Biological invasions have been attracting the attention of ecologists for the last thirty years, which has led to a rapid increase in the number of scientific papers and popular articles, as well as books, on the subject of invasions, therefore creating an impression that invasions are a relatively novel phenomenon. However, first articles concerning invasion come from the famous naturalist Darwin, who says „many European organisms are present in the territory of La Plata, and also in Australia on a smaller scale, and they have outcompeted native species to a certain extent…“. Despite the fact that biological invasions have also existed in the past, when describing the history of biological invasions, people usually start from the publication of the famous classical piece on invasions „The Ecology of Invasions by Animals and Plants“ (Elton, 1958)[1]. However, the contents of this publication were understood only in the 1980s, i.e. when it had become clear that invasions by alien species are one of the greatest dangers to native species and ecosystems.

Due to different points of view, certain disagreements and overlaps, which create confusion, the study of biological invasions requires a clarification of terms and simplification of definitions used in this branch of science. It is of great importance to stress that organisms can be divided into native and alien, by their origin. Native organisms are species present in areas which are part of their natural range of distribution, and alien are those organisms which have been introduced into areas which are not a part of their natural distribution range. There are also those species which cannot be classified into one of these categories with certainty, and they are called cryptogenic species. Alien species in the area where they have been introduced can threaten the survival of native species and have negative effects on the ecosystems, agriculture, health of people and domestic animals, as well as socio-economic aspects of the country.

Defining biological invasions according to the criterion of their influence on the community implies that the species must have a significant (positive or negative) impact on the community, i.e. ecosystem in which it spreads in order for it to be considered invasive. In general, the impact of biological invasions on the community, i.e. ecosystem depends on the biological characteristics of the species, and characteristics of the community, i.e. the ecosystem in which the species has been introduced. Therefore, if a certain species gets introduces into two different ecosystems, the effects of its invasion needn’t be (and they usually aren’t) the same. Different studies have shown that the intensity of the influence is greater in those cases when the differences in the characteristics of native and alien species are more pronounced. Also, there are different points of view on whether only introduced alien species can be considered invasive, or native species can also be classified as invasive. From the ecological point of view, an invasive species is always an “alien” for the environment into which it has arrived. Some scientists consider that the species which is native to a certain region can also be considered invasive when colonizing neighbouring and nearby habitats, spreading diffusely. This wider definition of biological invasions disregards  the differences between native and alien introduced species. It helps simplify the concept of invasions, by excluding the dilemma concerning the cryptogenic species for which it is not entirely clear whether they are native or not. Although there are points of view insisting that native species cannot be considered invasive, Richardson et al. (2000)[2], Pyšek and Richardson (2006)[3] and other phytoecologists consider that the functional similarity between native species, which become dominant suddenly, and introduced species, which colonize new areas, indicates the existence of the same spread mechanisms in both cases.

Groves (1986)[4] considers that the process of invasion can be divided into three phases: introduction, colonization and naturalization. He defines introduction as the process of spread of reproductive organs, which reach a place outside the previous range of the species’ distribution and the establishment of the mature plant population. Colonization implies that plants which form populations reproduce and that an independent population forms as a result of the increase in their numbers. Finally, the moment when the species has formed new populations capable of survival and spread, which become an integral part of the native flora is known as naturalization. Not all phytoecologists which study biological invasions agree with this way of defining the stages of invasion, because they find this type of division insufficiently clear. They think that the fundamental requirement for invasion is the introduction, while what others define as colonization is an integral part of the process of naturalization, while naturalization (from Groves’ definition) conforms with the term ”invasiveness”. The same authors define invasion as a process in which a taxon has to overcome different biotic and/or abiotic barriers. The concept of a ”barrier” (whether they are barriers which have been overcome or not) is very adequate for defining the phases of the invasion process. According to this concept, introduction means that the plant has managed to overcome a big geographic barrier, with human assistance. Many introduced taxa survive through reproduction, but don’t have the ability to maintain their populations over a longer period of time. Furthermore, it can be considered that the naturalization stage begins when the environment (as one of the barriers) is adequate for the individuals to survive and reproduce naturally. Therefore, a taxon can be considered naturalized only once it has overcome all three barriers. In this stage the population is big enough that the probability of its extinction due to unfavourable environmental conditions is low. According to the concept of ”barriers”, invasion entails the spread of the taxon outside its place of introduction, where the introduced plant must also overcome the barriers which prevent its spread into the new region. The introduction and spread of the species into new areas has begun in the time of the tribal movements, and due to the rapid disturbance of natural area it has been intensified with the development of the civilization, social, industrial and urban development, which has been followed by the rapid progress in technology, traffic and transport modernization, especially transoceanic and intercontinental, so that the number of intentionally or accidentally introduced species has increased significantly.

For a large portion of alien weed species exact time and introduction pathway are unknown, as well as the mode and way of their spread – there are just assumptions. Researchers are trying to reconstruct the introduction pathways and dispersal corridors for certain areas. One of the approaches in these types of research entails the analysis of museum collections, which are a rich source of information for ecologists and phytogeographers. The researchers mainly agree in the belief that some natural processes could never be explained and described without using these animal and herbarium collections. Biological collections also have an immensely important role in the monitoring of certain species extinctions and disruption of ecosystem biodiversity, due to anthropological influence. Herbarium collections very explicitly document and confirm the species’ presence or absence, thereby enabling rather reliable reconstruction of the process of invasive species spread. Maps (charts) which are based upon the analysis of available herbarium materials show the spatial distribution of ”conqueror plants” in function of time and provide evidence for the invasion pathways and the directions of their spread, as well as on the speed, i.e. dynamics of the alien invasive species spread.

