Membrane

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Membrane

Schematic of size-based membrane exclusion

A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Biological membranes include cell membranes (outer coverings of cells or organelles that allow passage of certain constituents); nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry (such as chemical plants).

This concept of a membrane has been known since the eighteenth century but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with microfiltration and ultrafiltration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.

The degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process.

Membrane processes classifications

Microfiltration (MF)

Main article: Microfiltration

Microfiltration removes particles higher than 0.08-2 µm and operates within a range of 7-100 kPa. Microfiltration is used to remove residual suspended solids (SS), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis.

Relatively recent developments are membrane bioreactors (MBR) which combine microfiltration and a bioreactor for biological treatment.

Ultrafiltration (UF)

Main article: Ultrafiltration

Ultrafiltration removes particles higher than 0.005-2 µm and operates within a range of 70-700kPa. Ultrafiltration is used for many of the same applications as microfiltration. Some ultrafiltration membranes have also been used to remove dissolved compounds with high molecular weight, such as proteins and carbohydrates. Also, they can remove viruses and some endotoxins.

The wall of an ultrafiltration hollow fiber membrane, with characteristic outer (top) and inner (bottom) layers of pores.

Nanofiltration (NF)

Main article: Nanofiltration

Nanofiltration is also known as “loose” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening.

Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are: (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.

Reverse osmosis (RO)

Main article: Reverse osmosis

Reverse osmosis is commonly used for desalination. As well, RO is commonly used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO excludes ions but requires high pressures to produce deionized water (850–7000 kPa). RO is the most widely used desalination technology because of its simplicity of use and relatively low energy costs compared with distillation, which uses technology based on thermal processes. Note that RO membranes remove water constituents at the ionic level. To do so, most current RO systems use a thin-film composite (TFC), mainly consisting of three layers: a polyamide layer, a polysulphone layer and a polyester layer.

Nanostructured membranes

An emerging class of membranes rely on nanostructure channels to separate materials at the molecular scale. These include carbon nanotube membranes, graphene membranes, membranes made from polymers of intrinsic microporosity (PIMS), and membranes incorporating metal–organic frameworks (MOFs). These membranes can be used for size selective separations such as nanofiltration and reverse osmosis, but also adsorption selective separations such as olefins from paraffins and alcohols from water that traditionally have required expensive and energy intensive distillation.

Membrane configurations

In the membrane field, the term module is used to describe a complete unit composed of the membranes, the pressure support structure, the feed inlet, the outlet permeate and retentate streams, and an overall support structure. The principal types of membrane modules are:

• Tubular, where membranes are placed inside a support porous tubes, and these tubes are placed together in a cylindrical shell to form the unit module. Tubular devices are primarily used in micro- and ultrafiltration applications because of their ability to handle process streams with high solids and high viscosity properties, as well as for their relative ease of cleaning.

• Hollow fiber membrane, consists of a bundle of hundreds to thousands of hollow fibers. The entire assembly is inserted into a pressure vessel. The feed can be applied to the inside of the fiber (inside-out flow) or the outside of the fiber (outside-in flow).

• Spiral wound, where a flexible permeate spacer is placed between two flat membranes sheet. A flexible feed spacer is added and the flat sheets are rolled into a circular configuration.

• Plate and frame consist of a series of flat membrane sheets and support plates. The water to be treated passes between the membranes of two adjacent membrane assemblies. The plate supports the membranes and provides a channel for the permeate to flow out of the unit module.

• Ceramic and polymeric flat sheet membranes and modules. Flat sheet membranes are typically built-into submerged vacuum-driven filtration systems which consist of stacks of modules each with several sheets. Filtration mode is outside-in where the water passes through the membrane and is collected in permeate channels. Cleaning can be performed by aeration, backwash and CIP.

Membrane process operation

The key elements of any membrane process relate to the influence of the following parameters on the overall permeate flux are:

• The membrane permeability (k)
• The operational driving force per unit membrane area (Trans Membrane Pressure, TMP)
• The fouling and subsequent cleaning of the membrane surface.

