point, some globally-important plants: Taro (Colocasia esculenta (L.) Schott), Neem (Azadirachta indica A. Juss), Guava (Psidium guajava L.) and ‘Ipil Ipil’ (Leucaena leucocephala (Lam.) De Witt), have been listed as ‘invasives’ within an Australian context (Groves et al., 2005). This listing tends to confuse the issue of which taxa should pose actual serious problems to humans. Nowhere in the world has these three species caused any major concern to societies for millennia. To say that these species are highly respected in most tropical and sub-tropical regions of the world would be an understatement. Often landholders and public officials do not take into account the full-published details or the rationale for listing such species by different Australian States, or countries of the region. Instead, the information is often misread and misquoted, leading to the summary condemnation of useful plants. Listing of such species is the result of a rather narrow, species-focused approach based on ‘Weed Risk Assessments’ (WRA). The Australian WRA approach is a product of the need for ‘bio-security’ that has been perpetuated “…as necessary to protect the continent’s borders from alien plants…”. Assessing the potential risk of a species becoming an invader by its invasiveness elsewhere is now widely supported in Australia, New Zealand, USA and Europe (Williams and West, 2000; Arcioni, 2003; Godden et al. 2006). However, this approach could lead to errors in judgement, because plant invasions are not taxonomic phenomena, but are often related to complex bio-geographical factors (Colautti and MacIsaac, 2004). The tendency to categorize individual taxa, or groups of taxa, as ‘invasives’, because of weed risk assessments, raises doubts in the public’s mind whether all non-native species are bad for the environment. It also leads to a negative perception of plant resources of potentially significant value, and this ‘branding’ tends to be perpetuated.
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The proliferation of descriptions like ‘invasives’, ‘aliens’, ‘noxious’ and ‘thugs’ in Australia has led to divergent interpretations, and subconscious associations of some plant taxa as undesirable. The use of emotive terms and phrases: ‘Aliens’ and ‘Garden Thugs’ (Randall, 2001; Glaznig et al., 2004b), ‘Damned Weeds!’ ‘Keeping Them at Bay’ (Arcioni, 2003), and ‘Jumping the Garden Fence’ (Groves et al., 2005) to highlight weed issues is common in Australia. The more serious issue is that, by attributing blame to imprecise plant characteristics, and weeds in general, the significance of human-mediated factors in the spread of weeds is constantly downplayed, or not given much recognition. Human activities are, directly or indirectly, the major factors implicated in plant or animal invasions in different regions or continents. Whilst some weed invasions are related to large-scale disturbances, such as land clearing, deforestation, or to the discharge of ships’ ballast, and the movement of ships, containers and machinery across oceans, others result from deliberate introductions (by various agencies and industry) or accidental introductions of organisms into new areas. One survey in Australia (Lonsdale, 1994) showed that 463 grasses and legumes were introduced between 1947 and 1985, and 60% of those were weeds from an agricultural or conservation perspective. Other activities, like expansion of farming in marginal lands, changing farming from sheep to cattle, and provision of artificial water holes in grasslands, where none existed before, are also implicated in the spread of invasive species. The latter, presumably to increase productivity from the CentralWest Queensland’s plains, has led to farm animals grazing over vast extents of land, and causing a range of significant negative impacts on the environment (James et al., 1999).
