Invasive+species

//**A look at Invasive Species in Riparian Systems**//

//**Background**//

 Exotic species were once considered a commodity of great value in the United States. The practice of importation, cultivation and domestication of non-native species was endorsed by Thomas Jefferson and the uncontrolled influx of exotics continued for the next 150 years (Sagoff, 2005). Since the late 1800’s, government agencies have spent large sums of money, time, and effort on international expeditions collecting seed and propagules of exotic species in order to domesticate them in the US. By 1933, the USDA alone had gathered over 100,000 specimens for study and propagation (Sagoff, 2005). However, by the mid-1900’s, the United States government realized the possible threat that non-native species posed on cultivated crops and proceeded to enact several laws that prohibited the importation of alien species. Such acts included the Plant Pest Act, the Plant Quarantine Act, and the Noxious Weed Act; all with the primary goal to protect the "cultivated from the domesticated" (Miller, 2007).

 Alien species have been transported to the US both intentionally and unintentionally. A large proportion of intentional introductions were the result of colonists and later immigrants bringing desired plants from their homeland for use here in the US. In a study looking at global similarities of weed migration, Seipel et al. (2011) found that non-native species richness was found to be highest in the New World regions, thus, reflecting the effects of colonization by European settlers. Other intentional releases occur when domesticated animals or cultivars escape the bounds of their new habitat and return to a wilderness setting.

 Many species have remarkable dispersal mechanisms that allow them to spread seed over a wide range or reproduce at high rate. Some invasive species have been imported as biological control agents to mitigate the effects of other exotic species, and then become invasive themselves (United States Fish and Wildlife Service, 2012), as is becoming the case of the tamarisk leaf beetle in the desert southwest of the United States. Unintentional introductions occur when vectors of dispersal unwittingly spread seed beyond the species’ native habitat. Common modes of unintentional introduction are commercial fishing vessels, international plane carriers, imported soil or nursery products, migratory birds, and range animals. Of the 22,000 plant species in the United States, approximately 23% are considered exotic (Zaimes, 2007).

 The potential for adverse consequences from non-native introductions has grown considerably over the last three decades due to a rapid rise in air-travel, increased ports of entry, escalation of international trade, and enhanced access to foreign ecosystems. Through commercial air-cargo, ship ballast water, and private travel, hundreds to thousands of exotic plant species are brought into the United States annually (Cohen, 1998; Mullin, 2000). However, most of the alien species that migrate to the US struggle to survive and fail to establish themselves as an invasive species (Miller, 2000).

 This paper will identify and examine the role that invasive plant species play in ecosystem function and why some alien species become more dominate than others. This paper will discuss the characteristics that make an alien species invasive and how land disturbance and degradation influence the dominant tendencies of invasive riparian plants, looking in detail at the “Driver vs. Passenger” model. A discussion of the beneficial contributions made by invasive species populations on ecosystem function including a summary of the //Resilience Theory// will follow, and lastly a short description of case studies from the United States will summarize the basic concepts explored in this paper.

//**Weed Ecology**//

 As per Presidential Executive Order 13112, "Invasive species" means an alien species whose introduction does, or is likely to cause, economic or environmental harm or harm to human health with "alien species" meaning; with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem (USDA, 2011). This paper will use the terms alien and exotic species interchangeably. "Invasive species" refers any class of organism; however, this paper will have a primary focus on plant invasive species in riparian and aquatic environments.

 The play between invasive exotic species and native flora and fauna can be detrimental. Invasive species can cause environmental impacts at both the population and ecosystem level. Impacts have often proven to be negative and have been shown to decrease the size of native populations and the extent of native ecosystems (Simberloff, 2005). These disturbances can occur on the local and global scale and invasions have at times proven to cause extinction of native species (Simberloff, 2005). It is generally assumed that population level impacts are related to compositional shifts in population mainly caused by the competition, predation, and hybridization of exotic species. Disease resistance is also a factor in the change of population dynamics. Many invasive species are often resistant to local diseases and pathogens that help control population size. Since alien species evolved independently of native flora and fauna they, in many cases, lack natural predators or ecosystem processes that would normally keep their populations in check. However, some alien species possess attributes that make them more invasive than others. Alien species that become invasive tend to have common characteristics that allow them to survive in their new environments and thrive. These common threads include: a rapid rate of reproduction and short generational turnover; this allows a species to naturally select and optimize offspring suitability for a specific location, coping mechanisms that provide flexibility in habitat preference; a species may thrive in both sunny and shady environments, and mechanisms of reproduction; proliferates by rhizomes or wind dispersed seed or may produce a large volume of seed to increase chances for seed survival (USDA, 2006).

