Alternanthera philoxeroides, commonly referred to as alligator weed, is a native species to the temperate regions of South America, which includes Argentina, Brazil, Paraguay and Uruguay. Argentina alone, hosts around 27 species that fall within the range of the genus Alternanthera. Its geographic range once used to cover only the Parana River region of South America, but it has since expanded to cover over 30 countries, such as the United States, New Zealand, China and many more. This invasive species is believed to have been accidentally introduced to these non-native regions through sediments trapped/attached to tanks and cargo of ships travelling from South America to these various areas.
The species of alligator weed can thrive in both dry and aquatic environments and is characterized by the following set of notable features: Whitish, papery flowers along its short stalks, Irregular, or sprawling hollow stems, and Simple and opposite leave pattern sprouting from its nodes. The aquatic plant is classified as a dioecious species  because it contains both male and female reproductive organs, which allows it to reproduce both sexually and asexually. It is also considered a herbaceous plant due to its short-lived shoot system. This plant produces horizontal stems, otherwise known as stolons, that can sprout up to 10 m in length and can easily float, hence the reason why they have hollow stems; this in turn leads to large clusters of stem to amass and create dense mats along the surface. The plant flowers from December to April and usually grows around 13 mm in diameter and tend to be papery and ball-shaped. The weed's intricate root system can either allow them to hang free in the water to absorb nutrients or directly penetrate the soil/sediment and pull their nutrients from below.
Alligator weed is considered a major threat to ecosystems because of the adverse effects it poses on both aquatic and terrestrial environments, as well as negatively influence both the ecosystem and society.
Impacts on vegetation
The presence of this invasive species disrupts the natural flow of water due to the dense mats created from its clusters of stems. It out competes the native vegetation for space and solar energy through these dense mats because they are large cluster and limit the amount of light that submerged vegetation receives. These compact clusters of stems also disturb the regular exchange of gases that occur underneath the surface that directly influences aerobic processes, such as photosynthesis. Aside from driving down the population of native aquatic vegetation, Alligator weed can also influence the growth and yield of crops in pastures and fields. These dense mats can affect the natural flow of water that is used in irrigation systems and as well as affect the quality of the water by increasing the sedimentation present in the water. Both factors are things that crops depend upon to not be disturbed, to provide a healthy yield for farmers, which is also falls within an impact it has on society that will be discussed later. On top of this, the likelihood of flooding is more like due to the drainage brought upon by the dense mats, which in turn can also damage the crops. Furthermore, this is a small insight into the negative impacts this invasive species has on the environment.
Impacts on animals
As mentioned, the compact mats formed by this species can drive down the population of native vegetation in the environments it invades, this is a major issue for the native herbivore because their food source is declining. In addition, the dense mats present a challenge for the native wildlife by acting as a barrier between them and the natural water source. However, even if they can reach the water, they are still at risk because the water quality can be contaminated by the increased sediments. Thus, just as it did with the native vegetation, Alligator weed is also driving down the population of the native wildlife as well.
Impacts on society
As been repeatedly stated in the previous two sections, the dense mats formed by this species influence the natural flow of water, which can impede various recreational activities, such as boating and fishing. The disruption of flow can also have a negative impact on infrastructure when it comes to energy, such as the use of hydro-electric dams to power generators. The dense mats also present suitable ecological conditions that mosquitoes can thrive off. This can be considered a commensal relationship between mosquitoes and alligator weed because mosquitoes receive a breeding ground and alligator weed gets nothing nor losing anything. The increases population of mosquitoes can lead to increase risks towards humans’ health, regarding the spread of disease. Furthermore, the dense mats produced by Alligator weed do not present suitable ecological conditions for native species or humans to thrive off.
Reproduction and dispersal
This invasive plant depends solely on vegetative means to reproduce and disperse itself in the area it has invaded and established its roots. In its native geographic range, the species spreads to through means of producing viable seeds; however, it has been observed within its non-native ranges that it rarely produces viable seeds. To accommodate this, the Alligator weed reproduces itself through fragmentation; the plant can regenerate itself from small portions of stems or small leaf cuttings. These small fragments of the plant can then be dispersed through human means and natural means; once its dispersed, the fragments can then find suitable ecological conditions and root itself and regenerate.
In effort to eradicate the species by manual means, such as mulching or pulling them out, if not removed efficiently, small stem fragments can be displaced to new areas. Soil movement caused by earthmoving machinery is another example of how humans influence the dispersal of the plant.
In aquatic environments, the Alligator weed can easily disperse its fragments by being sucked into the path of the waterways. In terrestrial environments, the small fragments of stems and leaf cuttings can be dispersed through the natural movement of soil caused by erosion. Thus, the geographic range of this invasive species can easily be expanded by any means necessary due to its ability to regenerate from practically nothing.
Methods of control
Early detection is the best bet to ensure that the invasive species does not successfully colonize a non-native region because of its persistent to regenerate and propagate from small portions of its stem or leaf cuttings. However, when that is not possible, the best that can be done is to limit and control the presence of Alligator weed in an area. Alligator weed can only establish itself in shallow waters no deeper than 2 meters, so one method of control is to erect barriers in shallower areas to limit the amount of suitable space the plant has. When it comes to terrestrial environments, by overpopulating the area with native species it can limit the suitable space available for it. However, this method is only effective before the invasive plant has asserted itself in an area. Lastly, as previously mentioned, this plant is only able to produce viable seeds in its native geographic range and not in the areas it has invaded. By getting a better understanding on what ecological conditions make it so that only sterile seeds are produced in non-native regions could be key to developing further preventative measures against it.
Insects have been released for the biological control of alligator weed. The most successful and widely used is Agasicles hygrophila commonly called the alligator weed flea beetle; it has been released for biocontrol in Australia, China, Thailand, New Zealand, and the United States. However, their effectiveness is limited due to their inability to survive through temperatures lower than 11 °C (52 °F). Amynothrips andersoni, the alligator weed thrips, and Vogtia malloi, the alligator weed stem borer, have also been released in the United States. These species result in immediate wilting and limit Alligator’s weed to reproduce by colonizing its stems. A variety of chemicals have been shown to be effective in controlling the plant, the most useful of which include glyphosate, triclopyr, fluridone, imazamox, and imazapyr; however, they must be constantly be applied to be successful. Furthermore, these are all the currently known methods for control against the invasive Alternanthera philoxeroides.
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- photograph, description and diagram from HerbiGuide
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- Alternanthera philoxeroides (Mart.) Griseb. Medicinal Plant Images Database (School of Chinese Medicine, Hong Kong Baptist University) (in traditional Chinese) (in English)