the biotic characteristics are mainly determined by the organisms that occur. for example, wetland plants may produce dense canopies that cover large areas of sediment—or snails or geese may graze the vegetation leaving large mud flats. aquatic environments have relatively low oxygen levels, forcing adaptation by the organisms found there. for example, many wetland plants must produce aerenchyma to carry oxygen to roots. other biotic characteristics are more subtle and difficult to measure, such as the relative importance of competition, mutualism or predation. there are a growing number of cases where predation by coastal herbivores including snails, geese and mammals appears to be a dominant biotic factor.