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2:29 AM
forest damaged by acid rain
forest damaged by acid rain

Acid precipitation is rain, snow, or mist which has a pH lower than unpolluted precipitation. Increased levels of acid precipitation have significant effects on food chains and ecosystems.

Precipitation—rain, snow, hail, sleet, or mist—is naturally acidified by carbonic acid (H2CO3). Carbon dioxide (CO2) in the atmosphere reacts with water molecules, lowering the pH of precipitation to 5.6. A pH scale is used to measure a solution’s acidity or alkalinity; pH is defined as the negative logarithm of the concentration of hydrogen ions, H+ . A solution with a pH of 7.0 is neutral. A pH lower than 7 is acidic, and a pH greater than 7 is alkaline.

Other acidic substances are also present in the atmosphere, causing "unpolluted" precipitation to have a pH approaching 5.0. Solutions with a pH of 5.0 or less have concentrations of hydroxyl ion, or OH– , and carbonate ion, or CO3– , approaching zero.


Acid precipitation is the name given to rain or snow contaminated with oxides of sulfur (SOx) and oxides of nitrogen (NOx). These chemicals combine with water droplets to form sulfuric acid and nitric acid. SOx is formed by combustion of materials containing sulfur, and NOx is formed by oxidation of molecular nitrogen in the atmosphere during combustion. SOx sometimes arises from natural sources such as volcanoes and geyser fields, and NOx is formed by lightning.

Downwind of smelting facilities, hydrochloric acid (HCl) and hydrofluoric acid (HF)may also contribute to acid precipitation. Acid precipitation may detrimentally change soil chemistry, either by stripping nutrients, especially magnesium and calcium, or mobilizing phytotoxic trace elements (elements toxic to plants).

Geographic Extent of Damage

Acid precipitation is a regional problem. SOx and NOx can travel many thousands of kilometers in the atmosphere after being emitted by large, stationary sources, especially those that have very high smoke stacks.

How acid rain is formed
How acid rain is formed

These pollutants are slowly transformed into sulfuric and nitric acid aerosols and are incorporated into precipitation, which eventually makes contact with the earth’s surface. Acid precipitation in the eastern United States contains more SOx than precipitation in the western United States, which contains more NOx.

In North America, acid precipitation and dry deposition (of acid aerosol particles) are major environmental problems in New England and New York State and in Ontario and Quebec. These regions attribute much of their acid precipitation to emissions from large coal-burning plants in the American Ohio Valley.

Scandinavian activists blame coal-burning power plants and factory emissions in the British Isles for that region’s acid rain problems. Central Europe—including Poland, the Czech Republic, Slovakia, and eastern Germany—has many power plants and factories that burn high-sulfur coal. Acid-laden pollution plumes stretch thousands of kilometers downwind from smokestacks in that region.

Controlled Studies

Controlled experiments on individual plant species have revealed short-term damage to a limited number of those species. Experiments using simulated acid rain (SAR) are difficult to extrapolate to field conditions, where the specific pollutants and pH levels vary widely over time.

Air Pollution
Air Pollution

In controlled conditions, studies showed no link between SAR and yield in Amsoy soybeans. However, field studies demonstrated that acid deposition does decrease yield in Amsoy soybeans.

Acid precipitation influences plant diseases by acting on both pathogens and host organisms. Seedlings of Pinus rigida, Pinus echinata, Pinus taeda, and Pinus strobus exposed to SAR of pH 3.0 had a 100 percent mortality rate because of fungal damping-off, a diseased condition of seedlings marked by wilting or rotting.

Red spruce seedlings subjected to dilute sulfuric acid mist developed brown lesions on their needles, followed by needle drop. Studies showed a reduction in the growth of sugar maple seedlings following exposure to low pH moisture, and that seedling survival decreased with increasing acidity.

Crop and Forest Decline

In field experiments, soybeans have shown reduced yields with decreasing pH (increasing acidity) of moisture applied. Yields of seed and seed protein are both reduced in soybeans exposed to high acidity. A lower number of seed pods were found in plants exposed to high acidity, compared to control plants.

Acid precipitation causes detrimental long-term effects in most ecosystems, especially forests. Root systems under acidic stress show great variability in tolerance and injury. Acidic stress on roots decreases root growth, measured by a reduction in root length, and severely damaged trees have more fine roots with opaque tip zones than do slightly damaged trees. Some scientists have suggested that the radical growth rate in yellow pines in the southeastern United States may be reduced by acid precipitation.

Since the 1960’s Central European soils have been progressively acidified, altering soil buffering capacities. Acid rain containing nitrates (which are not immobilized in soil) played an important role in this soil acidification.

Acidification has reduced the magnesium, calcium, and potassium available for nutrient uptake by plants and has affected root growth. One-quarter of European forests are moderately or severely damaged by acid precipitation, with dry deposition believed (by scientists and politicians) to be largely responsible.

This pattern of damage, first detected in the 1980’s, has been called neuartige Waldschäden (literally, "new-type forest decline"). It has been detected throughout Central Europe at all elevations and on all soil types. Waldschäden is most pronounced downwind of major air pollution sources.

Abnormally high numbers of red spruce have died in the high-elevation northern Appalachian Mountains since the 1960’s. This die-off has been attributed to high rates of acid deposition (up to 4 kilo equivalents of hydronium ions per hectare per year) and exposure to acid fog droplets for up to two thousand hours per year. Very high levels of trace metals (known to be phytotoxic) have accumulated in the region.

Aquatic Ecosystems

Most freshwater ecosystems range from pH 6.0 to pH 8.0. In limestone terrain, acid precipitation is neutralized by dissolution of calcium carbonate. As a freshwater environment becomes acidified, the number of species it supports declines. When conditions are more acidic than pH5.5, dissolved inorganic carbon exists only as dissolved carbon dioxide. Planktonic algae, which can use low levels of dissolved inorganic carbon, are favored in these environments.