 Ecosystem level impacts are confirmed through sometimes drastic alterations in nutrient cycles, hydrological or fire regimes, and habitat structure. It is thought that ecosystem impacts are more frequently caused by introduced plants, and they are usually more significant than single population impacts in terms of injury to the environment because, by changing the entire habitat, they affect multiple species at one time (Simberloff, 2005). However, pristine landscapes that have minimal disturbance or, have deviated slightly from ecosystem balance, experience very few endemic invasive species episodes. Degraded systems that no longer function on a level that maintains stability and sustainable ecosystem processes are most susceptible to infestations. Landscapes that lend themselves to invasion possess commonalities that increase their potential for colonization by nuisance species, these include: a lack of biotic constraints to keep populations under control, are likely to contain vacant niches that can be exploited; because there available niches, these landscapes often lack species diversity, are devoid of a multi-tiered canopy, and have been disturbed by: fire, agriculture, or a disruption to the hydrologic cycle (Mullin, 2000).

Aquatic and riparian ecosystems, in particular, are highly susceptible to invasion by exotic plants (Mullin, 2000). Landscapes with large volumes of free flowing water not only provide suitable habitats for a wide range of species but are much more exposed to the human activities that act as vectors of dispersal for many invasive species. People use oceans, rivers, and lakes for transportation and recreation and often move freely from one body of water to another.

//Riparian Zones//

 Interfaces between environmental patches occur where structural or functional system properties change discontinuously in space or time (Naiman, 1997). Specifically, r iparian zones form the boundary between aquatic and terrestrial ecosystems and, with the exception of wide floodplains, are often narrow, linear features that cross the landscape. These areas support unique vegetation that differs in structure and function from adjacent aquatic and terrestrial ecosystems (Holmes, 2005). They commonly serve as a basis for understanding the organization, diversity, and dynamics of communities associated with fluvial systems (Naiman, 1997). Riparian zones consist of the stream channel between the low and high watermarks, and the portion of the terrestrial landscape from the high watermark toward the uplands where vegetation is influencedby elevated water tables, flooding and sedimentation.

 Riparian zones are an extremely diverse medley of landforms, communities, and environments and are colonized by specialized disturbance-adapted species. Hydrological processes are the chief determinants of plant community structure and composition in riparian areas (Holmes, 2005) ; therefore, riparian plants are adapted to these types of fluctuations and disturbances. Vegetation in these areas provide habitat, food sources, stabilize riverbanks and filter sediments and nutrients from the surrounding catchment (Naiman, 1997). However, because river ecosystems are so dynamic and are often in a state of recovery from disturbance events, they are highly prone to invasion by exotic plant species. Their dynamic hydrology provides many opportunities for recruitment following floods. Efficient dispersal of alien propagules in water and continuous access to water resources thus facilitates invasive species colonization.

//The Importance of Riparian Areas in the Southwest Specifically//

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> In the arid western United States, riparian areas are estimated to be less than 2% of the total land area and despite their marginal acreage, riparian areas play a particularly important role in the semi-arid regions of North America (Zaimes, 2007). In semi-arid to arid environments, riparian areas support more productive and diverse vegetation assemblages and serve more ecological functions than their terrestrial upland counterparts. A large percentage of wildlife depends on riparian areas for foraging, nesting or cover during part or their entire life cycle. This is even truer for the southwestern United States where riparian areas are recognized as critical areas.

