Okano Jun-Ichi, Okuda Noboru
Center for Ecological Research, Kyoto University, Hirano, Otsu, Shiga, Japan.
Research Institute for Humanity and Nature, Motoyama, Kamigamo, Kyoto, Japan.
PLoS One. 2018 Feb 21;13(2):e0191925. doi: 10.1371/journal.pone.0191925. eCollection 2018.
Resource availability often determines the intensity of cannibalism, which has a considerable effect on population size distribution and individual life history. Larvae of the caddisfly Psilotreta kisoensis build portable cases from sedimentary sands and often display cannibalism. For this species, the availability of preferable case material is a critical factor that affects larval fitness, and material is locally variable depending on the underlying geology. In this study, we investigated how sand quality as a case material determines cannibalism frequency among larvae and, in turn, how the differential cannibalism frequency affects the body-size distribution and voltinism. Rearing experiments within a cohort revealed that a bimodal size distribution developed regardless of material quality. However, as the preferable material became abundant, the proportion of larger to smaller individuals increased. Consecutive experiments suggested that smaller larvae were more frequently cannibalized by larger ones and excluded from the population when preferable smooth material was abundant. This frequent cannibalism resulted in a bimodal size distribution with a significantly higher proportion of larger compared to smaller individuals. The size-dependent cannibalism was significantly suppressed when the larvae were raised in an environment with a scarcity of the preferable case material. This is probably because larvae cannot enjoy the benefit of rapid growth by cannibalism due to the difficulties in enlarging their case. At low cannibalism the growth of smaller individuals was stunted, and this was probably due to risk of cannibalism by larger individuals. This growth reduction in small individuals led to a bimodal size-distribution but with a lower proportion of larger to smaller individuals compared to at high cannibalism. A field study in two streams showed a similar size distribution of larvae as was found in the rearing experiment. The bimodal ratio has consequences for life history, since a size-bimodal population causes a cohort splitting: only larvae that were fully grown at 1 year had a univoltine life cycle, whereas larvae with a stunted growth continued their larval life for another year (semivoltine). This study suggests that availability of preferable case building material is an important factor that affects cannibalism, which in turn affects larval population size structure and cohort splitting.
资源可用性往往决定同类相食的强度，这对种群大小分布和个体生活史有相当大的影响。毛翅目昆虫基索扁脉石蛾的幼虫用沉积砂建造可携带的外壳，并且经常表现出同类相食的行为。对于这个物种来说，优质外壳材料的可用性是影响幼虫适应性的关键因素，而且材料因底层地质情况而在当地有所不同。在本研究中，我们调查了作为外壳材料的砂质如何决定幼虫之间的同类相食频率，以及反过来，不同的同类相食频率如何影响体型分布和化性。在一个群组内进行的饲养实验表明，无论材料质量如何，都会形成双峰体型分布。然而，随着优质材料变得丰富，较大个体与较小个体的比例增加。连续的实验表明，当优质光滑材料丰富时，较小的幼虫比较大的幼虫更频繁地被同类相食，并被排除在种群之外。这种频繁的同类相食导致了双峰体型分布，其中较大个体的比例明显高于较小个体。当幼虫在优质外壳材料稀缺的环境中饲养时，与体型相关的同类相食行为受到显著抑制。这可能是因为由于扩大外壳存在困难，幼虫无法通过同类相食获得快速生长的益处。在同类相食率较低时，较小个体的生长发育受阻，这可能是由于较大个体的同类相食风险所致。小个体的这种生长减缓导致了双峰体型分布，但与高同类相食率时相比，较大个体与较小个体的比例更低。在两条溪流中进行的野外研究显示，幼虫的体型分布与饲养实验中发现的相似。双峰比例对生活史有影响，因为体型双峰的种群会导致群组分裂：只有在1年时完全成熟的幼虫具有一化性生命周期，而生长发育受阻的幼虫则会继续幼虫生活一年（半化性）。本研究表明，优质外壳建造材料的可用性是影响同类相食的一个重要因素，而同类相食反过来又会影响幼虫种群大小结构和群组分裂。
