invasives

Invasive Species - Raphael

Invasive species are defined by the Midwest Invasive Plant Network (MIPN) as exotic species that dominate the native biota in the area in which they are introduced. Even with prevention measures, thousands of exotic species have introduced themselves in the last few centuries (Brown 2004). Invasive species sometimes cause short term but potent ecological changes in addition to being economically detrimental (Brown 2004). This causes the confusion between invasive species and exotics, where all exotic species are presumed as invasive. Due to this popular view of invasive species, people are generally more wary of exotic species. This perspective spans across different levels of academic involvement with the concept but also most who favor this perspective consider removal of exotic a silver bullet solution. This does not mean accepting every invasion is the best course of action (Brown 2004).
 * Introduction **

Concepts regarding invasive and exotic species are becoming broader with every passing moment. As a general sense because we live in the time of man we will likely find it more difficult to classify any non-manmade species introduction as invasive. Initially, exotic species were introduced to areas accidentally and likely unnoticed. Later, exotic species were introduced to an area likely for crop or aesthetics. The most modern form of introduction would involve an engineering solution and require a reasonable understanding of the species introduced. The latter are generally selected primarily for a characteristic that is often excessively aggressive towards the problem at hand. Perfect examples of this problem involve zebra mussels and salt cedar.

How do we define a pristine state? How far back in time would one have to look to satisfy pristine state conditions as pre-industrial, pre-anthropogenic, etc. This concept is highly debatable for most situations and varies from area to area as well as time. This complexity grows with the idea that an invasive species today could be classified as a native species tomorrow. Before human invasion, Hawaii accumulated approximately 1300 plant species and 100 bird species all of which could be considered exotic upon arrival (Brown 2004). From a restoration sense the pristine conditions are not definable and none the less achievable, but a general goal is to recreate a habitat more favorable to native species that are either currently threatened or endangered with the minimal threat to other native biota.
 * Human Impacts **

Over time mankind has evolved, adapted and more geographic barriers have been passed. When sticking to a strict concept of invasive species, humans are no exception. People have driven other species to extinction from expanding territory and hunting (Brown 2004). A more modern role where people contribute to species extinction involves changing the environment, habitat destruction and bridging paths to once isolated areas in favor of species invasion (Brown 2004). Weather humans are considered an invasive force or not there are still other factors that can be a more significant threat to native biota other than invasive species. Diseases can also contribute to extinction. For example, the American chestnut suffered from a fungus and Hawaiian Bird Species nearly died out from avian malaria (Brown 2004). None the less, human influence is currently considered the primary driver for species invasion regardless of intention. Taking this into account, one would assume that invasion could not occur in the past on the same level as it does today. But overall, human influence is not the most significant when compared to mass extinction events (Brown 2004). The asteroid impact from the Cretaceous whipped out more than half of all species in a single event in addition to severely altering the climate and landscape (Brown 2004). Eventually the ecosystem recovered even from an asteroid collision, so it is expected to recover from modern human intervention (Brown 2004). The biota as a majority will survive human assisted invasions but will bring long-term changes to it (Brown 2004). Human assisted invasions are more like unintentional uncontrolled experiments.

Species vary in richness and composition at local and global scales. When introducing an invasive species to a local area, that area becomes more species rich if fewer native species die off than the amount of exotics introduced. “America has more terrestrial bird and mammal species at present than when the ﬁrst Europeans arrived four centuries ago.” (Brown 2004) From a global perspective there are less species in existence indicating a decrease in species richness at this scale. However, there are exceptions. If invasion increases species richness that implies native species are not fully utilizing there resources suggesting that the area is under capacity. More desirable and successful invaders are able to utilize resources that aren’t available to the local biota. For example, an “exotic tree species is a successful invader on recent volcanic soils in Hawaii largely because it can ‘make its own fertilizer’ by ﬁxing nitrogen, a trait that none of the native tree species possess.” (Brown 2004) In some cases exotics coexist and share resources with the local biota (Brown 2004). The situation where an exotic species crosses the threshold to an invasive species by becoming dominant and aggressive, Brown (2004) considers an extreme case.
 * Invasive Aspects of Species Richness **