Trinajstić (1976)[5] has suggested a similar division, into:  boyletophytes – intentionally introduced adventive plant species and aboyletophytes – unintentionally, i.e. species introduced without our knowledge. The intentional introduction implies the introduction of a new species, mostly for ornamental purposes. If that species ”escapes the gardens” it can spread into the surrounding ecosystems very rapidly, causing severe damage. In natural habitats, introduction is allowed when the introduced species are used in biological control measures, or when the introduced species have almost identical functions in the ecosystem as the species which have disappeared, i.e. when the introduced species replaces the species which has disappeared. Unintentional introductions often remain unnoticed for long periods of time, and are registered too late, when the species have become a problem in the ruderal and agricultural areas. The pathways of unintentional introduction are different – insufficiently purified seed material, the use of unburned manure, inadequate soil preparation, wool industry, traffic, wind, animals, etc. Unintentional introduction of weed seeds most often happens through the deliveries of seed materials and seedlings, which pass unregistered by the quarantine authorities during the export and import of commodities. Ambrosia artemisiifolia has reached Europe from America in the XIX century in this way, with the seeds of corn and red clover, while Eleusine indica was imported into Europe at the end of the XIX century with the oilseeds, and  Lepidium virginicum was introduced with grass seeds and oilseeds.

Some authors think that the difference should also be made with regards to the time of introduction. According to this, introduced alien species can be divided into: archeophytes – species which have been introduced into Europe before 1492 (the discovery of America) and neophytes – species which have been introduced after this time. Trinajstić has proposed a more detailed division of adventive species according to their time of introduction, into:  archeophytes – introduced  between the paleozoic and neolitic period; paleophytes – introduced during the Ancient Times and Middle Ages, until the discovery of America (year 1492); neophytes – introduced after the discovery of America, and before the World War II, and neotophytes – introduced from the beginning of the World War II until this day.

The introduction and spread of introduced alien species is possible in many ways, and depending on the shape, morphology and size of the reproductive organs it can be: autochory, antropochory, hydrochory and zoochory. In some species, ripe fruits can, due to their specific morpho-anatomical properties, open rapidly or “invert” quickly, thereby spreading the seeds and dispersing them onto certain distances – this form of dispersal is known as autochory. The dispersal of plant fruits and seeds as a result of wind activity is called anemochory. Anemochory is one of the most common and most successful ways for plants to quickly and efficiently cross great distances and colonize new habitats. The dispersal of plant fruits and seeds by water is called hydrochory and this way of dispersal is far less significant for weed species when compared to anemochory, except with the aquatic invasive species, like Elodea canadensis. Zoochory, as a way of plant dispersal, encompasses all the ways in which plants, and therefore also weeds, fruits and seeds are dispersed by animals. The fruits and seeds can be dispersed by animals through: endozoochory – the dispersal through feces when the seeds and/or fruits pass through the animals’ digestive system undamaged, synoic zoochory – the dispersal through the storage of fruits and seeds when they are gathered by animals as food reserves for the winter period and epizoochory – dispersal by attaching to the body hair of animals (wool, hair and feathers). Among the ways of vertebrate dispersal of seeds and fruits, the most important form is ornithochory (dispersal by birds) and teriochory (dispersal by mammals). The dissemination of plant fruits and seeds by humans is called anthropochory. Anthropochory has been inseparable from the agriculture since its inception and it can be said that the scale and distance range of anthropochory have become more clear with the passage of time. It has been, to a great extent, a result of the increase in migrations, travel, intensive traffic development, wars, trade and also tourism, to a certain extent.

The influence of biological invasions on ecosystems is a subject of numerous debates within the scientific community. Many researchers “blame” the phenomenon of invasions for the loss of biodiversity, but when it is taken into consideration that the number of naturalized species far surpasses the number of extinct species it can be concluded that the biodiversity level has actually increased as a result of invasion processes. Therefore, invasions should not be considered as a priori negative with regards to biodiversity. The fact that the central European flora has become more diverse due to many newly introduced plant species also speaks in favor of this, and in the Mediterranean region there are numerous examples of positive influence the invasions have had on the species’ biodiversity. However, it is clear that it is difficult to determine and predict different impacts the introduced species have on natural habitats and biological communities.

In order to estimate the impact of introduced species on the new habitat more objectively, it is necessary to perceive:

• behaviour of the introduced species/population,
• its effect on the biotic and abiotic factors,
• the impact of the environment on these populations, and
• the genotypic and phenotypic changes of these populations in the new environment.

Changes in the ecosystem, caused by invasive species, which have no significance for human welfare usually do not attract great attention from the general public, despite the fact that their influence can be very important for the ecosystem. However, the influence of invasive species on the ecosystem which also addresses human interests, attracts the public attention worldwide. The ecosystem effects are visible through primary production, the functioning of matter and water cycles, soil formation and fertility retention, as well as through the production of food, fresh water, fuels, genetic resources, biochemicals and pharmaceutical products, folk medicines, ornamental plants, etc. Besides, many alien species are plant pathogens or parasites, and invasive weed species and other invasive organisms present a serious problem in agriculture. Invasive organisms also modify and affect the air quality, the climate, water cycle (time and intensity of floods, water drainage, etc.), the quality and quantity of water, disease regulation, natural pests control, pollination, erosion control and storm protection. Through the influence on the ecosystem, invasive organisms also affect different ecosystem aspects, along with recreation, tourism, its spiritual, religious, educational and scientific values, the values of its cultural heritage, etc. The public attention is especially drawn by the economic influence of the invasive species on the ecosystems, which entail the costs of control and eradication measures for the invasive species. Also, great significance is given to weed species invasions, which have different negative effects on agriculture. Also, certain groups of invasive organisms have significant impact on the human health, whether because they directly cause diseases and allergies or by being the vectors of plant pathogens. Many invasive species can cause allergic reactions in humans and animals. The problems caused by invasive species can also be of a mechanical or traumatic nature. Aside from all the negative effects on human and animal health, many invasive species contain certain medicinal ingredients, and as such can be used in folk medicine, pharmaceutical, cosmetic and food industry, etc.

