1、V1 稀有物种、生境损失和灭绝要 点稀 有 种 稀有性的类型取决于该种的三个属性 (Rabionwitz,1981)：（ i） 地理分布区的大小（大或小）；（ ii） 栖息地特异性（宽或窄）；（ iii） 地方种群大小（高或低）。对于任何物种，这些特征中有一个或几个的属性低，证明其某些稀有性。窄的栖息地要求、低的种群密度和地方性都是稀有性的各个方面。许多种是由于其生态学而成为稀有的，另一些种则由于人类活动的结果。扩散能力弱或有固着行为的物种，很可能由于人类因素而成为稀有的。The type of rarity depends on three attributes of the species
2、 in question (Rabinowitz, 1981)(i)size of geographical range (large v. small);(ii)habitat specificity (wide v.narrow); (iii)local population size (high v. low).Any species for which one or more of these attributes is low population density and endemism are all different facets of rarity. Many specie
3、s are rare by virtue of their ecology while others are rare as a result of human activities.Species with poor dispersal ability of sedentary behavior are likely to become rare as a result of anthropogenic factors.Key NotesRare speciesV1 RARE SPECIES, HABITAT LOSS AND EXTINCTION稀有种的遗传多样性数量下降得很低的种会遭受到
4、遗传多样性丧失和杂交衰退。为了保证遗传多样性得以维持，要求种长期生存，需要有最小可存活种群（ MVP）。 人们接受的避免杂交衰退的最好估计值是 250500个体的 MVP。 精确的确定 MVP要求有详细和长期的研究，优先保护栖息地的考虑，常常使得对于遗传多样性的关心有所失色。A species which has declined to low numbers will suffer from a loss of genetic diversity and inbreeding depression. A minimum viable population (MVP) is required
5、for the long-term survival of a species to ensure genetic variability is maintained. An MVP of 250-500 individuals is accepted as the best estimate for inbreeding avoidance. Detailed and lengthy study is required for the accurate determination of MVP and frequently the protection of habitat is the c
6、onservation priority, overshadowing concern over genetic diversity.Genetic diversity in rare species生境损失和片断化数目众多的生境，如湿地、草地和森林，面积剧烈地减少，或者被改变，或者受破坏，通常是为了农业。生境的退化导致物种的减少或损失。生境片断化减少了生境面积，使留下斑块之间的距离扩大。通常生境质量较差的这种小斑块，遭受边缘效应，支持与连续生境不相同的种。物种损失的原因是生境面积的大量损失和鸟屿化过程的加剧。新近分割出的斑块含有的种数，开始可能比长期保持的地区还要多一些，但物种将会从斑块中丧
7、失，这个过程叫做松弛 (relaxation)。昌都地区的森林破坏 日喀则地区的水土流失Numerous habitats, such as wetlands, grasslands and forests, have been either drastically reduced in area, modified or destroyed, usually for agriculture. Habitat degradation results in the decline or loss of species. Habitat fragmentation reduces the habi
8、tat area and increases the distance between remaining patches. These smaller patches of usually poorer quality habitat suffer from edge effects and support different species to continuous habitats. Species are lost because of the overall loss of habitat area, and as a result of increasing insulariza
9、tion. Recently isolated patches may initially contain more species than the area can sustain over the long term and species will be lost from the patch a process known as relaxation.Habitat loss and fragmentation灭 绝 灭绝是一个自然过程，但灭绝的步伐和方式由于人类活动而加速。过去的灭绝聚集在地质时间中（如二叠纪和更新世出现的重大灭绝）。灭绝的人为原因包括捕猎、生境破坏和人工引种。 IUCN分类的面临灭绝大风险的物种叫做 “濒危物种（ endangered） ”。 小种群更易于由于直接或间接的日常种群波动的结果而成为濒危物种。种群数量变动和遗传因素相互作用导致灭绝。种群灭绝的概率可以通过种群生存力分析（population viability analysis, PVA） 进行预测。相关主题 遗传变异（见 O1） 保育对策（见 V2）岛屿群落与移植（见 Q2）
