Underwood A J
School of Biological Sciences, University of Sydney, Zoology Building, 2006, N.S.W., Australia.
Oecologia. 1980 Jan;46(2):201-213. doi: 10.1007/BF00540127.
The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure.Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime.In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock.Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels.The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers.Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards.The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.
在新南威尔士州许多海岸的低水位藻类带之上，叶状藻的覆盖稀疏甚至不存在，岩池区域除外。在这个藻类带之上，壳状藻类，主要是原希氏藻，占据了隐蔽海岸大部分的主要基质。在潮间带中部进行了实验操作，以检验关于软体动物啃食以及退潮时物理因素对这种藻类群落结构模式影响的假设。使用围栏和笼子来排除啃食者：软体动物在屋顶下和开阔区域啃食。笼子和屋顶提供了遮荫，并降低了退潮时环境的恶劣程度：围栏和开阔区域具有正常的环境状况。在没有啃食者的情况下，所有实验区域都出现了石莼的快速定殖以及其他叶状藻较慢的定殖。石莼幼苗的定殖率最初在现有的壳状藻类上受到抑制，但几个月后，石莼的覆盖度与清理后的岩石上的覆盖度相当。大多数藻类物种仅在笼子内生长至成熟，并在围栏内保持为幼苗草皮。在开阔的、有啃食的区域，没有叶状藻生长到可见大小。因此，啃食阻止了叶状藻在海岸正常上限以上的定殖，但退潮时物理因素的影响阻止了在去除啃食者后定殖的藻类生长。因此，物理因素限制了潮间带中部叶状藻的丰度。原希氏藻对清理区域的重新定殖不受幼苗草皮存在的影响，也不受屋顶遮荫的影响，但在成熟藻类形成冠层的笼子中受到抑制。即使在这样的冠层下，原希氏藻最终覆盖的主要基质与开阔的、有啃食的区域一样多。这种壳状藻类能够通过具有坚韧的叶状体、通过营养生长使个体植物覆盖大量基质以及新个体倾向于从岩石上明显啃食者无法触及的小裂缝和凹坑开始生长来逃避啃食的影响。成熟的叶状藻被海浪从基质上冲走，许多个体植物在炎热天气期间死亡。在移除实验围栏后，草皮中的幼苗被大型藻类啃食者（它们侵入这些区域）和微型藻类啃食者（它们从边缘向内啃食草皮）的综合作用消除。这里获得的结果结合关于潮间带大型藻类分布限制的其他研究以及啃食在潮间带藻类群落多样性和结构中的作用进行了讨论。还讨论了这些实验处理的一些问题。
