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 factors, such as salinity and flooding, often are considered to be the determining factors controlling the establishment and zonational patterns of species in salt marsh and salt desert environments. Sharp boundaries commonly are found between halophyte communities even though there is a gradual change in the physicochemical environment, which indicates that biotic interactions may play a significant role in determining the distribution pattern of species and the composition of zonal communities. Competition is hypothesized to play a key role in determining both the upper and lower limits of species distribution along a salinity gradient. Field and laboratory experiments indicate that the upper limits of distribution of halophytes into less saline or nonsaline habitats is often determined by competition. There appears to be a reciprocal relationship between the level of salt tolerance of species and their ability to compete with glycophytes in less saline habitats. Halophytes are not competitive in nonsaline habitats, but their competitive ability increases sharply in saline habitats. Allelopathic effects have been reported in salt desert habitats, but have not been reported along salinity gradients in salt marshes. Some species of halophytes that are salt accumulators have the ability to change soil chemistry. Chemical inhibition of intolerant species occurs when high concentrations of sodium are concentrated in the surface soils of salt desert plant communities that are dominated by salt-accumulating species. Establishment of less salt-tolerant species is inhibited in the vicinity of these salt-accumulating species. Herbivory is reported to cause both an increase and a decrease in plant diversity in salt marsh habitats. Heavy grazing is reported to eliminate sensitive species and produce a dense cover of graminoids in high marsh coastal habitats. However, in other marshes, grazing produced bare patches that allowed annuals and other low marsh species to invade upper marsh zonal communities. A retrogression in plant succession may occur in salt marshes and salt deserts 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. Removal of geese from areas by use of inclosures caused an increase in species richness in subarctic salt marshes. Invertebrate herbivores could also inhibit the survival of seeds and the ability of plants to establish 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 parasitized species.