In his report, (Brown 2004) describes three cases that account for species richness, 1) Near maximum capacity, 2) Well under capacity, and 3) Equilibrium Theory. All of these cases are related to the principals that diversity is reduced by extinction and increased by speciation and differentiation. Scale, weather local or global must also be taken into account (Brown 2004). 1) Near maximum case, is where, the biota is near carrying capacity for individuals and species for most places on Earth (Brown 2004). Because this is an ecosystem where resources are at their limit, it will either resist or the invasive species will kick out native species and eventually replace those (Brown 2004). In the case of a mass extinction event, this concept is referred to as secondary succession where species richness rapidly recovers and slows as it gets closer to its prior state (Brown 2004). 2) The well under capacity case states that the biota is well below its carrying capacity for individuals and species (Brown 2004). This is caused by the setbacks from extinction events and other natural disturbances such as landslides, floods, etc. These disturbances are also considered to have decreased the rate of speciation (Brown 2004). If it weren’t for these events species richness would be greater today (Brown 2004). Because of this, ecosystems should welcome colonization of exotic species as a driver for species richness (Brown 2004). 3) Equilibrium theory is an intermediate view where species richness is determined by the balance between colonization and speciation countered by the rate of extinction (Brown 2004). Equilibrium theory also suggests that larger areas support larger populations in addition to having more environmental conditions both of which favor a lower rate of extinction (Brown 2004). As mentioned by (Brown 2004), Equilibrium theory is most applicable to islands where less isolation implies that source of colonization will be greater thus leading to more speciation and species richness (Brown 2004).

Invasive species are typically known for the detrimental impacts they have on the ecosystem into which they are introduced and also there adaptability. Some species take this to the next step by causing massive damage to infrastructure while others have become a nuisance on the global scale. Zebra mussel and giant African snail have become a perfect example of this.
 * Extreme Cases **

In the Great Lakes, zebra mussels (//Dreissena polymorpha//) are a nightmare for any mechanism involving water transport. Zebra mussels impair flow by growing on intake screens, pipe walls and pretty much any surface with which they come into contact (Pejchar and Mooney 2009). The pipe in Figure 1 clearly illustrates the extreme degree of density these organisms accumulate. In addition to making it difficult to see the interior pipe's surface, zebra mussels appear to develop on top of each other. Municipalities, hydroelectric companies and “339 water-dependent facilities reported total zebra mussel-related expenses of $69,070,780 from 1989 to 1995; control costs of average large water user: $400 000-460 000 yr-1” (Pejchar and Mooney 2009). Prior to the incurred damage, zebra mussels were actually the chosen remediation to increase the water clarity by removing pollution and restoring the area to a more habitable state (Pejchar and Mooney 2009). An additional benefit is the food source, mussels provide to some fishes, crayfish and water fowl (Pejchar and Mooney 2009). Zebra mussels were an effective remediation method for filtering out pollution, and overall, provided a positive net benefit toward the ecosystem but grew out of hand, creating considerable damage to resources essential to local communities.
 * Zebra Mussels **

The Giant African Snail ( //Lissachatina fulica // ) appears as a nuisance in many articles throughout the world. They pose a variety of threats to humans as well as the environment. “ The Giant African land snail is one of the most damaging snails in the world because they consume at least 500 different types of plants, can cause structural damage to plaster and stucco, and can carry a parasitic nematode that can lead to meningitis in humans.” (Florida Department of Agriculture and Consumer Services)
 * Giant African Snail **

Saltcedar (//Tamarisk// ssp.) is one of the most prominent invasive species along the riparian corridor of the Rio Grande. Saltcedar was initially introduced from Asia more than a century ago in order to control soil erosion and to provide landscaping (Dennison et al. 2009). Saltcedar is well adapted to surviving severe drought conditions. Saltcedar’s phreatophyte characteristics allow it to draw moisture from saturated zones above the water table (FWS 2014). Its extensive root structure has allowed access to deeper water tables in less saturated soils. When soil moisture is more abundant saltcedar consumes great volumes of water (FWS 2014). This form of water consumption creates more favorable conditions for floods and wildfires and also separates native species all of which contribute environmental and economic problems (Dudley 2005).
 * Saltcedar **

