Ecology of Weeds and Invasive Plants : Relationship to Agriculture and Natural Resource Management

The classic reference on weeds and invasive plants has been revised and updated. The Third Edition of this authoritative reference provides an in-depth understanding of how weeds and invasive plants develop and interact in the environment so you can manage and control them more effectively. The guide includes an introduction to weeds and invasive plants in various environments and an overview of their ecology and evolution. With extensive examples, this book: Focuses on the biological features of weeds and invasive plants, especially as they exist in agriculture, forests, rangelands, and natural ecosystems. Includes coverage of exotic invasive plants. Discusses a variety of methods and tools for managing weeds and invasive plants, including physical, cultural, biological, and chemical approaches. Examines systems approaches for management, including modern Integrated Pest Management. Addresses future challenges for scientists, farmers, and land managers. This is the definitive, hands-on reference if you're a land manager or professional in plant sciences, agronomy, weed science, and horticulture. The book is also an excellent textbook for senior undergraduate or graduate students studying agriculture, ecology, natural resources management, environmental management, or related fields.

Excerpt

CHAPTER 1

WEEDS AND INVASIVE PLANTS

Weeds exist as a category of vegetation because of the human ability to select desirable traits from among various members of the plant kingdom. Just as some plants are valued for their uses or beauty, others are reviled for their apparent lack of these characteristics. Weeds are recognized worldwide as an important type of undesirable, economic pest, especially in agriculture. However, the value of any plant is unquestionably determined by the perceptions of its viewers. These perceptions also influence the human activities directed at this category of vegetation.

Harlan, in the middle of the last century, described how vegetation evolved under the impacts of humans. He suggested that vegetation, in relation to the degree of human involvement with it, exists as three categories: wild plants, weeds, and crops. Crops were domesticated from wild plants while weeds evolved from wild plants as an unintentional consequence of growing crops. Some crops also were once weeds and some have again escaped from domestication. In Harlan's concept neither weeds nor crops can permanently displace wild plants from wild habitats over time (DeWet and Harland 1975).

Invasive plants, unlike agricultural weeds, are those that can successfully establish and spread to new habitats after their introduction, seemingly without further assistance from humans. These plants can spread into new areas already occupied by a native flora and displace those species. Such invasions from the intentional or unintentional transport of plants to new regions now seriously threaten the biodiversity, structure, and function of many of the world's ecosystems. Invasive plants are thus weeds in the broadest sense because they evoke human dislike and often some form of management to eradicate or contain them in their new environments. Not all weeds are invasive, however. In this text, the term weed will be used in the broad sense and to describe undesirable plants in agricultural systems, while invasive plant will be used for those weeds that can spread beyond their point of introduction, often in natural ecosystems.

WEEDS

A "plant growing out of place," that is, plants growing where they are not wanted, at least by some people, is a common, accepted explanation for what weeds are. This notion of undesirability imparts so much human value to the idea of weediness that it is usually necessary to recognize who is making the determination as well as the characteristics of the plants themselves. For example, certain plants growing in a cereal field or pasture or along a fence row may be unwanted by a farmer or rancher, but they also may be wildflowers or a valuable wildlife cover to other people. Vine maple, Acer circinatum, is a valued source of deer browse in the spring and a spectacular source of coloration in the Cascade Mountains of Oregon and Washington in the United States, during autumn, but it also is known to hamper forest regeneration. It can be argued that many weeds in agricultural fields, forest plantations, and rangelands are not "out of place" at all but are simply not wanted there by some people.

In Table 1.1 we list many of the "human" characteristics that have been used to describe weeds. Most of these characteristics are based on some judgment of worth, success, or other human attribute, like aggressiveness, harmfulness, or being unsightly or ugly. Since this anthropomorphic view of weeds is so prevalent (Table 1.1), it may be that weeds are little more than plants that have aroused a level of human dislike at some particular place or time. Unfortunately, the anthropomorphic view of weeds provides little insight into why and where they exist, their interactions and associations with crops, native plants, and other organisms, or even how to manage them effectively. Weeds are found worldwide and have proven to be successful organisms in the environments that they inhabit. Therefore, it is important to explore whether weeds posses common traits that distinguish them from other plants or whether they are only set apart by local notions of usefulness.

