China is one of the countries with particularly rich species diversity in the world. There are 2,554 species of terrestrial vertebrates, 3,862 species of fish and about 3, species of higher plants, among which invertebrates including insects, lower plants, fungi, bacteria and actinomycetes are more diverse. According to literature records and preliminary investigation, there are more than 6 species of exotic naturalized plants in China, including 18 species of exotic weeds, belonging to 23 families and 76 genera, and 15 species are considered as national or regional. At present, there are about 4 species of exotic animals that seriously harm our country, including American white moth, Hemiberlesia pitysophila, Pinus elliottii, rice water weevil, Liriomyza sativae, Bursaphelenchus xylophilus, sugarcane moth, Myzus avenae, grape nodule aphid, Tetranychus urticae, potato beetle, Dendroctonus valens, etc. Other exotic animals, as well as the big bottle snail originated in South America, the brown cloud agate snail originated in East Africa, the muskrat originated in North America, the squirrel originated in the former Soviet Union, and the nutria originated in South America. The introduction of exotic fish also poses a great threat to the local fish species and ecosystem of the lake. The biggest threat to the biodiversity of Yunnan waters comes from the invasive fish, such as grass carp, silver carp, bighead carp, new whitebait in Taihu Lake, wheat-eared fish, goby-nosed tiger fish in Ziling and goby-nosed tiger fish in Bo's mouth. At present, exotic microorganisms or diseases that are harmful to agriculture, such as Xanthomonas oryzicola, Synchytrium endobi-oticum, Phytophthora megasperma, Verticillium alboatrum, Citrus Huanglongbing, Citrus canker (Xanthomonas citri), cassava bacterial wilt (Xanthomonas campestris PV. manihotis), tobacco ring spot virus disease, tomato canker (Clavibacter michigan subsp. Mishiganen), scale bulb stem nematode (Ditylenchus spp.), etc.
However, it is still unknown how many alien species there are among the numerous biological species in China. At the same time, China is vulnerable to alien species. Because China spans 55km from north to south and 52km from east to west, spanning 5 latitudes and 5 climatic zones (cold temperate zone, temperate zone, warm temperate zone, subtropical zone and tropical zone), most exotic species from all over the world may find suitable habitats in China.
The problem of alien species invasion in China has the following characteristics:
A. It involves a wide range: invasive species have been found in 34 provinces, municipalities and autonomous regions. By May 22, 1,5 nature reserves had been established in China, covering about 9% of the country's total area. Except for a few remote nature reserves, invasive species can be found more or less.
B. There are many ecosystems involved: almost all ecosystems can be seen from forests, agricultural areas, waters, wetlands, grasslands and urban residential areas. Among them, the ecosystem in low altitude areas and tropical islands is the most seriously damaged.
C. There are many species involved: examples can be found from vertebrates (mammals, birds, amphibians and reptiles, fish), invertebrates (insects, crustaceans, mollusks), plants, bacteria and viruses.
D. brings serious harm: in many places of our country, under the condition that the original forest cutting is stopped and further ecological damage is forbidden, alien invasive species have become an important cause of ecological degradation and biodiversity loss, especially for the aquatic ecosystem and the tropical and subtropical regions in the south, and have risen to become the first important influencing factor.
2. Main ways (channels) of alien species invasion
The most fundamental reason of biological invasion is that human activities have brought these species to places where they should not appear. Therefore, we call these species "harmful", which is actually unfair to these species. They just stay in the wrong place, and the reason for this mistake is often some irresponsible activities of human beings. The key to the problem of alien invasive species is man-made. Introduction refers to the transfer of taxonomic units of species, subspecies or below (including all parts, gametes or propagules that may survive and then reproduce) to areas beyond their (past or present) natural distribution range and diffusion potential by human beings. This transfer can be within countries or between countries. Introduction can be divided into intentional introduction and unintentional introduction.
Intentional introduction refers to the intentional introduction by human beings, which purposefully moves a species out of its natural distribution range and diffusion potential (this kind of introduction can be authorized or unauthorized).
Unintentional introduction refers to the unintentional introduction of a species by using human or human transmission system as a medium and spreading to places outside its natural distribution range.
Exotic species are introduced to their non-native areas in these two ways.
intentional introduction
China has a long history of introducing excellent varieties from other places or abroad. Early introduction is often achieved through ethnic migration and trade between regions. The seeds of sour beans (Tamarindus indica) originating in Africa, grapes (Vitis vinifera), Medicago sativa, pomegranate (Punica granatum) and safflower (Carthamus tinctorius) originating in Central Asia passed through the famous ancient "Shu-Toxoplasma Road" and "Silk Road" in the 4th century BC and the 1st century BC respectively. Since then, in the Northern Song Dynasty, Aloe barbdensis (or A. vera var. chinensis) (which originated in northeastern Africa and is now naturalized in the coastal areas of South China), and in the 17th century, economic plants such as Ipomoea batatas, Nicotiana tabacum and Passiflora coerulea were introduced to China. Earlier, the Dutch introduced some American species, such as Leucaena leucocephala, Acacia farnesiana and Hylocereus undatus, to Taiwan Province, and most of these species were later naturalized as wild plants. After the Opium War in 1842, Hongkong, Guangzhou, Xiamen, Shanghai, Qingdao, Yantai and Dalian became the main entrances for exotic weeds to enter China. Weeds such as Conyza bonariensis)(1857), Conyza canadensis)(1862) and Erigeron annuus)(1886) landed in Hong Kong, Yantai and Shanghai successively, and gradually spread to the mainland. The introduction time and escape place of some cultivated plants are irregular. For example, French missionaries introduced European Vitex europaeus to the vicinity of Chengkou Church in Sichuan, and then a large number of Vitex europaeus escaped.