Acid environments greatly reduce the numbers of herbivores that graze on aquatic plants; this is thought to explain why filamentous green algae are found in most acidified lakes. Scientists point out that it is difficult to separate the effects that acidification alone produces in an aquatic ecosystem.
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12:48 AM
Water lilies, a kind of adaptations
Water lilies, a kind of adaptations

The results of natural selection in which succeeding generations of organisms become better able to live in their environments are called adaptations. Many of the features that are most interesting and beautiful in biology are adaptations. Specialized structures, physiological processes, and behaviors are all adaptations when they allow organisms to cope successfully with the special features of their environments.

Adaptations ensure that individuals in populations will reproduce and leave well-adapted offspring, thus ensuring the survival of the species. Adaptations arise through mutations—inheritable changes in an organism’s genetic material.

These rare events are usually harmful, but occasionally they give specific survival advantages to the mutated organism and its offspring. When certain individuals in a population possess advantageous mutations, they are better able to cope with their specific environmental conditions and, as a result, will contribute more offspring to future generations than those individuals that lack the mutation.


Over time, the number of individuals that have the advantageous mutation will increase in the population at the expense of those that do not have it. Individuals with an advantageous mutation are said to have a higher fitness than those without it, because they tend to have comparatively higher survival and reproductive rates. This is natural selection.

Natural Selection

Growing on poisonous environtment
Growing on poisonous environtment

Over very long periods of time, evolution by natural selection results in increasingly better adaptations to environmental circumstances. Natural selection is the primary mechanism of evolutionary change, and it is the force that either favors or selects against mutations.

Although natural selection acts on individuals, a population gradually changes as those with adaptations become better represented in the total population. Most flowering plants, for example, are unable to grow in soil containing high concentrations of certain elements (for example, heavy metals) commonly found in mine tailings.

Therefore, an adaptation that conferred resistance to these elements would open up a whole new habitat where competition with other plants would be minimal. Natural selection would favor the mutations, which confer specific survival advantages to those that carry them and impose limitations on individuals lacking these advantages.

Thus, plants with special adaptations for resistance to the poisonous effects of heavy metals would have a competitive advantage over those that find heavy metals toxic. These attributes would be passed to their more numerous offspring and, in evolutionary time, resistance to heavy metals would increase in the population.

Types of Adaptations

Although natural selection serves as the instrument of change in shaping organisms to very specific environmental features, highly specific adaptations may ultimately be a disadvantage. Adaptations that are specialized may not allow sufficient flexibility (generalization) for survival in changing environmental conditions.

The degree of adaptative specialization is ultimately controlled by the nature of the environment. Environments, such as the tropics, that have predictable, uniform climates and have had long, uninterrupted periods of climatic stability are biologically complex and have high species diversity.

Tropical rain forest
Tropical rain forest, complex competition for resources
and intense predator-prey relationships

Scientists generally believe that this diversity results, in part, from complex competition for resources and from intense predator-prey relationships. Because of these factors, many narrowly specialized adaptations have evolved when environmental stability and predictability prevail.

By contrast, harsh physical environments with unpredictable or erratic climates seem to favor organisms with general adaptations, or adaptations that allow flexibility. Regardless of the environment type, organisms with both general and specific adaptations exist because both types of adaptation enhance survival under different environmental circumstances.

Metabolism is the sum of all chemical reactions taking place in an organism, whereas physiology consists of the processes involved in an organism carrying out its function. Physiological adaptations are changes in the metabolism or physiology of organisms, giving them specific advantages for a given set of environmental circumstances.

harsh physical environments, enhance survival under different environmental circumstances
harsh environments, enhance survival under different environmental circumstances

Because organisms must cope with the rigors of their physical environments, physiological adaptations for temperature regulation, water conservation, varying metabolic rate, and dormancy allow organisms to adjust to the physical environment or respond to changing environmental conditions.

Adaptations and Environment

Desert environments, for example, pose a special set of problems for organisms. Hot, dry environments require physiological mechanisms that enable organisms to conserve water and resist prolonged periods of high temperature.

Evolution has favored a specialized form of photosynthesis in cacti and other succulents inhabiting arid regions. Crassulacean acid metabolism (CAM) photosynthesis allows plants with this physiological adaptation to absorb carbon dioxide at night, when relative humidity is comparatively high and air temperatures relatively low.

✯ A Beavertail cactus in Henderson Canyon - CA
Adaptations and Environment

Taking in carbon dioxide during the day would dehydrate plants, because opening the pores through which gas exchange takes place allows water to escape from the plant. CAM photosynthesis, therefore, allows these plants to exchange the atmospheric gases essential for their metabolism at night, when the danger of dehydration is minimized.

Because organisms must also respond and adapt to an environment filled with other organisms— including potential predators and competitors— adaptations that minimize the negative effects of biological interactions are favored by natural selection. Often the interaction among species is so close that each species strongly influences the others and serves as the selective force causing change.

Under these circumstances, species evolve together in a process called coevolution. The adaptations resulting from coevolution have a common survival value to all the species involved in the interaction. The coevolution of flowers and their pollinators is a classic example of these tight associations and their resulting adaptations.

Speciation

Adaptations can be general or highly specific. General adaptations define broad groups of organisms whose lifestyles are similar. At the species level, however, adaptations are more specific and give narrow definition to those organisms that are more closely related to one another.

Slight variations in a single characteristic, such as bill size in the seed-eating Galapágos finches, are adaptive in that they enhance the survival of several closely related species. An understanding of how adaptations function to make species distinct also furthers the knowledge of how species are related to one another.

Why so many species exist is one of the most intriguing questions of biology. The study of adaptations offers biologists an explanation. Because there are many ways to cope with the environment, and because natural selection has guided the course of evolutionary change for billions of years, the vast variety of species existing on the earth today is simply an extremely complicated variation on the theme of survival.
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12:17 AM
Active transport is the process by which cells expend energy to move atoms or molecules across membranes, requiring the presence of a protein carrier, which is activated by ATP. Cotransport is active transport that uses a carrier that must simultaneously transport two substances in the same direction. Countertransport is active transport that employs a carrier that must transport two substances in opposite directions at the same time.

