An unintended consequence of the widespread dispersal of plants and seeds worldwide is the displacement of ‘invasive’ species. When plants are introduced to foreign shores with favorable environmental conditions and a lack of natural predators, they often adapt quickly and become established in the new region. Some of these plants have found ready pollinators or suitable environments for seed dispersal, enabling them to spread rapidly. In certain instances, the new conditions have actually made these plants more successful in their new habitat compared to their native environment.
What Is an Invasive Plant Species?
A plant is deemed invasive if it disrupts the natural flora of a specific region. One example is Clematis vitalba, a harmless climber native to southern, western, and central Europe. However, since its natural spread in New Zealand in the 1930s, it has caused extensive damage to scrublands and forests, even toppling 20-meter trees in its path.
Plant hunters, as well as botanical gardens and nurseries collaborating with them, have played a pivotal role in introducing alien plants to different regions. For instance, the Australian viburnum Pittosporum undulatum was initially disseminated through botanical garden networks in the British colonies. Introduced to Jamaica’s Cinchona Botanical Gardens in 1870, it now invades relatively undisturbed mountain rainforests.
This plant flowers and fruits earlier than most native trees on the island, thus facing minimal competition for pollinators. These factors have contributed to its remarkable success in new environments. Besides Jamaica, the plant has also become a concern beyond its natural range, impacting regions such as South Africa, parts of Australia, and Hawaii.
Invasive Plant Species
Lantana camara’s brightly colored flowers made it a popular garden flower upon its arrival in Europe from Central and South America. As colonial forces expanded into the tropics, the plant spread extensively. Today, it poses a significant problem in over fifty countries. Since its introduction to South Africa in 1880, it has invaded native forests, nurseries, overgrazed or burned steppes, vineyards, rocky slopes, and fields. In 1938, it was inadvertently introduced to Floreana Island, one of the Galapagos Islands, as an ornamental plant. Since 1970, Scalesia pedunculata and Croton have replaced the dry vegetation of Macraea and Darwiniothamnus.
Two out of the three populations of Leocarpus pinnatifdus, which are endemic to Floreana, the smallest island in the Galapagos, as well as one population of Scalesia villosa, are currently under threat of extinction if the invader continues to advance. If Lantana reaches the crater area of Cerro Pajas, it will pose a direct threat to the world’s last nesting site for dark-sided petrels in the Galapagos Islands. The dense thorn bushes of Lantana would hinder the birds from nesting in their breeding grounds.”
Rhododendron ponticum was also introduced as an ornamental plant. Originating in Southeast Europe and Western Asia, it arrived at Kew in 1793 and was subsequently spread by a gardener named Charles Loddiges. Wealthy Victorian aristocrats utilized the plant to screen pheasants on their estates. However, this plant quickly outgrew these controlled gardens and began to invade semi-natural woodlands with acidic soils.
While members of the genus Rhododendron are typically not invasive, Rhododendron ponticum is an exception. It readily infiltrates mixed oak forests, creating dense shade that suppresses mosses, herbaceous plants, and dwarf shrubs. A single Rhododendron ponticum plant can cover an expansive area of 100 square meters as its branches root upon touching the soil. On the British island of Lundy, the presence of Rhododendron ponticum poses a threat to the native Lundy cabbage and the flea beetle that relies on it as a food source. Clearing areas infested with this invasive species proves to be exceptionally challenging due to its ability to grow on steep cliffs and in river hollows. In fact, it required 226 hours of work in 1997 to clear just one hectare of land from this invader.
Imperata cylindrica (Cogon grass)
Reported as invasive in 73 countries, this species, native to Southeast Asia, has the ability to invade various habitats including pine forests, dunes, wetlands, and meadows. While the wild, green variety is known to be invasive, there is a cultivated form called Japanese bloodroot, which is believed to be less invasive. However, scientists express concerns that hybridization between the two forms could result in the development of a resilient, invasive version capable of thriving in colder regions.
Reynoutria japonica (Fallopia japonica, Japanese knotweed)
This plant has emerged as a significant pest in various regions including the UK, Europe, Australia, New Zealand, and North America. It forms dense arrays and, when disturbed, extends its underground stems for several meters. This unique ability enables the plant to establish new populations far away from the original source, making it exceptionally challenging to eradicate, even with chemical methods.
Pontederia crassipes (Water hyacinth, formerly Eichhornia crassipes)
Although considered one of the world’s most noxious weeds, this Brazilian native plant is often cultivated as an ornamental species in garden ponds. It features large mauve flowers on spongy, floating stems. This versatile plant can thrive in various freshwater environments, ranging from lakes to canals. Its rapid reproduction rate is so significant that it can even impede boat traffic.
The attractive and fragrant flowers of this plant conceal its perilous nature. This climber thrives in tree canopies and suppresses the growth of native vegetation. It has been reported to invade regions such as La Reunion, Mauritius, Hawaii, and Florida. The International Union for Conservation of Nature (IUCN) recognizes it as one of the world’s 100 most notorious invasive organisms.
Impatiens glandulifera (Himalayan balsam)
Himalayan Balsam (Impatiens glandulifera) is an invasive plant species that poses significant challenges to ecosystems and native plant communities across many parts of the world. Native to the western Himalayas, this plant has gained notoriety due to its rapid growth, high seed production, and ability to outcompete native plants. Himalayan Balsam is a tall annual herb that can reach heights of up to 10 feet (3 meters). It features attractive pink or purple flowers and long, narrow leaves. The plant’s hollow stem is succulent and tends to have a reddish hue. Himalayan Balsam is known for its explosive seed pods, which, when mature, can burst open and scatter seeds over a wide area.
Heracleum mantegazzianum (Giant hogweed)
This plant distinguishes itself through its gigantic leaves and flower stalks that can reach heights of up to 3 meters. It exhibits a remarkably high seed yield and dispersal rate. As it grows, it forms a dense canopy that displaces native plants in forests and along waterways. This plant is considered invasive in the United Kingdom and the United States, and it contains chemicals that can cause skin irritation and leave permanent scars.