River Ecology and Restoration - River Dynamics and Integrated River Management - page 662

Alien invasive species Eupatorium adenophorum originates from Mexico and was introduced into
south China from south Asia in the 1940s. The species has spread quickly in southwestern China and
eliminated local species. The species is toxic and many cattle and sheep have been killed. The area
occupied by the species has increased to about 30 million ha (Li and Wang, 2003). To remove the species
from grassland is very difficult. Millions of dollars have been lost due to husbandry loss and control of
spreading of the species in Sichuan and Yunnan Provinces. Figure 10.39(b) shows the Eupatorium
adenophorum in Yunnan Province in southwestern China.
Eichoimia crassip was introduced to control eutrophication in streams and lakes. The species adsorb
pollutants and nutrients in the water and may enhance the purification capacity of the stream or lake.
Nevertheless, the species spread too fast and fishery and water surface recreation have been affected.
Humans have to remove them from waters, which has caused economic losses up to several tens of
millions of dollars. Figure 10.39(c) shows Eichoimia crassip spreading quickly in a polluted stream in
Beijing.
Ambrosia artemisia L. and Ambrosia tritida L. entered China in the 1930s and spread quickly since the
1980s and 1990s. The species produce a lot of pollen. In Shenyang in northeastern China the density of
pollen in air in 1987 was 38 times of that in 1983 because of introduction of the species. About 1.5% of
the local people suffer from pollinosis. Millions of dollars have been lost due to introduction of the
species. Figure 10.39(d) shows the Ambrosia artemisia L. in northeastern China.
Introduction of exotic species is not always bad for the ecosystem. Hong Kong has become a paradise
of exotic species and most of these species have been naturalized in the island. Hong Kong and the island
of Dominica, in the Caribbean, probably had no inland plant species in common 500 years ago. Today
they share more than a hundred weeds of human-dominated open habitats. The terms 'alien' is used to
refer to species that originated elsewhere but have become established in Hong Kong. Although people
have introduced many alien species by accident, others have been brought to a location deliberately, as
crops, ornamentals, livestock, or pets. Not all introduced species are aliens. In fact, reintroduction of
species to parts of their former range is an important conservation tool. Hong Kong's total vascular plant
flora of approximately 2,100 species includes at least 150 naturalized aliens: that is, species introduced
from other parts of the world which have run wild in Hong Kong (Dudgeon and Corlett, 2004). For
faunal species, most of these aliens were brought to Hong Kong by people, but some have spread on their
own. Some of these have established wild populations when they escaped, were abandoned or released.
In Hong Kong the majority of introduced species are confined to those areas where human influence is
strongest and most persistent. Indeed, in most residential and industrial areas, as well as the few sites still
used for intensive farming, alien species dominate the biota. In contrast, recognizable aliens are rare or
absent in most upland streams and hillside communities. Thus, the majority of aliens are found in those
places where the native flora and fauna has already suffered most as a result of human activities. At
present, the impact of the numerous alien plant and animal species established in Hong Kong is, in most
cases, hard to distinguish from the direct impact of human activities on the habitats they occupy
(Dudgeon and Corlett, 2004).
The introduction of alien species into Hong Kong has increased the biodiversity and has resulted in no
serious impacts on the local ecology. However, invasions by alien species are a potential conservation
management problem that has received almost no attention in Hong Kong. Even if we ignore the risk
posed by aliens to the ecology of Hong Kong, we have an obligation to ensure that the territory does not
become a stepping-stone for invasion elsewhere.