Biologists in nearly every field of study have discovered that one of the major methods by which organisms regulate their metabolisms is by controlling the movement of molecules into cells or into organelles such as the nucleus.

This regulation is possible because of the semipermeable nature of cellular membranes. The membranes of all living cells are fluid mosaic structures composed primarily of lipids and proteins. The lipid molecules are aliphatic, which means that their molecular structure exhibits both a hydrophilic (water-attracted) and a hydrophobic (water-repelling) portion.

These aliphatic molecules form a double layer: The hydrophilic heads are arranged opposite one another on the inner and outer surfaces, and the hydrophobic tails are aligned across from one another within the interior, sandwiched between the hydrophilic heads. The protein in the membrane is interspersed periodically throughout the lipid bilayer.


Some of the protein, referred to as peripheral protein, penetrates only one of the lipid layers. The integral protein, as the remaining protein is called, extends through both layers of lipid to interface with the environment on both the internal and external surfaces of the membrane. These integral proteins can serve as transport channels and carriers.

Cellular Energy

Transport across the membrane is accomplished by three different mechanisms: simple diffusion, facilitated diffusion, and active transport. The first two mechanisms are referred to as passive processes because they do not require the direct input of cellular energy, and they involve transport down a concentration gradient, that is, from the side with a higher concentration to the side with a lower concentration of the substance being transported.

In many instances, however, substances are transported across a membrane from the side with a low concentration to the side containing a greater concentration. This "uphill" movement across membranes is called active transport, and it requires the expenditure of cellular energy.

Cellular energy, produced by the biological oxidation of fuels such as carbohydrates, is stored as adenosine triphosphate (ATP). When this high-energy phosphate is hydrolyzed, the stored energy is released to drive cellular reactions such as active transport. The ATPase protein located in membranes belongs to one of the groups of enzymes which hydrolyze ATP.

The mechanism has not been completely deciphered, but it appears as though a protein carrier molecule binds with the substance to be transported at the surface on one side of the membrane. This binding occurs at a specific activated region on the carrier protein called the active site. After combining with the carrier, the substance is moved across the membrane and released at the surface on the other side of the membrane.

ATP is then hydrolyzed by an ATPase, and the energy released in this reaction prepares the protein carrier for attachment to another molecule to be transported by reactivating the active site. There is some question as to whether ATPase is a component of the carrier molecule or functions separately from it. Regardless of the spatial arrangement, the two molecules are intimately related in the active transport process.

Cotransport and Countertransport

There are two important modifications of the active transport process: cotransport and countertransport. Cotransport, or symport, involves a specialized protein molecule referred to as a symport carrier. Asymport carrier has two attachment sites. One site is for the attachment of the molecule to be transported, and the other is for the attachment of a second molecule, which can be referred to as the synergist.

Both the molecule to be transported and the synergist must be bound to the symport carrier before transport across the membrane can take place. The synergist is moved down a concentration gradient, and this downhill flow of the synergist drives the carrier to transport both molecules.

In order to keep the synergist moving down a concentration gradient when attached to the symport carrier, the synergist must be pumped back across the membrane. This movement of the synergist in the opposite direction is mediated by a protein carrier activated by the energy released from the hydrolysis of ATP by an ATPase.

Countertransport, or antiport, also utilizes a specialized carrier with two attachment sites. This antiport carrier binds with the molecule to be transported at one of the attachment sites, and a second molecule, which can be called the antagonist, binds at the other.

The carrier moves the molecule to be transported across the membrane while simultaneously moving the antagonist in the opposite direction. Again, both molecules must be attached to the antiport carrier before either can be transported, and the flow of the antagonist down a concentration gradient drives the transport by the carrier in both directions.

The antagonist is pumped back across the membrane by a protein carrier activated by the energy released from the hydrolytic action of an ATPase on ATP. This action maintains a concentration gradient favorable for transport when the antagonist is attached to the antiport carrier.

Transport in Action

The presence of these three active transport mechanisms has been well documented. Calcium, for example, has been shown to be pumped from the cell by a carrier protein activated by the hydrolysis of ATP. Sugars for energy and carbohydrate structure must be cotransported into the cell by a symport carrier that utilizes the sodium ion as a synergist. At least two countertransport ion pumps have been identified.

One pumps the potassium ion into the cell at the same time that it pumps the hydrogen ion out. The second pumps the potassium ion into the cell while the antagonist, sodium, is moved in the opposite direction. It is likely that numerous other active transport systems exist that have not yet been positively identified.

A protein carrier is one of the basic components of any active transport mechanism. Although no specific carrier molecule has yet been positively identified, there is ample indirect evidence to support the presence of such a protein. Much of this evidence comes from studies showing that active transport exhibits saturation kinetics.

This means that the transport of a specific ion will increase as the concentration increases, up to a certain point. At this point, further increases in concentration will have no effect on transport. These results strongly suggest that the ion is binding with another molecule in the membrane, such as a carrier protein, which is limited in concentration and becomes saturated.

Studies have also shown the transport of some substances to be competitively inhibited by the presence of another, very similar, substance. This indicates that both substances are competing for the same site on a membrane molecule, such as a protein carrier.

Role of Active Transport

The ability to accumulate substances against a concentration gradient is necessary for the normal function and survival of cells. There are numerous examples, however, of active transport being intimately involved in the regulation of some important biological processes. In the plant kingdom, sugar is produced by photosynthesis in the green leaves.

This sugar must be transported out of the leaves and into nonphotosynthetic tissues, such as roots or fruit, through specialized transport cells in the phloem. The loading of sugars into the phloem is dependent on an active cotransport mechanism.

Almost every field of life science is concerned with gene regulation. Genes are continually being induced (activated) or repressed (deactivated) as organisms develop and change from the time of their conception until their death.

Repression is usually caused by the presence of a protein molecule in the cell nucleus, but induction may very often be the result of metabolites being actively transported into the cell or nucleus. Hence, the active transport mechanisms may be a very important component of gene regulation.
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11:07 PM
In adaptive radiation, numerous species evolve from a common ancestor introduced into an environment with diverse ecological niches. The progeny evolve genetically into customized variations of themselves, each adapting to survive in a particular niche.

