The term invasive species refers to a subset of introduced species or non-indigenous species that are rapidly expanding outside of their native range. Invasive species can alter ecological relationships among native species and can affect ecosystem function and human health. A species is regarded as invasive if it: (1) has been introduced by human action to a location where it did not previously occur naturally, (2) becomes capable of establishing a breeding population in the new location without further intervention by humans, and (3) spreads widely throughout the new location. In simple terms, an invader has to (1) arrive, (2) survive, and (3) thrive.
The species must find a vector that will bring it to a new environment. This new habitat must be a close enough match to its native range that it is able to survive and reproduce here without human cultivation (Williams and Meffe, 1998). To actually become invasive, the introduced species has to be able to out-compete native species, to reproduce effectively enough to start spreading geographically through its new habitat, and to negatively impact the ecosystems in its introduced range.
Conditions that lead to invasion
Scientific literature proposes several mechanisms to explain the success of invasive species. These mechanisms generally fall into two different categories: species-based mechanisms and ecosystem-based mechanisms. More likely, it is a combination of several mechanisms that cause an invasive situation to occur.
Species-based characteristics focus on plant competition. While all plants are able to compete in some manner to survive and oppose, invasive species appear to have specific traits or combinations of specific traits that make them especially good competitors. In some cases it can be as simple as having the ability to grow and reproduce more rapidly than native species. Other situations are more complex, such as allelopathy, which is a common mechanism whereby the invader directly or indirectly prevents other plants from growing nearby.
The life history of an organism describes the different stages of life an organism will go through during its lifetime. Such traits are tempting to study because life history is a quantifiable trait that could lead to very predictive models.
Several traits have been singled out by researchers as predictors of invasive ability in plants. For example, the ability to reproduce both asexually (vegetatively) as well as sexually, rapid growth, early sexual maturity, high reproductive output, the ability to disperse young widely, tolerance of a broad range of environmental conditions, and high phenotypic plasticity are all abilities that might aid an invasive plant in establishing and proliferating in a new environment.
The majority of studies agree in a general sense on which kind of traits mark an invasive species, but there are differences in how invasive each trait can make a species. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found that invasive species tended to only have a small subset of the invasive traits, and that many of these invasive traits were found in non-invasive species as well. This is one of the great difficulties in invasive species research: while many generalities can be made about invasive species, there are always exceptions to these observations (Kolar and Lodge 2001, Thebaud et al. 1996, Reichard and Hamilton 1997).
A common trait of invasive species is great competitive ability, which can be stronger against plants in a new habitat than plants in their native habitat. There can be huge differences between how an invasive species interacts with its native ecosystem, and with the ecosystem it is invading. Often, the invading species has a better chance at acquiring resources, such as light, water, space, or nutrients. Ecosystems where all available resources are being used to their full capacity by native plants can be modeled as zero-sum systems, where any gain for the invader is a loss for the native. However, such unilateral competitive superiority (and instant, equivalent extinction of native plants with increased populations of the invader) is not the rule (Stolgren 2003, Sax et al. 2002). Invasive plants can coexist with native plants for an extended time, and only gradually does the superior competitive ability of an invasive species become apparent, as its population grows larger and denser, and slowly increases the risk of extinction to other species.
An invasive species might be able to use resources previously unavailable to native plants, such as very deep water sources accessed by a long taproot, or an ability to live on previously uninhabited soil types. For example, barb goatgrass (Aegilops triuncialis), can be found in its introduced range in California on serpentine soils, which have low water-holding capacity, low nutrients, high Mg/Ca ratio, and possible heavy metal toxicity. Few plants have adapted to grow on them, which may explain why they have had so few plant invasions. However, since goatgrass can take advantage of these soils, it can also use the light, water, and space resources that other plants are restricted from using (Huenneke et al. 1990). By using these resources, goat grass can become so dense as to exclude other species and form monospecific swards.
There are other reasons that an invader might be a superior competitor. For example, an invasive plant may be inedible to local herbivores, allowing it to flourish unmolested where the native species are constantly held in check. The herbivores would find themselves in increasing competition with each other over fewer native plants, while the invader is taking the place of the native species. As the invader comes to dominate its new habitat, the local food webs are changed from the bottom up, since their foundation of native plants has been altered. (Petren and Case 1996, Gray 1986).
