Fincke Ola M
Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama.
Oecologia. 1994 Nov;100(1-2):118-127. doi: 10.1007/BF00317138.
The relative importance of intraspecific, interspecific, and seasonal causes of larval mortality were investigated for aquatic larvae of the giant damselfly Megaloprepus coerulatus in Panama. These larvae live in water-filled holes in fallen and living trees, where they and three other common odonate species are the top predators. By mid wet season, M. coerulatus larvae were found in nearly half of all tree holes that harbored odonates. Although M. coerulatus were typically, but not always, eliminated from holes inhabited by larger hetero-specifics, M. coerulatus were more likely to encounter conspecifics than other odonate species. Hole with less than 11 of water rarely contained more than a single larva. In large holes where M. coerulatus was the only odonate species present, multiple larvae coexisted at a density of one larva per 1-21 of water. There the absence of 2-4 of the 5 larval size classes, despite a continuous input of eggs, suggested that cannibalism was a common cause of mortality. The size of the final instar, which determined adult body size, was correlated positively with tree hole volume for male, but not female, larvae. Experiments showed that when two larvae were placed together in 0.4-1 holes with abundant tadpole prey, the larger larva killed the smaller one. Often the larva that was killed was not eaten. Small larvae were more tolerant of each other than were pairs of medium or large larvae. Before killing occurred, the presence of larger larvae reduced the growth of smaller individuals, relative to controls. 'Obligate' killing was density-dependent. In 3.0-1 holes with ad libitum prey, conspecific killing occurred until the larval density stabilized at one larva per 1-1.5 I, similar to the density found in large holes under field conditions, For M. coerulatus, cannibalism functions to reduce the number of potential competitors for food in addition to providing nutrition. When interactions between paired larvae in small holes were experimentally prevented, competition for food reduced the growth of one or both larvae relative to controls. Holes that were watered during the dry season supported larval densities similar to those in the wet season. Thus, dry season mortality could not be attributed to a decrease in available prey. Rather, M. coerulatus larvae could not survive more than 1 month of complete drying. Because the dry season typically lasts more than 6 weeks, habitat drying is a secondary source of mortality, affecting second- or third-generation larvae that fail to emerge before tree holes dry out completely.
针对巴拿马巨型豆娘（Megaloprepus coerulatus）水生幼虫，研究了种内、种间和季节性因素对幼虫死亡的相对重要性。这些幼虫生活在倒下的树木和活树中充满水的树洞里，它们与其他三种常见的蜻蜓目物种是顶级捕食者。到雨季中期，在几乎一半有蜻蜓目的树洞都发现了巨型豆娘幼虫。虽然巨型豆娘通常（但并非总是）会被较大的异种从栖息的树洞中排挤出去，但与其他蜻蜓目物种相比，巨型豆娘更有可能遇到同种个体。水深不足11厘米的树洞很少容纳超过一只幼虫。在巨型豆娘是唯一存在的蜻蜓目物种的大树洞中，多个幼虫以每1至21升水一只幼虫的密度共存。在那里，尽管有持续的卵输入，但5个幼虫大小类中有2至4个类别的缺失，这表明同类相食是死亡的常见原因。决定成虫体型的末龄幼虫的大小，对于雄性幼虫而言与树洞体积呈正相关，而对于雌性幼虫则不然。实验表明，当将两只幼虫一起置于有丰富蝌蚪猎物的0.4至1升的树洞中时，较大的幼虫会杀死较小的幼虫。通常被杀死的幼虫不会被吃掉。小幼虫比中等或大幼虫对彼此更具耐受性。在发生杀戮之前，相对于对照组，较大幼虫的存在会降低较小个体的生长速度。“必然”的杀戮是密度依赖性的。在有充足猎物的3.0升树洞中，同类相残会一直发生，直到幼虫密度稳定在每1至1.5升水一只幼虫，这与野外条件下大树洞中的密度相似。对于巨型豆娘来说，同类相食除了提供营养外，还起到减少潜在食物竞争者数量的作用。当通过实验阻止小树洞中配对幼虫之间的相互作用时，食物竞争相对于对照组会降低一只或两只幼虫的生长速度。旱季浇水的树洞支持的幼虫密度与雨季相似。因此，旱季的死亡率不能归因于可获得猎物的减少。相反，巨型豆娘幼虫在完全干燥的情况下存活时间不能超过1个月。由于旱季通常持续超过6周，栖息地干燥是死亡率的次要来源，影响那些在树洞完全干涸之前未能羽化的第二代或第三代幼虫。
