IntroductionAn Invasive plant is a plant which grows aggressively, spreads, and displaces other plants. Identification of invasive plants can have positive and negative impacts on ecosystems. There are many developed processes used to determine potential invasiveness in plant species and these various processes are typically known as methods and models. These models will be evaluated in how well they screen for certain characteristics in known invasive species.
Groups, such as the Massachusetts Invasive Plant Advisory Group (MIPAG) and (IPANE) The Invasive Plant Atlas of New England use various methods to identify invasive plants. Using these efficient models contribute to faster classification and extermination; thereby increasing the chance to preserve threatened habitats. The focus is in the evaluation and some comparisons of two existing weak models and two strong models to successfully classify potential invasive and non-invasive plants.
Depending on how they are classified, plants can have a significant impact on ecosystems, economics, and agriculture. An invasive plant species is one that is non-native to the habitat and has the potential to cause economic or ecological harm. These exotic species aggressively compete with and displace the native organisms (IPANE 2015).
To determine the likelihood of a plant becoming invasive, flow chart diagrams, data analysis tests, and other models are used (Higgins et al, 1996). Identifying plants as invasive species depends on factors such as: exploring the potential threat levels of plants being introduced into a new habitat, ability to invade natural flora, current ecological range, and the extent of invasion within ecosystems already affected (Warner et al, 2003). More specifically, some common traits that help to identify threat level are native range, invasion outside of specified range, germination rate, dispersal mechanisms, and relation to a family or genus of already known invasive species, temperature tolerance, growth rate, and growing conditions (Reichard & Hamilton, 1997).
Process of determining the invasive nature of species
The process of determining if a species is invasive can be at times somewhat difficult because it has direct regulatory and ecological consequences. Once a species escapes the boundaries of cultivation, it is tough to get back under control (US Forest Service, 2014).
Choosing accurate models to determine invasiveness allows earlier detection and a more rapid response. The earlier the species is identified as invasive, the better it is to protect or save the natural habitat. By taking steps to contain or eradicate the harmful species, environmental and economic impacts can be reduced (US Forest Service, 2014). Invasive plants are introduced to an area, either accidentally or for cultivation in agriculture, and negatively impact the ecosystem (Rejmanek, 2000).
Invasive species either kill or leave native species displaced and out-competed. To declare species as invasive and determine its threat level, it is typically run through a series of models and tests (Higgins et al, 1996). Some of the main characteristics or attributes that classify a plant as invasive are toleration of various habitat conditions, rapid growth and reproduction, aggressive competition for resources, and the lack of natural enemies and pests in the new habitat (US Forest Service, 2015).
Models using a decision tree combined with the use of statistical data have demonstrated to be at least 80% accurate in identifying invasive species, signifying that models predict invasiveness better than relying on chance (Reichard & Hamilton, 1997). To ensure the validity of such models, tests and controls should be run using already known invasive plants and native plants related to the species under evaluation. If the models identify the plants correctly, the models are most likely effective. The first model evaluated is from Marcel Rejmanek and this is based on many factors and the list quite extensive. This includes:
Depends on initial inoculums size, residence time, and a number of introduction attempts.
Is it invasive in other regions or biomes?
Learn about individual invasiveness of species as well as ecological and economic
Evaluation of biological characters
Understand how and why certain biological characters promote invasiveness in species
The major predictions made by an emerging theory of plant invasiveness based on biological characters by ten points (p. 501).
Evaluation of habitat compatibility.
4) Climate difference
First, four can be tested in quarantine field trials but can be time-consuming.
Extensive experimental testing will be conducted more often when introduction of a taxon under question is highly desirable.
This is extremely detailed and confusing but may need to be explained better or even simplified before being put into use. The criteria that IPANE lists is somewhat short and should be expanded upon, they use criteria and address questions such as:
Is the plant a generalist?
Does is out-compete other plants?
Does it reproduce easily?
Does it contain abundant seeds which can be dispersed by birds?
Is the plant not affected by native pests/diseases?
These models that I just listed have similarities but are lacking in clarity and decisiveness. The next is a highly developed model examined and is very similar to IPANE in format but asks more questions.
The MIPAG criteria
The Massachusetts Invasive Plant Advisory Group (MIPAG) has used certain invasive-determining criteria for identifying plants as invasive, potentially invasive, or likely invasive. During the evaluation, herbarium samples, peer-reviewed papers, and published records are scientifically investigated (MIPAG, 2005). In Massachusetts, MIPAG is responsible for classifying and determining whether a plant fall in the category of intensive plants.
