Neotropical coastal mangrove forests are usually"zonal,"with certain mangrove species found predominantly in the seaward portion of the habitat and other mangrove species on the more landward portions of the coast. The earliest research on mangrove forests produced descriptions of species distribution from shore to land, without exploring the causes of the distributions.

The idea that zonation is ca used by plant succession. was first expressed by J. H. Davis in a study of Florida mangrove forests. According to Davis' scheme, the shoreline is being extended in a seaward direction because of the"land-building"role of mangroves t which l by trapping sediments over time, extend the shore. As a habitat gradually becomes more in land as the shore extends, the"land-building"species are replaced. This continuous process of accretion and succession would be interrupted only by hurricanes or storm flushings.

Recently the universal application of Davis' succession paradigm has been challenged. It appears that in areas where weak currents and weak tidal energies allow the accumulation of sediments, mangroves will follow land formation and accelerate the rate of soil accretion;succession will proceed according to Davis' scheme. But on stable coastlines, the distribution of mangrove species results in other patterns of zonation;"land building"does not occur.

To find a principle that explains the various distribution patterns, several researchers have looked to salinity and its effects on mangroves. While mangroves can develop in fresh water,they can also thrive in salinities as high as 2. 5 times that of seawater. However, those mangrove species found in freshwater habitats do well only in the absence of competition, thus suggesting that salinity tolerance is a critical factor in competitive success among mangrove species. Research suggests that mangroves will normally dominate highly saline regions, although not because they require salt. Rather, they are metabolically efficient(and hence grow well)in portions of an environment whose high salinity excludes plants adapted to lower salinities. Tides create different degrees of salinity along a coastline. The characteristic mangrove species of each zone should exhibit a higher metabolic efficiency at that salinity than will any potential invader, including other species of mangrove.


The primary purpose of the passage is to


refute the idea that the zonation exhibited in mangrove forests is caused by adaptation to salinity

describe the pattern of zonation typically found in Florida mangrove forests

argue that Davis' succession paradigm cannot be successfully applied to Florida mangrove forests

discuss hypotheses that attempt to explain the zonation of coastal mangrove forests

establish that plants that do well in saline forest environments require salt to achieve maximum metabolic efficiency

考题讲解

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正确答案是 D. discuss hypotheses that attempt to explain the zonation of coastal mangrove forests。原因是文章通过提出和检验不同的假说,来解释滨海红树林的分带现象,其中包括丹斯的继承范式,以及盐度对树种的影响。文章在最后指出,盐度对于红树林生态系统的分带现象起着重要作用,但是并不是因为树种需要盐才能够生存,而是因为树种能够在高盐环境下以更高的代谢效率发育。因此,为了解释滨海红树林的分带现象,这篇文章讨论了不同的假说,而 D 选项正是描述这一点的最佳选择。

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