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> The higher stem densities of riparian vegetation increase their sediment trapping capacity and allow the buildup of soil. As a result, these areas can quickly develop stream banks and floodplains. As in most environments, higher vegetation density leads to a higher level of microbial activity resulting in more efficient rates of nutrient cycling and greater storage of non-point source toxins (Zaimes, 2007). Riparian areas slow and spread flood waters that crest over the banks, allowing more water to infiltrate the soil, thus recharging groundwater and extending stream base-flow; two vital contributions of rivers in desert environments.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Throughout the world, riparian zones have historically been the focus of human establishment. Areas adjacent to, or occurring in, riparian zones have experienced a great deal of development over the past several centuries and have undergone extreme derivational changes and are largely degraded (Holmes, 2005). These systems have been directly affected by anthropogenic alterations such as: clearance for agriculture, grazing, implementation of dams upstream, pollution from populations within the catchment, and the planting of exotic species (Holmes, 2005). The damming of rivers has so altered the hydrologic cycle of many riverine systems that the current hydrology is no longer able to support native vegetation, further exacerbating the spread and colonization of invasive species.

<span style="color: #231f20; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//**Invasive Species: drivers or passengers in degraded ecosystems?**//

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Invasive species are widely accepted as one of the leading direct causes of biodiversity loss (Simberloff, 2005). The dominance of invasive species is assumed to reflect their competitive superiority over displaced native species; this theory is often based on the simple fact that when degraded systems experience a spike in exotic species populations, there is often a corresponding decline in native species populations. This hypothesis attempts to explain causation; however, it only describes a correlative relationship. A more plausible alternative is that exotic species dominance is the direct consequence of habitat modification driving native species loss (MacDougall, 2005).

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Hundreds to thousands of exotic plant species are brought into the United States annually, yet most of these species fail to develop invasive tendencies and become a species of concern. In an attempt to explain why some alien species become more dominant than others and how their presence impacts the abundance of other species, scientists have described two primary pathways for dominance: superior access to limiting resources by competition (highly interactive model) or limited susceptibility to noncompetitive processes that are more restrictive to one species than the other (weakly interactive model) (MacDougall, 2005). Based on this assumption, ecologists are able to better understand the cause and effect of invasive species colonization.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> The “Driver vs. Passenger” model is a contrasting hypothesis for the causes and consequences of plant invasion. Andrew S. MacDougall and Roy Turkington developed this concept in 2005 and were the first to directly test whether invasive species are the drivers of community shifts, or merely the “passengers” along for the environmental ride. The ‘driver’ model predicts that invaded communities are highly interactive, with native species being limited or excluded by competition from the exotic dominants. The “passenger” model predicts that invaded communities are primarily structured by non-interactive factors (environmental change, dispersal limitation) that are less constraining on the exotics, which thus dominate (MacDougall, 2005). Invasive species may be abundant because of their greater tolerance to anthropogenic impacts accompanying their introduction. Thus, invasive species can either be the drivers or passengers of change.

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//**Riparian Disturbance in the Southwest and Invasive Plant Infestations**//

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Human disturbances such as: the limited frequency and duration of flooding events through the implementation of dams, increased groundwater pumping and the subsequent decrease in water table, and fire suppression can give a competitive edge to invasive exotic species (Zaimes, 2007; Stromberg 2007). By altering the natural ecosystem, exotic species that are better equipped to handle a variety of stresses are more fit than the native species and quickly win dominance. Some of the most threatening invasive exotic plants in the Southwest include: tamarisk (t//amarix// spp.), Russian olive (//angustifolia// L.), silktree (//Albizia julibrissin// Durazz.), tree of heaven (//Ailanthus altissima// (Mill.) Swingle), Russian knapweed (//Acroptilon repens// (L.), purple loosestrife (//Lythrum salicaria// L.), Bermudagrass (//Cynodon dactylon// (L.), Johnsongrass (//Sorghum halepense// L.), buffelgrass (//Pennisetum ciliare// L.), Lehmann lovegrass (//Eragrostis lehmanniana// Nees), Eurasian watermilfoil (//Myriophyllum spicatum// L.), water hyacinth (//Eichhornia crassipes// Mart.) and cheatgrass (//Bromus tectorum// L.) (USDA, 2006).