The MIPAG model consists of various qualitative criteria. Any plant that is reviewed by MIPAG must first satisfy the following requirements: be nonindigenous to Massachusetts, have potential for large scale expansion and establishment in poorly managed habitats, have potential for dispersing large distances away from introduction site and have potential for existing in large quantities (MIPAG, 2005). Once the first four criteria are met, the species can be reviewed and identified as either invasive, likely invasive or potentially invasive.
MIPAG has a fifteen question step by step criteria to evaluate plants in the three categories mentioned above. It has to be non-native to Massachusetts, be able to grow rapidly, disperse large distances away from the parent plant, have the potential to exist in large quantities, has to be naturalized that is to freely and regularly reproduce in the wild whether sexually or asexually and have persistent occurrences over time.
The MIPAG criteria appear to be difficult to follow and interpret the data. Some of the criteria were missing some key issues that should be researched when assessing invasiveness. The model also contains multiple characteristics within the same criteria, making it difficult to accurately answer especially for the non-botanist.
The MIPAG model includes many subjective criteria that can lead to different outcomes when running a species through. For example, number three addresses the biologic potential for dispersing over spatial gaps away from site of the introduction (MIPAG, 2005). This question is addressing both seed dispersal mechanisms and how far they have dispersed. Both of these traits combined in a single question make it difficult to answer objectively. If the criteria asked more questions and addressed one trait in each, it would keep the criteria objective and clear.
The Decision Tree Model
The second major or strongly developed model analyzed is a decision tree model that was created by Reichard and Hamilton in 1997 and is based on various plant attributes to determine invasiveness. Many other plant models have been based on this. The criteria are listed as follows.
Do the species invade elsewhere, outside of N.A?
Is it an interspecific hybrid with known seed sterility?
Is it native to parts of North America other than the region of the proposed introduction?
Is the juvenile period usually less than five years (trees), three years (shrubs/vines), or does it grow very rapidly in its first two years?
Does it reproduce quickly vegetatively?
Do the seeds require pretreatment for germination?
This decision tree model includes many relevant traits and is considered to be a strong base model for future models. One of the flaws within this model is the open-endedness of some of the questions and results do not provide definitive answers. When a list of criteria is established for invasive ecological identification, there should be additional questions to provide researchers with a straight-forward accept or reject the conclusion.
Overall, both of the primary models analyzed have strengths as well as weaknesses. The strength in the Reichard & Hamilton decision tree is that it incorporates many appropriate traits correlated with invasiveness. The main weakness of their model is the lack of a clear outcome by simply accepting a plant into the habitat under consideration, rejecting it, or requiring further analysis and research. Within the MIPAG model, the strength has a clear outcome by identifying a plant as invasive, likely invasive, and potentially invasive. The main weakness is the complexity of the criteria, making it difficult to follow through to the end. This model is extremely subjective in the questions being asked. Subjectivity does not guarantee a clear and accurate outcome because more results can vary depending on a researchers opinions.
Elements to consider when using a model to predict the level of invasiveness a species can have on a habitat should include a significant number of biotic factors and abiotic factors which some of these models failed to address. Abiotic properties to consider are the ability for plants to flourish in various habitats by having soil, the wind, drought, and extreme temperature tolerance (Moles et al, 2008).
Another element to consider is if the species in question, if invasive or not can out-compete other plants in the area. When the plant begins to grow aggressively, can it block out light that prevents other plant life from growing. An effective model should include more abiotic factors related to the method of seed dispersal, serotiny (ecological adaptation exhibited by some seed plants, in which seed release occurs in response to an environmental event), and the resistance to fire and herbicides. Seed size can also be crucial to the method of dispersal and cause higher establishment in other areas (Moles et al, 2008). Invasive plant models need to utilize varied statistical methods to include plant attribute databases with information on the abiotic factors under which the traits happen (Moles et al, 2008). There are many factors, both biotic and abiotic, that can be included in an efficient model, and more come about through further research. With current models in use by botanists and researchers, one can utilize a similar, yet revised approach and apply it to most plant species for correct identification.
Higgins S, Richardson D, & Cowling R (1996). Modeling invasive plant spread: the role of plant-environment interactions and model structure. Ecology: Vol. 77, No. 7, pp.
MIPAG. The evaluation of non-native plant species for invasiveness...
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