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. The primary of the purpose of the passage is to______