At present, almost all planting and breeding units are introducing from other places or foreign countries. These departments or units include agriculture, forestry, gardens, aquatic products, animal husbandry, special aquaculture and various breeding bases. Most of them are introduced to improve economic benefits, ornamental and environmental protection, but some species have become harmful species due to improper introduction. Among the known alien harmful plants in China, more than 5% species are the result of artificial introduction.
The purpose of intentional introduction is varied, which can be mainly divided into the following aspects:
As pasture or feed
There are many examples of invasion caused by introduction as pasture or feed. For example, Alternanthera philoxeroides, purple alfalfa, Melilotus albiflora, Malvastrum coromandelianum, Panicum maximum, Phleum pratense, Pennisetum setosum, Sorghum sudanense, Lolium persicum, Pistia stratiotes, Hordeum jubatum, Eichhornia crassipes, etc. Long-term overgrazing and grassland degradation in China's animal husbandry have increased the demand for new high-quality fast-growing pastures, which provides an excellent opportunity for foreign grass seed companies to dump grass seeds in China. A large number of new forage experiments are being carried out now, mostly to test whether the varieties provided by foreign grass seed companies can grow rapidly on our land. What is worrying is that some of them have become dangerous alien invasive species. Alternanthera philoxeroides, commonly known as "Alternanthera philoxeroides", was introduced and spread as pig feed in many places in south China after 195s, and later it became wild. In 1986, it was found that the natural occurrence area of Alternanthera philoxeroides was about 889 6 hm2, which had become the main weeds in vegetable, sweet potato and citrus orchards.
As an ornamental plant,
the pursuit of exotic flowers and plants has prompted people to introduce exotic or foreign flower varieties. These flowers and plants inevitably escape from the garden, and under natural growth, some exotic ornamental plants become dangerous alien invasive species after escaping. Such as Ageratum houstonianum, Coreopsis lanceolata, Cosmos bipinnata, Helium Autumn, Tagetes erecta, Solidago canadensis, Pharbitis nil, Petunia rotundifolia (P. purpurea), Lantana camara, Mimosa pudica, Oxalis corymbosa, Allium tuberosum, Vitex negundo (Ulex europaeus), Spider Orchid (Hymenocallis littoralis).
The popularity of aquariums and family aquariums has also made some exotic aquatic plants become invasive species. For example, Cabomba caroliniana, which originated in the United States, has now appeared in rivers in Zhejiang; Myriophyllum aquaticum from South America has been naturalized in Taiwan Province. Although the former only blooms but does not bear fruit, and the latter only has female plants, neither of them produces seeds, but they can spread rapidly through asexual reproduction. For example, the green algae (Caulerpa taxifolia) introduced from the Adriatic Sea to the French aquarium formed a dominant population in the French local marine ecosystem by releasing spores, which caused a large number of local marine plants to die out. Lantana camara is native to tropical America. It was introduced to Taiwan Province by the Dutch in 1645 and cultivated as an ornamental plant. Now it is spreading in tropical and south subtropical areas of China, crowding out local plants and blocking roads. Its plants have a bad smell and its stems have thorns, so it is a harmful shrub. Its fleshy fruit is further spread by local birds pecking. Scavengers often kept in aquariums are a species of the genus Plecostomus punctatus, which is native to Latin America. Recently, it was reported that it was collected in Nanchanghe River in Beijing, Pearl River and Hanjiang River in the south. It is reported in Dongshan River, Yilan County, Taiwan that this species has no natural enemies in Yilan, but it is very fertile, laying 3-5 eggs at a time, and the hatching rate is almost 1%. Feeding on the eggs of other fish gradually reduced the local species in Taiwan Province. To this end, Yilan also launched a fish cleaning operation.
As medicinal plants
More than 12, species of organisms used in traditional Chinese medicine in China are mostly native to China, and some are exotic species, some of which have become invasive species. Such as Saponaria officinalis, Cassia mimosoides, Cassia tora, talinum, Wangjiangnan, Phytolacca americana, Datura metel, Solanum laciniatum, etc.
As a plant to improve the environment
In order to quickly solve the problems that have been bothering us for a long time, such as ecological environment degradation, vegetation destruction, soil erosion and water pollution, people often look at some characteristics of exotic species unilaterally, which provides an excellent opportunity for the invasion of exotic species. At present, many areas are actively carrying out vegetation restoration, but some of the species used are dangerous alien species. At present, some species have invaded, and typical cases are Spartina alterniflora, Mikania micrantha and Eichhornia crassipes. Spartina alterniflora was introduced from the salt marsh vegetation on the east coast of America in 1979. First, it was planted in Fujian coastal areas in October 198, and then it was widely publicized. In 1982, it was expanded to Jiangsu, Guangdong, Zhejiang and Shandong. The purpose of introduction was to protect beaches and revetment, improve soil, green beaches and improve the ecological environment of beaches. Now this species has escaped in Zhejiang, Fujian, Guangdong and Hong Kong. In 199, the loss of aquaculture in Dongwuyang, Ningde, Fujian alone reached more than 1 million yuan a year. This species has become a serious problem in coastal areas that affects the local fishery output and threatens mangroves.
Exotic species are also widely used in urban landscape construction and landscaping, which often causes complete changes in local ecosystems and landscapes. Taking the lawn industry as an example, with the large-scale construction of lawns with different functions (golf courses, football fields, parks and green spaces, etc.) in cities all over the country, the rapid development of lawn industry in China has been promoted, and the demand for lawn grass seeds has increased sharply. At present, the grass seeds used are mainly excellent lawn varieties from abroad. Except Zoysia materlla seeds, almost all other grass species are imported, with the import volume reaching more than 2, tons in 1997 alone. By 199,