Flux, pressure, permeability

The total permeate flow from a membrane system is given by following equation:

${\displaystyle Q_p=F_w\cdot A}$

Where Qp is the permeate stream flowrate [kg·s⁻¹], F_w is the water flux rate [kg·m⁻²·s⁻¹] and A is the membrane area [m²]

The permeability (k) [m·s⁻²·bar⁻¹] of a membrane is given by the next equation:

${\displaystyle k=\frac{F_w}{P_{TMP}}}$

The trans-membrane pressure (TMP) is given by the following expression:

${\displaystyle P_{TMP}=\frac{(P_f+P_c)}{2}-P_p}$

where P_TMP is the trans-membrane pressure [kPa], P_f the inlet pressure of feed stream [kPa]; P_c the pressure of concentrate stream [kPa]; P_p the pressure if permeate stream [kPa].

The rejection (r) could be defined as the number of particles that have been removed from the feedwater.

${\displaystyle r=\frac{(C_f-C_p)}{C_f}\cdot 100}$

The corresponding mass balance equations are:

${\displaystyle Q_f=Q_p+Q_c}$

${\displaystyle Q_f\cdot C_f=Q_p\cdot C_p+Q_c\cdot C_c}$

To control the operation of a membrane process, two modes, concerning the flux and the TMP, can be used. These modes are (1) constant TMP, and (2) constant flux.

The operation modes will be affected when the rejected materials and particles in the retentate tend to accumulate in the membrane. At a given TMP, the flux of water through the membrane will decrease and at a given flux, the TMP will increase, reducing the permeability (k). This phenomenon is known as fouling, and it is the main limitation to membrane process operation.

Dead-end and cross-flow operation modes

Two operation modes for membranes can be used. These modes are:

• Dead-end filtration where all the feed applied to the membrane passes through it, obtaining a permeate. Since there is no concentrate stream, all the particles are retained in the membrane. Raw feed-water is sometimes used to flush the accumulated material from the membrane surface.
• Cross-flow filtration where the feed water is pumped with a cross-flow tangential to the membrane and concentrate and permeate streams are obtained. This model implies that for a flow of feed-water across the membrane, only a fraction is converted to permeate product. This parameter is termed "conversion" or "recovery" (S). The recovery will be reduced if the permeate is further used for maintaining processes operation, usually for membrane cleaning.

${\displaystyle S=\frac{Q_{permeate}}{Q_{feed}}=1-\frac{Q_{concentrate}}{Q_{feed}}}$

Filtration leads to an increase in the resistance against the flow. In the case of the dead-end filtration process, the resistance increases according to the thickness of the cake formed on the membrane. As a consequence, the permeability (k) and the flux rapidly decrease, proportionally to the solids concentration and, thus, requiring periodic cleaning.

For cross-flow processes, the deposition of material will continue until the forces of the binding cake to the membrane will be balanced by the forces of the fluid. At this point, cross-flow filtration will reach a steady-state condition, and thus, the flux will remain constant with time. Therefore, this configuration will demand less periodic cleaning.

Fouling

Main article: Membrane fouling

Fouling can be defined as the potential deposition and accumulation of constituents in the feed stream on the membrane. The loss of RO performance can result from irreversible organic and/or inorganic fouling and chemical degradation of the active membrane layer. Microbiological fouling, generally defined as the consequence of irreversible attachment and growth of bacterial cells on the membrane, is also a common reason for discarding old membranes. A variety of oxidative solutions, cleaning and anti-fouling agents is widely used in desalination plants, and their repetitive and incidental exposure can adversely affect the membranes, generally through the decrease of their rejection efficiencies.

Fouling can take place through several physicochemical and biological mechanisms which are related to the increased deposition of solid material onto the membrane surface. The main mechanisms by which fouling can occur, are:

• Build-up of constituents of the feedwater on the membrane which causes a resistance to flow. This build-up can be divided into different types:

Pore narrowing, which consists of solid material that it has been attached to the interior surface of the pores.

Pore blocking occurs when the particles of the feed-water become stuck in the pores of the membrane.

