Menge Bruce A
Department of Zoology, Oregon State University, 97331, Corvallis, OR, USA.
Oecologia. 1978 Jan;34(1):1-16. doi: 10.1007/BF00346237.
Knowledge of predation intensity and how and why it varies among communities appears to be a key to understanding of community regulation. Along the rocky shores of New England, predation intensity in the mid intertidal zone appears to be low with exposure to severe wave shock, low desiccation stress, and a sparse cover of canopy algae, and high at areas protected from waves, with high desiccation potential and a dense cover of algae. As a result, predators at exposed headlands have no controlling influence on community structure, while at protected sites, they exert a strong and controlling effect on community structure.Experimental-observational studies of the effects of wave shock and desiccation on survival, foraging range and activity of the primary predator in this community (Thais lapillus) indicate that: (1) wave shock is a continuous and actual source of mortality at exposed sites but is relatively unimportant at protected sites; (2) mortality rates from desiccation at protected sites are potentially high and greater than at exposed sites; however, (3) actual desiccation stress is greatly reduced at protected sites by a dense algal canopy; (4) mortality from desiccation is greater in the higher mid intertidal than in the lower mid intertidal. Comparisons of activity patterns of Thais from April through November (these snails are usually active from May to early October) at an exposed and a protected site suggest snails at the former site restrict their active feeding to crevices while those at the latter site forage throughout the habitat. Field experiments support this hypothesis. Hence, differences in predator effectiveness at exposed and protected communities are probably due in part to the influence of wave shock. Exposed areas receive frequent severe wave shock in all seasons, even summer. Thus, the risk of being swept off the shore for snails foraging away from the shelter of a crevice at such areas is apparently great and exerts a strong selective force on foraging range. The importance of waves as a selective agent is further reinforced by the fact that crevices are nearly barren of prey, while just a few cm beyond the limits of the crevice, prey occur in great abundance.In contrast, at protected sites wave shock is never as severe as at exposed sites and is a relatively minor factor among several which might affect the foraging activity of a Thais. A major factor which varies among protected sites is the algal canopy. The influence of this factor is considered in a companion paper.
了解捕食强度以及它在不同群落中如何变化和为何变化，似乎是理解群落调节的关键。在新英格兰的岩石海岸，潮间带中部的捕食强度在受到强烈海浪冲击、低干燥胁迫且藻冠覆盖稀疏的区域似乎较低，而在免受海浪影响、干燥潜力高且藻类覆盖密集的区域则较高。因此，在暴露的岬角处，捕食者对群落结构没有控制作用，而在受保护的地点，它们对群落结构产生强烈的控制作用。对海浪冲击和干燥对该群落主要捕食者（荔枝螺）的生存、觅食范围和活动的影响进行的实验观察研究表明：（1）海浪冲击在暴露地点是持续且实际的死亡来源，但在受保护地点相对不重要；（2）受保护地点因干燥导致的死亡率可能很高，且高于暴露地点；然而，（3）受保护地点的密集藻冠大大降低了实际的干燥胁迫；（4）潮间带中部较高区域因干燥导致的死亡率高于较低区域。对4月至11月（这些蜗牛通常在5月至10月初活跃）在一个暴露地点和一个受保护地点的荔枝螺活动模式的比较表明，前者地点的蜗牛将其主动觅食限制在缝隙中，而后者地点的蜗牛则在整个栖息地觅食。野外实验支持了这一假设。因此，暴露群落和受保护群落中捕食者有效性的差异可能部分归因于海浪冲击的影响。暴露区域在所有季节，甚至夏季都会频繁受到强烈的海浪冲击。因此，在这些区域，远离缝隙庇护觅食的蜗牛被海浪冲下海岸的风险显然很大，这对觅食范围施加了强大的选择压力。海浪作为一种选择因素的重要性进一步得到加强的事实是，缝隙中几乎没有猎物，而在缝隙范围外几厘米处，猎物大量存在。相比之下，在受保护地点，海浪冲击从未像在暴露地点那样严重，并且是可能影响荔枝螺觅食活动的几个因素中相对较小的一个因素。受保护地点之间变化的一个主要因素是藻冠。在一篇配套论文中考虑了这个因素的影响。
