原帖由 jasonpoon 於 2017-5-15 22:33 發表
I once thought about the effect of typhoon in S. China on dispersal and genetic remixing of species especially for freshwater fishes which are easily isolated especially in hillstreams. Isolating mechanisms maybe alleviated by the frequent blown out individuals of small sizes.
Typhoons might be important in genetic remixing. However, I suppose that is more relevant to insects, plants, and birds? It is hard to imagine fish being blown off from one stream to another. I actually think that freshwater fish, especially primary freshwater fish that never leaves the river basin it is in, are quite genetically isolated. There has been quite a lot of examples of impressive adaptive radiation and the evolution into different morphs in some freshwater fish taxa. Examples include the three-spine sticklebacks in North America showing high regional variation, guppies in Trinadad with different morphs below and above the waterfalls, and the amazing cichlids in Lake Victory and Lake Malawi. Even if there is gene mixing between populations, I believe birds carrying fish around and human activity might actually be a more plausible explanation? I wonder if anyone looked into the Fst values of primary freshwater fish in Hong Kong?
Sources reported that early British colonists described HK as a "barren rock". However, I think this might be an overstatement as I doubt any Englishman had actually walked or charted the inland of NT. Systematic surveying of lands in NT only begin after the 1898 Convention and was done mainly by indian surveyors. There were history of early resistances from villagers silenced only by cannon blows from British warships. Throughout early to mid 20th century, urbanization happened mainly in HK Island and Kowloon areas. It is also a matter of question about the extend of deforestation during WWII. Major influx of people from China occurred in around 1949 and 1967. While the country parks were started around 1977.http://twpcentre.weshare.hk/oceandeep3000/articles/756909
Photos of new territories from 1950 clearly shows hillslopes in New Territories are quite bare with only short vegetation, and the slopes that are actually forested are mainly covered with sparse pine trees. Clearly not the broadleaved subtropical rainforest one would expect in the absence of heavy human disturbance. At that time, the economy of New Territories was still heavily agricultural, and the deforestation is probably caused by villagers going uphill to collect fuelwood or directly using the slopes for 梯田 as some of the photos show. In fact agricultural activity in the New Territories could be dated back to settlements in Ho Chung Valley 4000 years ago. Evidence suggested that people in the Late Neolithic Period already started planting rice in the valley. Hence, vegetative change by agriculture in New Territories must have occurred very very long time ago. The main question is, how severe was it? When did most of our forests disappear?
I found a very detailed government account of changes in landscape in Hong Kong that may give us some answers:
The basic idea is that, settlements from Neolithic period till 遷界令 in Qing Dynasty were only focused in flood plains and coastal areas. Agriculture, salt industry, and lime industry came into play in different times during this period. These industries, especially the lime industry, needs quite a lot of fuel wood from hillslopes. The resulting deforestation was gradual but substantial, with pollen data showing the dominance of grasses and the decline of pine trees/Fagaceae after the settlements are established. (The fact that other large tree families such as Lauraceae 樟科 is not represented in pollen data does not indicate that pine forests are dominant. It is probably because these families are insect pollinated and leave only a faint pollen record). The end of the 遷界令 in 1669 seems to be a very important point in the vegetative history in Hong Kong. In addition to the return of the people who left Hong Kong during the 遷界令, Hakka people moved into Hong Kong and created new settlements. Hakka people converted slopes to 梯田 and occupied even hilly areas.
Based on the information provided, I speculate that megafauna such as elephants and rhinos probably went locally extinct one by one since the Neolithic period. Gradual deforestation to provide fuel wood for salt and lime industries wiped out large animals dependent on large patches of forests, with Hakka settlements on mountains giving them a final hit. Finally, other more resilient predators such as tigers and foxes went locally extinct when rural population surged once again under British rule, with habitats heavily fragmented by roads and railways. Urbanization might actually be beneficial to preserving forests since people living in cities are no longer involved in agricultural activity, and does not have to depend on trees on hillslopes for fuel wood.
The main message is, the ecosystem in Hong Kong is degraded very early in history.