http://education.seattlepi.com/dominant-species-diverse-ecosystem-3936.html

by Kari Norborg Carter, Demand Media 

In the temperate rainforests of Olympic National Park in Washington, the coast Douglas fir is the dominant plant species. 

Natural areas such as a redwood forest, mangrove swamp, coral reef or cypress slough did not come about their names accidentally. A dominant species is a plant, animal or functional group of different species most commonly or conspicuously found in a particular ecosystem. It is generally the most populous species or comprises the greatest biomass in an ecosystem. It also influences the distribution of other organisms and helps define the ecosystem and its characteristics. A dominant species might be better at obtaining resources, resisting diseases or deterring competitors or predators than other species. 

Dominant Species in Forests 

Not surprisingly, the dominant plants in any kind of forest are trees. With their height, resource use and shade production, they greatly influence what and where other vegetation can grow and also influence what kinds of animals will live in the forest. Just as there are many different types of forests in the world, the dominant species in each can vary. In a northern boreal forest, with its long, frigid winters, conifers including spruce, fir and larch dominate. Their fallen needles help create acidic soil conditions that influence what other kinds of vegetation can grow. In the very wet, temperate coastal rainforests in Washington, Canada and Alaska, the coast Douglas fir is king. Just south of that, in northern California, fog replaces high precipitation, and coastal redwoods dominate old-growth forests. Even further south, in southern California, shrubby forests with small, evergreen, aromatic, leathery leaves inhabit dry, fire-prone areas called Mediterranean scrub or chaparral. Dominant species in temperate deciduous forests of North America include oak and hickory in well-drained areas and beech and maple in loamy, poorly drained areas. 

Dominant Species in Grasslands 

Grasslands around the world include tropical savannas, temperate grasslands and steppes. Grasses and other herbaceous plants generally dominate for one or more reasons: the ecosystem is very dry for at least part of the year; it is frequently disturbed by fire, flooding or grazing; or it lacks the kind of soil to support forests. In Venezuela and Colombia, annual flooding inhibits trees in the llanos savannas of the Orincoco basin, while in Belize and Honduras, pine savannas grow in sandy soils. In the humid Pampas of Argentina and Uruguay, showy pampas grass thrives in the rich soil. Native peoples of central North America might have helped maintain the tallgrass prairies, dominated by deeply rooted grasses like big bluestem and switchgrass, by setting periodic fires. In tropical savannas in East Africa, occasional trees — mostly acacia — dot the horizon, while nutrient-rich volcanic sands but only seasonal rains keep short grasses dominant in Serengeti volcanic grasslands. The most populous animals in African grasslands include large grazing ungulates such as wildebeest, plains zebras and many species of antelope; other species of grazing animals are common in grasslands of the world. 

Dominant Species in Hot Deserts 

Heat and lack of precipitation characterize hot deserts, so dominant plants and animals have developed adaptations to preserve water and survive hot temperatures. In the Sonoran desert of North America, saguaro, prickly pear and barrel cacti are some of the most common vegetation. A type of yucca, the Joshua tree, monopolizes the landscape in parts of the Mohave Desert from 2,000 to 6,000 feet. Acacia trees and eucalyptus populate Australian deserts, while the Sahara Desert salt flats host halophytic vegetation, and extremely drought-hardy gerbils are the most abundant mammal. Few plants can survive in the parched white sand dunes of the Namib; leathery welwitschia is one of them. Reptiles, birds of prey and mammals specially adapted to hot, dry conditions — including nocturnal and burrowing mammals — tend to be the dominant fauna in desert ecosystems. 

Dominant Species in Wetlands 

Water-logged conditions during at least part of the year define wetlands, which can be dominated by either woody or herbaceous plants. Saltwater mangrove swamps in tropical and subtropical coastal areas feature their namesake, various species of mangrove trees, while cypress sloughs carry bald cypress trees. Reeds, grasses, cattails and other herbaceous plants predominate in marshes. In northern bogs, notable vegetation can include sphagnum moss, heather, reeds, sedges, cranberry, blueberry, Labrador tea and tamarack in the acidic peat mats of slowly decaying vegetation. Because ombrotrophic bogs, which receive all their water from precipitation, have very few nutrients, insectivorous plants such as sundews, venus flytraps and pitcher plants have evolved to trap and digest insects for their minerals. In many wetlands in North America, the non-native purple loosestrife has taken over as a dominant species as it has been able to aggressively out-compete other plants.