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//Tamarisk//

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Tamarisk, also known as salt cedar, was introduced in the late 1800s from its native territory of Asia for control of soil erosion and landscaping (Sromberg, 2007, Doody, 2011, USDA, 2006). From 1920 to 1987, tamarisk population size has increased from about 10,000 acres to 1.5 million acres in Arizona alone and is now the third most prevalent woody riparian species in the western United States (Stromberg, 2007) occupying several million acres. It has proliferated rapidly due to its high tolerance of soil salinity and alkalinity and its ability to adapt to long periods of drought; the tamarisk has been known to have deep taproots that extend more than 90 feet below ground (Taylor, 1998). Furthermore, the trees produce seeds for longer periods than native riparian species such as cottonwood, willow, or ash.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Human activities that have led to the degradation of riparian areas often began with the intentions of benefiting society; however s hifts in hydrologic flow regimes have severely altered the levels and timing of resource availability and disturbance patterns and have in turn altered competitive hierarchies, typically favoring one species to another (Stromberg, 2007). The conditions favoring tamarisk have been established in many riparian areas due to ground water pumping that has dropped the water table significantly, thus the benefit of having an extensive root system. Native cottonwood and willow species, often assumed to be displaced by the tamarisk, do not have the acquired ability to grow deep roots and therefore lack access water that is well below their root zone. The alteration of flow regimes from dams along with livestock that prefer to graze on native trees has also been detrimental for the native willow and cottonwood populations. These activities have given a competitive edge to the tamarisk. Flow alterations have had significant impacts in the degradation of riparian areas in the Southwest U.S. and are especially important (Zaimes, 2007).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> <span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Tamarisk has been designated as one of the 10 worst noxious weeds in the U.S. (USDA, 2011). Some research has suggested that tamarisk uses more groundwater than the native plants it displaces, leading the U.S. to explore potential water savings through the removal of the non-native, while disregarding the possible side effects to the ecosystem health and function. The removal of tamarisk is a massive undertaking that often requires significant disturbances that may contribute to further degradation. However, Doody et al. ( 2011 ) found that although the colonization of tamarisk has altered riparian ecosystem and ecohydrological processes, including trophic interactions, fire regimes and stream-channel geomorphology, their presence has little impact on water quantity and flow rates. Doody tested this by measuring the ET rates of tamarisk and found that the average ET in mm/yr of a representative native species population overlapped the rates measured for tamarisk. Therefore, the research showed that the removal of tamarisk might not produce the expected results of making more water available for human consumption.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> In 2007, a study conducted by Stromberg et al showed that river reaches with perennial flow and a natural flood regime had a high abundance of cottonwood and willow species, the historically dominant pioneer riparian trees in the region, and a low abundance of tamarisk. In contrast however, river reaches showing evidence of degradation through hydrologic alterations caused by dam-regulated flows, were dominated or co-dominated by tamarisk a more stress-adapted and more reproductively opportunistic species. Scientists were then able to conclude that the change in the physical environment, which is wide- spread in riparian corridors of the southwestern United States, was the primary driver of the high abundance of tamarisk and that tamarisk was therefore the passenger of environmental change, not the driver.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Other studies have shown that tamarisk has a beneficial impact on the ecosystem. As mentioned previously, the abundance of tamarisk in the arid Southwest is a result of the loss of hydrologic function. The hydrology of the contemporary Southwest U.S. can no longer support the native species that once depended on a higher water table, the cyclical nature of floods, and routine disturbance (J. C. Stromberg, 2007). As a result, tamarisk is often the dominant woody species of riparian systems in the desert and is the primary stabilizer if these types of systems. Often, these environments are extremely saline and have little vegetation to begin with. Tamarisk provides habitat for the endangered Southwestern Willow flycatcher, stabilizes banks reducing erosion and turbidity, shades and cools water in perennial systems, attracts pollinators and other insects that may act as a food source for aquatic species, is palatable to Black-footed jack-rabbits and beaver, and can be used medicinally.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Although research suggests that in some cases, the costly removal of tamarisk populations had little effect on in-stream flows and that tamarisk proliferation is the result of ecosystem degradation, not the cause, the eradication of tamarisk is still an ongoing battle. The war on tamarisk can be expensive. It has been stated that the eradication efforts along the Bosque Del Apache in New Mexico have cost anywhere between $1,500-$2,800/acre (Taylor, 1998). Congress has also recognized the importance of the tamarisk “problem” and has pending legislation that could provide $25 to $50 million per year throughout the Western U.S. for tamarisk control, revegetation, and research ([|Tamarisk Coalition, 2011]).