In 1898 Henry F. Osborn identified and developed the evolutionary phenomenon known as adaptive radiation, whereby different forms of a species evolve, quickly in evolutionary terms, from a common ancestor.

According to the principles of natural selection, organisms that are the best adapted (most fit) to compete will live to reproduce and pass their successful traits on to their offspring. The process of adaptive radiation illustrates one way in which natural selection can operate when members of one population of a species are cut off or migrate to a different environment that is isolated from the first.

Such isolation can occur from one patch of plantings to another, from one mountain top or hillside to another, from pond to pond, or from island to island. Faced with different environments, the group will diverge from the original population and in time become different enough to form a new species.


Genetic Changes

In a divergent population, the relative numbers of one form of allele (characteristic) decrease, while the relative numbers of a different allele increase. New environmental pressures will select for favorable alleles that may not have been favored in the old environment.

Over successive generations, therefore, a new gene created by random mutation (change) may replace the original form of the gene if, for example, the trait encoded by that gene allows the divergent group to cope better with environmental factors, such as food sources, predators, or temperature.

The result in the long term is that deoxyribonucleic acid (DNA) changes sufficiently through the growth of divergent populations to allow new generations to become significantly different from the original population. In time, they are unable to reproduce with members of the original species and become a new species.

Galápagos Islands Case Study

Adaptive radiation occurs dramatically when a species migrates from one landmass to another. This may occur between islands or between continents and islands. A classic example of adaptive radiation is the evolution of finches noted by Charles Darwin during his trips to the Galápagos Islands off the west coast of South America.

Several species of plants and animals had migrated to these islands from the South American mainland by means of flight, wind, ocean debris, or other means of transport. Finches from the mainland—perhaps aided by winds—settled on fifteen of the islands in the Galápagos group and began to adapt to the various unoccupied ecological niches on those islands, which differed.

Over several generations, natural selection favored a variety of finch species with beaks adapted for the different types of foods available on the different islands. As a result, several species of different finches evolved, roughly simultaneously, on these islands.

Hawaiian Silversword Alliance

Although plants seem unable to "migrate" as birds and other animals do, adaptive radiation occurs in the plant world as well. In the Hawaiian Islands, for example, twenty-eight species of the Asteraceae family are known together as the Hawaiian silversword alliance. The entire group appears to be traceable to one ancestor, thought to have arrived on the island of Kauai from western North America.

The silverswords—which compose three genera, Argyroxiphium, Dubautia, and Wilkesia— have since evolved into twenty-eight species, and this speciation came about due to major ecological shifts. These plants are therefore prime examples of adaptive radiation.

Within the silversword alliance, different species have adapted to widely varying ecosystems found throughout the islands. Argyroxiphium sandwicense, for example, is endemic to the island of Maui and grows at high elevations from 6,890 to 9,843 feet (2,100 - 3,000 meters) on the dry, alpine slopes of the volcano Haleakala.

This species has succulent leaves covered with silver hairs. It is thought that the hairs lessen the pace of evaporative moisture loss and protect the leaves from the sun. In contrast, species of the genus Dubautia that grow in wet, shady forests have large leaves that lack hairs.

Despite their "customized" physiologies, the silverswords that have evolved in Hawaii are all closely related to one another, so much so that any two can hybridize. Studies of the silverswords have provided what geneticist Michael Purugganan called a "genetic snapshot of plant evolution". Adaptive radiation is one window into how new plant structures arise.
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10:48 PM
African Agriculture
African Agriculture

Soil and climatic conditions throughout Africa determine not only agricultural practices, such as which crops can be grown, but also whether plant life is capable of sustaining livestock on the land and enabling fishing of the oceans.

Rainfall—the dominant influence on agricultural output—varies greatly among Africa’s fifty-six countries. Without irrigation, agriculture requires a reliable annual rainfall of more than 30 inches (75 centimeters). Portions of Africa have serious problems from lack of rainfall, such as increasing desertification and periods of drought.

Food output has declined, with per capita food production 10 percent less in the 1990’s than it was in the 1980’s. In most African countries, however, more than 50 percent, and often 80 percent, of the population works in agriculture, mostly subsistence agriculture. Large portions of the continent, such as Mali and the Sudan, have the potential of becoming granaries to much of the continent and producing considerable food exports.


Traditional African Agriculture

Traditionally, agriculture in Africa has been subsistence farming in small plots. It has been labor-intensive, relying upon family members. New land for farming was obtained by the slash-and-burn method (shifting cultivation). The trees in a forested area would be cut down and burned where they fell.

The ashes from the burned trees fertilized the soil. Bothmen andwomenworked at such farming. Slash-and-burn agriculture is common not only in Africa but also in tropical areas around the world. In areas of heavy rainfall, the rainswash out the nutrients from soil and burned trees in a period of two to three years.

The crops grown depend upon the region. In the very dry, yet habitable, parts of Africa—such as the Sudano-Sahelian region that stretches from Senegal and Mali in the west of Africa to the Sudan in the east—a key subsistence crop is green millet, a grain. Ground into a type of flour, it can be made into a bread-like substance.

In moister areas, traditional crops are root and tuber crops, such as yams and cassava. Cassava has an outer surface or skin that is poisonous, but it can be treated to remove the poison. The tuber then can be ground and used tomake a bread-like substance.Other important traditional crops are rice and corn, which were introduced by Europeans when they came to Africa.

Animal husbandry, or seminomadic herding, is another form of traditional agriculture. Problems that have arisen with this type of agriculture are the availability of water and grass or hay for cattle. Regions that are very moist, such as the Gulf of Guinea, which has rain forest, are not good for cattle because of the tsetse fly, which carries diseases such as sleeping sickness.

Crops

Rice field in Africa
Rice field in Africa

The most widely grown crop is rice, which is grown on more than one-third of the irrigated crop area in Africa. Cultivated mostly in wetlands and valley bottoms, rice is the most common crop in the humid areas of the Gulf of Guinea and Eastern Africa. It is also grown on the plateaus of Madagascar.

