Underwood A J
Department of Zoology, School of Biological Sciences, University of Sydney, 2006, N.S.W., Australia.
Oecologia. 1984 Oct;64(2):211-222. doi: 10.1007/BF00376873.
Grazing by the snail Nerita atramentosa and the limpet Cellana tramoserica caused similarly great reductions in abundance of microalgae - measured by direct counts and by estimation of chlorophyll analyses. A smaller snail, Bembicium nanum, caused smaller reductions of microalgal resources, compared with ungrazed areas. These results were consistent with the competitive abilities of these three species. Chlorophyll concentrations in samples of grazed rocks were reliable estimates of the nature and abundance of food available to the grazers.Inter- and intra-specific competition amongst Nerita and Cellana were investigated at various densities in experimental cages. To examine the effects of different availability of food resources, the experiments were repeated at three heights on the shore (abundance of food decreases with height) and during autumn/winter and spring/summer periods of the year (less food is available during summer).Density-dependent mortality of Cellana was caused by the presence of other limpets, or of Nerita. Mortality was greatest at higher levels and during the spring/summer and was significantly, negatively correlated with mean chlorophyll concentration in the experimental cages. The only exception was that all limpets suddenly died in cages at the highest level during the summer period, which cannot be explained solely by competitive interactions. Nerita showed no density-dependent mortality during the short periods of these experiments. Tissue-weights of Nerita declined with increased density, but the effect of Cellana was not as great as the intraspecific effect of Nerita. Snails retained weight better at lower than at higher levels, and during the autumn/winter which is consistent with the availability of food. Tissue weights of both Nerita and Cellana were positively correlated with chlorophyll concentrations inside the cages in both seasons investigated.These experimental results demonstrate that intensity of competition will vary from place to place and time to time according to the densities and mixtures of the grazers, and according to the availability of microalgal food.
黑唇玉螺（Nerita atramentosa）和细纹蜒螺（Cellana tramoserica）的啃食导致微藻数量出现了同样大幅的减少——这是通过直接计数以及叶绿素分析估算得出的。与未被啃食的区域相比，体型较小的玉黍螺（Bembicium nanum）造成的微藻资源减少幅度较小。这些结果与这三个物种的竞争能力相符。啃食过的岩石样本中的叶绿素浓度是对食草动物可获得食物的性质和数量的可靠估计。
在实验笼中，研究了不同密度下黑唇玉螺和细纹蜒螺种间及种内的竞争情况。为了检验不同食物资源可利用性的影响，实验在海岸的三个高度（食物数量随高度降低）以及一年中的秋冬和春夏季节（夏季食物较少）重复进行。
细纹蜒螺的密度依赖性死亡率是由其他蜒螺或黑唇玉螺的存在导致的。死亡率在较高高度以及春夏季节最高，并且与实验笼中的平均叶绿素浓度显著负相关。唯一的例外是，在夏季，最高层笼子里的所有蜒螺突然死亡，这无法仅用竞争相互作用来解释。在这些实验的短时间内，黑唇玉螺未表现出密度依赖性死亡率。黑唇玉螺的组织重量随密度增加而下降，但细纹蜒螺的影响不如黑唇玉螺的种内影响大。蜗牛在较低高度比在较高高度能更好地保持体重，并且在秋冬季节也是如此，这与食物的可获得性一致。在所研究的两个季节中，黑唇玉螺和细纹蜒螺的组织重量都与笼内的叶绿素浓度呈正相关。
这些实验结果表明，竞争强度会因食草动物的密度和混合情况以及微藻食物的可获得性，随地点和时间而变化。