Facilitation is the mechanism by which some species can alter their environment through chemicals or manipulation of abiotic factors, usually to make it more favorable to their growth or reproduction. Sometimes, neighboring species may benefit by another’s facilitation, but often the facilitation actually benefits the target species to the detriment of its neighbors. One such facilitative mechanism is allelopathy, also known as chemical competition. In allelopathy, a plant will secrete chemicals which make the surrounding soil uninhabitable, or at least inhibitory, to other plant species.
One example of this is the knapweed (Centaurea diffusa). This Eastern European weed has spread its way through the western United States. Experiments show that 8-hydroxyquinoline, a chemical produced at the root of C. diffusa, has a negative effect only on plants that have not co-evolved with C. diffusa. Such co-evolved native plants have also evolved defenses, and C. diffusa does not appear in its native habitat to be an overwhelmingly successful competitor. This result shows how difficult it can be to predict whether a species will be invasive just from looking at its behavior in its native habitat, and demonstrates the potential for novel weapons to aid in invasiveness (Vivanco et al. 2004, Hierro and Callaway 2003).
Changes in fire regimes are another form of facilitation. Bromus tectorum, originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning, but actually increases the frequency and intensity (heat) of fires, by providing large amounts of dry detritus during the height of the fire season in western North America. In areas where it is widespread, it has altered the local fire regime so much that native plants cannot survive the frequent fires, allowing B. tectorum to further extend and maintain dominance in its introduced range (Brooks et al. 2004).
Facilitation also occurs when one species physically modifies a habitat and that modification is advantageous to other species. For example, zebra mussels increase habitat complexity on lake floors providing nooks and crannies in which invertebrates live. This increase in complexity, together with the nutrition provided by the waste products of mussel filter-feeding increases the density and diversity of benthic invertebrate communities (Silver Botts et al. 1996).
Ecosystem based mechanisms
When examining an ecosystem, it is important to look at two things: the amount of resources available, and how much of those resources are being used. In a stable ecosystem, all of the available resources are used up, and all of the species there have enough to continue to survive.
Yet what happens if spare resources happen to appear in an ecosystem? For example, lets say a forest fire has cleared an area, leaving only hardy brush and established trees. The resources used by the cleared species (e.g. grasses) are no longer being utilized, and may be taken up by other organisms. While these resources are most likely to be used by the nearby species, there is an opportunity for exotic species to establish themselves. The data shows that nitrogen and phosphorus are often the limiting factors for a situation such as this (Davis et al 2000).
Every species has a role to play in its native ecosystem; some are general while others are highly specialized. These roles are known as niches. This mechanism describes a situation where the invaded ecosystem in question has unfilled niches, which are promptly filled by an invader.
Imagine a situation involving a group of people who have mastered a game, and are able to beat anyone outside of their group. They know the rules inside and out, and are constantly perfecting their strategies to win. Few, if any, have a chance against this group. Then suddenly the rules change, and this group is at a loss, since their strategies no longer work the way they used to, and other players with vastly different strategies are able to come in, and win where they could not before. This is how the familiar ecosystem mechanism works.
This mechanism describes a situation where the ecosystem in question has suffered a disturbance of some sort, which changes the fundamental nature of the ecosystem (Byers 2002). Examples of this type of disturbance can range from the loss of an important predator to the eutrophication of an aquatic environment. In situations such as these, the specialized evolution of the native plant species becomes wasted, and non-native species can gain a foothold.
Ecology of invasive species
Ecological circumstances of invasive species
Although an invasive species is often defined as an introduced species that has spread widely and causes harm, some species native to a particular area can, under the influence of natural events such as longterm rainfall changes or human modifications to the habitat, increase in numbers and become invasive. The Pied Currawong of south-east Australia is an example; as a result of human changes to the landscape, Pied Currawongs increased greatly in range during the 20th century and have caused substantial declines in the populations of the smaller birds whose nestlings they prey on.