Offering minimal ecosystem benefits in a state where drought is a primary concern, saltcedar offers an additional list of detriments that only add to already local problems. The leaves of saltcedar deposit salt making less favorable soil surface conditions for other vegetation (FWS 2014). The presence of saltcedar can also increase fire frequency (FWS 2014). Around 0.5-0.7 million acres of cottonwood-willow area has been lost to saltcedar (Dudley 2005). From an economic perspective, the invasion can even affect the financial performance of a national park. An estimated $2,000,000 a year was lost at the Grand Canyon due to saltcedar reducing optimal flow for boating (Penny 1991; Dennison et al. 2009). In 1998, total economic loss in the US from saltcedar was estimated between $133-285 million (Zavaleta 2000; Dennison et al. 2009).
 * Issues**

There are five methods known for removing; burning, flooding, mechanical, chemical, and biological. If used alone, most of these methods will be ineffective in removing the saltcedar or may even do more harm than good.
 * Methods of Removal**

Burning and flooding are better suited as a finishing touch for mechanical and herbicide removal. Even though the native vegetation is adapted to burns, saltcedar is even better adapted. Using the burning method alone will only clear away native vegetation for the faster recovering saltcedar to come in and replace it. Burning is better used to better dispose of saltcedar killed off by other removal methods. Areas that were flooded regularly were less susceptible to saltcedar invasion (Dudley 2005). If timed correctly, flooding can be used after removal efforts because it is essential for reestablishing native vegetation (FWS 2014). However, this is unlikely with urbanization and agricultural practices occurring on the flood plain.

The mechanical method involves using machinery, such as a bulldozer, chainsaw, or other physical means to remove the tree (FWS 2014). Because saltcedar can grow back from root fragments and seeds, this approach alone will likely make more trees in the long run (FWS 2014). The chemical method which uses Triclopyr (Garlon 3A and Garlon 4) as the herbicide and has been considered successful by Martin (2001). The herbicide can be applied through backpack or aerial dispersal (FWS 2014). The herbicide will either defoliate the top layer or kill the saltcedar altogether (FWS 2014).

The biological approach involves introducing a species predatory to saltcedar to the ecosystem. A biological approach was considered because mechanical and chemical methods would be extremely costly over the same scale of area and may require maintenance (Dudley 2005). Two insects were approved in 1996, which were the leaf beetle (//Diorhabda elongata//) from central Asia and the mealy bug (//Trabutina mannipara//) from the eastern Mediterranean (Dudley 2005). More attention was placed on the leaf beetle because it selectively feeds on saltcedar and overwinters in litter under the saltcedar (Dudley 2005). A female leaf beetle can produce up to 750 eggs in its lifetime (USBR 2014). Introducing an exotic solution is looked upon with more caution nowadays. Saltcedar itself was an exotic solution for controlling erosion. Governments from local to federal levels, recognized the importance of saltcedar removal (Dennison et al. 2009). The leaf beetle was approved for use as a biological control agent in 1996 by the United States Department of Agriculture (USDA) and the Animal and Plant Health inspection Service (APHIS). (DeLoach et al. 2000; Dudley 2005) The saltcedar leaf beetle was released to multiple areas as a biological control. Dennison says that no program is monitoring the spread of leaf beetle since the initial release and suggests that there should be long term programs that observe saltcedar defoliation in relation to water resources, recreation and riparian habitat.

One of the earliest studies conducted by Dudley only had project duration from 2002-2005. A Nevada site in the Lower Humboldt River served as Dudley’s (2005) base for comparing defoliation rates overtime. Initially, the beetles were not very effective at defoliating larger areas of saltcedar until their population expanded. Roughly 2ha were affected in 2002, 200ha by 2003 and 20000ha by 2004 (Dudley 2005). Saltcedar recovers roughly 60% of its leaves after a defoliation event (Dudley 2005). These events are based upon the generations of beetles which appear to increase with population growth and years after release. After three years of defoliation saltcedar lives but is limited to a 10% recovery (Dudley 2005). Ground water loss caused by saltcedar was reduced by 75% after the first defoliation (Dudley 2005). Reduction in canopy shading allows for new vegetation to come in (Dudley 2005). The leaf beetles serve as a food source for some birds as beetle remains appear in their fecal matter. Small mammals have been observed searching through litter for over-wintering leaf beetles (Dudley 2005).