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//**Beneficial Uses for Non-native Species in Restoration:**//

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> There are situations where the use of alien species might be appropriate and may even prove useful. Exotic species used as a tool in restoration is often looked upon as a detrimental practice and a poor use of judgment (Zaimes, 2007). However, there are times when they may be of use. Often, restoration practitioners look to eradicate alien species populations and spend large sums of money to do so when in fact, they could be using those funds to better support the ecosystem services they are trying to recover (Taylor, 1998).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Alien species can act as nurse plants in newly restored areas. Shrubby or unpalatable alien species can serve to protect more vulnerable early successional plants that may be desirable as forage for large grazers that often disturbed newly restored areas. Alien species can also add multiple tiers to an ecosystem and increase biodiversity by providing pockets of shade for fragile seedlings.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Trees and shrubs that exist in open areas provide perching and nesting habitat for avian and terrestrial species, and may include some designated as "Threatened or Endangered". These areas also attract seed dispersing animals that help the regeneration of the landscape, which can better provide desirable habitat for native species populations. Therefore, removing these woody species may not benefit the system to a degree that justifies their costly removal (Ewel, 2004).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;">

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Many invasive, now naturalized, grass species started as forage material for cattle, horses and livestock. As the grasses migrated throughout the landscape, ecosystems adapted and took advantage of these fine fuels. The abundance of herbaceous species such as exotic pasture grasses and “invasive” shrubs that tend to be resistant to fire, help facilitate light burns that help rejuvenate forest ecosystems (MacDougall, 2005).

<span style="color: #141413; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> From a rehabilitation standpoint, exotic plants can sometimes be used to restore soils and waters that have been degraded or polluted by acid mine drainage, disposal of toxic wastes or nutrient enrichment. For example, the Chinese ladder brake fern (Pteris vittata), hyperaccumulates arsenic, and cattails are often used to help bioaccumulate other heavy metals. Many plants are adept at accumulating unwanted materials in soil and water. Also, alien plants, such as hydrilla, that proliferate in eutrophic lakes and marshes consume high levels of phosphorus and other nutrients from non-point source pollutants. Such plants sequester the target substance and can then be harvested and removed for safe disposal elsewhere (Ewel, 2004).

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//Hydrilla//

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> //Hydrilla verticillata,//also known as Esthwaite Waterweed, is native to the cool and warm waters of Asia, Europe, Africa and Australia. Hydrilla was first found in the United States in 1960 and is believed to have been released into the Florida waterways as byproduct of the aquarium industry. However, it was not considered a nuisance until the early 1980’s when it began rapidly colonizing the Potomac River and eventually the Chesapeake Bay. By the 1990’s the control of hydrilla was costing Americans millions of dollars annually (Cayuga Hydrilla Task Force, 2011).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Hydrilla is said to be one of the most aggressive aquatic plants to invade North America. Hydrilla forms dense mats of vegetation that can interfere with recreation and it is often believed to devastate fish and wildlife habitat. Hydrilla has several advantages over other plants: it will grow with less light, is more efficient at taking up nutrients than native species, and has extremely effective methods of propagation. Besides making seeds, it can sprout new plants from root fragments or stem fragments containing as few as two whorls of leaves (Cayuga Hydrilla Task Force, 2011). Recreational users can easily spread these small fragments from one water body to another.