The influence of invasive species on the society and economy can be:  (i) negative, (ii) negative with occasional small positive effects, and (iii) positive, and their significance is valued differently depending on the perception, which can be very diverse, changeable and dependent on the context in which it is being observed. One of the key problems in the valorization and characterization of invasion processes is the lack of knowledge. The fact is that the impact of biological invasions in a socio-economic concept is analyzed with regards to the question whether a certain species undermines the ecosystem values for human livelihood or not. Another example of a double meaning of the term invasiveness is the case of Aegilops triuncialis and Acacia mearnsii, which the ecologists consider unwanted, while the agronomists find that the value of these species is greater (they increase the nitrogen content in the soils) than the negative effects which they have from the socio-economic standpoint. In general, the social consequences of biological invasions can be divided into four categories: ethical, aesthetic, historical and recreational. The ethical significance comes from the conviction that different species have an intrinsic value and deserve the protection from anthropogenic destruction. The aesthetic significance of different species lies in the natural beauty of these organisms and their habitats, and the historical value relates to the historical significance of these species in a certain area or region. Unlike the North America, Australia or New Zealand, which pay great attention to the economic consequences of biological invasions, in Europe this matter has received more attention only in the last ten or so years, and the estimates are that the yearly losses from alien invasive species amount to about 12 billion Euros. Only the losses coming from pollen of the weed species Ambrosia artemisiifolia, due to medical expenses, reach up to 10 million Euros in Hungary, and 88 million Euros in Australia. Besides, this species can cause losses in tourism, as is the case on the Dalmatian coast, which is overrun by this species. Also, this species leads to huge losses in agriculture, especially in the sunflower, corn, sugar beet and soybean fields, with the losses reaching up to 130 million Euros in Hungary alone.

Despite the great scientific and technological progress, it is impossible to completely solve the problem of biological invasions, due to the fact that humanity is still unable to control all the processes occurring in nature. However, it is possible to take different measures in order to limit biological invasions and reduce their negative impacts. European and Mediterranean Plant Protection Organization (EPPO) has been trying for the past 60 years to prevent the introduction and spread of organisms which are harmful to plants in the area of Europe and Mediterranean, giving priority to those organisms which have adverse effects on agriculture. As invasive species can seriously threaten native species and their natural communities, EPPO puts special emphasis on the analysis of risks which invasive plant species pose in the EPPO region and recommended measures for the prevention of their introduction and spread through international trade routes.

The Commission on Invasive Species was founded in 2002, with a mission to:

• gather information on invasive species in the EPPO region;
• conduct studies to assess the risks of specific invasive species;
• recommend measures for the prevention of their introduction and spread; and
• recommend measures for the eradication and/or control of the already introduced invasive species.

One of the very important EPPO missions is to encourage the exchange of information on invasive species through its publications (EPPO Bulletin), databases and international conferences, and additionally, EPPO cooperates with a number of European organizations dealing with the issues of environmental and biodiversity conservation.

The study of genetic diversity between the individuals of the same species, essentially resulting from the presence of different alleles and gene pool, thereby different genotypes within a single population, is of critical importance for the reliable monitoring, risk assessment and control of invasive species. Assessment of genetic diversity of invasive weed species demands the analysis of a population along a certain gradient of the environment. It is possible to track different levels of the species’ genetic structure along a different spatial gradient, however, the relative signifficance of the environmental factors influencing the genetic structure also varies along the continuum of the spatial gradient. For example, unequal gene transfer is a key factor defining the genetic structure of a population in a wider area. On a global, i.e. continental and/or intercontinental level, humans have a great influence on the genetic structure of invasive weed species and the adaptability of their populations.

Recombination, hybridization, introgression and mutation are the main drivers of genetic diversity. In this way, for example, mutations lead to changes in the DNA sequence. Therefore, every change in the structure of the genetic material that cannot be a result of the recombination of genes or chromosomes is considered to be a mutation. It is a change in one of the traits which wasn’t inherited from the parents, but once it occurs it will be passed further on. Oscillations, i.e. fluctuations of genetic variability, which are a result of the changes in the alleles and the frequency of genotypes over time, are the basis of population genetics. Factors initiating these changes and influencing the genetic diversity levels within and between plant populations are: selection, gene transfer, genetic drift, crossing-over system, etc. Therefore, if the level of genetic variability is known, and the variability within and between the populations is „decoded“, it can be determined in which way the factors which have resulted in this variability have acted on this population/species in that environment, and it is possible to reconstruct the phylogenetic tree of the species and its lower and higher taxa in a reliable manner. However, the relative influence of each individual factor (e.g. temperature, humidity, salinity, soil trophic regime, etc.), which leads to  population variability, differs in time and space, therefore making it very difficult to define what exactly has influenced the genetic structure of that population.