All species on Earth go through periods of increasing and decreasing population numbers, in many cases accompanied by expansion and contraction of range. Human “alterations” on the landscape are especially significant. Anthropogenic alteration of an environment may enable the expansion of a species into a geographical area where it had not been seen before and thus that species could be described as invasive because the range expansion results in the species occurring where it was not before native. In essence, one must define "native" with care, as it refers to some natural geographic range of a species, and is not coincident with human political boundaries. Whether noticed increases in population numbers is sufficient reason to regard a native species as "invasive" requires a broad definition of the term—but it seems reasonable to consider that some native species in disrupted ecosystems can spread widely and cause harm and in that sense become invasive.
Traits of invasive species
Many features have been attributed to invasive species and invaded ecosystems, but none are universal and invasive species tend to have a suite of traits rather than all of them. Common invasive species traits include fast growth, rapid reproduction, high dispersal ability, phenotypic plasticity (the ability to alter one’s growth form to suit current conditions), tolerance of a wide range of environmental conditions, ability to live off of a wide range of food types, asexual reproduction, and association with humans (Williams and Meffee 1998). The single best predictor of invasiveness, however, is whether or not the species has been invasive elsewhere (Ewel et al. 1999).
A study done by Marcel Rejmanek and David Richardson (1996) on invasive Pine species in South Africa found three main traits that successfully separated invasive from non-invasive species. These are: small seed sizes, early reproduction, and short periods between large seed sets.
Before an invasive species can even put some of the aforementioned “invasive traits” to work and thrive, it must survive at a very low population density when it may be difficult to find mates or cross-fertilize or when random changes in the environment could easily wipe out the entire population (Tilman 2004). This is why successful invasive species are often associated with humans. Our repeated patterns of movement, such as ships sailing to and from ports, or cars driving up and down highways, allow for species to have multiple opportunities of establishing (also known as a high “propagule pressure”) (e.g., Verling et al. 2005).
Traits of invaded ecosystems
In 1958, Charles S. Elton argued that ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Since then, other ecologists have pointed to highly diverse, but heavily invaded ecosystems and have argued that ecosystems with high species diversity seem to be more susceptible to invasion (Stohlgren et al. 1999). In the end, this debate seems largely to hinge on the spatial scale at which invasion studies are performed, and the issue of how diversity affects community susceptibility to invasion remains unresolved. Small-scale studies tend to show a negative relationship between diversity and invasion, while large-scale studies tend to show a positive relationship. The latter result may be an artifact of invasive or non-native species capitalizing on increased resource availability and weaker overall species interactions that are more common when larger samples are considered (Levine 2000, Byers and Noonberg 2003)
Invasion is more likely if an ecosystem is similar to the one that the potential invader evolved (Williams and Meffee 1998). Island ecosystems may be prone to invasion because their species are “naïve” and have faced few strong competitors and predators throughout their existence, or because their distance from colonizing species populations makes them more likely to have “open” niches (Stachowicz and Tilman 2005). An example of this phenomenon is the decim ation of the native bird populations on Guam by the invasive brown tree snake (Fritts and Leasman-Tanner 2001). Alternately, invaded ecosystems may lack the natural competitors and predators that keep introduced species in check in their native ecosystems, a point that is also seen in the Guam example. Lastly, invaded ecosystems have often experienced disturbance, usually human-induced (Williams and Meffee 1998). This disturbance may give invasive species, which are not otherwise co-evolved with the ecosystem, a chance to establish themselves with less competition from more adapted species (Tilman 2004).
Non-native species have many vectors, including many natural ones, but most of the species that we consider "invasive" are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and the distances that species can travel to colonize are also often much greater with human agency (Cassey et al. 2005).
One of the earliest human influenced introductions involves prehistoric humans introducing the Pacific rat (Rattus exulans) to Polynesia (Matisoo-Smith et al. 1998). Today, non-native species come from horticultural plants either in the form of the plants themselves or animals and seeds carried with them, from animals and plants released through the pet trade. Invasives also come from organisms stowed away on every type of transport vehicle imaginable, to name a few unintentional vectors. For example, ballast water taken up at sea and released in port is a major source of exotic marine life.
Species have also been introduced intentionally. For example, to feel more "at home", American colonists formed "Acclimation Societies" that repeatedly released birds that were native to Europe until they finally established along the east coast of North America. As a result, several native bird species were pushed to extinction.