In the northern and southern regions, rice represents only a small portion of the total crops under water management. Wheat and corn are cultivated and irrigated, mostly in Egypt, Morocco, South Africa, Sudan, and Somalia.

Vegetables, including root and tuber crops, are present in all regions and almost every country. Vegetables are grown on about 8 percent of the cultivated areas under water management. In Algeria, Mauritania, Kenya, Burundi, and Rwanda, they are the most widespread crops under water management.

Arboriculture (growing of fruit trees), which represents 5 percent of the total irrigated crops, is concentrated in the northern region and consists mostly of citrus fruits. Commercial crops (for cash and export) are grown mostly in the Sudan and in the countries of the southern region and consist mostly of cotton and oilseeds.

Other commercial crops in Africa are sugarcane, coffee, cocoa, oil and date palm, bananas, tobacco, and cut flowers. Sugarcane is grown in all countries except in the northern region. The other commercial crops are concentrated in a few countries.

North Africa

olives in african market
olives in african market
In Morocco, Algeria, Tunisia, Libya, and Egypt, the region’s agricultural resources are limited by its dry climate. Its products are those typical of the Mediterranean, steppe, and desert regions: wheat, barley, olives, grapes, citrus fruits, some vegetables, dates, sheep, and goats.

Agriculture employs less than 20 percent of the working population in Libya and as much as 55 percent in Egypt. From about the middle of the twentieth century, North Africa’s production failed to keep pace with its population growth and remained susceptible to large annual fluctuations.

Cropland occupies about 33 percent of Tunisia but less than 3 percent of Algeria, Egypt, and Libya. Some export crops, such as citrus fruits, tobacco, and cotton, have suffered from strong international competition.

The northern region is not a major contributor to the continent’s fish catch. Morocco, however, with its cool, plankton-rich Atlantic waters and access to the Mediterranean Sea, is one of the world’s largest fish producers.

Sudano-Sahelian Region

This region comprises Mauritania, the western Sahara, Senegal, Gambia, Mali, Burkina-Faso, Niger, Chad, and the Sudan. Because of the region’s extreme dryness, mostly subsistence farming and seminomadic herding are practiced. Millet is the primary crop.

In the late twentieth century, this region was devastated by long droughts that caused famine and starvation. Mali and the Sudan have the Niger and Nile Rivers flowing through them. These great rivers provide plenty of water for irrigation of fields.

During the rainy season in Mali—typically June through September—the Niger River widens into a great, extensive floodplain. This area is good for the growing of rice. Similarly, in the Sudan the Blue and White Niles meet at Khartoum to form the Nile River.

Gulf of Guinea

This region comprises Guinea-Bissau, Cape Verde, Guinea, Liberia, Sierra Leone, Côte d’Ivoire, Togo, Ghana, Benin, and Nigeria. With the exception of Nigeria, agriculture there is dominated by rice cultivation. The percentage of total land area that is under cultivation ranges from 60 percent in Liberia to just 9 percent in Sierra Leone.

The total cultivable area of Ghana is 39,000 square miles (100,000 square kilometers), or 42 percent of its total land area. Only 4.8 percent of the total land area was under cultivation at the end of the twentieth century. Much of the cultivation is subsistence farming of yams and other crops.

Ghana’s efforts in agriculture have been hampered by droughts. Additional problems are that organic matter has been leached out of the soils by heavy rainfall and that increasing deforestation has led to additional erosion. This is the situation in much of the Gulf of Guinea and the central regions.

About half of Nigeria’s available land is under cultivation. Increasing rainfall from the semiarid north to the tropically forested south allows for great crop diversity. Principal food crops are corn, millet, yams, sorghum, cassava, rice, potatoes, and vegetables.

Nigeria was the world’s fourth-largest exporter of cocoa beans in 1990-1991, accounting for about 7.1 percent of world trade in this commodity. However, Nigeria’s share of the world cocoa market has been substantially reduced because of aging trees, low prices, black pod disease, smuggling, and labor shortages.

Central Region

This region comprises the Central African Republic, Cameroon, Congo-Brazzaville, Congo-Kinshasa, Gabon, Equatorial Guinea, Burundi, Rwanda, and São Tomé and Príncipe. Cameroon has 14.7 million acres of arable land.

In 1997, 55,000 tons of rice were produced, but the country imported 124,000 tons in 1995. In the central region, the percentage of arable land ranges from 0.4 percent for the Congo-Brazzaville to 47 percent for Rwanda.

Cassava is harvested in Congo-Brazzaville, Congo-Kinshasa, Equatorial Guinea, and Gabon. Corn is harvested in Congo-Brazzaville, Congo-Kinshasa, and Burundi. In Rwanda, 17 percent of the harvested land is used to grow sweet potatoes. Agriculture is not important in the economy of São Tomé and Príncipe.

Eastern Region

This region comprises Eritrea, Djibouti, Ethiopia, Somalia, Kenya, Uganda, and Tanzania. Agriculture employs about 80 percent of the labor force in Uganda and Ethiopia. Approximately 2.5 million small farms dominate agriculture in both countries.

About 84 percent of Uganda’s land is suitable for agriculture—a high percentage compared to the majority of African countries, such as Ethiopia with only 12 percent. Food crops account for about 74 percent of agricultural production.

Only one-third is marketed; the rest is for home consumption. In four years out of five, the minimum needed rainfall may be expected in 78 percent of Uganda but in only 15 percent of Kenya. Somalia and Ethiopia receive almost none of the needed minimum.

Tanzania has almost four million farms. Traditional export crops include coffee, cotton, cashew nuts, tobacco, and tea. Major staple foods (corn, rice, and wheat) are exported in times of surplus.

Tanzania’s climatic growing conditions are favorable for the production of a wide range of fruits, vegetables, and flowers. Drought-resistant crops (sorghum, millet, and cassava) and other substaples such as onions, Irish potatoes, sweet potatoes, bananas, and plantains are also produced.