A list of biological characteristics that describe weeds was proposed in the 1970s and continues to be used today (Table 1.2) (Baker 1974), but it seems unlikely that any plant species could possess all of those "ideal" weedy traits. However, Herbert Baker, botanist and originator of the list, suggests that a species might possess various combinations of the characteristics in Table 1.2, resulting in a range of weediness from minor to major weeds (Baker 1974). In the latter case, Baker believes that evolutionary processes would compound specific adaptations into highly successful (weedy) individuals, which constitutes an "all-purpose genotype." It must be stressed, however, that ecological success in the form of weediness cannot be measured solely from the perspective of noxiousness. The number of individuals, the range of habitats occupied, and the ability to continue the species through time must be considered foremost when evaluating success of a species. The obvious limitation of the list in Table 1.2 is that almost every plant species has some "weedy" characteristics, but, of course, not all plants are weeds.

Definitions

As we have just observed (Tables 1.1 and 1.2), weeds can be described in either anthropomorphic or biological terms. Weeds emerge from such descriptions as organisms that may possess a particular suite of biological characteristics but also have the distinction of negative human selection. Thus, a definition of a weed as any plant that is objectionable or interferes with the activities or welfare of man (Weed Science Society of America 1956) seems to describe sufficiently this category of vegetation. A sample of definitions of weeds published over the past century was presented by Randall (1997), who also argued that the most important criterion was problem-causing plants that interfere with land use.

Other authors, for example Zimmerman (1976), Aldrich (1984), and Rejmánek (2000), define weeds in more specific terms than the simple definition given above. Zimmerman believes that the term "weed" should be used to describe plants that (1) colonize disturbed habitats, (2) are not members of the original plant community, (3) are locally abundant, and (4) are economically of little value (or are costly to control). Aldrich defines weeds as plants that originated under a natural environment and, in response to (human) imposed or natural conditions, are interfering associates of crops and human activities. Each of these definitions implies that weeds have some common biological traits but also a level of relative undesirability as determined by particular people. Whether or not a plant is a weed depends on the context in which someone finds it and on the perspectives and objectives of those involved in dealing with it. Rejmánek, on the other hand, believes that weeds, colonizers, and naturalized species (including invasive plants) reflect three overlapping concepts. In his view (Figure 1.1), weeds are plants growing where they are not desired (anthropomorphic definition), colonizers occur early in succession (ecological definition), and invasive plants are plants that become locally established and spread to areas where they are not native (biogeographical definition).

The most important criterion for weediness is interference at some place or time with the values and activities of people—farmers, foresters, land managers, and many other segments of human society. However, the abundance of weeds is often of more concern than the mere presence of them. For instance, farmers and land managers are usually less concerned about the occurrence of a few isolated plants in a field, even noxious ones, than the occupation of land by vast numbers of weeds. Therefore, the relative abundance of plants, their location, and the potential use of the land they occupy should also be considered in weed definitions. When abundance is applied as a criterion for weediness, it implies a condition of the land as well as a class of vegetation (Table 1.2) and a form of human discrimination (Table 1.1). Weed abundance also may be an indicator or symptom of land mismanagement or neglect.

Agrestals. Agrestals are weeds of tilled, arable land. They require the nearly continual disturbance of agriculture to occupy the land. Holzner et al. (1982) indicate that every cropping system, for example, cereals, root crops, and orchards, also has its special complement of weeds, which may be either native plants or exotics that have been naturalized into the local flora. A list of the 76 worst agricultural weeds in the world was developed by Holm and his associates (1977) and has become the standard by which agrestals are compared. The top 18 weeds on this list are given in Table 1.3. An additional 104 of the weeds that cause the greatest impacts on agriculture was reviewed by Holm et al. in 1997. As a group these 180 agricultural weeds are estimated to cause over 90% of the loss of crop productivity worldwide (Holm et al. 1997).

