Norwegian Forest and Landscape Institute, Fanaflaten 4, N-5244 Fana, Norway.
Ecology. 2012 Apr;93(4):749-59. doi: 10.1890/11-1018.1.
Colonization studies may function as natural experiments and have the potential of addressing important questions about community assembly. We studied colonization for a guild of epiphytic lichens in a former treeless heathland area of 170 km2 in southwest Norway. We investigated if epiphytic lichen species richness and composition on aspen (Populus tremula) trees corresponded to a random draw of lichen individuals from the regional species pool. We compared lichen communities of isolated young (55-120 yr) and old (140-200 yr) forest patches in the heathland area to those of aspen forest in an adjacent reference area that has been forested for a long time. All thalli (lichen bodies) of 32 selected lichen species on trunks of aspen were recorded in 35 aspen sites. When data for each site category (young, old, and reference) were pooled, we found the species richness by rarefaction to be similar for reference sites and old sites, but significantly lower for young sites. The depauperated species richness of young sites was accompanied by a skew in species composition and absence of several species that were common in the reference sites. In contrast, genetic variation screened with neutral microsatellite markers in the lichen species Lobaria pulmonaria showed no significant differences between site categories. Our null hypothesis of a neutral species assembly in young sites corresponding to a random draw from the regional species pool was rejected, whereas an alternative hypothesis based on differences in colonization capacity among species was supported. The results indicate that for the habitat configuration in the heathland area (isolated patches constituting < 0.4% of the area) lichen communities may need a colonization time of 100-150 yr for species richness to level off, but given enough time, isolation will not affect species richness. We suggest that this contradiction to expectations from classical island equilibrium theory results from low extinction rates.
定殖研究可以作为自然实验，并有可能解决关于群落组装的重要问题。我们研究了挪威西南部一个 170 平方公里的前无树石南荒原地区的一个附生地衣真菌群的定殖情况。我们调查了在白杨树上（Populus tremula）的地衣物种丰富度和组成是否与从区域物种库中随机抽取地衣个体相对应。我们将荒地中孤立的年轻（55-120 年）和古老（140-200 年）森林斑块中的地衣群落与相邻的长期森林化的白杨林参考区中的地衣群落进行了比较。在 35 个白杨林中，对 32 种选定地衣物种的所有（地衣体）进行了记录。当将每个站点类别（年轻、古老和参考）的数据汇总时，我们发现通过稀有化获得的参考站点和古老站点的物种丰富度相似，但年轻站点的物种丰富度显著降低。年轻站点物种丰富度的减少伴随着物种组成的倾斜和几种在参考站点中常见的物种的缺失。相比之下，在 Lobaria pulmonaria 等地衣物种中筛选出的中性微卫星标记的遗传变异在站点类别之间没有显示出显著差异。我们对年轻站点中性物种组装对应于从区域物种库中随机抽取的假设的零假设被拒绝，而基于物种间不同的定殖能力的替代假设得到了支持。研究结果表明，对于荒地中的生境配置（孤立的斑块构成面积的<0.4%），地衣群落可能需要 100-150 年的定殖时间才能使物种丰富度稳定下来，但只要有足够的时间，隔离就不会影响物种丰富度。我们认为，这种与经典岛屿平衡理论的预期相矛盾的结果是由于低灭绝率所致。