Other research focused on the diversity of invasive populations, aiming to clarify in which way the alien plant populations adapt and spread in the newly colonized environment, through the study of genetic structures. That is, which genetic features enable the adaptability, i.e. make the species eurivalent and more plastic in the new environment. Regarding this, it is a popular opinion that populations with a stronger genetic diversity have by definition a greater level of invasiveness, which in A. artemisiifolia species means 6 to 20 km on a yearly basis, making it one of the extremely invasive weed species.

The degree and speed of invasive weed species spread, as well as in other living organisms, is to a great extent influenced by ecological adaptations, regardless of whether it is in an environment where this species is native, or an environment into which it has been introduced, i.e. has a status of an alien species. Measurable ecological adaptations can be linked to genetic diversity of the population, based on the degree of changes resulting from natural selection. Also, the level of ecological adaptations of alien plant species, i.e. their populations, is to a great extent influenced by the capacity for the initial colonization (group of individuals which have been introduced first). Namely, the process of adaptation to conditions of the new environment can lead to a significant reduction in the genetic diversity of the alien species. In theory, it means that at the beginning there can be a reduction in adaptive capacity of the alien species in the new environment, but it is not necessarily always the case. Over the course of the period of ecological adaptation the introduced species can go through multiple phases – ephemerous, naturalized and invasive, and over the course of these phases, there is a weaker or stronger reduction in adaptive capacity. However, as time goes by the species/population “regenerates” with regards to this quality. There are a few possible ways in which the genetic diversity of an introduced species can be restored during the adaptation period, and those are: multiple introductions of the species/population, resulting in the introductions of new, i.e. additional genotypes, from the natural area of distribution of the species; and in the processes of intra- and inter-specific hybridization.

Clonal reproduction can lead to a reduction in the genetic diversity within the population, so that the better adapted clonal genotypes spread faster, and make better use of the available resources, making them more successful competitors for space and natural resources. Clones, in the genetic sense, represent a group of genetically identical cells or organisms originating from the same ancestral cell or organism. Some of the alien invasive weed species which are clones are:  Fallopia japonica, Alternanthera philoxeroides, Egeria densa, Pilosella officinarum, etc.

Adaptive mechanisms of alien invasive weed species are based on two groups of factors: phenotypic plasticity and selection, resulting in the survival of those ecotypes which can adapt to new environmental conditions. Phenotypic plasticity is a phenomenon in which the same genotype can portray a series of different phenotypes in different environmental conditions. This characteristic becomes evident in alien weed species’ adaptations to local conditions. In contrast to that, many researchers studying biological invasions highlight the importance of the process of adaptation of an invasive species to local environmental conditions in the process of selection within the intra-specific variability. They have determined a significant genetic variability within the native populations of Common St John’s wort (Hypericum perforatum), growing in Europe and introduced (alien) populations which are widespread as invasive plants in America.

Polyploidy and hybridization can be important factors in the development of new adaptive features in weed species, leading to a quick colonization of novel habitats in a new environment. Certain phylogenetic groups of plants are biologically predisposed to create hybrid individuals and/or to persist as hybrids. Polyploidy is a state in which an organism has more chromosome sets than a diploid. Polyploid populations originate in the process of hybridization of related species and, by definition, have better fitness than the diploid individuals of the same species, most likely due to the increase in heterozygosity and reduction in the inbreeding depression (the reduction of the adaptive value and vigor resulting from the effects of inbreeding in individuals which are normally open-pollynated (alogamy). Weed hybrids can result from sexual reproduction between the individuals of different species, known as the interspecific hybridization, or between the different populations within the same species known as the intraspecific hybridization. The first generation (F1) of plants which originates in the process of hybridization, usually contains a wider set of genetic material than its parents. The ensuing offspring can mutually reproduce sexually and create new generations of new hybrids. A repeated backcrossing of the hybrid offspring with the same parental line can transfer individual genes and embed the traits from one parent from the population (species) onto the others. In reality, this means that many invasive hybrid plants represent a continuity of hybrid types, ranging from the novel F1 hybrid to the highly introgressed individuals which are more alike one of the parents, i.e. look very little like the other. The onset of introgression represents the incorporation of genes of one species into the genepool of another speices through hybridization and backcrossing.

The hybridization between native and alien plants can enable the introduced species to inherit adaptive features from the native species. Also, the reverse is also possible, i.e. that the resulting hybrid can inherit a larger portion of the genetic material from the introduced plant, thus enabling it to infiltrate, i.e. to asimilate into the native population more easily. Up until now more hybrids originating in the process of hybridization between a native and an introduced species which represent examples of a potential genetic assimilation are known, and these hybrids have become aggressive invasive species, e.g. Taraxacum officinale and Taraxacum ceratophorum in America, Eucaliptus spp. in Tasmania, Spartina alterniflora in California, etc.

By comparing the genetic diversity of populations in their introduced and native ranges it is potentially possible to show which native populations were the source of invasion. In the same manner, based on the comparative analysis of the genetic diversity of native and alien populations it is possible to evaluate how much of the genetic diversity gets lost during the process of invasion, which on the other hand confirms whether or not there has been multiple introductions of populations.

