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  3. Neotropical coastal mangrove forests are usually “zonal,” with certain mangrove species foundpredominantly in the seaward portio

Neotropical coastal mangrove forests are usually “zonal,” with certain mangrove species foundpredominantly in the seaward portio

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问题     Neotropical coastal mangrove forests are usually “zonal,” with certain mangrove species foundpredominantly in the seaward portion of the habitat and other mangrove species on the morelandward portions of the coast. The earliest research on mangrove forests produced descriptions ofspecies distribution from shore to land, without exploring the causes of the distributions.
    The idea that zonation is caused by plant succession was first expressed by J. H. Davis in a studyof Florida mangrove forests. According to Davis’ scheme, the shoreline is being extended in aseaward direction because of the “land-building” role of mangroves, which, by trapping sedimentsover time, extend the shore. As a habitat gradually becomes more inland as the shore extends, the“land-building” species are replaced. This continuous process of accretion and succession wouldbe interrupted only by hurricanes or storm flushings.
    Recently the universal application of Davis’s succession paradigm has been challenged. It appearsthat 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 willproceed according to Davis’s scheme. But on stable coastlines, the distribution of mangrovespecies results in other patterns of zonation; “land building” does not occur.
    To find a principle that explains the various distribution patterns, several researchers have lookedto salinity and its effects on mangrove. While mangroves can develop in fresh water, they can alsothrive in salinities as high as 2.5 times that of seawater. However, those mangrove species found infreshwater habitats do well only in the absence of competition, thus suggesting that salinitytolerance is a critical factor in competitive success among mangrove species. Research suggeststhat mangroves will normally dominate highly saline regions, although not because they requiresalt. Rather, they are metabolically efficient (and hence grow well) in portions of an environmentwhose high salinity excludes plants adapted to lower salinities. Tides create different degrees ofsalinity along a coastline. The characteristic mangrove species of each zone should exhibit ahigher metabolic efficiency at that salinity than will any potential invader, including other speciesof mangrove.
选项 A、refute the idea that the zonation exhibited in mangrove forests is caused by adaption to salinity
B、describe the pattern of zonation typically found in Florida mangrove forests
C、argue that Davis’ succession paradigm cannot be successfully applied to Florida mangrove forests
D、discuss hypotheses that attempt to explain the zonation of coastal mangrove forests
E、establish that plants that do well in saline forest environments requre salt to achieve maximum metabolic efficiency
答案D
解析 This question asks you to identify the primary purpose of the passage as a whole. The best answer is D. The passage discusses two hypothesis concerning zonation of mangrove forests. Choice A can be eliminated because the passage presents as a viable hypothesis the idea that mangrove zonation may be caused by adaptation to salinity. Choice B is incorrect because the passage is concerned with the causes of zonation patterns, not simple with a description of those patterns. Both C and E contradict information that is presented in the passage.
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