As advantageous may seem to introduce the leaf beetle to an area dominated by its most preferable forage, there are other environmental factors that need to be addressed in order for this remediation to perform optimally. The leaf beetle needs at least 14.5 hours of daylength in order to reproduce most effectively (Bean et al. 2001; Dudley 2005). This daylength also needs to be sustained for at least 3 months or else the beetles may end up dormant when the saltcedar starts to grow (Dudley 2005). This is known to limit beetle population and the time necessary to store energy for the next season (Dudley 2005). Because daylength is influenced by latitude, latitude is a geographic boundary for the leaf beetle (Dudley 2005). Leaf beetles have difficulty reproducing in latitudes below 38 degrees (Dudley 2005; Dennison et al. 2009). The effectiveness of leaf beetles on saltcedar also varies with the species of saltcedar hosted upon (Dudley 2005). At least, along the Rio Grande is the species of salt cedar (//Tamarisk// ssp.) that is one of the listed hosts suitable for the leaf beetle. Predation can also be the factor that is the difference between a habitable area and an uninhabitable one. Dense populations of harvester-type ants appear more effective at eating the leaf beetle than the leaf beetle is at eating saltcedar (Dudley 2005). Establishing beetles in any of these conditions would be less than ideal.

However, Dudley (2005) has listed possible solutions to daylength, differing tamarisk species, and predation issues. Leaf beetles from different latitudes in Eurasia and Africa could be imported to North America (Dudley 2005). The solution is similar for different species of saltcedar where importing a different species of Diorhabia could be an easy fix (Dudley 2005). To deal with ants predation, Dudley (2005) recommends Cryptocephalus Sinatra, a beetle very similar to the leaf beetle except that it builds a defensive case around its larva (Dudley 2005. The drawback to these solutions is amount of testing necessary to determine what new effects of introducing these species could have on the ecosystem (Dudley 2005).

The Tamarisk Coalition and Dennison et al. (2009) have attempted long term monitoring solutions of the leaf beetle. The Tamarisk Coalition has tracked the expansion of leaf beetle on an annual basis. This expansion is mapped and the data is an accumulation of every possible source the coalition can get their hands on. The Coalition’s map (Figure 1) indicates that the leaf beetle arrived in the Middle Rio Grande in 2012. Dennison et al. (2009) used a remote sensing method to determine the impact leaf beetles have on saltcedar evapotranspiration. Vegetation indices and multispectral imagery have been used interpret defoliation by other insects in the past. Less leaf implies less Chlorophyll to be detected by NIR wavelength. Saltcedar polygons are first created from GPS points and are used to mark known locations of saltcedar. These polygons will later be used to determine a baseline for the vegetation color index. Imagery from Advanced Spaceborne Thermal Emission Radiometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) are used to compare saltcedar before and after beetle release scenes. Lastly, an equation is applied to derive evapotranspiration to the before and after images. The biggest drawback to this approach with the satellites are a limited maximum resolution of 15m per pixel for (ASTER) and a 30m resolution for (MODIS). This becomes a problem when saltcedar is mixed with other vegetation types affecting the average pixel color limiting this technique to saltcedar dominated areas.
 * Future Monitoring**

Russian Olive (//Elaeagnus augustifolia//) was introduced to America for the same reasons as salt cedar (soil erosion and ornamental). Russian olive has silvery leaves, olive shaped fruits and waxy branches with thin long thorns. Russian Olives prefer open moist riparian zones, and are shade tolerant (DOA 2008). Like saltcedar, Russian Olive has an extensive root system. Unlike Saltcedar, Russian olive can grow on bare mineral substrates because it is capable of fixing nitrogen in its roots (DOA 2008). Russian Olive can reproduce by root suckers or seeds that last for 5 years (DOA 2008). Mechanical and chemical removal works effectively the same as it would for saltcedar using the same excavation techniques and the same herbicides. The biological controlling agent Tubercularia canker shrivels up and deforms the plant. This may stress the tree enough to kill it. Tubercularia overwinters on stems where it can be transmitted by rain splash and animals onto exposed bark (DOA 2008). For the most part most issues related to saltcedar are likely to apply to Russian Olive.
 * Russian Olive **