Reconstructing the invasion history of a certain species, including weed species, requires, just like the genetic diversity research, for a genetic analysis of the plant material to be done. Several general approaches to the history of invasions have been described so far, but models of plant invasions haven’t been precisely defined yet. In scientific circles there are a few existing models explaining the history of biological invasions. One of the approaches is based on the fact that there is an initial reduction in the genetic diversity of introduced populations due to the loss of a certain set of individuals in the colonizing plant populations, although this is yet not an absolute barrier, therefore enabling a possible later spread of surviving populations. The other approach in explaining the history of biological invasions starts with the assumption that there have been multiple introductions of the populations, which have positively reflected on the phenomenon and process of biological invasion. Regarding this, according to the previous research done, the appearance of certain invasive species gets initiated by a very small number of introduced individuals, or even by introducing only solitary individuals. The third approach in explaining the history of invasions is based on the genetic analysis showing that men have unintentionally (for example Ambrosia artemisiifolia and Ambrosia trifida from America into Europe) or intentionally (for example Fallopia japonica from the Far East into Europe) introduced many species, i.e. brought them in for certain purposes.

Hybridization as a method of creating plants tolerant to herbicides is especially significant in those plant species where it is possible to come to a hybridization with the weed species or wild congeners which possess genes for resistance to a certain herbicide. Through the hybridization of the weed species Brassica campestris, resistant to atrazine, with a number of cultivated species of the genus Brassica commercially important genotypes, resistant to atrazine, were created. Although the yield of these genotypes was reduced, on average, by 20% due to the unfavorable chloroplast mutation, these genotypes have found their place in highly weedy areas where classic cultivation method was not economically justified. Many genotypes were obtained through direct insertion of desirable alien genes into the host plant’s genome. DNA hybridization (Southern blotting), polymerase chain reaction (PCR) with the application of specific DNA and enzyme activity analysis are generally accepted methods for screening of the foreign gene insertion into the host cell’s genome.

Today there are genetically modified corn, soy, cotton, oilseed rape, sugar beet and fodder beet plants which are tolerant to herbicide gliphosate, but also corn, soybean, cotton, oilseed rape, sugar and fodder beet, tomato and wheat plants tolerant to gluphosinate. Also, there are sunflower hybrids which are tolerant to imazamox and imazetapir and to tribenurol-methile also.

The risks linked to the cultivation of genetically modified (GM) plants tolerant to herbicides are not entirely known and fully clarified. They can be of a different nature and duration. The biggest fear with GM plants is the possibility of gene transfer into their wild congeners and development of resistant weeds. Due to the genetic variability of crops and weeds, and chemical variability of herbicides, many phenomena cannot be generalized. As in previous cases, it is necessary to observe each case in particular (plant, herbicide, wild congeners, etc.) when considering potential risks. Gene transfer from the cultivated plant tolerant to a certain herbicide into its wild congeners is an existing problem. The adversaries of GM plants have most frequently pointed out this problem, which is felt the most in the centre of origin of the crops. In the case of corn, the risk is present in central America, while in the other regions it has no such signifficance. Gene transfer is possible only between sexually compatible plant species. Up to now, out of 60 cultivated species worldwide, only 11 have no wild congeners. In twelve out of thirteen leading cultivated plant species, natural hybridization with wild congeners has been proven. Also, it has been said that as a result of gene transfer, the wild congener of the cultivated plant can take on the invasive species’ traits. The possibility of the transfer of the gene responsible for tolerance/resistance to imidazoline, from cultivated sunflower into the wild sunflower has already been confirmed several times worldwide. Likewise, the tolerance gene to imidazoline in wheat can be transferred into a weed species  Aegilops cylindrica through hybridization in natural conditions.

GM plants tolerant to herbicides as volunteer plants behave as weeds in subsequent crops and can therefore acquire the status of an invasive species. Also, there is a real possibility for a hybridization between a volunteer crop plant and its wild congener, thereby creating hybrids which can also possess the invasive species’ traits. These risks are definitely a problem in the zones of intensive cultivation of GMs, i.e. tolerant crops, and some of the cases have already been confirmed. Therefore, this phenomenon needs to be studied and risks of spread of resistant weed populations monitored in order for the risks to be minimized. Faced with this problem, the global companies are searching for a solutions to the problem of creation of hybrids/varieties tolerant to dicamba herbicide. In this new biotechnological endeavor there is considered to be a certain kind of a security mechanism, which is a gene taken from a bacteria, which is resistant to dicamba herbicide. As chloroplast DNA is inherited only through mothers it means that the gene of this GM plant will not be transferred through pollen (male plants) – therefore creating a reproductive barrier (and thereby preventing the development of weed resistance to dicamba herbicide). Monsanto company has already protected “the dicamba technology” through a license and is now working on creating crops which are resistant to a number of different herbicides, the so-called OMICS (“genomics, “proteomics”, “metabolomics”, “transcriptomics”, “weedomics” etc.) technologies (combining the multiple-resistance genes in one plant).

