The main focus of the research described in the opening chapter of Salt Marshes: Formation, Ecological Functions and Threats focuses on the study of the plant heritage of the Fuerteventura salt marshes and their surroundings, together with the analysis of their ecological value, threats to the marshes, and of use and recent management practices. The methodology used predominantly consists of field work, photo-interpretation4, and implementation of a GIS from the information obtained and the database generated. The Fuerteventura salt marshes are of great importance because of their vegetal wealth and role as a refuge for fauna, especially birds and invertebrates. The subtropical saltmarshes of Brazil are located in the South of Brazil, between the north-central coast of São Paulo state and the central coast of Santa Catarina state, and have been studied over the course of many years by the researchers M.R. Bornschein and B.L. Reinert, resulting in their recognition as a new ecosystem. Despite several ornithological researches in these marshes, little is known about detailed aspects of their ecology and other groups of animals. Thus, the authors provide a general description of this recently recognized ecosystem and the ecological influences over the species that live in these marshes, resulting in the fragile ecological balance point of Borschein-Reinert, and propose a mathematical index to this balance. Salt marshes develop in estuaries where there is reduced wave action, which allows for a source of sediment and suitable conditions for marsh plants to grow. The vegetation generally provides a structural habitat on featureless soft-sediment bottoms, and so it is utilized by a wide and diverse range of fish and invertebrates as their physical home, food supply and shelter from predators. The authors discuss the way in which species such as insects and crabs demonstrate variable levels of adaptation to life in this intertidal habitat, as well as how species composition and abundance of benthic invertebrates are strongly influenced by changes in the vegetation.