Biology, SS 1
Large biotic communities corresponding to vegetation belts or zones are called biomes. Biomes are described as any large natural terrestrial ecosystem. The type of any biome is largely determined by climatic factors, especially rainfall and temperature. Thus, regions of the world with similar climates have similar biomes.
The local biomes in Nigeria can be divided into:
– Forest zone: The forest zone is made up of vegetation having mainly trees and consists of the following local biotic communities:
- Mangrove swamp forest (salt and fresh water swamp)
- Tropical rain forest
– Savanna Zone: The savanna zone is made up of mainly grasses and is further sub-divided into three savanna belts or biotic communities which are:
- Southern guinea savanna
- Northern guinea savanna
- Sahel savanna
- Sudan savanna
However, the most important factors that determine the distribution of forest and savanna are climatic factors. These are the;
- Mean annual rainfall
- Length and severity of the dry season;
- Minimum relative humidity in the driest months
Diagnostic Features Of Nigeria’s Biomes
Mangrove Swamps: These are found in the tropics, along coastal regions and river months. In Nigeria, mangrove swamps are found in the delta regions of Lagos, Delta, Edo, Rivers, Ondo and Cross Rivers States.
- The climate is hot and wet throughout the year.
- The total annual rainfall is heavy, usually above 2500mm, and the average monthly temperature is around 26oC for nearly all months of the year.
- They are forests of small, evergreen, broadleaved trees growing in shallow, brackish waters or wet soils. Many mangrove trees have prop roots and breathing roots. These features enable them to grow well in the soft swampy areas of this region of Nigeria. However, evaporation from rivers in the fresh water swamp forests is low.
- Shrubs and climbers form clumps of thick bush. The floating plants in standing waters include species of water lettuce, Lemna and Salvina. Trees occurring near the waters include Alstonia and Spondiathus.
- These occur in regions that lie between the equator and latitude 5 – 10oN and S. The forests areas are mainly in the lowlands and they also extend up hill slopes to a height of 600 or 1000 metres.
- In Nigeria, rainforest regions are hot and wet throughout the year. The mean annual temperature is 27oC while the mean total annual rainfall is 2000mm. Relative humidity is atleast 70%. Rainy season is long and the dry season is short and very severe.
- The tropical rainforest is dense and made up of many types of broad-leaved trees that are mostly evergreen i.e. the trees drop their leaves gradually throughout the year, and new leaves grow continuously to replace them.
- The trees form three layers; the tree tops form a thick canopy that prevents sunlight from reaching the forest floor. As a result, the vegetation on the forest floor is sparse. It is also possible to identify five layers, called strata, forming stratification, in a forest.
- Epiphytic plants and woody climbers (Lianas) are common features of these forests. The animal species found here are diverse and. Examples include monkeys, gorillas, chimpanzees, leopards, bats, beetles, bees, ants, termites, spiders, cobras, squirrels, lizards, birds, frogs, insects and scorpions.
Savanna: Tropical grasslands are often called savanna. The savanna is usually flat for miles at a stretch. The main savanna region is in Africa. It occurs in tropical and subtropical regions. It contains scattered trees and shrubs. The West African savanna has three belts (Guinea savanna, bordering the rainforest, the Sudan and Sahel savannas, bordering the desert). The savanna regions have hot, wet season, which alternates with cool, dry season. The average monthly temperatures are around 29oC during the hot season and around 18oC during the cold season. The total annual rainfall varies a great deal from around 500mm in regions on the semi desert fringes to about 1500mm in regions bordering the rainforest. Most of the rain falls during the hot season, i.e. from May to October in the West African savanna regions. The rainfall of the savanna is sufficient to support a rich growth of trees but is enough to prevent deserts from forming. Thus, perennial grasses with chimps of trees mostly dominate the savanna. In the moist Guinea savanna, the grasses grow tall during the rainy season. Trees occur quite close together especially along the rainforest fringe.
Southern Guinea Savanna: Southern Guinea Savanna occurs in parts of Oyo, Ondo, Osun and Kwara states.
- The mean rainfall is above 500mm.
- Dry season is shorter and less intense than in the northern Guinea savanna.
- The vegetation is an open savanna wood land type, with tall grasses up to 3 meters high.
- The trees have short, large and broad leaves.
- Major characteristics of the savanna are the common occurrence of fires in the dry season. Several of the trees are adapted to fires by the possession of thick, fore resistant barks. Trees include Daniellia oliveri; Hypmemocardia, alida, vitex doniana and Afzelia Africana. Species of grasses include Andropogan, Hypharrenia and Pennisetum.
- The animal forms include guinea fowl, deer, rats, grasshoppers and snakes.
Northern Guinea Savanna: In Nigeria, Northern Guinea Savanna is found in parts of Kaduna, Kwara, Benue and Kogi states.
- Grasses with scattered trees and shrubs dominate the vegetation.
- Trees are shorter and thorny and are fewer in numbers than in the southern Guinea savanna. Tree species found in the Northern Guinea Savanna include Isoberlina doka, Uapola somon and Monotes kerstingii.
Sudan Savanna: The Sudan savanna is found in Kano and parts of Borno, Yobe, Sokoto, Kebbi, Zamfara, Niger, Taraba, Adamawa, Bauchi and Gombe states. In the drier Sudan savanna, the grasses are shorter and the trees are fewer and more scattered.