The research of community invasibility has a biological approach, which implies the study of species composition (e.g. the number of parasites or predators) in the recipient habitat, as well as of the conditions of the environment, making the given community or habitat susceptible or resilient to a biological invasion. According to the available literature, there are 16 hypothesis mentioned in the Ecology of biological invasions, which try to explain the success or failure of invasions. They do not exclude each other, and the relative significance of each of them can be different in different habitats, for different species and in different stages of invasion. The hypothesis are: Biotic Resistance Hypothesis (BRH) – which explains the invasion models, but does not predict the possible invasion success, i.e. explain the reasons of the invasion’s failure. The concept of the existence and occupancy of ecological niches, on which this hypothesis is based upon implies that the communities with a higher diversity are less sensitive to invasion by new species; the  Resource Fluctuation Hypothesis (RFH) – according to this hypothesis the changes in the resources promote invasions by creating favorable conditions for new species, or by reducing the potential competition with native species in a certain timeframe; the Superior Competitor Hypothesis (SCH) – according to which a successful naturalization of an invasive species is possible if it uses the limited natural resources more efficiently (they are marked with an R*) than the resident species, while consequently suppressing or eliminating the competitively weaker resident species; the  Enemy Release Hypothesis (ERH) – based on the rapid increase in the numbers and spread of plant species in the newly-colonized area, as a consequence of reduction in the number of natural enemies, including herbivores; the Invasional Facilitation Hypothesis (IFH) – which unlike the BRH and ERH hypotheses foresees an increase in the degree of invasibility with the passage of time; the  Evolution of Increased Competitive Ability (EICA) which foresees that an invasive species, which has been released from its natural enemies over a longer period of time (from its native environment), can allocate resources (which it has used for its defence earlier) for growth, reproduction and other needs which strengthen its competitive ability;  General-Purpose genotype Hypothesis (GPG) – which relates to the species which possess such traits which enable them to colonize very different habitats (traits such as the growth adaptation, r-selected life cycle and tolerance to  highly different environmental conditions); the Selection for Invasive Ability Hypothesis (SIA) – based on the fact that invasive species can rapidly adapt to new conditions of the environment, or are in some way selected by anthropogenic activities; the  Invasiveness as an Evolutionary Strategy Hypothesis (IES) – based on the view that there is a greater risk from invasion of those plant species which are related to an invasive species, i.e. that the distribution of invasive species is not phylogenetically random; Human Commensal Hypothesis (HCH) – by which the most successful invasive species are those whose survival is “linked to humans” and which benefit from the anthropogenic disturbance or other human activities; Weapon of Mass Destruction Hypothesis (WMD), or as Inderjit et al. (2005) suggest: Environmental Manipulation Hypothesis (EMH) – by which invasive species are successful because they can alter the conditions in the immediate environment so that it suits their needs, and this on the expense of native species; and the Neutral Community Dynamics Hypothesis (NCD) – which can be considered the zero hypothesis, and according to which there is no single direct mechanism of invasion, other than random chance, i.e. that the outcomes of native and alien species interactions are so complex that they can be stochastically modeled.

There are two approaches which clearly stand out as part of the research conducted with the aim to define traits which are of importance to invasion: (i) studies which include a large number of species, and (ii) studies of related species within a single genus (in certain cases this category also includes the so-called confamiliar research (the studies of related species, different genera within the same family). Over the last year the increase of comparative studies between two or more species has been evident, resulting in a more efficient availability of data. Research strategies in studies which include large species numbers are based upon: different approaches (with regards to the study area), different comparisons (native-alien or alien-alien species), different scopes of research (local – habitat level, regional or continental), different levels of data, different parameters (presence, abundance, frequency, area of distribution, history), different analytical methods (simple comparisons, phylogenetic corrections), etc.

Based on a number of studies it is considered that the invasibility success depends on:

• complex species traits (growth form, life form, life strategy according to Grime, area of origin);
• morphological characteristics of a species (the plant’s height, mass, number of leaves, leaf morphology, spatial vegetative growth);
• physiological characteristics of the species (intensity of photosynthesis and the efficacy of water, nitrogen and phosphorus use, chlorophyll and nitrogen content in the leaves, leaf longevity and the consumption for tissue formation, specific and total leaf area, relative growth rate of the seedlings);
• reproductive characteristics of the species (type of reproduction, pollen quality, pollinators, flowering period, reproductive maturity, the size of reproductive organs, dispersal method, seed dispersion, germination ability, formation and survival of seedlings, seed dormancy, seed size, seed longevity and seedbank); and
• the reaction of the species to abiotic and biotic conditions (habitat adequacy, tolerance to low levels of nutrients, drought and fire tolerance, tolerance to grazing (herbivores), tolerance to shading).

Many scientists have dealt so far with the predictions of invasions and their success for certain plant species, with the aim, among other things, to find a universally acceptable model for their prediction and control. Methods for the invasibility risk assessment are necessary, especially in the cases when it is necessary to make a decision on the introduction of a certain alien plant species into a new region. In the late 90s of the XX century three screening systems for predicting the invasibility of alien plants have independently been developed:

• for invasive species of North America
• for woody invasive species of South African regions known as fynbos (a characteristic type of vegetation exclusive for the southern part of Africa which includes a wide array of plant species, and especially low-growing trees, scrub and bush); and
• for invasive plant species of Australia.

All three systems are applicable on a wider array of taxonomic groups, but they are all mostly focused on specific ecosystems or geographical regions. All three systems are based upon different types of information (life cycle, phytogeography, habitat characteristics, weed species history) with the aim to classify the studied species into those which will most likely be invasive, which will probably not be invasive and those which demand further studies. In regions for which they were designed these systems have a 70-90% rate of success. On the other hand, several risk assessment systems have been developed in Europe, and the most famous one is the Pest Risk Assessment Scheme which is used by all country members of the EPPO (European and Mediterranean Plant Protection Organization). This protocol relates to all pest organisms, including weeds, and upon it is also based the following classification into: (i) harmful species, which are further managed according to the adopted regulations; and (ii) quarantine and harmful species which could present a risk. Unlike the previously stated screening systems, this procedure, based upon an expert assessment, does not require the species to be ranked.

Therefore, the EPPO has established a prioritization process for invasive plant species with the aim to:

• define the list of invasive species forming communities, or which can potentially form them, in the EPPO region, and
• to determine, i.e. set up which of the species are of the biggest priority for risk assessment.

