Hills and Streams: An Ecology of Hong Kong

8. Aliens

8 Aliens When people first started clearing patches of forest for cultivation and settlement, an entirely new type of habitat for plants and animals was created. The most distinctive characteristic of this new habitat was that it combined high light intensities with adequate water and nutrients. Tree-fall gaps in forest have these characteristics but they are usually small and always short-lived, with regeneration dominated by woody species. Before the arrival of people, permanent open habitats were confined largely to sites such as cliffs and beaches, where soil conditions prevented the formation of a closed woody canopy, and eroding river-banks, where disturbance kept the site open. At first, the newly-cleared habitat would have been occupied by plants and animals from the surrounding natural habitats. Previouslyrare inhabitants of cliffs, riverbanks and other open areas expanded into the man-made sites to which they were at least partly pre-adapted. Intense natural selection probably led to rapid evolution of distinct, 'weedy' varieties, better adapted to living with man. As human impact spread and trading links developed, the plant and animal weeds of different areas also spread and mixed. The regional floras and faunas of man-made open habitats became increasingly homogenized until, from the sixteenth century onwards, the process of globalization began. The ultimate limits to this process are set not by natural dispersal abilities, but by the climatic tolerance of the species concerned. Globalization has proceeded at different rates in different groups of organisms. It has been most dramatic with plant weeds, where now 143 Hills and Streams: An rrr)lnrlll even a trained botanist would find it hard to identify what continent he or she was on from the plant life of a tropical city. For example, 500 years ago Hong Kong and the island of Dominica, in the Caribbean, probably had no inland plant species in common; today they share more than a hundred weeds of man-made open habitats. Invertebrates have also participated in the globalization of the earth's biota, because many smaller forms are carried undetected among other goods and so have dispersed over the planet to wherever man has spread. Indeed, many stored-products pests and insects associated with widely-cultivated crops have almost cosmopolitan distribution patterns. In contrast, there have been very few intercontinental exchanges of birds and mammals, although several species of freshwater fishes have become quite widespread during the last century. Plants Hong Kong's total vascular plant flora of approximately 2000 species includes at least 145 naturalized aliens or exotics: that is, species introduced from other parts of the world which have run wild in Hong Kong. Of these, slightly over half were probably introduced from tropical America (South and Central America, and the Caribbean), 180/0 from northern Eurasia (including Europe), 15% from Africa or Madagascar, and 100/0 from tropical and subtropical Asia. The apparently small proportion from Asia is misleading, however, as weeds of Asian origin, unless they became established recently, would not usually be recognized as introductions. Hong Kong's flora includes many weedy species, such as Emilia sonchifolia and Vernonia cinerea, which are undoubtedly of Asian origin but are unlikely to have found a suitable habitat in the primeval, forested landscape. Moreover, there are a number of species, such as Cynodon dactylon, which spread throughout the tropics so early in man's recent expansion that their region of origin is obscure and they are treated as natives. The real figure for naturalized exotics in Hong Kong should probably be 250 to 300 species. The exotic flora must be able to tolerate local climatic conditions in the same way as the natives. The presence of at least 26 species which have apparently been introduced from temperate northern Eurasia - most probably, Europe and the Mediterranean - is an indication of the transitional nature of Hong Kong's climate. In contrast, only 144 Aliens one of these Eurasian species (Plantago major) has successfully established itself in equatorial Singapore. Elsewhere in the tropics, aliens of temperate Eurasian origin are usually confined to high altitudes where mean temperatures are less than those in the lowlands. In Hong Kong, many of these species grow most actively in the cooler, winter months. It is more difficult to identify species with the opposite distribution, that is, widespread in tropical lowlands but excluded from Hong Kong by cold. The 60 or so species naturalized in Singapore but absent from Hong Kong...