Departamento de Botânica, Instituto de Biologia, Universidade Estadual de Campinas, Campinas, Brasil.
PLoS One. 2013 Jun 11;8(6):e65836. doi: 10.1371/journal.pone.0065836. Print 2013.
Mistletoes are aerial hemiparasitic plants which occupy patches of favorable habitat (host trees) surrounded by unfavorable habitat and may be possibly modeled as a metapopulation. A metapopulation is defined as a subdivided population that persists due to the balance between colonization and extinction in discrete habitat patches. Our aim was to evaluate the dynamics of the mistletoe Psittacanthus robustus and its host Vochysia thyrsoidea in three Brazilian savanna areas using a metapopulation approach. We also evaluated how the differences in terms of fire occurrence affected the dynamic of those populations (two areas burned during the study and one was fire protected). We monitored the populations at six-month intervals. P. robustus population structure and dynamics met the expected criteria for a metapopulation: i) the suitable habitats for the mistletoe occur in discrete patches; (ii) local populations went extinct during the study and (iii) colonization of previously non-occupied patches occurred. The ratio of occupied patches decreased in all areas with time. Local mistletoe populations went extinct due to two different causes: patch extinction in area with no fire and fire killing in the burned areas. In a burned area, the largest decrease of occupied patch ratios occurred due to a fire event that killed the parasites without, however, killing the host trees. The greatest mortality of V. thyrsoidea occurred in the area without fire. In this area, all the dead trees supported mistletoe individuals and no mortality was observed for parasite-free trees. Because P. robustus is a fire sensitive species and V. thyrsoidea is fire tolerant, P. robustus seems to increase host mortality, but its effect is lessened by periodic burning that reduces the parasite loads.
槲寄生是一种气生半寄生植物，占据有利生境（宿主树）斑块，周围是不利生境，可能被建模为一个复合种群。复合种群被定义为由于离散生境斑块中定居和灭绝之间的平衡而持续存在的细分种群。我们的目的是使用复合种群方法评估巴西三个稀树草原地区槲寄生 Psittacanthus robustus 及其宿主 Vochysia thyrsoidea 的动态。我们还评估了火灾发生的差异如何影响这些种群的动态（研究期间有两个地区发生火灾，一个地区受到火灾保护）。我们每六个月监测一次种群。P. robustus 种群结构和动态符合复合种群的预期标准：i）槲寄生的适宜栖息地以离散斑块的形式存在；（ii）当地种群在研究期间灭绝；（iii）先前未被占据的斑块发生了定居。随着时间的推移，所有地区的有斑块比例都在下降。由于两个不同的原因，局部槲寄生种群灭绝：无火地区的斑块灭绝和燃烧地区的火灾杀死。在燃烧地区，由于一次杀死寄生虫而不杀死宿主树的火灾事件，有斑块比例的最大下降。V. thyrsoidea 的最大死亡率发生在没有火灾的地区。在这个地区，所有死树都支持槲寄生个体，而无寄生虫的树木没有观察到死亡率。由于 P. robustus 是一种对火敏感的物种，而 V. thyrsoidea 是对火耐受的物种，因此 P. robustus 似乎增加了宿主的死亡率，但周期性燃烧减少了寄生虫的负荷，从而减轻了其影响。