Gel/cake layer formation takes places when the solid matter in the feed is larger than the pore sizes of the membrane.

• Formation of chemical precipitates known as scaling
• Colonization of the membrane or biofouling takes place when microorganisms grow on the membrane surface.

Fouling control and mitigation

Since fouling is an important consideration in the design and operation of membrane systems, as it affects pre-treatment needs, cleaning requirements, operating conditions, cost and performance, it should prevent, and if necessary, removed. Optimizing the operation conditions is important to prevent fouling. However, if fouling has already taken place, it should be removed by using physical or chemical cleaning.

Physical cleaning techniques for membrane include membrane relaxation and membrane backwashing.

• Back-washing or back-flushing consists of pumping the permeate in the reverse direction through the membrane. Back-washing removes successfully most of the reversible fouling caused by pore blocking. Backwashing can also be enhanced by flushing air through the membrane. Backwashing increase the operating costs since energy is required to achieve a pressure suitable for permeate flow reversion.

• Membrane relaxation consists of pausing the filtration during a period, and thus, there is no need for permeate flow reversion. Relaxation allows filtration to be maintained for a longer period before the chemical cleaning of the membrane.

• Back pulsing high frequency back pulsing resulting in efficient removal of dirt layer. This method is most commonly used for ceramic membranes 

Recent studies have assessed to combine relaxation and backwashing for optimum results,.

Chemical cleaning. Relaxation and backwashing effectiveness will decrease with operation time as more irreversible fouling accumulates on the membrane surface. Therefore, besides the physical cleaning, chemical cleaning may also be recommended. It includes:

• Chemical enhanced backwash, that is, a low concentration of chemical cleaning agent is added during the backwashing period.
• Chemical cleaning, where the main cleaning agents are sodium hypochlorite (for organic fouling) and citric acid (for inorganic fouling). Every membrane supplier proposes their chemical cleaning recipes, which differ mainly in terms of concentration and methods.

Optimizing the operation condition. Several mechanisms can be carried out to optimize the operating conditions of the membrane to prevent fouling, for instance:

• Reducing flux. The flux always reduces fouling but it impacts on capital cost since it demands more membrane area. It consists of working at sustainable flux which can be defined as the flux for which the TMP increases gradually at an acceptable rate, such that chemical cleaning is not necessary.
• Using cross-flow filtration instead of dead-end. In cross-flow filtration, only a thin layer is deposited on the membrane since not all the particles are retained on the membrane, but the concentrate removes them.
• Pre-treatment of the feed water is used to reduce the suspended solids and bacterial content of the feed-water. Flocculants and coagulants are also used, like ferric chloride and aluminium sulphate that, once dissolved in the water, adsorbs materials such as suspended solids, colloids and soluble organic. Metaphysical numerical models have been introduced in order to optimize transport phenomena 

Membrane alteration. Recent efforts have focused on eliminating membrane fouling by altering the surface chemistry of the membrane material to reduce the likelihood that foulants will adhere to the membrane surface. The exact chemical strategy used is dependent on the chemistry of the solution that is being filtered. For example, membranes used in desalination might be made hydrophobic to resist fouling via accumulation of minerals, while membranes used for biologics might be made hydrophilic to reduce protein/organic accumulation. Modification of surface chemistry via thin film deposition can thereby largely reduce fouling. One drawback to using modification techniques is that, in some cases, the flux rate and selectivity of the membrane process can be negatively impacted.

Recycling of RO membranes

Waste prevention

Main article: Waste prevention

Once the membrane reaches a significant performance decline it is discarded. Discarded RO membrane modules are currently classified worldwide as inert solid waste and are often disposed of in landfills; although they can also be energetically recovered. However, various efforts have been made over the past decades to avoid this, such as waste prevention, direct reapplication, and ways of recycling. In this regard, membranes also follows the waste management hierarchy. This means that the most preferable action is to upgrade the design of the membrane which leads to a reduction in use at same application and the least preferred action is a disposal and landfilling 

RO membranes have some environmental challenges that must be resolved in order to comply with the circular economy principles. Mainly they have a short service life of 5–10 years. Over the past two decades, the number of RO desalination plants has increased by 70%. The size of these RO plants has also increased significantly, with some reaching a production capacity exceeding 600,000 m3 of water per day. This means a generation of 14,000 tonnes of membrane waste that is landfilled every year. To increment the lifespan of a membrane, different prevention methods are developed: combining the RO process with the pre-treatment process to improve efficiency; developing anti-fouling techniques; and developing suitable procedures for cleaning the membranes. Pre-treatment processes lower the operating costs because of lesser amounts of chemical additives in the saltwater feed and the lower operational maintenance required for the RO system.