Giant reed (//Arundo donax L.//) is a member of the grass family can be found in riparian areas throughout the Southwest (USDA 2012). Giant reed is also known as phragmites, carrizo, arundo grass, donax, elephant grass, Spanish cane, wild cane, and oboe cane (USDA 2012). This plant was originally native to Asia but was introduced to California as an ornamental species and also for erosion control along drainage ditches (USDA 2012). The reeds grows 20-30ft tall with tough, fibrous and deep growing roots (USDA 2012). Giant reed prefers elevations below 5000ft, it does well in saline soils and typically occurs with salt cedar and Russian olive (USDA 2012). The primary form of reproduction occurs when in stems, which can sprout, even when buried up to 10 ft deep, unlike the seeds, which have a low chance of successfully germinating (USDA 2012).
 * Giant Reed **

Giant reeds impacts and threaten the native biota in a variety of ways. Consuming moisture, nutrients and light, the giant reed crowds out native vegetation to the point where it establishes monocultural stands (USDA 2012). The USDA (2012) claims the giant reed transpires three times as much water in the course of a year, as native vegetation. When reeds become dry they increase the risk for greater fire intensity. Giant reed can grow at rates up to 4 inches per day, much quicker than the native vegetation giving it the advantage to invade and take over (USDA 2012). Giant reeds also cast less shade than the willow it most likely replaced, creating higher water temperatures which may contribute a less ideal habitat for aquatic life and also to evaporation losses. Decaying reeds can also convert ammonium (NH4+) to the toxic Ammonia (NH3), thus reducing soil and water quality (USDA 2012).
 * Issues**

The Management methods are grouped into physical, chemical and biological. Giant reed has many traits of resilience that make it challenging to remove. Giant reed can resprout from nodes in the root structure and allowing soil disturbance to helps spread it to new areas (USDA 2012). Herbicide is the USDA’s recommended treatment when giant reed is growing but admits that complete removal is rarely guaranteed.
 * Management**

Physical Controls offer a variety of mechanical removal options ranging from removal by hand to mulching and excavating. In order to be successful, root structures must be unearthed properly in order to prevent spread and regrowth. Manual methods include removal by hand and hand tools. This can be done, but it is generally not a very efficient approach even at small scales (USDA 2012). Mulching can be used to remove the top growth followed by herbicide treatments for three years. These machines have difficulty in rugged areas and slope percent’s greater than 30 degrees (USDA 2012). The method of excavating can be used to uproot patches of giant reed by pulling (USDA 2012). This is not recommended near streams because root material can end up getting washed downstream further encouraging spread and distribution of this invasive species (USDA 2012).

Chemical control could be composed of herbicides and burning methods. The herbicides, imazapyr and glyphosate are considered effective tools for managing giant reed (USDA 2012). Downsides for this treatment include 3-5 years of repetition in addition to the possibility of damaging other types of vegetation (USDA 2012). Although in most cases chemical sprays are applied on foot, larger areas composed of 80 percent giant reed may be more practical using a helicopter sprayer (USDA 2012). Burning is not recommended as an independent solution because it may promote the replacement of giant reed over the native vegetation (USDA 2012). Burning is expected to be more successful after a mechanical or other control is used as a means of better disposal (USDA 2012).

Biological controls for giant reed are limited to grazing animals and one type of insect. Animals can only graze young shoots where the rest of the giant reed isn’t as palatable (USDA 2012). Arundo scale (Rhizaspidiotus donacis) is the only known insect interested in the giant reed (USDA 2012). It attacks the roots structure and is considered for future release (USDA 2012).

A well-documented complexity that has arisen is the Southwestern flycatcher an endangered species nests in Saltcedar in place of a plant that was long out competed. Viable solutions are rarely an ultimatum and most often involve a balanced degree of moderation. A recommendation for approaching the area with the southwestern willow flycatcher would be to apply treatments to upstream areas until a less detrimental replacement flora can be established for the Southwestern Willow Fly Catcher.
 * Conclusion **


 * Sources **

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Brown, J. H. and Sax, D. F. (2004) [|An Essay on some Topics Concerning Invasive Species]. Austral Ecology **29:**530-536.

Dallas, D. (2008) [|Invasive Species Action Plan]. USDA. (Accessed 20 Feb, 2014)

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DOA (2008) [|Russian Olive: Identification and Management]. Colorado Dept. of Agriculture.

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USDA (2013) [|Giant African Snail]. (Accessed 20 Feb, 2014)