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> The Chesapeake Bay, one of the United States' most polluted water bodies had evidence of invasion by hydrilla in the early 1980s. At that time, hydrilla was feared to be an aggressive, destructive invasive species that undoubtedly caused severe degradation to the native ecosystem. It was feared that the expansion of hydrilla would impair the reemergence of native species; a valuable nutrient source to aquatic and avian species. this theory, however, was disproven by a seventeen-year study conducted by Nancy Rybicki and Jurate Landwehr of the USGS. A ccording to thei r study published in the journal //Limnology and Oceanography,// the exotic species that was deemed a ruthless nuisance when it began rapidly colonizing the Potomac River, has instead benefited the watershed's ecosystem//.//

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">**//" Shortly after it first appeared in the Potomac in 1983, hydrilla produced dense vegetation masses and, in some areas, impeded boat traffic and water sports. More significantly, it was feared that hydrilla would interfere with native vegetation, which is important for waterfowl, such as black duck, a signature species in this area… (Rybiki, 2007)"//**

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Using data from annual field surveys and aerial photographs, Rybicki and Landwehr created a database to document, plot-by-plot, which species of vegetation were found in different sections of the Potomac River system. They recorded the percentage of total coverage and biomass each species achieved annually. In comparing species coverage with water quality composition, they discovered that, with the reduction of nitrogen concentration in Potomac River, hydrilla coverage expanded along with the diversity of plant species found throughout the River. Hydrilla did not outcompete or over-crowd native species; native species populations and diversity actually increased. In addition, hydrilla is a good winter food source for waterfowl communities, which have increased significantly over this period.

<span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">**//"This research is the only long-term, quantitative study of aquatic plant biodiversity following the colonization of an exotic species in an estuary where millions of dollars are spent annually to reduce nutrient input and it demonstrates that exotics are not always harmful to an ecosystem… (Rybiki, 2007) "//**

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">Not only was hydrilla found to be a beneficial addition to the ecosystem, but it clearly was an asset to the state in helping to comply with federal mandates to improve water quality. The findings supported federal and state management strategies to increase water clarity and reduce nutrient loads in order to enhance aquatic vegetation coverage, increase waterfowl habitat, and protect biodiversity of the existing native community. <span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> One recent case of a hydrilla infestation occurred in the Cayuga Lake Inlet in Ithaca, NY in August 2011. If not contained, there is a threat that hydrilla may spread into an immense network of interconnected water bodies not solely in New York State but span further into the Great Lakes. According to the //Cayuga Inlet Hydrilla Task Force,// it will cost about $91,000 (plus staff time and volunteer time) to contain hydrilla for this year only. To prevent a widespread infestation, substantial investments will be needed for monitoring and management efforts over the next five years. Left unchecked, the cost to eradicate hydrilla in the Inlet will rise significantly. States with widespread hydrilla infestations may spend up to $30 million per year to contain hydrilla //(//Cayuga Inlet Hydrilla Task Force, 2012//)//. In concert with a comprehensive monitoring plan and an in-depth assessment of ecosystem health, restoration ecologists and land managers could avoid the extreme costs of hydrilla eradication, and instead, learn how this invasive species may enhance environmental quality.

<span style="color: #141413; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">**//The Resilience Theory//**

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Resilience Theory was first introduced by Canadian ecologist C.S. “Buzz” Holling in 1973. Holling characterized stability as persistence of a system at or close to an equilibrium state ; by contrast, resilience was introduced to indicate behavior of dynamic systems not at equilibrium, by defining resilience as the amount of disturbance that a system can undergo without changing state (Gunderson, 2000). This science focuses on tipping points of ecosystem processes and species composition and today should include the idea that humans and nature are strongly coupled and co-evolving, and shall be conceived of as one “social-ecological” system (Montenegro, 2010). Ecologists often attempt to restore ecosystems to a state that meets historical benchmarks and fail to recognize the inseparable connection between human population dynamics and ecosystem function. However, ecosystems are in constant flux and are always in a state of change; t his theory works to disprove the long-held assumption that systems respond to change in a linear, predictable fashion.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> A key feature of complex adaptive systems is that they can settle into a number of different equilibria (Montenegro, 2010)**. ** Ecosystems are highly unpredictable and self-organizing, with feedbacks across time and space and can react to anthropogenic alterations in various timescales depending on the tipping point or threshold of when a severe shift will take place. Resilience science looks at gradual stresses, such as climate change, as well as chance events like; storms, fires, groundwater mining, and river channelization that can tip a system into another equilibrium state from which it is difficult, if not impossible, to recover (Montenegro, 2010). How much stress can a system experience before it transforms into something fundamentally different is the question and is, in a sense, the essence of resilience.