Holzner and his associates (1982) suggest that agrestals have evolved as either specialists or colonizers during the course of agricultural history. Specialized weeds (specialists) have evolved a narrow adaptation to a single crop or sometimes crop cultivar and its particular growing conditions. Perhaps the most extreme example of how human activities influence weed species distribution and composition are crop mimics. These are weeds that have evolved life cycles or morphological features so similar to a crop that the two species cannot be distinguished or separated easily. Chapter 4 considers the influence of humans on the evolution of weed species, including crop mimicry, in much more depth. Since agrestals that are specialists have evolved along with the cultural practices of a particular crop, any change in practices usually disfavors the weed. Colonizers, on the other hand, are plants with characteristics that allow them to rapidly occupy and dominate disturbed areas. These species follow the general characteristics listed in Table 1.2 and Figure 1.1.

Weeds are major constraints to crop production, yet as primary producers, they also can be important components in an agroecosystem. It is in this context that weeds are sometimes perceived as an ecological "good" (Gerowitt et al. 2003). Awareness of the importance of weeds on arable land for their role in other trophic levels is growing as natural landscapes become rare or disappear due to the expansion of human-occupied landscapes. The weed flora in many parts of the world has changed over the past century, with some species declining in abundance while others have increased (Haas and Streibig 1982, Marshall et al. 2003, de la Fuente et al. 2006). These changes in the weed flora reflect improved agricultural efficiency, the use of different crops in arable rotations, and the use of more broad-spectrum herbicide combinations (Marshall et al. 2003, de la Fuente et al. 2003). Many weed species of arable land support a high diversity of insects, so the reduction in abundance of weed host plants can affect associated insects and, therefore, the abundance of other taxa. For example, in the United Kingdom a number of insect groups and farmland-associated birds (notably the grey partridge, Perdix perdix) have undergone marked population decline, which is associated with changes in agricultural practices over the past 30 years (Marshall et al. 2003). Thus, it seems that weeds may have a general role in supporting biodiversity within agroecosystems.

Invasive Plants. Invasive plants, unlike agricultural weeds, are generally defined as those that can successfully establish, become naturalized, and spread to new natural habitats apparently without further assistance from humans (Randall 1997). They are also generally nonnative or exotic in the new habitat and are often relatively new introductions to an ecoregion (Mashhadi and Radosevich 2003). Invasive plants respond readily to human-induced changes in the environment such as disturbance but also may initiate environmental change through their dominance on the landscape (Pyke and Knick 2003, Hobbs et al. 2006). In addition, the spatial and temporal extent of their impact may be expressed at scales ranging from local to global. Some ecological impacts believed to be caused by invasive plants are as follows (Parker et al. 1999, Alien Plant Working Group 2002):

• Reduction of biodiversity

• Loss or encroachment upon endangered and threatened species and their habitats

• Loss of habitat for native insects, birds, and other wildlife

• Loss of food sources for wildlife

• Changes to natural ecological processes such as plant community succession

• Alterations to the frequency and intensity of natural fires

• Disruptions of native plantanimal associations such as pollination, seed dispersal, and hostplant relationships

It is widely believed that the most effective way to limit plant invasions is to prevent the introduction of exotic species, which may be difficult because of the ongoing expansion in global travel and trade, changes in environments at all scales (local to global), and increasing development of land for human use (Kolar and Lodge 2001).