The seeds of different alien species are continuously being introduced into new areas, but only a small portion of them is capable of developing populations in their new habitats. Some of the introduced species are only temporarily present in the new environment (ephemorous plants), but they do not form stable populations, some are present and they reproduce, but are not of an expansive character (naturalized), while some establish strong connections to their new habitat, go through a complete life cycle and expand successfully colonizing vast areas (invasive). Upon the establishment of stable populations the invasive species have the ability to spread either to long (with or without human assistance) or short distances (dispersion and locally).

According to some phytoecologists the spread of invasive plant species can be divided into four stages:

• the arrival of a new species into a novel environment and its establishment in the new habitat,
• formation of long-lasting populations whose individuals can grow unimpeded and can reproduce successfully,
• the spread of these populations into new, suitable habitats, and
• the expansive spread of species through the increase in the number and size of its

The spread of invasive weeds depends on different factors: the characteristics of the species, recipient vegetation, habitat and climate, but also on the socio-economic conditions in the recipient area. The characteristics of the species significant for the spread of invasive weeds are: the capability of the species to propagate both by seeds and vegetatively, short life cycle, the ability of the species to adapt to stressful conditions (phenotypic plasticity) and increased tolerance (plasticity) to changing environmental conditions. However, many species are very different in their level of invasiveness although they have the aforementioned characteristics, while many invasive species have just some of those features. It is considered that the mode of reproduction of alien species plays a key role in their spread, which further affects their invasiveness. The production of a great amount of seeds and the ability to propagate sexually are often highlighted as important qualities for the invasiveness of a species. The spread of invasive species does not depend solely on the quantity of produced seeds, but also on its characteristics. Also, the morphology of the seed coat can be more or less adapted for dispersal by different agents. So, for example, the species  Erigeron canadensis is primarily spread by wind due to its pappus. Wind dispersal of seeds does not depend solely on the presence and morphology of pappus, but also on the speed and direction of the wind. Intensive vegetative reproduction can also enable successful spread of species. By rule, plants which reproduce vegetatively are more successful invaders than those which are exclusively propagated by seeds. Such is the case of the invasive species Fallopia japonica which spreads very aggressively by its rhizome growth, thus hampering the attempts for its eradication. However, the mechanism of spread of a single species can be different on a local scale from the mechanism of its spread to great distances. Thereby, the species Spartina alterniflora spreads through the growth of its rhizome on the local scale, and through seeds over great distances. When considering the characteristics of the recipient vegetation it is thought that the most important thing for the sensitivity of a plant community to the spread of invasive species is the existence of empty or half-empty ecological niches, followed by the recipient species’ resilience or the level of habitat degradation. In agrophytocoenosis, crop species have a significant influence on the spread of invasive weeds. One of typical examples is the spread of Ambrosia artemisiifolia in sunflower crops which is a result of a small choice of herbicides available for its suppression due to the phylogenetic relatedness to the crop. The characteristics of the habitat in combination with other factors also affect the speed and success of the spread of alien species. Degraded habitats are the most common recipients of alien invasive species. Besides, it has been observed that these species are typical for habitats whose purpose has been changed. Other than the state of the habitat, ecological conditions of the habitat also affect the spread of invasive species. Hence, the successful spread of A. artemisiifolia is usually attributed to its high tolerance of environmental conditions, including drought and soil salinity, as well as the weak nourishment of the soil, and also to its symbiosis with arbuscular mycorrhizal fungi, etc. The characteristics of the climate also have a strong effect on the spread of invasive species. Namely, the temperature, precipitation, CO₂ concentration in the air and availability of nutrients in the soil are main factors affecting the survival of the species, and therefore, the changes in these factors can be stressful to the ecosystem and influence the process of biological invasion. Apart from this, societal and socio-economic relations can affect the spread of invasive species. With the development of transportation and the establishment of intensive commodity exchange between the continents, after the discovery of America, the spread of invasive organisms has intensified. Wars have also played an important role in the spread of invasive species, because the army has introduced many plant species in order to cover up its military establishments and armories.

The rate of spread (index of spread) of invasive weeds differs significantly among the species from different habitats and in different regions and can be analyzed based on the distances (m or km) which invasive species cross within a certain timeframe or based on areas which they colonize during their spread over a certain time period. Namely, as a result of comparative analysis of data referring to the spread of invasive plant species in space and time, from the studies which have been done worldwide for over a 100 taxa, an average rate of spread of invasive species has been determined on a local scale to be from 2 to 370 m per year, while the average rate of spread over great distances is at least twice as this, up to 167 km per year, documented for the species Wedelia trilobata over a period of 15 years.

The influence of invasive species on the environment, as well as the consequences resulting from their spread, aside from the distance which these species cross and areas they colonize, also depends on the speed of their spread. As a rule, the species spreading more slowly also express their ecological effects on the environment more slowly, while for those with a faster rate of spread this is reversed. However, there isn’t always a positive correlation between the spread of invasive species and the influence they have in the newly colonized environment.

The harmful effects ensuing from invasive processes can manifest themselves through: the changes in the vegetation, changes in the seedbank, changes in the soil, competitive interactions between native and alien invasive plant species, allelopathic interactions and/or changes in the soil microflora. The problems resulting from invasive species in agriculture mustn’t always be evident straight after the introduction of an alien species. Serious problems are usually a result of the lack of timely and adequate measure for the eradication of alien species. With regards to biological invasions, more importance is being given to their influence on agriculture in the context of growing genetically modified (GMO) plants, the emergence of invasive species resistance to herbicides, organic products and other current topics. It is important to remark that tolerant crops (to herbicides) can become invasive on their own due to the spread of their volunteer populations, and it can also happen that a certain weed species after hybridizing with these plants can become invasive. Namely, as a result of the transfer of genes responsible for tolerance from crops to their wild congeners and weed species, crop-weed hybrids are created, which can then be significantly more invasive and aggressive when compared to their wild parents.

