Wissinger Scott A, Whissel John C, Eldermire Charles, Brown Wendy S
Biology Department, Allegheny College, Meadville, PA 16335, USA.
Oecologia. 2006 Apr;147(4):667-78. doi: 10.1007/s00442-005-0303-1. Epub 2006 Feb 7.
Species replacements along freshwater permanence gradients are well documented, but underlying mechanisms are poorly understood for most taxa. In subalpine wetlands in Colorado, the relative abundance of caddisfly larvae shifts from temporary to permanent basins. Predators on caddisflies also shift along this gradient; salamanders (Ambystoma tigrinum nebulosum) in permanent ponds are replaced by predaceous diving beetles (Dytiscus dauricus) in temporary habitats. We conducted laboratory and field experiments to determine the effectiveness of caddisfly cases in reducing vulnerability to these predators. We found that larvae of a temporary-habitat caddisfly (Asynarchus nigriculus) were the most vulnerable to salamanders. Two relatively invulnerable species (Limnephilus externus, L. picturatus) exhibited behaviors that reduced the likelihood of detection and attack, whereas the least vulnerable species (Agrypnia deflata) was frequently detected and attacked, but rarely captured because cases provided an effective refuge. Vulnerability to beetle predation was also affected by cases. The stout cases of L. externus larvae frequently deterred beetle larvae, whereas the tubular cases of the other species were relatively ineffective. Two of these vulnerable species (A. nigriculus and L. picturatus) often co-occur with beetles; thus, case construction alone is insufficient to explain patterns of caddisfly coexistence along the permanence gradient. One explanation for the coexistence of these two species with beetles is that they develop rapidly during early summer and pupate before beetle larvae become abundant. One species (L. picturatus) pupates by burying into soft substrates that serve as a refuge. The other (A. nigriculus) builds stone pupal cases, which in field experiments, more than doubles survival compared to organic pupal cases. The combined results of these experiments suggest that caddisfly distributions along permanence gradients depend on a suite of primary and secondary predator defenses that include larval and pupal case structure, predator-specific escape behaviors, and the phenology of larval development.
沿淡水持久性梯度的物种替代现象已有充分记录，但对于大多数分类群而言，其潜在机制仍知之甚少。在科罗拉多州的亚高山湿地中，毛翅目幼虫的相对丰度从临时性流域向永久性流域转变。捕食毛翅目的捕食者也沿此梯度发生变化；永久性池塘中的蝾螈（虎纹钝口螈星云亚种）被临时性栖息地中的捕食性龙虱（东方龙虱）所取代。我们进行了实验室和野外实验，以确定毛翅目虫茧在降低对这些捕食者的易感性方面的有效性。我们发现，一种临时性栖息地毛翅目昆虫（暗黑阿氏石蛾）的幼虫最易受到蝾螈的攻击。两种相对不易受攻击的物种（外栖长跗石蛾、彩长跗石蛾）表现出降低被发现和攻击可能性的行为，而最不易受攻击的物种（膨胀夜蛾石蛾）虽经常被发现和攻击，但由于虫茧提供了有效的避难所，很少被捕食。对龙虱捕食的易感性也受虫茧影响。外栖长跗石蛾幼虫厚实的虫茧常常能威慑龙虱幼虫，而其他物种的管状虫茧则相对无效。这两种易受攻击的物种（暗黑阿氏石蛾和彩长跗石蛾）中有两种常与龙虱共存；因此，仅虫茧构建不足以解释沿持久性梯度的毛翅目共存模式。这两个物种与龙虱共存的一种解释是，它们在初夏迅速发育，并在龙虱幼虫大量出现之前化蛹。其中一个物种（彩长跗石蛾）通过埋入作为避难所的软质基质中化蛹。另一个物种（暗黑阿氏石蛾）构建石质蛹茧，在野外实验中，与有机蛹茧相比，其存活率提高了一倍多。这些实验的综合结果表明，毛翅目沿持久性梯度的分布取决于一系列主要和次要的捕食者防御机制，包括幼虫和蛹茧结构、针对捕食者的逃避行为以及幼虫发育的物候学。