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> There are two widely accepted definitions for resilience in the ecological arena depending on the behavior one is trying to describe. The first defines resilience as the time required for a system to return to a steady-state (equilibrium) post diturbance. This definitions is most used in systems that contain one underlying or 'global' equilbrium, and is measured by how far a system has moved from equilibrium and how quickly it is able to rebound (Ives, 1995). The alternate use of the term resilience describes a system with multiple levels of equilibria and emphasizes conditions that are no longer near or at a steady-state condition, and where instibilities often cause a shift in system behavior and regime. Here, resilince is measured by the magnitude of disturbance that is absorbed before the system redifines itself by altering ecosystem processes and behavior (Gunderson, 2000).

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> When assessing invasive species colonization, it is important to keep this concept in mind and recognize that ecosystem shifts are a response to a disruption of ecosystem processes. When an exotic species is able to become invasive it is likely due to the fact that the system is at a tipping point. The use of resilince theory further demonstrates the role of invasive species in systems that experience extreme peturbations. As described in the passenger vs. driver model, exotic species become invasive when the system is no longer functioning at a historic level and is shifted into a new equilibria with accompaning changes in ecosystem processes and behaviors. Very rarely can these systems be restored to pre-invasion function and therefore might be better viewed as being in a new state of equilibrium (Montenegro, 2010).

<span style="color: #141413; font-family: Arial,Helvetica,sans-serif; font-size: 18px;">//**Conclusion**//

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Invasive species cost the United States several billion dollars per year in management costs and loss of industry, however, with proper assessment and study of local systems, these costs can be significantly reduced and, in some cases, eliminated all together. Research suggests that invasive species are not always the drivers of ecosystem degradation, but often the passengers of the ecosystem’s response to climate change and environmental shifts (MacDougall, 2005). As growing populations and urbanization continue to manipulate the environment, ecosystems must adapt to sustain health and function. Since invasive species are better equipped to handle stress and adapt more easily to disturbance, they often fill vacant niches. Therefore, invasive species can play an important role in maintaining ecosystem processes and thus the health of many ecosystems.

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> Many management agencies and ecologists would like to see invasive species eradicated, however, the time, money and further disruption to natural systems may not always pay-off. Many exotic species are utilized by the fauna of the surrounding area and sometimes provide vital habitat for Threatened and Endangered Species (Ewel, 2004).Tamarisk and hydrilla are examples of how invasive species not only fill vacant niches but actually help ecosystems rebound from the degenerative effects of human establishment. Tamarisk acts as an invasive species when the hydrological setting can no longer support native species. Drawdown of the local water table, the building of dams, and the ever-increasing pressure on native riparian zones have given tamarisk a competitive advantage over its native counterparts. Tamarisk is filling niches that have become too difficult for native vegetation to fill, and as a result: help stabilize banks, provide habitat for threatened or endangered species, and reduce in-stream temperatures by providing shade. Hydrilla is another case of an invasive helping support ecosystem function. Hydrilla an exotic species found in freshwater waterbodies throughout the US is considered an invasive species. In the Eastern U.S. especially, hydrilla ranks high on the list of management priorities and has a considerable budget dedicated to its eradication. Eradication is nearly impossible; hydrilla proliferates at astonishing rates and thrives in recreational areas that see great influxes of users in the summer months. However, this paper suggests that hydrilla eradication may not be the best use of limited funds as it has been shown to increase water quality in severely degraded systems and offers a nutritional food source to aquatic species.

<span style="color: black; font-family: Arial,Helvetica,sans-serif; font-size: 16px;"> The case studies highlighted in this paper show how exotic invasive species can play a significant role in ecosystem health and function and yet land-use managers pay millions every year in eradication efforts. This paper suggests that land managers survey, study, and assess the cause of invasive species infestations and adaptively manage infestations from the bottom up; approaching ecosystem restoration through a holistic lens.

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