Are Weeds ‘Plants Out of Place’? The complexity of agreeing on a universally accepted definition for weeds is evident from the large body of Weed Science literature. One of the commonest definitions of weeds is that ‘a weed is a plant out of place’. This notion - that weeds are plants ‘out of place’ - is very American, as explained by the historian Zachary Falck (2010). The incongruous, highly subjective definition arose in the 1850s out of the American ‘middleclass’ dream of creating spaces and cities, which are ‘sanitary’ and ‘orderly’. Untamed growth of a group of plants was blamed for being ‘unsightly’ and for ‘disfiguring’ cities, as opposed to the ‘gentility’ (upper class refinement) offered by attractive and colourful wild flowers , which could be planted to beautify parks, sidewalks and median strips. Although, inherently flawed, the phrase: ‘out of place’ is bandied around in the popular media. Tim Cresswell (1996; 1997) analyses the innate flaw in this dubious concept as follows: “…the notion that everything “has its place” and that things (e.g., people, actions) can be “inplace” or “out-of-place” is deeply engrained in the way we think and act. Such is our acceptance of these ideas that they have achieved the status of common sense or second nature. Common sense produces the strongest adherence to an established order...” “…People act as they think they are supposed to; they do what they think is appropriate in places that are also appropriate. It is therefore essential for powerful groups in any given context to define 5
‘common sense’ and that which goes unquestioned. When individuals or groups ignore this sociallyproduced common sense, they are said to be “out-of-place” and defined as deviant...”
We brand some plants ‘out of place’, because, we have firstly ourselves defined in some abstract way, elements of our immediate environment as ‘proper places’, and these would demand ‘appropriate behaviour’. Arguably, such a notion may be satisfactory for some of our living spaces, such as home gardens, turfed lawns kept manicured, neat and tidy, and in which weeds may be accused of de-spoiling the tidiness. One may also call agricultural fields ‘proper places’, because we use them to produce our food. Extending the same argument, we may call all natural areas and wilderness with little human interference as ‘proper places’ from a human point of view. What is “out of place” will always depend on the context. Thus, we have lists of plants we call ‘environmental weeds’, ‘horticultural weeds’, ‘agricultural weeds’, ‘ruderal weeds’, ‘urban weeds’, and so on. From a conservation point of view, agricultural crops could be viewed as weeds; and from a farmer’s perspective, native plants growing in his fields could be deemed ‘unwanted weeds’.
Definitions and Confusions This word ‘weed’, an epithet of human invention, and a dubious ‘cultural construct’ at that, has caused so much confusion within the field of Weed Science. In the world of plants, it simply does not exist. The term belongs to an emotional or psychological category than to Botany. However, the vast collection of journal papers and books are a testament that weeds have left an impressive and indelible mark in the halls of academia across the globe. During the last 50-60 years, the world has seen countless numbers of institutions, professional scientists and farmers take up the challenges that weeds pose to mankind. This Weed Science ‘corpus of knowledge’ has been extremely useful to face some of the challenges, and indeed, overcome them, from time to time. But, weeds, one could argue, are unremarkable in many ways; they are ubiquitous, always present in the background of ‘humanised spaces’, and become problems only under certain conditions. The negative impacts of weeds are simply not as dramatic as those brought about by a disease or plague, directly attributed to a pest organism, such as a mosquito, rat, or armies of locusts. Hence, many people ignore them (unless you are a farmer of some kind) until a significant increase in weed populations raises a concern. My contention is that weeds, as a group, have been given a bad name, and are poorly understood by not just the common man, but also a large proportion of the academe. Mis-information is rife regarding the negative role weeds have in the environment or on biodiversity, and of late, the negative publicity has been on the increase. It is extremely rare to find an academic, a biology teacher, or an ecologist, even within the Asian-Pacific Region, who would expound the virtues of weeds; they are either scared; or unsure, because the opposite view has been so powerfully advocated, by influential voices, and is deeply entrenched in the affluent, industrialized, western societies. It is quite easy and expedient for the real culprit – man himself – to pass the blame to another party (weeds, in this case) for our inability to correctly approach 6