Economics play a major role in exotic species introduction. The scarcity and demand for the valuable Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.
Impacts of invasive species
Biological species invasions can negatively impact ecological systems in a multitude of ways. Worldwide an estimated 80% of endangered species could suffer losses due to competition with or predation by invasive species (Pimentel et al. 2005). As highly adaptable and generalized species are introduced to environments already impacted by human activities, native species are put at a distinct disadvantage to survive. There are many examples of decreases in biodiversity in such areas. For example, Purple loosestrife (Lythrum salicaria) changed the ecology of wetlands by reducing the abundance of native plants and endangering several species of ducks and a species of turtle that depend on the native plants. Clearly, a primary threat to biodiversity is the spread of human activity into once pristine areas.
Land clearing and human habitation put significant pressure on local species and disturbed habitat is often prone to invasions that can have adverse effects on local ecosystems, changing ecosystem functions. A species of wetland plant known as Template:OkinaaeTemplate:Okinaae in HawaiTemplate:Okinai (the indigenous, Bacopa monnieri) is regarded as a pest species in artificially manipulated waterbird refuges because it quickly covers shallow mudflats established for endangered Hawaiian stilt (Himantopus mexicanus knudseni), making these undesirable feeding areas for the birds. Sometimes, multiple successive introductions of different nonnative species can have interactive effects, where the introduction of a second non-native species can enable the first invasive species to flourish. Examples of this are the introductions of the amethyst gem clam (Gemma gemma) and the European green crab (Carcinus maenas). The gem clam was introduced into California's Bodega Harbor from the East Coast of the United States a century ago. It had been found in small quantities in the harbor but had never displaced the native clam species (Nutricola spp.). In the mid 1990s, the introduction of the European green crab, found to prey preferentially on the native clams, resulted in a decline of the native clams and an increase of the introduced clam populations (Grosholz, 2005).
Invasive plants can arise from human clearing (such as slash-and-burn) or cattle grazing actions, such that the altered land is more hospitable to the invasive species than the original plant palette. The invasive plant can even be a pre-existing species that has attained dominance from the disturbance. For example, in the Waterberg region of South Africa, cattle grazing over the past six centuries has allowed invasive scrub and small trees to displace much of the original grassland, resulting in a massive reduction in forage for native bovids and other grazers. Since the 1970s large scale efforts have been underway to reduce invasives; partial success has led to re-establishment of many species that had dwindled or left the region. Examples of these species are giraffe, Blue Wildebeest, impala, kudu and White Rhino.
Invasive species can change the functions of ecosystems. For example invasive plants can alter the fire regime (cheatgrass, Bromus tectorum), nutrient cycling (smooth cordgrass Spartina alterniflora), and hydrology (Tamarisk) in native ecosystems (Mack et al. 2000). Invasive species that are closely related with rare native species have the potential to hybridize with native species. Harmful effects of hybridization have lead to a decline and even extinction of native species (Hawkes et al. 2005, Rhymer and Simberloff 1998). For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass (Spartina foliosa) in San Francisco Bay (Ayres et al 2004).
Economic costs due to invasive species can be separated into direct costs due to production loss in agriculture and forestry, and management costs of invasive species. Estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually (Pimentel et al. 2005). In addition to these costs, economic losses can occur due to loss from recreational and tourism revenues (Simberloff 2001). Economic costs of invasions, when calculated as production loss and management costs, are low because they do not usually consider environmental damages. If monetary values could be assigned to the extinction of species, loss in biodiversity, and loss of ecosystem services, costs from impacts of invasive species would drastically increase (Pimentel et al. 2005). The following examples from different sectors of the economy demonstrate the impact of biological invasions.
Agricultural weeds cause an overall reduction in yield. Most weed species are accidental introductions with crop seeds and imported plant material. Many introduced weeds in pastures compete with native forage plants, are toxic (e.g., leafy spurge, Euphorbia esula) to cattle or non palatable due to thorns and spines (e.g., yellow star thistle, Centaurea solstitialis). Forage loss due to invasive weeds on pastures amounts to nearly $1 billion in the U.S. alone (Pimentel et al. 2005). A decline in pollinator services and loss of fruit production has been observed due to the infection of honey bees (Apis mellifera another invasive species to the Americas) by the invasive varroa mite. Introduced rodents (rats, Rattus rattus and R. norvegicus) have become serious pests on farms destroying stored grains (Pimentel et al. 2005).