Four types of fouling are found on RO membranes: (i) Inorganic (salt precipitation), (ii) Organic, (iii) Colloidal (particle deposition in the suspension) (iv) Microbiological (bacteria and fungi). Thereby, an appropriate combination of pre-treatment procedures and chemical dosing, as well as an efficient cleaning plan that tackle these types of fouling, should enable the development of an effective anti-fouling technique.

Most plants clean their membranes every week (CEB – Chemically Enhanced Backwash). In addition to this maintenance cleaning, an intensive cleaning (CIP) is recommended, from two to four times annually.

Reuse

Main article: Reuse

Reuse of RO membranes include the direct reapplication of modules in other separation processes with less stringent specifications. The conversion from the RO TFC membrane to a porous membrane is possible by degrading the dense layer of polyamide. Converting RO membranes by chemical treatment with different oxidizing solutions are aimed at removing the active layer of the polyamide membrane, intended for reuse in applications such as MF or UF. This causes an extended life of approximately two years. A very limited number of reports have mentioned the potential of direct RO reuse. Studies shows that hydraulic permeability, salt rejection, morphological and topographical characteristics, and field emission scanning electron and atomic force microscopy were used in an autopsy investigation conducted. The old RO element's performance resembled that of nanofiltration (NF) membranes, thus it was not surprising to see the permeability increase from 1.0 to 2.1 L m-2 h-1 bar-1 and the drop in NaCl rejection from >90% to 35-50%.

On the other hand, In order to maximize the overall efficiency of the process, it has lately been common practice to combine RO elements of varying performances within the same pressure vessel, which is called Multi-membrane vessel design. In principle, this innovative hybrid system recommends using high rejection, low productivity membranes in the upstream segment of the filtration train, followed by high productivity, low energy membranes in the downstream section. There are two ways in which this design can help: either by decreasing energy use due to decreased pressure needs or by increasing output. Since this concept would reduce the number of modules and pressure vessels needed for a given application, it has the potential to significantly reduce initial investment costs. It is proposed to adapt this original concept, by internally reusing older RO membranes within the same pressure vessel.

Recycle

Main article: Recycling

Recycling of materials is a general term that involves physically transforming the material or its components so that they can be regenerated into other useful products. The membrane modules are complex structures, consisting of a number of different polymeric components and, potentially, the individual components can be recovered for other purposes. Plastic solid waste treatment and recycling can be separated into mechanical recycling, chemical recycling and energy recovery.

Mechanical recycling characteristics:

• A first separation of the components of interest is needed.
• Previous washing to avoid property deterioration during the process.
• Grinding of the polymeric materials into suitable size (loss of 5% of the material).
• Possible posterior washing.
• Melting and extrusion process (loss of 10 % of material).
• Membrane components than can be recycled (thermoplastics): PP, polyester, etc.
• Membrane sheets: constructed from a number of different polymers and additives and therefore inherently difficult to accurately and efficiently separate.
• Main advantage: it displaces virgin plastic production. • Main disadvantages: need to separate all components, large-enough amount of components to be viable.

Chemical recycling characteristics:

• Break down the polymers into smaller molecules, using depolymerisation and degradation techniques.
• Cannot be used with contaminated materials.
• Chemical recycling processes are tailored for specific materials.
• Advantage: that heterogeneous polymers with limited use of pre-treatment can be processed.
• Disadvantage: more expensive and complex than mechanical recycling.
• Polyester materials (such as in the permeate spacer and components of the membrane sheet) are suitable for chemical recycling processes, and hydrolysis is used to reverse the poly-condensation reaction used to make the polymer, with the addition of water to cause decomposition.