Although the traits of an "ideal weed" (Baker 1974) have also been ascribed to invasive plants, few empirical studies have tested this concept (Kolar and Lodge 2001). The biological characteristics of invasive plants appear in many cases to be dependent upon the habitat in which they occur (Sakai et al. 2001). Thus, general descriptions of invasive plants remain inconclusive. Some useful generalizations have been made, however, from reviews of empirical evidence or broadscale analyses of floras or databases. For example, Reichard and Hamilton (1997), using a regression tree analysis of biological and environmental traits of invasive plants, suggest that species known to be invasive elsewhere should be limited in introduction to a new area with a similar environment, where they might also be invasive. Reichard and Hamilton further suggest that a species related to one that is already "invading" a site may share invasive traits through a common ancestor. From a retrospective review of literature, Rejmanek (2000) lists several biological characteristics related to invasiveness, including constant fitness, small genome size, effective dispersal and vegetative propagation, and absence of strong interactions with other taxa (e.g., natural enemies, pollinators, seed dispersers) (Table 1.4). Sutherland (2004) reviewed databases for nearly 20,000 plant species in the United States and concluded that invasive exotic species were more likely to be perennial, monoecious, self-incompatible, and trees than noninvasive exotic species. A broad-scale analysis of the flora of the Czech Republic over 500 years showed that life-form and competitiveness were related to invasiveness (Pysek et al. 1995). Similarly, an analysis of global datasets revealed some common traits of invasive plants, including nitrogen fixation and clonal growth (Daehler 1998). Other traits that have been shown to be related to invasiveness are described in later chapters.

Terminology

Massive amounts of money, time, and energy are expended on weeds and invasive plants because of their economic and ecological costs and impacts on agricultural and natural systems. Because of the magnitude of these effects, it is important that scientists and land managers consider carefully the metaphors they use to describe these two categories of vegetation. Larson (2005) points out that metaphors allow people to understand abstract or perplexing subjects in term of something they already know about, a common referent. Thus, weeds and especially invasive plants are often described in militaristic terms, which probably date to Elton's (1958) classic The Ecology of Invasions by Animals and Plants. Davis (2005) points out that such terms as alien, exotic, invader, and invasion commonly used by invasion ecologists contrast markedly to the less evocative terms such as colonizer, founding population, introduced plant, nonnative, spread, or migration, which could be used to describe weeds and "invasive" plants. It should be noted that a similarly militaristic terminology has been used for decades in the pest management field.

From a management point of view, there is little doubt that the "invasion" terminology and metaphors have been useful in pointing out the significance of weeds to land managers and policymakers. From a strictly scientific point of view, however, it is difficult to argue against returning to the more value-neutral terminology used by Baker and Stebbins (1965) in their early classic, The Genetics of Colonizing Species (Davis 2005). Since this text is designed to fulfill a dual role for both scientists and land mangers and because the notion of "weed" is itself value laden, we have chosen to use the language of both scientists and managers that is in conventional use to discuss this important class of vegetation.
(Continues...) Excerpted from Ecology of Weeds and Invasive Plants by Steven R. Radosevich. Copyright © 2007 by John Wiley & Sons, Ltd. Excerpted by permission of John Wiley & Sons.
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The classic reference on weeds and invasive plants has been revised and updated.

The Third Edition of this authoritative reference provides an in-depth understanding of how weeds and invasive plants develop and interact in the environment so you can manage and control them more effectively. The guide includes an introduction to weeds and invasive plants in various environments and an overview of their ecology and evolution. With extensive examples, this book:

• Focuses on the biological features of weeds and invasive plants, especially as they exist in agriculture, forests, rangelands, and natural ecosystems.
• Includes coverage of exotic invasive plants.
• Discusses a variety of methods and tools for managing weeds and invasive plants, including physical, cultural, biological, and chemical approaches.
• Examines systems approaches for management, including modern Integrated Pest Management.
• Addresses future challenges for scientists, farmers, and land managers.

This is the definitive, hands-on reference if you're a land manager or professional in plant sciences, agronomy, weed science, and horticulture. The book is also an excellent textbook for senior undergraduate or graduate students studying agriculture, ecology, natural resources management, environmental management, or related fields.