The unintentional introduction of forest pest species and plant pathogens can change forest ecology and negatively impact timber industry. The Asian long-horned beetle (Anoplophora glabripennis) was first introduced into the U.S. in 1996 and is expected to infect and damage millions of acres of hardwood trees. Thirty million dollars have already been spent in attempts to eradicate this pest and protect millions of trees in the affected regions (Pimentel et al. 2005).
The woolly adelgid inflicts damage on old growth spruce fir forests and negatively impacts the Christmas tree industry (Forest Pests: Insects, Diseases & Other Damage Agents 2005). The chestnut blight fungus (Cryphonectria parasitica) and Dutch elm disease (Ophiostoma novo-ulmi) are two plant pathogens with serious impacts on forest health.
Tourism and Recreation
Invasive species can have impacts on recreational activities such as fishing, hunting, hiking, wildlife viewing, and water-based recreation. They negatively affect a wide array of environmental attributes that are important to support recreation, including but not limited to water quality and quantity, plant and animal diversity, and species abundance (Eiswerth 2005). Many invasive species have thorns and spikes than can prevent easy access to hiking trails. Aquatic invasive species, such as hydrilla (Hydrilla verticillata) and Eurasian watermilfoil (Myriophyllum spicatum), affect water-based recreation by impeding human access, interfering with the operation of boats and fishing lines, lowering water quality, and negatively altering aquatic ecosystems, including the abundance and diversity of fishes.
An increasing threat of exotic diseases exists due to increased transportation and encroachment of humans into previously remote ecosystems that can lead to new associations between a disease and a human host (e.g., AIDS virus in human host; Pimentel et al. 2005). Introduced birds (e.g. pigeons), rodents and insects (e.g. mosquitoes, fleas, lice and tsetse fly) can serve as vectors and reservoirs of human diseases. Throughout recorded history epidemics of human diseases such as malaria, yellow fever, typhus, and plague have been associated with these vectors (Elton 1958). A recent example of an introduced disease is the spread of the West Nile virus across North America resulting in human deaths and in the deaths of many birds, mammals, and reptiles (Lanciotti 1999). Waterborne disease agents, such as Cholera bacteria (Vibrio cholerae), and causative agents of harmful algal blooms are often transported via ballast water (Hallegraeff 1998). The full range of impacts of invasive species and their control goes beyond immediate effects and can have long term public health implications. For instance, pesticides applied to treat a particular pest species could pollute soil and surface water (Pimentel et al. 2005).
The threat to global biodiversity
- Main article: Biodiversity
The impact on global biodiversity of human introduction of non native species that have subsequently become invasive is subjective. Climate change and the movement of the continents through the ages have created divisions and changes to species over the long history of this planet. Limited information on the circumstances and impact at the time makes it difficult to directly compare this to the advent of international travel for people and goods which has made the introduction of new species increasingly easy, and the direct ecological impacts are now far more evident and measurable.
Historically the deliberate introduction of non native species has been done with little or no consideration of the impact outside of having a favored animal, fish, or plant available locally, or perhaps an ill-conceived attempt to control a native pest. In areas with highly endemic, specialised and isolated flora and fauna such as Australia, New Zealand, Madagascar, the Hawaiian Archipelago, and the Galapagos Islands, introduced species that successfully establish themselves in habitats utilized by natives compete for limited resources or prey on the native species, some of which are unable to adapt to the more competitive environment and gradually die out.
As more adaptable and generalized species are introduced to environments impacted adversely by human activities, native species are put at a disadvantage to survive in what previously was a unique, balanced ecosystem. There are many examples of decreases in biodiversity in those areas. One of the primary threats to biodiversity is the spread of humanity into what were once isolated areas with land clearing and habitation putting significant pressure on local species. Agriculture, livestock and fishing can also introduce changes to local populations of indigenous species which may result in a previously innocuous native species becoming a pest due to a reduction of natural predators.