Areas that have 20-30 inches (50-75 centimeters) of rainfall per year rely on a mixture of agriculture and livestock herding. Regions with a smaller annual rainfall or a long dry season can support only drought-resistant crops such as sorghum, millet, and cassava. Over large areas of eastern Africa, rainfall is inadequate for crop cultivation.

The whole of Somalia and 70 percent of Kenya receive less than 20 inches (50 centimeters) of rain four years out of five. In these areas, the only feasible use of land is for raising livestock. Agriculture is not an important factor in the economies of Eritrea and Djibouti.

Southern Region

This region comprises Angola, Namibia, Zambia, Zimbabwe, Malawi, Mozambique, Botswana, Lesotho, Swaziland, and South Africa. The arable percentage of the total land area ranges from 14 percent in Malawi to just 1 percent in Namibia. With the exception of Mozambique, where cassava predominates, corn is the major crop in the countries in this region.

About 13 percent of South Africa’s land area can be used for crop production. Rainfall varies across the country, and varied climatic zones and terrains enable the production of almost any kind of crop.

The largest area of farmland is planted with corn, followed by wheat, then oats, sugarcane, and sunflowers. The nation is well known for the high quality of its fruits, such as apples and citrus.

Agriculture is the predominant economic activity in Zimbabwe, accounting for 40 percent of total export earnings—about 22 percent of the total economy—and employing more than 60 percent of the country’s labor force.

The main export crops are tobacco, cotton, and oilseeds. Zimbabwe is usually self-sufficient in food production. Its main food crops are corn, soybeans, oilseeds, fruits and vegetables, and sugar.

Mozambique’s agriculture has been badly hindered by civil war. However, the country has considerable potential for irrigation due to the Zambezi and Limpopo Rivers. The irrigation potential is estimated to be 7.5 million acres. In the 1990’s, only 110,000 acres were irrigated, growing rice, sugarcane, corn, and citrus.

Agriculture and livestock production employ about 62 percent of Botswana’s labor force. Most of the country has semidesert conditions with erratic rainfall and poor soil conditions, making it more suitable to grazing than to crop production. The principal food crops are sorghum and corn.

Namibia’s cultivated area is only 506,000 acres—only 0.8 percent of the cultivable area. Agriculture makes up approximately 10 percent of the economy but employs more than 80 percent of the population. The major irrigated crops are corn, wheat, and cotton.

Indian Ocean Islands

This region comprises Madagascar, Mauritius, the Comoros, and the Seychelles. During the 1990’s an estimated 8.7 million people lived in the rural areas, 65 percent of whom lived at the subsistence level.

Only 5.2 percent of Madagascar’s total land area (7.4 million acres) was under cultivation. Of the total land area, 50.7 percent supported livestock production, while 16 percent (1.2 million acres) of the land under cultivation was irrigated.

Cassava, planted almost everywhere on the island, is grown as well as corn and sweet potatoes, with smaller quantities of cotton, bananas, and cloves. The fisheries sector, especially the export of shrimp, has been the most rapidly growing area of the agricultural economy in the Indian Ocean Islands region.

Mauritius has 30,000 acres of sugarcane plantations that have had one of the highest sugarcane and sugar yields in the world. The Seychelles have a total land area of only 72 squaremiles (187 square kilometers), of which only 3,000 acres are cultivated.

This 3 percent of the land area accounts for only 4 percent of the island nation’s economy. The Comoros’ agriculture is heavily weighted toward rice, the staple food of the populace.

7:54 AM
African Flora
African Flora

With few exceptions, Africa’s flora (vegetation) is tropical or subtropical. This is primarily because none of the African continent extends far from the equator, and there are only a few high-elevation regions that support more temperate plants.

Listed in order of decreasing land area, the three main biomes of Africa are subtropical desert, tropical savanna, and tropical forest. The flora in southern Africa has been most studied. The flora of central and northern Africa is less known.

The subtropical desert biome is the driest of the biomes in Africa and includes some of the driest locations on earth. The largest desert region is the Sahara in northern Africa. It extends from near the west coast of Africa to the Arabian Peninsula and is part of the largest desert system in the world, which extends into south central Asia.


A smaller desert region in southern Africa includes the Namib Desert, located along the western half of southern Africa, especially near the coast, and the Kalihari Desert, which is primarily inland and east of the Namib Desert.

Where more moisture is available, grasslands predominate, and as rainfall increases, grasslands gradually become tropical savanna. The difference between a grassland and a savanna is subjective but is in part determined by tree growth, with more trees characterizing a savanna. The grassland/ tropical savanna biome forms a broad swath across much of central Africa and dominates much of eastern and southern Africa.

Tropical forests make up a much smaller area of Africa than the other two biomes. They are most abundant in the portions of central Africa not dominated by the grassland/tropical savanna biome and are not far from the coast of central West Africa. Scattered tropical forest regions also occur along major river systems of West Africa, from the equator almost to southern Africa.

Subtropical Desert

Subtropical Desert - Looking for a warm escape? Check out the beautiful desert oases of Libya.
Subtropical Desert

The subtropical deserts of Africa seem, at first, to be nearly devoid of plants. While this is true for some parts of the Sahara and Namib Deserts that are dominated by sand dunes or bare, rocky outcrops, much of the desert has a noticeable amount of plant cover.

The Sahara is characterized by widely distributed species of plants that are found in similar habitats. The deserts of southern Africa have more distinctive flora, with many species endemic to specific local areas.

Succulents of the Subtropical Desert

To survive the harsh desert climate, plants use several adaptations. Mesembryanthemum, whose species include ice plant and sea figs, is a wide-spread genus, with species occurring in all of Africa’s deserts. It typically has thick, succulent leaves.

Such succulents store water in their leaves or stems, which they retain by using a specialized type of photosynthesis. Most plants open their stomata (small openings in the leaves) during the day to get carbon dioxide from the surrounding air.

Succulents of the Subtropical Desert - Euphorbia echinus
Succulents of the Subtropical Desert, Euphorbia echinus

This would lead to high amounts of water loss in a desert environment, so succulents open their stomata at night. Through a biochemical process, they store carbon dioxide until the next day, when it is released inside the plant so photosynthesis can occur without opening the stomata.