The development of invasive weed species resistance to herbicides can lead to the changes in the fitness of these species which can further reflect on their biological production, spread, competitive capability and other traits significant for invasiveness, as well as on the possibility of their eradication.

Starting from all the findings listed above, from the period 2007-2009 as part of the project “Identification and monitoring of alien invasive weeds in the region of Serbia with proposed measures for eradication” (No. 401-00-16422/2007-11/29-4), financed by the Ministry of agriculture, forestry and water management of the Republic of Serbia, registering, evaluation of presence and mapping of 19 alien invasive weed species was done in the area of our country. The project holder was the Faculty of Agriculture of the University of Belgrade. In the realization of this project professors and scientific researchers from the following institutions were involved: University of Belgrade, Faculty of Agriculture, Zemun-Belgrade: Prof Dr Sava Vrbničanin (project leader), Prof Dr Ibrahim Elezović, Assistant prof  Dr Dragana Božić and Assistant prof  Dr Katarina Jovanović-Radovanov; from the Institute for Pesticides and the Environment, Zemun-Belgrade:  academic Dr Vaskrsija Janjić, Dr Radmila Stanković-Kalezić, research associate, Dr Ljiljana Radivojević, senior research associate, B.Sc. Jelena Gajić Umiljendić, assistant; from the Institute for Plant Protection and Environment, Belgrade: Dr Danijela Pavlović, research associate; from the Institute of Field and Vegetable Crops, Novi Sad: Dr Goran Malidža; and from the University of Novi Sad, Faculty of Technical Sciences, Novi Sad: Assistant prof Dr Milan Gavrić.

Evaluating the presence, abundance and cover of invasive weed species was done in the period from April to October of the years 2007 and 2008 in certain types of crops (row crops, small grain criops, perennial legumes, perennial crops, strubble) and non-agricultural (rural and urban) areas. The basic unit for mapping of invasive weed species in all areas was a 100 km² quadrant of the Universal Transverse Mercator (UTM) map of the Republic of Serbia. In each (active) quadrant all present crop types and non-agricultural areas were registered (an assessment of quantitative presented was done) twice during the vegetation season. On average, 36 plots were mapped in each quadrant. The mapping included the following alien invasive weed species: Abutilon theophrasti, Ambrosia artemisiifolia, Ambrosia trifida,  Asclepias syriaca, Cannabis sativa, Conyza canadensis, Cuscuta spp., Eleusine indica, Helianthus tuberosus,  Iva xanthifolia, Kochia scoparia, Orobanche cumana, Portulaca oleracea, Reynoutria japonica, Solidago canadensis, Solidago gigantea, Stenactys annuua, Xanthium strumarium and Xanthium spinosum (results of the mapping are given in Chapter 5). The distribution and quantitative presented are shown on UTM maps (10×10 km ratio) and in addition to an original photograph of each species, its synonyms, taxonomy, common names in several world languages (English, German, French, Russian), Bayer code, species status regarding the time of its introduction, life form, ecological indices, floral elements, phytogeographical origin, chromosome number, basic morphological (seed, seedling, adult plant), ecological and reproductive characteristics of the species, data on its presence and abundance in crop types in Serbia and proposed control, i.e. eradication, measures were given.

Also an analysis was done for the invasive weed species presence, depending on the type of the area mapped, altitude of the area, the species life form and whether the area mapped did or did not have any herbicide application. A total of 43 796 points (field/releve´s) were mapped in the territory of the Republic of Serbia, and 41 733 field/releve´s (some were rejected for objective reasons) were included in the calculation, analysis and making of UTM charts/maps of the distribution of 19 invasive weed species.

And finally, with regards to the possibilities of invasive weed plant species eradication, as with other invasive organisms, the prevention is often highlighted as the first and most cost-effective control measure for invasive species. However, the expectations that the invasion process can be stopped only through prevention are unrealistic, because each new invasion is a new problem. Yet, very good results are achieved as a result of early detection of the presence of invasive species and their subsequent eradication. The choice of eradication measure, and thus the expenses, depend on multiple factors, including the biological-ecological characteristics of the species, and especially its reproduction type, pesticide sensitivity, the existence of natural enemies suitable for biological control, etc. Despite modern tendencies directed towards environmental protection and healthy and safe food production, herbicide application is still a necessary measure in invasive weed control. Following the current trends, and aiming to preserve the life-balance on our planet, a generally accepted strategy for weed control, and thus for the invasive weed species, entails the application of integral measures which can popularly be defined as the „Application of many little hammers“.

[1] Elton, C. S.: The ecology of invasions by animals and plants. Methuen, London, 1958.

[2] Groves, R. H.: Invasion of mediterranean ecosystems by weeds. In: Resilience in Mediterranean-type Ecosystems (Dell, B., Hopkins, A. J. M., Lamont, B. B., eds.). Junk, Dordrecht, 129-145, 1986.

[3] Pyšek, P., Richardson, D. M.: The biogeography of naturalization in alien plants. Journal of Biogeography, 33: 2040-2050, 2006.

[4] Richardson, D. M., Pyšek, P., Rejmanek, M., Barbour, M. G., Panetta, F. D., West, C. J.: Naturalization and invasion of alienplants: concepts and definitions. Diversity and Distribution, 6: 93-107, 2000.

[5] Trinajstić, I.: Hronološka klasifikacija antropohora. Fragmenta herbologica Jugoslavica, Zagreb, 2: 27-31, 1976.