ARE BIOTIC FACTORS SIGNIFICANT IN INFLUENCING THE DISTRIBUTION OF HALOPHYTES IN SALINE HABITATS

The influence of biotic factors on the distribution and establishment of halophytes is being considered in this review. Physicochemical fact ors, such as salinity and flooding, often are considered to be the det ermining factors controlling the establishment and zonational patterns of species in salt marsh and salt desert environments. Sharp boundari es commonly are found between halophyte communities even though there is a gradual change in the physicochemical environment, which indicate s that biotic interactions may play a significant role in determining the distribution pattern of species and the composition of zonal commu nities. Competition is hypothesized to play a key role in determining both the upper and lower limits of species distribution along a salini ty gradient. Field and laboratory experiments indicate that the upper limits of distribution of halophytes into less saline or nonsaline hab itats is often determined by competition. There appears to be a recipr ocal relationship between the level of salt tolerance of species and t heir ability to compete with glycophytes in less saline habitats. Halo phytes are not competitive in nonsaline habitats, but their competitiv e ability increases sharply in saline habitats. Allelopathic effects h ave been reported in salt desert habitats, but have not been reported along salinity gradients in salt marshes. Some species of halophytes t hat are salt accumulators have the ability to change soil chemistry. C hemical inhibition of intolerant species occurs when high concentratio ns of sodium are concentrated in the surface soils of salt desert plan t communities that are dominated by salt-accumulating species. Establi shment of less salt-tolerant species is inhibited in the vicinity of t hese salt-accumulating species. Herbivory is reported to cause both an increase and a decrease in plant diversity in salt marsh habitats. He avy grazing is reported to eliminate sensitive species and produce a d ense cover of graminoids in high marsh coastal habitats. However, in o ther marshes, grazing produced bare patches that allowed annuals and o ther low marsh species to invade upper marsh zonal communities. A retr ogression in plant succession may occur in salt marshes and salt deser ts because of heavy grazing. Intermediate levels of grazing by sheep, cattle, and horses could produce communities with the highest species richness and heterogeneity. Grazing by geese produced bare areas that had soils with higher salinity and lower soil moisture than vegetated areas, allowing only the more salt-tolerant species to persist. Remova l of geese from areas by use of inclosures caused an increase in speci es richness in subarctic salt marshes. Invertebrate herbivores could a lso inhibit the survival of seeds and the ability of plants to establi sh in marshes. Parasites could play a significant role in determining the species composition of zonal communities, because uninfected rarer species are able to establish in the gaps produced by the death of pa rasitized species.