''Alternanthera philoxeroides'', commonly referred to as alligator weed, is a native species to the temperate regions of
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, which includes
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
,
Brazil
Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...
,
Paraguay
Paraguay, officially the Republic of Paraguay, is a landlocked country in South America. It is bordered by Argentina to the Argentina–Paraguay border, south and southwest, Brazil to the Brazil–Paraguay border, east and northeast, and Boli ...
and
Uruguay
Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...
.
Argentina alone hosts around 27 species that fall within the range of the genus ''
Alternanthera
''Alternanthera'' is a genus of flowering plants in the family Amaranthaceae. It is a widespread genus with most species occurring in the tropics, tropical Americas,Sánchez-Del Pino, I., et al. (2012)Molecular phylogenetics of ''Alternanthera'' ...
''.
Its geographic range once covered only the
Parana River region of South America, but it has since expanded, having been introduced to over 30 countries, such as the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
,
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
,
China
China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
,
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
,
New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
and many more.
This
invasive species
An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...
is believed to have been accidentally introduced to these non-native regions through sediments trapped by, or attached to, tanks and cargo of ships travelling from South America to these various areas.
Description
''Alternanthera philoxeroides'' can thrive in both dry and aquatic environments
and is characterized by whitish, papery flowers along its short stalks, irregular, or sprawling hollow stems, and simple and opposite leaf pattern sprouting from its nodes.
It is also considered a
herbaceous plant
Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials.
Definitions of "herb" and "herbaceous"
The fourth edition o ...
due to its short-lived shoot system.
It produces horizontal stems, otherwise known as
stolon
In biology, a stolon ( from Latin ''wikt:stolo, stolō'', genitive ''stolōnis'' – "branch"), also known as a runner, is a horizontal connection between parts of an organism. It may be part of the organism, or of its skeleton. Typically, animal ...
s, that can sprout up to in length and thanks to its hollow stems, floats easily. This results in large clusters of stem amassing and create dense mats along the surface.
Bisexual flowers are small, white, and born on dense, axillary spikes (type of
raceme
A raceme () or racemoid is an unbranched, indeterminate growth, indeterminate type of inflorescence bearing flowers having short floral stalks along the shoots that bear the flowers. The oldest flowers grow close to the base and new flowers are ...
).
Presence of a
peduncle is a key trait that distinguishes this species from the sessile
inflorescences
In botany, an inflorescence is a group or cluster of flowers arranged on a plant's stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a main axis ( pe ...
of ''
Alternanthera sessilis''.
While seed production has been observed in its native range, there is currently no record of viable ''A. philoxeroides'' seeds in the introduced range. 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.
As an invasive species
''Alternanthera philoxeroides'' is considered a major threat to ecosystems because of the adverse effects it poses on both aquatic and terrestrial environments, as well as the negative influence it has on society.
The species features on the list of invasive alien species of Union Concern since 2017. This means that import and trade of this species is forbidden in the whole of the European Union.
Impacts on vegetation
The presence of this invasive species disrupts the natural flow of water due to the dense mats created by its clusters of stems.
It out competes the native vegetation for space and solar energy through these dense mats because they form large clusters and limit the amount of light that submerged vegetation receives.
These compact clusters of stems also disturb the regular exchange of gases that occurs underneath the surface that directly influences aerobic processes, such as
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
.
Aside from driving down the population of native aquatic vegetation, ''A. philoxeroides'' 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 of these factors must remain undisturbed for crops to grow well, and thus to provide a healthy yield for farmers, which will be discussed in the later section on its impacts on society. On top of this, the likelihood of flooding is higher due to the impaired drainage caused by the dense mats, which in turn can also damage crops.
This invasive species also has other negative impacts 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 then becomes a major issue for native herbivores because their food source declines.
In addition, the dense mats present a challenge for the native wildlife by acting as a barrier between them and natural water sources.
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, ''A. philoxeroides'' is also driving down the populations of the native wildlife as well.
Impacts on society
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 of.
This is a
commensal relationship between mosquitoes and ''A. philoxeroides'' because mosquitoes receive a breeding ground and the plant gains nothing nor loses anything. The increased population of mosquitoes can bring an increased risk of
mosquito-borne diseases in humans.
Furthermore, the dense mats produced by ''A. philoxeroides'' may 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 ''A. philoxeroides'' reproduces 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 it is dispersed, the fragments can then find suitable ecological conditions and root themselves and regenerate.
Human dispersal
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.
Natural dispersal
In aquatic environments, the ''A. philoxeroides'' 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
Preventive measures
Early detection is the best bet to ensure that the invasive species does not successfully colonize a non-native region because of its ability 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 ''A. philoxeroides'' in an area. ''Alternanthera philoxeroides'' can only establish itself in shallow waters no deeper than , 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, overpopulating the area with native species 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. Learning why that is - what ecological conditions make it produce only sterile seeds - could be key to developing further preventative measures against it.
Biological measures
Insects have been released for the biological control of ''A. philoxeroides''. The most successful and widely used is ''
Agasicles hygrophila'' commonly called the alligator weed flea beetle; it has been released for biocontrol in
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
,
China
China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
,
Thailand
Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...
,
New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
, and the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
. However, their effectiveness is limited due to their inability to survive through temperatures lower than .
''
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 ''A. philoxeroides'' reproduction 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
Glyphosate (IUPAC name: ''N''-(phosphonomethyl)glycine) is a broad-spectrum systemic herbicide and crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by EPSP inhibitor, inhibiting the plant enzyme 5-en ...
,
triclopyr
Triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid) is an organic compound in the pyridine group that is used as a systemic foliar herbicide and fungicide.
History
Triclopyr triethylammonium (TEA) was first registered in 1979 in the United ...
,
fluridone,
imazamox, and
imazapyr; however, they must constantly be applied to be successful.
Legality of sale and shipment in the United States
In 1956, ''A. philoxeroides'' was banned for sale or shipment in the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, subject to a fine and/or imprisonment.
This law was repealed by H.R.498
16th Congress (2019-2020)on January 23, 2019.
See also
*''
Ludwigia grandiflora''
References
External links
The evolutionary history of an invasive species: alligator weed, ''Alternanthera philoxeroides'' XII International Symposium on Biological Control of Weeds
Pest Plant Risk Assessment: Alligatorweed Biosecurity Queensland, Department of Employment, Economic Development and Innovation
Species Profile - Alligatorweed (''Alternanthera philoxeroides'') National Invasive Species Information Center,
United States National Agricultural Library
The United States National Agricultural Library (NAL) is one of the world's largest agricultural research libraries, and serves as a national library of the United States and as the library of the United States Department of Agriculture. Locate ...
. Lists general information and resources for Alligatorweed.
Alternanthera philoxeroides (Mart.) Griseb.Medicinal Plant Images Database (School of Chinese Medicine, Hong Kong Baptist University)
{{Taxonbar, from=Q1472735
philoxeroides
Aquatic plants
Flora of Southern America