Control of invasive species
The control of invasive species can involve their eradication or their containment within a specified area. In both cases, the goal is to prevent further spread to un-invaded systems (National Biological Information Infrastructure 2004). This type of management can be implemented at several scales, from a homeowner working in his or her own backyard to large government agencies taking a national approach. The decision to eradicate a species versus contain it can depend on several factors, including, but not limited to, the type of habitat, characteristics of the organism, the spatial dimensions of the spread, time available to dedicate to control, and cost. These factors also play a role in determining which specific control technique(s) to utilize.
Mechanical control involves the removal of invasive species by hand or with machines. Often, these methods are effective in controlling small populations and can be target specific, minimizing harm to non-invasive plants and animals (The Nature Conservancy 2005). Mechanical control, however, is extremely labor intensive and requires a large time investment, as treatments must often be applied several times to ensure success (The Nature Conservancy 2005). Commonly implemented control methods for plants include hand pulling, mowing, girdling, and burning (The Nature Conservancy 2005). For invasive animal control, techniques such as hunting, trapping, and the construction of physical barriers like fences or nets, are used (Hengeveld 1989).
Chemical compounds can be used to prevent the spread of invasive species. This method of control can be very effective in both large and small areas, but is often criticized due to the possible contamination of land and water resources and a lack of target specificity that can result in the killing of desirable plant and animal species (National Park Service 2004). Also, the target species may develop a resistance to the chemicals over time, rendering this method ineffective. Herbicides are chemicals used to control invasive plants and, depending on the target species, can be applied directly to a plant, in the soil at a plant’s base, or even to the soil before seeds develop (The Nature Conservancy 2005). For animals, other pesticides are used to restrict growth and reproduction or to kill invasive insect pests (National Biological Information Infrastructure 2004). Another form of chemical control is the use of attractant pheromones to lure mate-seeking insects into traps (Myers et al. 2000).
Biological control involves the release of a specific species to restrict the spread of the invasive species. With the proper research, this method of control can be both environmentally safe and successful. However, it can be ineffective if the released spe cies do not survive or if their impact on the invasive species is not as great as predicted (The Nature Conservancy 2005). Also, the species chosen for release is not always a native organism, increasing the possibility of even more invasive species. Predatory insects, called weed feeders, can be released to control invasive plants. Similarly, plants can be infected by disease causing organisms, such as fungi, bacteria, and viruses, killing them or reducing their reproductive output (Cornell University 2005). Invasive animals can be controlled with the release of predatory or parasitic organisms (especially in the case of invasive insects) or with the transmission of diseases in a similar manner as with plants (Cornell University 2005). Additionally, sterile insect or fish males of the invasive species can be released so that after mating, a female will lay “dead” eggs or eggs that will develop into sterile adults (Myers et al. 2000).
Preventing the establishment of invasive species is always the best method of control (The Nature Conservancy 2005). Stopping harmful species at this stage can be difficult. Many governments try to limit the entry of invasive species into their lands with thorough inspections of international shipments, customs checks, and proper quarantine regulations. The creation of list of safe and potentially harmful species can be helpful in regulation. The general public can also participate in invasive species prevention by educating themselves about invasive species and by making informed decisions.
- List of invasive species
- Introduced species
- Invasion biology terminology for a review of the terminology used in invasion biology.
- Invasive species in New Zealand
- Invasive species in Australia
- Island restoration
- Weed Science
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- Alien Plants in Ireland Database of Alien and invasive plants in Ireland
- City forests in peril: Invasive plants are killing native trees
- Global Invasive Species Database
- Hawaiian Ecosystems at Risk project (HEAR)
- INVASIVE .org The Source for Information and Images of Invasive & Exotic Species
- Invasive alien species in Belgium The Belgian Forum on Invasive Species (BFIS) is an informal structure animated by the Belgian Biodiversity Platform where in scientists interested in biological invasions are involved.
- Invasive Species Specialist Group
- Invasive Species Weblog
- National Invasive Species Information Center
- Pacific Island Ecosystems at Risk project (PIER)
- The Nature Conservancy Wildland Invasive Weeds
- USDA NRCS Plant Materials Program
- UK Plant Health service
- U.S. Coast Guard Ballast Water Management Program