Passage 41
Neotropical coastal¹ mangrove² 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 caused 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, which, by trapping sediments⁴
over 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 accretion⁵ and succession would
be interrupted only by hurricanes or storm flushings.
Recently the universal application of Davis’s succession paradigm⁶ 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’s 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 salinity⁷ and its effects on mangrove. 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 metabolically⁹ 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 metabolic⁸ efficiency at that salinity than will any potential invader¹⁰, including other species
of mangrove.
 
253. The primary of the purpose of the passage is to
(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 applied¹¹ to Florida mangrove forests
(D)    discuss hypotheses that attempt to explain the zonation of coastal mangrove forests (D)
(E)     establish that plants that do well in saline forest environments requre salt to achieve maximum metabolic efficiency
 
254. According to the passage, the earliest research on mangrove forest produced which of the
following?
(A)   Data that implied random¹² patterns of mangrove species distribution
(B)    Descriptions of species distribtutions suggesting zonation
(C)    Descriptions of the development of mangrove forests over time
(D)    Reclassification of species formerly¹³ thought to be identical (B)
(E)     Data that confirmed the “land-building” role of mangroves
 
255. It can be inferred from the passage that Davis’ paradigm does NOT apply to which of the
following?
(A)   The shoreline of Florida mangrove forests first studies by Davis
(B)    A shoreline in an area with weak currents
(C)    A shoreline in an area with weak idal energy
(D)    A shoreline extended by “land-building” species of mangrove (E)
(E)     A shoreline in which few sediments can accumulate
 
256. Information in the passage indicates that the author would most probably regard which of
following statements as INCORRECT?
(A)   Coastal mangrove forests are usually zonal.
(B)    Hurricanes interrupt the process of accretion and succession that extends existing shorelines.
(C)    Species of plants that thrive in a saline habitat require salt to flourish.
(D)    Plants with the highest metabolic efficiency in a given habitat tend to exclude other plants from that habitat. (C)
(E)     Shoreline in areas with weak currents and trides are more likely to be extended through the porocess of accumulation of sediment³ than are shorleines with strong currents and tides.

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