Lythrum salicaria - Global Invader

CHAPTER 1

DARWIN'S NIGHTMARE

In general, nature keeps troublemakers in check

The purple loosestrife (Lythrum salicaria L.) is a familiar, widespread, and highly valued wildflower that has also been deliberately propagated by moving and planting abundant and long-flowering plants in gardens. This is the position in Europe, but the species is not considered a beauty on the other side of the Atlantic Ocean. In the New World, the plant is considered - and widely deemed - a monster.

The plant, which migrated and settled in North America alongside humans, has become a despised invasive species on the New Continent, and every effort is being made to eradicate it. However, the newcomer is stronger than the means of human control; therefore, the stranger conquers more territories while destroying the native wetland flora of the continent.

As an explanation for the success of the purple loosestrife, the primary factor is the absence of natural enemies in the New World. In its native range, the plant remains in check due to leaf-eating insects, particularly beetles.

In North America, no group of herbivorous animals has begun to use the newcomer as a food source. Therefore, being free of enemies, the alien has gained a competitive advantage over the region's local flora. Leaf beetles are so effective in curbing L. salicaria growth that these insects have been deliberately introduced to the United States for the biological control of the invasive species.

With the absence of plant-eating animals and high seed production, L. salicaria has become a textbook model for describing the rapid progression and harm of alien species. The background to these explanations is rooted in the theories and works of Charles Darwin, the foundation of evolutionary biology. As early as 1859, Darwin outlined in The Origin of Species how any plant can fill the Earth and displace other species with its established seed production if some external factor - a competing species, a predator, or environmental conditions - does not limit the production of new organisms advancing exponentially.

Jonathan Silvertown, professor of ecology at the Open University, cites Lythrum salicaria as a model organism for the unrestricted spread of plants in an enemy-free natural environment. According to Silvertown, the purple loosestrife behaves on the New Continent like a typical Darwinian Demon, which can spread uncontrollably, destroying other species found in similar places, even to local extinction. The newcomer has become a nightmare in North America due to its ability to escape its natural enemies in the Old World.9

Escaping from herbivores is undoubtedly essential, but by no means the only reason for the extraordinary success of the purple loosestrife on the New Continent. The success of alien species may be based on one or more biological or environmental factors acting simultaneously.

In the summary of published scientific literature on the characteristics of newcomer plants conquering new territories, U.S. scientists Jonathan Fleming and Eric Dibble listed 13 traits that enable aliens to achieve dominance in their new habitats. The extensive review reveals the diverse reasons for the success of alien organisms in conquering new territories.

The purple loosestrife, which has effectively colonized North America, has practically all the means of vegetation available in its arsenal.10

Hundreds of articles in scientific journals, newspapers, and magazines have been published about the appearance and distribution of L. salicaria in North America. The tone and conclusions have been largely negative, especially in recent decades, following the initial interest in the beautiful plant, which has since subsided. The rapidly spreading alien has gained a questionable reputation as a scourge, even a plague, the transfer and peddling of which is prohibited in many U.S. states. But does the beautiful stranger deserve the discrimination and contempt the plant has received? There is less scientifically valid evidence of the harms caused by purple loosestrife than might be inferred from the species' questionable reputation.

In 1995, Mark Anderson, a researcher at the University of New Hampshire, analyzed 71 published studies on the effects of Lythrum salicaria on the native flora and fauna of North America. The result of the summary is surprising: A total of 29 species of organisms were found to utilize L. salicaria to their advantage, and not a single plant species was found to have regressed at the national or state level due to the colonization of purple loosestrife. However, hardly anyone disputes the negative influences of the newcomers on the local scale.

For example, the relationships between the alien purple loosestrife and the native broad-leaf cattail (Typha latifolia) growing in the same wetland colonies have been studied experimentally. In gardens and small-scale plantings, the purple loosestrife has proven to be a superior competitor to cattails. However, in nature, the ability of the newcomer to displace tall native species has not been observed in the same proportion. The diversity of flora may even increase in some locations after L. salicaria has settled in communities.11

A familiar plant with many names

Lythrum salicaria grows in a wide range of habitats. The species is typically found at the water's edge, where it inhabits both salty seawater and inland freshwater, often with its roots and shoot bases submerged below the waterline, as well as in ditches and wetlands. However, in terms of its natural habitats, the species is unpretentious. At the other end of the spectrum, the purple loosestrife can be found high above the waterline in the crevices of cliffs, colonizing inland sand fields or peat soil depressions that remain dry for long periods.

The plant's scientific name, Lythrum, is derived from the Greek word lythron, meaning "bloody" or "blood-stained." The species name refers to willows (Salix), as the leaf form of the plant resembles the shape typical of willows.

In English, Lythrum salicaria is known as purple loosestrife, and what could be more apt than to name the plant after its large flowers, which shine bright purple for months in mid-to-late summer?

Besides its role in natural history and gardening, the plant has also found its place in British literature and art, gaining worldwide fame. In William Shakespeare's Hamlet (1599- 1601), several plants are listed in the act when Ophelia drowns herself in a river. The plants are mentioned when Gertrude describes the scene of Ophelia's watery demise with the words "long purples that liberal shepherds gave a grosser name/ But our cold maids dead men's fingers call them" (Shakespeare, Hamlet: 4.7., 168-171). The name Long Purples was also associated with Lythrum salicaria in John Clare's The Shepherd's Calendar (1821).

It is not guaranteed that the name Long Purples represents Lythrum salicaria. However, several analyses consider this plant to be the most prominent candidate. The early purple orchid (Orchis mascula) is an alternative species to the long purples. Based on the occurrence and prevalence of these two plants in the riverside meadows, L. salicaria appears to be the most probable.

The same conclusion was also made by the artist John Everett Millais, whose painting Ophelia (1851-1852) - one of the most admired paintings in the permanent collection of Tate Britain in London - shows erect plants with bright purple spikes lining the river where Ophelia's plant-covered body is resting. According to art historians, Millais made excursions to study typical riverine plant communities when sketching and painting Ophelia12 .

In German, the name of Lythrum salicaria is Blut Weiderich. The name refers to the flower's colour and the leaf shape, as Blut denotes blood, and the remainder of the species name denotes willow-like. In French, the name is Salicaire commune; in Spanish, the plant is known as Lysimaquia roja.

Immigrants took the Beauty along into the New World

An old-world plant in origin, the purple loosestrife is native to Europe and Asia; however, the plant is now present on all continents except Antarctica. Its range extends from the British Isles in the west to central Russia in the east, reaching the Arctic Circle in the north (in Eurasia, up to 67°N, and, as an alien newcomer, in Canada, up to 51-56°N).

In addition, the species is found in North Africa, Australia, New Zealand, Japan, and the Korean Peninsula, as well as in India and many Southeast Asian countries and parts of the northern Himalayan Mountains. In the Asian parts of Russia, the species' northern limit is at 61°N; in China, the range extends to 50°N. The southern limit of Lythrum salicaria in Eurasia is 24°N in China and 33°N in Afghanistan and Iran. In Australia, the range spans between 23°S and 42°S and 137°E and 153°E13 .

The Old-World herb has already reached South America as well. In 2015, the first spontaneous population of the purple loosestrife on the continent was reported in Chubut Province, Argentina14 .

The purple loosestrife's latest conquest was South Africa, where the alien has established seven small-scale...