To prevent water loss, many succulents have no leaves at all. Anabasis articulata, found in the Sahara desert, is a leafless succulent with jointed stems. Cacti are found only in North and South America, but a visitor to the Sahara would probably be fooled by certain species in the spurge family that resemble cacti.

For example, Euphorbia echinus, another Saharan plant, has succulent, ridged stems with spines. The most extreme adaptation in succulents is found in the living stones of southern Africa. Their plant body is reduced to two plump, rounded leaves that are very succulent.

They hug the ground, sometimes partially buried, and have camouflaged coloration so that they blend in with the surrounding rocks and sand, thus avoiding being eaten by grazing animals. Other succulents, such as the quiver tree, attain the size and appearance of trees.

Water-Dependent Plants of the Subtropical Desert

Water-dependent plants are confined to areas near a permanent water source, such as a spring. The most familiar of these plants is the date palm, which is a common sight at desert oases.

Tamarind and acacia are also common where water is available. A variety of different sedges and rushes occur wherever there is abundant permanent freshwater, the most famous of these being the papyrus, or bulrush.

Ephemerals of the Subtropical Desert

Ephemerals of the Subtropical Desert
Ephemerals of the Subtropical Desert
Annuals whose seeds germinate when moisture becomes available and quickly mature, set seed, and die, are called ephemerals. These plants account for a significant portion of the African desert flora.

A majority of the ephemerals are grasses. Ephemerals are entirely dependent on seasonal or sporadic rains. A few days after a significant rain the desert turns bright green, and after several more days flowers, often in profusion, appear.

Some ephemerals germinate with amazing speed, such as the pillow cushion plant, which germinates and produces actively photosynthesizing seed leaves only ten hours after being wetted. Reproductive rates for ephemerals, and even for perennial plants, are rapid. Species of morning glory can complete an entire life cycle in three to six weeks.

Tropical Savanna

Tropical savanna ranges from savanna grassland, which is dominated by tall grasses lacking trees or shrubs, to thicket and scrub communities, which are composed primarily of trees and shrubs of a fairly uniform size.

The most common type of savanna in Africa is the savanna woodland, which is composed of tall, moisture-loving grasses and tall, deciduous or semi deciduous trees that are unevenly distributed and generally well spaced. The type of savanna familiar to viewers of African wildlife documentaries is the savanna parkland, which is primarily tall grass with widely spaced trees.

Savanna Grasses and Herbs

African Savanna Grasses
African Savanna Grasses

Grasses represent the majority of plant cover beneath and between the trees. In some types of savanna, the grass can be more than 6 feet (1.8 meters) high. Although much debated, two factors seem to perpetuate the dominance of grasses: seasonal moisture with long intervening dry spells and periodic fires.

Given excess moisture and lack of fire, savannas seem inevitably to become forests. Activities by humans, such as grazing cattle or cutting trees, also perpetuate, or possibly promote, grass dominance.

A variety of herbs exist in the savanna, but they are easily overlooked, except during flowering periods. Many of them also do best just after a fire, when they are better exposed to the sun and to potential pollinators.

Plants such as hibiscus and coleus are familiar garden and house plants popular the world over. Vines related to the sweet potato are also common. Many species from the legume or pea and sunflower families are present. Wild ginger often displays its showy blossoms after a fire.

Savanna Trees and Shrubs

Savanna Trees and Shrubs
Savanna Trees and Shrubs

Trees of the African savanna often have relatively wide-spreading branches that all terminate at about the same height, giving the trees a flattopped appearance. Many are from the legume family, most notably species of Acacia, Brachystegia, Julbernardia, and Isoberlinia. With the exception of acacias, these are not well known outside Africa.

There is an especially large number of Acacia species ranging from shrubs to trees, many with spines. A few also have a symbiotic relationship with ants that protect them from herbivores. The hashab tree, a type of acacia that grows in more arid regions, is the source of gum arabic.

Although not as prominent, the baobab tree is renowned for its large size and odd appearance and occurs in many savanna regions. It has an extremely thick trunk with smooth, gray bark and can live for up to two thousand years. Many savanna trees also have showy flowers, such as the flame tree and the African tulip tree.

Tropical Forest

Tropical Forest
Tropical Forest
The primary characteristics of African tropical forests are their extremely lush growth, high species diversity, and complex structure. The diversity is often so great that a single tree species cannot be identified as dominant in an area.

Relatively large trees, such as ironwood, iroko, and sapele, predominate. Forest trees grow so close together that their crowns overlap, forming a canopy that limits the amount of light that falls beneath them. A few larger trees, called emergent trees, break out above the thick canopy.

A layer of smaller trees live beneath the main canopy. A few smaller shrubs and herbs grow near the ground level, but the majority of the herbs and other perennials are epiphytes, that is, plants that grow on other plants.

On almost every available space on the trunks and branches of the canopy trees there are epiphytes that support an entire, unique community. All this dense plant growth is supported by a monsoon climate in which 60 inches (150 centimeters) or more of rain often falls annually, most of it in the summer.

Lianas and Epiphytes

Lianas are large, woody vines that cling to trees, many of them hanging down near to the ground. They were made famous by Tarzan movies. Many lianas belong to families with well-known temperate vine species, such as the grape family, morning glory family, and cucumber family. Other, related plants remain intimately connected to the trunks of trees. One of these, the strangler fig, is a strong climber that begins life in the canopy.

The fruits are eaten by birds or monkeys, and the seeds are deposited in their feces on branches high in the canopy. The seeds germinate and send a stem downward to the ground. Once the stem reaches the ground, it roots; additional stems then develop and grow upward along the trunk of the tree.

After many years, a strangler fig can so thoroughly surround a tree that it prevents water and nutrients from flowing up the trunk. Eventually, the host tree dies and rots away, leaving a hollow tube of mostly strangler fig. Other climbers include members of the Araceae family, the most familiar being the ornamental philodendron.