and address environmental challenges. It would not be unreasonable to say that most people fail to understand nature, and how it works, let alone engage with it, in a sensible and practical way. One could safely assume that the concept of undesirable species in a farmer’s field (weeds) and the need for their control is as old as agriculture itself. Although the exact references are hard to quote, there is archaeological evidence of weed control in China from the tenth to the sixth century BC. The websites on English words (Word Reference, 2015) indicate that in the 13th Century, the word “woed” or “weodian” had been used as a verb “to weed” or clear the ground; or to mean “grass, herb”. From the Middle Ages (5th to 12th Century), there is also an Old Dutch word “wieden”, which means “weed”. It seems reasonable to conclude that the use of word “weod” may have had the connotations of “ugly, worthless plant” that needs to be ‘cleared from the ground’, and this may be the origin of the current usage of the word. In any case, the word was widely used in the 16th Century. Weeds were frequently used as a metaphor by Great Bard William Shakespeare, demonstrating a brilliant understanding. Shakespeare (1591; 1595) referred to weeds, sometimes in profoundly deep appreciation, and sometimes in reckless derision, but always as allegory to convey messages on real life situations, as shown in the following, well-known passages: “…Cut off the heads of too fast growing sprays, that look too lofty in our commonwealth: All must be even in our government. You thus employ'd, I will go root away the noisome weeds, which without profit suck the soil's fertility from wholesome flowers…” (Act 3: Scene 3; Richard II); “…The freckled cowslip, burnet and green clover, wanting the scythe, all uncorrected, rank, conceives by idleness and nothing teems but hateful docks, rough thistles, kecksies, burs, losing both beauty and utility. And as our vineyards, fallows, meads, and hedges defective in their natures, grow to wildness; Even so our houses, ourselves, and children, have lost, or do not learn, for want of time, the sciences that should become our country…” (Act 5: Scene 2; King Henry V); "…Sweet flowers are slow and weeds make haste..." (Richard III); "…Now 'tis spring, and weeds are shallow-rooted; suffer them now and they'll o'er grow the garden…" (Henry IV) Published literature indicates the word ‘weed’ was used to refer to tobacco in the 17th Century; and to marijuana in the 20th Century (Falck, 2010). The word became a common term in the 18th Century, and was widely used by Jethro TuIl (1731) in his seminal book - The New Horse-Hoeing Husbandry. At the start of Chapter VII - “Of Weeds”, Jethro Tull (1674-1740) muses as follows: “….Plants that come up in any land, of a different kind from the sown, or planted Crop, are Weeds….That there are in Nature any such things as inutiles Herbae, the Botanists deny; and justly too, according to their meaning...But the Farmer, who expects to make profit of his land from what he sows or plants in it, finds not only Herbae inutiles, but also noxiae, unprofitable and hurtful
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Weeds; which come like Muscae or uninvited Guests, that always hurt, and often spoil his Crop, by devouring what he has, by his labour in dunging and tilling, provided for its sustenance...” “…All weeds, as such, are pernicious, but some much more than others; some do more injury, and are more easily destroyed; some do less injury, and are harder to kill; others there are, have both these bad qualities. The harder to kill are such as will grow and propagate by their seed, and also by every piece of their roots, as Couch-grass, Coltsfoot, Melilot, Fern, and such-like. Some are hurtful only by robbing legitimate (or sown) plants of their nourishment, as all weeds do; others both lessen a legitimate crop by robbing it, and also spoil that crop, which escapes their rapine, when they infect it with their nauseous scent and relish, as Melilot, Wild Garlick…”
Overall, Jethro Tull’s his views on weeds are rather sympathetic, as he described these problematic plants as Herbae inutiles (useless plants) or Herbae noxiae (noxious plants). He acknowledged that Botanists would disagree with this view, which largely reflected the way farmers looked at these plants. He also likened weeds to ‘muscae’ - flies, who appear as ‘uninvited guests’ that can become pests, under certain conditions. Throughout Tull’s book, there are serious discussions on the nature of weeds: their seeds, vegetative propagules, seedling emergence, dispersal and negative impacts on crops. As indicated by its sub-title, the book was: “…Designed to introduce a new method of culture; whereby the produce of land will be increased, and the usual expense lessened...Together with accurate descriptions…of the instruments employed in it…” Way back in 1863, the American Naturalist Ralph Waldo Emerson (1863) reflected sympathetically on values of weeds: “…What is a weed? A weed is a plant whose virtues have not yet been discovered…” A century later, the crop breeder Jack Harlan (1975) pointed out that: “...Weeds have been constant and intimate companions of man throughout his history and could tell us a lot more about man, where he has been and what he has done, if only we knew more about them...Weeds are adapted to habitats disturbed by man. They may be useful in some respects and harmful in others. They may be useful to some people and hated and despised by others...” The perceptions contained in these views warrant renewed and serious considerations. Ecologically-speaking, Bunting (1960) referred to weeds as: ‘pioneers of secondary succession’. Baker (1965) defined a weed as ‘a plant …if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed by man, (without, of course, being a deliberately cultivated plant)’. Radosewich et al. (1997) pointed out that: “…any plant can be a weed and no plant is always a weed...” As a consequence, some plants may be considered weeds, and hence, undesirable to have at particular places, and at particular times. Zimdahl (1999) favoured the view that weeds are: ‘…those plants that are successful in disturbed environments, are fast growing, and, are often, but not always herbaceous…’ These viewpoints are important to understand in managing the tension in the relationship between weeds and men. The consensus is that weeds are essentially ‘colonizing plants’, which become objectionable under certain circumstances that allow them to rapidly colonize habitat disturbed by humans or by natural causes. 8
After much debate in the 1980s, the Weed Science Society of America adopted the definition of a weed as: ‘a plant growing where it is not desired’. The European Weed Science Society extended this idea to include: ‘any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people’. The Australian definitions have a strong slant towards the European version, i.e. ‘a weed is a species that adversely affects biodiversity, the economy or society’ (Groves et al., 2005) or ‘a weed as a plant, which has, or has the potential to have, a detrimental effect on economic, conservation, or social values in Australia’ (ARMCANZ, 1999). These definitions have effectively removed the accepted view that weeds are generally a symptom of a man-made crisis, but not the cause of it. Part of the negative attitude towards weeds is due to the lack of clarity on man’s own role and culpability in creating disturbed habitats. As Bunting said, ‘an essential feature of all of man’s activities, in agriculture or otherwise, is the production of open, or at least disturbed, habitats’. The downplaying of man’s part in creating much of the disturbance to which ‘colonizing plants’ naturally respond has inevitably led to misconceptions, and, over time, a hardened attitudes towards ‘weedy’ taxa. There are many examples (see Kim et al., 2007 and references therein; Chandrasena, 2007; 2014) which indicate that even plants with strong colonizing attributes are of value in various situations, at different times, or to different people. Given this, it may be worthwhile to broaden the common definition of weeds to capture the idea that weeds present problems to some people, and certainly not at all times or at all places. In this regard, Kloot’s definition from Australia (Kloot, 1987) that a weed, ‘is a plant that may interfere with human activity in one way or another and, thus, has come to be regarded negatively by at least part of the society’ is a reasonable one to consider. It is evident that there is a strong human element in defining what a weed is, based on the likes or dislikes of men. Despite this, in an ecological sense, what determines a ‘weed’ is related to habitat disturbances and the local climatic, hydrologic, soil and geomorphologic factors, as well as biotic interactions in nature. All of the above factors strongly affect ecological processes and the abundance and distribution of species at any location. Variations in local environmental conditions reflect location of a species along gradients of elevation, longitude and latitude. Ecology teaches us that a multitude of micro-scale physical, chemical and edaphic factors vary within these gradients, and such factors define the dominant, resident species (whether colonizers or slow-growing species), which can occupy a location, and processes. The type, intensity, and duration of disturbances also shape the characteristics of plant populations, communities, and ecosystems. Disturbances are events that disrupt ecological systems; they may occur naturally [e.g., wildfires, storms, or floods] or be induced by human actions, such as clearing for agriculture, clear-cutting in forests, building roads, or altering stream channels. The effects of disturbances are controlled largely by their intensity, duration, frequency, timing, as well as the size and shape of the area affected. Disturbances may affect both above- and below-ground processes and can impact communities and ecosystems by changing the number and kinds of species present; change the nutrient dynamics that affect productivity and habitat structure. Large disturbances can also cause movements of species and change functional attributes of ecosystems, including changes in species abundance and distribution, community 9
composition, and ecosystem function. Colonizing species (weeds) thrive under disturbances, and in some cases will dominate a local ecosystem for varying periods of time. Weeds have attributes that make them are efficient invaders and opportunistic, exploiters of disturbed soil in our agricultural land, cities and suburbs and their existence often has adverse impacts on agricultural productivity, rangelands, nature reserves, as well as in urban and rural environments and human aspirations (Naylor and Lutman, 2002; Zimdahl, 1999; Falck, 2010). It is also clear that a plant species that may be valuable in one situation, such as a horticultural or food plant, could escape cultivation and become a problem weed in another, often in environmental areas. In my view, the conflict between weeds and men cannot be won, except intermittently, in a very local setting. Colonizing plants were here on earth before man; and they will be here after man. Their adaptations far exceed the capabilities of man. On the other hand, instead of continuing on the pathway of engaging in an unwinnable war, there is a possibility for humans to judiciously adapt further to live with weeds, and perhaps even use them to our advantage. To do this, humans need to rein in our prejudices and biases, and appreciate what weeds actually tell us about landscapes.