Sahel Savanna: The Sahel savanna is found around Lake Chad. It extends across West Africa from Senegal in the West to Chad in the east and from Sahara in the north to the Northern Guinea Savanna belt in the south. In Nigeria, it occurs only at the northeast corner, in Borno and Yobe states near Lake Chad. Rainfall is very variable. The mean annual rainfall is 629mm. The zone is arid.
- Typical vegetation is an open thorn savanna, with trees up to 10 metres tall, having small leaves and thorns. In the Sahel, there are clumps of short grasses and a few isolated shrubs and trees. During the dry season, the grasses are usually dry and brown, and bush fires are often are of common occurrence.
- The underground parts of the grasses survive the dry season and fires, and grow again when the rain comes. Common grasses are; Pennisetum, Andropogan, Panicum, Aristida, Stipoides etc. While common trees include Afzelia, Acalia, Balanites, Leptodenia, date palsete.
- The fauna include gazelle, antelope, buffalo, giraffe, elephant, zebra, eland, bush cow, lion, hyena etc.
Major Biomes of the World
Tropical Rain Forest
Tropical rain forests are home to more species than all other land biomes combined. The leafy tops of tall trees – extending up to 70 meters above the forest floor – form a dense covering called a canopy. In the shade below the canopy, a second layer of shorter trees and vines forms an understory. Organic matter that falls to the forest floor quickly decomposes and the nutrients are recycled.
Abiotic factors: hot and wet year-round; thin, nutrient-poor soils
Dominant plants: broad-leaved evergreen trees; ferns; large woody vines and climbing plants; orchids and bromeliads
Dominant wildlife: herbivores such as sloths, tapirs, and capybaras; predators such as jaguars; anteaters; monkeys; birds such as toucans, parrots, and parakeets; insects such as butterflies, ants, and beetles; piranhas and other freshwater fishes; reptiles such as frogs, caymans, boa constrictors, and anacondas
Geographic distribution: parts of South and Central America, Southeast Asia, parts of Africa, southern India, and northeastern Australia
Receiving more seasonal rainfall than deserts but less than tropical dry forests, tropical savannas, or grasslands, are characterized by a cover of grasses. Savannas are spotted with isolated trees and small groves of trees and shrubs. Compact soils, fairly frequent fires, and the action of large animals such as rhinoceros prevent some savanna areas from turning into dry forest.
Abiotic factors: warm temperatures; seasonal rainfall; compact soil; frequent fires set by lightning
Dominant plants: tall, perennial grasses; sometimes drought-tolerant and fire resistant trees or shrubs
Dominant wildlife: predators such as lions, leopards, cheetahs, hyenas, and jackals; aardvarks; herbivores such as elephants, giraffes, antelopes, and zebras; baboons; birds such as eagles, ostriches, weaver birds, and storks; insects such as termites
Geographic distribution: large parts of eastern Africa, southern Brazil, northern Australia
Characterized by a rich mix of grasses and underlaid by some of the world’s most fertile soils, temperate grasslands – such as plains and prairies – once covered vast areas of the Midwestern United States. Since the development of the steel plow, however, most have been converted to agricultural fields. Periodic fires and heavy grazing by large herbivores maintain the characteristic plant community.
Abiotic factors: warm to hot summers; cold winters; moderate, seasonal precipitation; fertile soils; occasional fires
Dominant plants: lush, perennial grasses and herbs; most are resistant to drought, fire, and cold
Dominant wildlife: predators such as coyotes and badgers — historically included wolves and grizzly bears; herbivores such as mule deer, pronghorn antelope, rabbits, prairie dogs, and introduced cattle — historically included bison; birds such as hawks, owls, bobwhite, prairie chicken, mountain plover; reptiles such as snakes; insects such as ants and grasshoppers
Geographic distribution: central Asia, North America, Australia, central Europe, and upland plateaus of South America
All deserts are dry; in fact, a desert biome is defined as having annual precipitation of less than 25 centimeters. Beyond that, deserts vary greatly, depending on elevation and latitude. Many undergo extreme temperature changes during the course of a day, alternating between hot and cold. The organisms in this biome can tolerate the extreme conditions.
Abiotic factors: low precipitation, variable temperatures; soils rich in minerals but poor in organic material
Dominant plants: cacti and other succulents; creosote bush and other plants with short growth cycles
Dominant wildlife: predators such as mountain lions, gray foxes, and bobcats; herbivores such as mule deer, pronghorn antelope, desert bighorn sheep, and kangaroo rats; bats; birds such as owls, hawks, and roadrunners; insects such as ants, beetles, butterflies, flies, and wasps; reptiles such as tortoises, rattlesnakes, and lizards
Geographic distribution: Africa, Asia, the Middle East, United States, Mexico, South America, and Australia
Temperate Woodland and Shrubland
This biome is characterized by a semiarid climate and a mix of shrub communities and open woodlands. In the open woodlands, large areas of grasses and wildflowers such as poppies are interspersed with oak trees. Communities that are dominated by shrubs are also known as chaparral. The growth of dense, low plants that contain flammable oils makes fires a constant threat.
Abiotic factors: hot, dry summers; cool, moist winters; thin, nutrient-poor soils; periodic fires
Dominant plants: woody evergreen shrubs with small, leathery leaves; fragrant, oily herbs that grow during winter and die in summer
Dominant wildlife: predators such as coyotes, foxes, bobcats, and mountain lions; herbivores such as blacktailed deer, rabbits, squirrels, and mice; birds such as hawks, California quail, western scrub jay, warblers and other songbirds; reptiles such as lizards and snakes; butterflies; spiders
Geographic distribution: western coasts of North and South America, areas around the Mediterranean Sea, South Africa, and Australia.