The most common epiphytes are bryophytes, lower plants related to mosses, and lichens, a symbiotic combination of algae (or cyanobacteria) and fungus. The most abundant higher plants are ferns and orchids. As these plants colonize the branches of trees, they gradually trap dust and decaying materials, eventually leading to a thin soil layer that other plants can use.

Accumulations of epiphytes can be so great in some cases that tree branches break from their weight. Epiphytes are not parasites (although there are some parasitic plants that grow on tree branches); they simply use the host tree for support.

Tropical Forest Floor Plants

Grasses are almost entirely absent from the forest floor; those that grow there have much broader leaves than usual. Some forest-floor herbs are able to grow in the deep shade beneath the canopy, occasionally being so highly adapted to the low light that they can be damaged if exposed to full sunlight.

Some popular house plants have come from among these plants, because they do not need direct sunlight to survive. Still, the greatest numbers of plants occur beneath breaks in the canopy, where more light is available.
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5:25 AM
Experimental Crops
Experimental Crops

Experimental crops are foodstuffs with the potential to be grown in a sustainable manner, produce large yields, and reduce people’s reliance on the traditional crops wheat, rice, and corn.

Shifting from a hunter-gatherer society to an agrarian society led to increasingly larger-scale agricultural production that involved selecting local crops for domestication. In recent history there has been a reduction in the number of agricultural crops grown for human consumption.

There are estimated to be at least 20,000 species of edible plants on earth, out of more than 350,000 known species of higher plants. However, only a handful of crops feed most of the world’s people.


These include wheat, rice, corn, potatoes, sugar beets, sugarcane, cassava, barley, soybeans, tomatoes, and sorghum. Rice, wheat, and corn together account for a majority of calories consumed. In the effort to develop experimental crops, agricultural goals include expanding the diversity of plant food in the human diet.

Recent Successes

Soybeans (Glycine max) are a relatively newcrop that gained worldwide acceptance and widespread cultivation in the second half of the twentieth century. Originally cultivated in China, soybeans gradually spread throughout Asia and became a staple food there.

High in protein, soybeans were first grown in the Western world as animal feed. Concerted breeding efforts have resulted in many locally adapted varieties. Today, soybeans as both meal and oil are common place. Worldwide soybean production is now the greatest of any legume.

Triticale (x Triticosecale) is a hybrid created to combine the ruggedness and high protein content of rye (Secale cereale) with the high yield of wheat (Triticumaestivum). Triticale has not replaced wheat or rye in bread-making due to its rather low gluten content but is used to supplement bread flours. Triticale is also adaptable to marginal agricultural soils.

Kiwifruit (Actinidia deliciosa) is another recent success story. Apreviously little-known fruit originally called Chinese gooseberry, it was introduced to New Zealand at the turn of the twentieth century and renamed kiwifruit. The name change was a marketing strategy that led to worldwide popularity.

Kiwifruit farm
Kiwifruit farm

Today kiwifruit cultivation and consumption are increasing worldwide. Kiwifruit grows on a de- ciduous vine, much like grapes. It can be harvested and then stored for several months without loss of quality.

Grains and Cereals

Quinoa (Chenopodium quinoa) is a grain native to the Andes Mountains of South America. It has been a staple in the diets of people living in that region for centuries. Although the leaves are edible, it is principally the tiny seed which is consumed.

The seeds contain high amounts of protein, calcium, phosphorus, and the essential amino acid lysine, which is typically lacking in other cereals such as wheat, rye, and barley.

Quinoa seeds must be washed or otherwise processed to remove the bitter saponins contained in the pericarp and can then be cooked and eaten much like rice. Quinoa can also be ground into flour as a supplement for bread making. Cultivation and use of quinoa have increased steadily since the 1980’s.

Grain amaranths (Amaranth) are being rediscovered and developed as a potential new source of grain. Amaranth was a staple crop for centuries in Mexico, Central America, and South America. Amaranth is grown as an annual and yields thick, heavy seed heads containing numerous tiny seeds.

The hard seed coat is removed by heating or boiling and can be prepared much like corn. Amaranth is comparable to other grains in protein, contains high amounts of lysine, and can be consumed by those allergic to typical grains.

Breeding efforts over the last few decades involving A. hypochondriacus, A. cruentus, and A. hybridus have greatly increased seed yield as well as desirable plant growth habits. Another important characteristic is amaranth’s drought resistance.

Legumes

Legumes
Legumes
Members of the Leguminosae family are particularly valuable as food sources because they contain high levels of protein. This is in part due to their ability to fix atmospheric nitrogen in root nodules that contain nitrogen-fixing bacteria. This symbiotic relationship with the bacteria means relatively little nitrogenous fertilizer is required for agricultural production of legumes.

Tarwi (Lupinus mutabilis) is a legume native to the South American Andes that has a high protein and oil content, similar to the soybean. Tarwi is also high in the essential amino acid lysine. It grows well in poor soils and is drought-resistant. Current breeding efforts focus on reducing the bitter alkaloids, which can be removed by rinsing in water.

The winged bean (Psophocarpus tetragonolobus), native of tropical Asia, is entirely edible—leaves, flowers, seeds, pods, and tuberous roots. Like most legumes, the winged bean has a high protein content. This species could have tremendous potential in many tropical regions of the world, rivaling the success of the soybean.

A native of North America, the groundnut (Apios americana)was a major food source of many American Indian tribes. It is purported to have been offered to the Pilgrims to avert starvation. The numerous underground tubers can be prepared (cooking is necessary) like potatoes yet have a much higher protein content.

Several other legumes whose use and acceptance are likely to increase include the tepary bean (Phaseolus acutiflius), the pigeon pea (Cajanus cajan), and the bambara groundnut (Voandzeia subterranea).

Other Crops

There are many other potential food crops. Most have been cultivated on a small scale for years and are being rediscovered and researched for commercial production. Some of these include potato-like oca tubers (Oxalis tuberosa), fruits such as cherimoya (Annona cherimola), pepino (Solanum muricatum), and feijo (Acca sellowiana), and nuts such as egg nut (Couepia longipendula).
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