Large concentrations of roosting or nesting seabirds and semiaquatic mammals such as seals and walrus are found on relatively isolated islands and shores. These animals consume large amounts of marine resources but spend many weeks and months on land, accumulating high concentrations of nitrogen, phosphorous and other nutrients. The abundance and relatively large body size of individuals disrupt the growth of vegetation. The combination of these factors means that microbial activity is high and soil invertebrates are abundant, but plant diversity is usually low, and land based grazers and predators are usually absent.