The Way Forward: Befriending Weeds There is abundant literature that shows not all weeds are bad all the time, although some may be problematic more often than not. Also, a ‘weed’ to one person may be a joy to another. Ehrenfried Pfeiffer (1950) elegantly put forward this side of the story in the 1950s, as follows: ‘…Weeds are only weeds from our egotistical point of view, because they grow where we do not want them. In nature, however, they play an important and interesting role. They resist conditions, which cultivated plants cannot resist, such as drought, acidity of soil, lack of humus and mineral deficiencies, as well as one-sidedness of minerals. They represent human beings’ failure to master the soil, and they grow abundantly wherever people have made mistakes – they simply indicate our errors and nature’s corrections...” Humans have an opinion on just about everything, whether they understand it or not, and much of the time, these assertions are wrong, and it is in human nature to blame someone or something else for their failures. Edward Salisbury (1961) a well-respected pioneer Weed Scientist and a learned Professor, working in London, ill-advisedly chose words that likened a weed to a criminal! ‘…When not engaged in their nefarious activities both may have admirable qualities; a thief may be an affectionate husband and father outside business hours, while an aggressive weed in one environment may be a delightful wildflower in another…’ In the broader context of describing weeds this way, Professor Salisbury’s intentions were unquestionably honourable. However, the choice of simile was unfortunate; and the lay person could be excused for being confused! It is truly unfortunate that many Weed Scientists continue to use unnecessarily provocative metaphors to convey messages on weeds and their management. 10
Richard Mabey, in his recent book on Weeds (Mabey, 2010), began as follows: ‘Plants become weeds when they obstruct our plans, or our tidy maps of the world. If you have no such plans or maps, they can appear as innocents, without stigma or blame’. The words capture in a naively romantic way (his words) the essence of weeds. Until about the 1970s, weed issues were discussed only from the perspective that they were problems to crop production. In subsequent decades, attention turned to weeds as environmental problems affecting landscapes. Weeds are now projected as a major problem affecting all aspects of our daily lives, and much energy and resources are spent fighting them. However, given the evidence, is the problem really weeds or is it our perception of them? As Auld (2004) states, weed occurrence is inevitable. This is because man’s activities will continue to disturb environments, and movement of people across continents will exacerbate introductions into new areas. There is no simple remedy for the weed problem in its many manifestations. Therefore, whilst continuing to study the reasons why colonising species come to dominate landscapes, the best management strategy would be to use several control tactics in an integrated manner. Management approaches need to be designed to prevent new introductions to disturbed areas, and to minimise the undesirable impacts where the conflict exists between man and the colonising species. However, this must be done with a proper ecological understanding, and with a balanced view of economic implications, but without dramatising weed issues, and certainly avoiding messages that create a visceral dislike for some plant taxa. Given the magnitude of the negative publicity attached to weeds, it seems paradoxical to consider that these plants, so much ‘maligned’, may be beneficial as resources. Nevertheless, if we are concerned about man’s relentless impact on the environment, perhaps we should question the validity of the summary condemnation of plant taxa. If an attitude change can be stimulated, it will be for the benefit of both the environment and humans (both present and future generations); but it would need debunking a few myths on the way, and also evidence-based arguments and information. Globally, the utilization of weeds has been patchy over the past five decades. The most significant uses have been largely restricted to aquatic weeds (Little, 1979). These have most often been used for nutrient removal, with the plant residues largely utilized as mulches. Some species have been used as sources of food, fibre and medicines, mostly in developing countries. However, there is a renewed interest in focusing on utilization of weeds in various other productive ways, such as remediation of polluted waterways and land (see references in Kim, 2001).
