Biotic interactions among plants are an important determinant of structure and dynamics. Competition is recognized as one of the most important forces structuring ecological communities. Competition is the interaction of organisms or species such that, for each, the birth or growth rate is depressed and the death rate increased by the presence of the other organisms (or species). Well-known examples of competition between plants growing on coastal dunes are grass and shrub encroachment and the invasion of exotic species.
Grass encroachment occurs when aggressive and competitive grasses spread over dune areas, reducing biodiversity because of the dominance of a few species. Grass encroachment is found in many dune areas, where grasslands become the dominant community type. Among the more aggressive species are Calamagrotis epigejos, Ammophila arenaria, and Schizachyrium scoparium. Shrub encroachment is also common, for example, in the Caribbean (Coccoloba uvifera).
Species introduction has been a common practice in dunes, both for dune stabilization and for cattle ranching activities. European marram grass was widely dispersed in other regions that were quite different from its native Europe, mainly to fix sand dunes. Several conifers have also been used for example in Dofiana's dune system in southern Spain. African grasses (e.g., Panicum maximum) have been brought to America and used to replace local grass species because they have been considered better fodder.
Neither the effects of fauna nor those of grazing animals (especially rabbits) on dunes have received the attention they deserve. The importance of herbivory by rabbits was seen in Great Britain during the outbreak of myxomatosis, a viral disease which infects rabbits. The disappearance of rabbits led to profound changes in the structure of the vegetation, mainly the development of scrub in several dune areas. Rabbits also produce nitrogen and phosphorus enrichment beneath the scrub species under which they find shelter, causing N-fixing root nodules to invade.
Lethal yellowing is a specialized bacterium, an obligate parasite that attacks many species of palms, including the coconut palm (which has become the symbol of tropical beaches). Extensive coconut plantations in the Tropics have been abandoned because of coconut die-back, and shrub encroachment has taken place.
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