An Ethno-biological perspective- Link between Plants and Humans In discussing the relative variety and intensity of uses of Phragmites by human groups, Kiviat and Hamilton (2001) suggested that the utility of a plant to humans is related to: (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, fiber characteristics, nutritional composition); and (6) availability and quality of alternate taxa. These ideas reveal 11
why some taxa are much valued, and others much disliked. Discussion of such ethno-biological perspectives is important in building better relationships of plants by humans, particularly in developed countries where the conflicts between the two are more profound. The importance of traditional cultures, their wisdom and sustainable interactions with plants and animals is a routine subject in Anthropology, and Social Science. Interactions between the humanities and a discipline like Weed Science are not strong and hence both sides may gain from a closer exchange of views. Journals dedicated to Ethnobbotany and Economic Botany often carry articles relating to human uses of colonizing plants. Increased appreciation of plant resources can be achieved by studying these ethnobiological appraisals, as well as by exercising more common sense. Improved 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. The presumption here is that ‘branding’ of taxa tends to stick in the minds of the public. Applying ‘a guilty until proven innocent’ approach to taxa with colonising abilities, as widely practiced in some countries, belies common sense, is disrespectful to nature, and may not be tenable for long. In a recent study in semi-arid areas of Brazil (Dos Santos et al., 2013) the authors posed the question: Are invasive species considered useful in traditional societies? How are they useful? Are they more or less useful than non–invasive species? Is there a relationship between use categories and taxonomic groups (families, genera, and species)? What plant parts are preferentially used and how are they distributed by categories of usage? And are there differences in the perceived usefulness of native vs. exotic invasive plants? In the study, a total of 56 invasive species were recorded, of which 55 were considered useful. Overall, invasive species were considered useful more often than non–invasive species. The predominant use was as animal fodder, followed by medicine, food, and technology. 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 those that are most used locally. This study underscored the value of invasive species in semi–arid Brazil, as well as the need to include local people in regional and national strategies to address invasive species management.
Weeds as ‘Healers’ There is a great deal of evidence that weeds are some of the most useful medicinal plants in traditional medicine, as well as the sources of many pharmacological drugs. There is a general belief that undisturbed, mystical, primary tropical forests are the most likely habitat to discover new pharmaceuticals, due to high biodiversity and endemism. This may not be quite so true. The evidence from many traditional cultures and indigenous groups is that they rely on non-forested, disturbed environments for medicinal plants (Voeks, 1996; Stepp, 2004; Stepp and Moerman, 2001, and references therein). Stepp’s review (2004) of drug discovery suggests that disturbed environments may also harbor potentially useful medicinal compounds. An analysis of the 101 plant species from which 119 contemporary 12
pharmaceuticals are derived showed that at least 36 of these plants are considered weeds (χ2=343.34, P