Energetic recovery characteristics:

• Volume reduction by 90–99%, reducing the strain on landfill.
• Waste incinerators can generally operate from 760 °C to 1100 °C and would therefore be capable of removing all combustible material, with the exception of the residual inorganic filler in the fiberglass casing.
• Heat energy can be recovered and used for electricity generation or other heat related processes, and can also offset the greenhouse gas emissions from traditional energy.
• If not properly controlled, can emit greenhouse gases as well as other harmful products.

Applications

Distinct features of membranes are responsible for the interest in using them as additional unit operation for separation processes in fluid processes. Some advantages noted include:

• Less energy-intensive, since they do not require major phase changes
• Do not demand adsorbents or solvents, which may be expensive or difficult to handle
• Equipment simplicity and modularity, which facilitates the incorporation of more efficient membranes

Membranes are used with pressure as the driving processes in membrane filtration of solutes and in reverse osmosis. In dialysis and pervaporation the chemical potential along a concentration gradient is the driving force. Also perstraction as a membrane assisted extraction process relies on the gradient in chemical potential. A submerged flexible mound breakwater as a type of using membrane can be employed for wave control in shallow water as an advanced alternative to the conventional rigid submerged designs.

However, their overwhelming success in biological systems is not matched by their application. The main reasons for this are:

• Fouling – the decrease of function with use
• Prohibitive cost per membrane area
• Lack of solvent resistant materials
• Scale-up risks

Columbian exchange

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Columbian_exchange 

 

New World native plants Clockwise, from top left:
1. Maize (Zea mays);
2. Tomato (Solanum lycopersicum);
3. Potato (Solanum tuberosum);
4. Vanilla (Vanilla);
5. Pará rubber tree (Hevea brasiliensis);
6. Cacao (Theobroma cacao);
7. Tobacco (Nicotiana rustica)

 

Old World native plants Clockwise, from top left:
1. Citrus (Rutaceae);
2. Apple (Malus domestica);
3. Banana (Musa);
4. Mango (Mangifera);
5. Onion (Allium);
6. Coffee (Coffea);
7. Wheat (Triticum spp.);
8. Rice (Oryza sativa)

The Columbian exchange, also known as the Columbian interchange, was the widespread transfer of plants, animals, precious metals, commodities, culture, human populations, technology, diseases, and ideas between the New World (the Americas) in the Western Hemisphere, and the Old World (Afro-Eurasia) in the Eastern Hemisphere, in the late 15th and following centuries. It is named after the Italian explorer Christopher Columbus and is related to the European colonization and global trade following his 1492 voyage. Some of the exchanges were purposeful; some were accidental or unintended. Communicable diseases of Old World origin resulted in an 80 to 95 percent reduction in the number of Indigenous peoples of the Americas from the 15th century onwards, most severely in the Caribbean. The cultures of both hemispheres were significantly impacted by the migration of people (both free and enslaved) from the Old World to the New. European colonists and African slaves replaced Indigenous populations across the Americas, to varying degrees. The number of Africans taken to the New World was far greater than the number of Europeans moving to the New World in the first three centuries after Columbus.

The new contacts among the global population resulted in the interchange of a wide variety of crops and livestock, which supported increases in food production and population in the Old World. American crops such as maize, potatoes, tomatoes, tobacco, cassava, sweet potatoes, and chili peppers became important crops around the world. Old World rice, wheat, sugar cane, and livestock, among other crops, became important in the New World. American-produced silver flooded the world and became the standard metal used in coinage, especially in Imperial China.

The term was first used in 1972 by the American historian and professor Alfred W. Crosby in his environmental history book The Columbian Exchange. It was rapidly adopted by other historians and journalists.

Etymology

In 1972, Alfred W. Crosby, an American historian at the University of Texas at Austin, published the book The Columbian Exchange, and subsequent volumes within the same decade. His primary focus was mapping the biological and cultural transfers that occurred between the Old and New Worlds. He studied the effects of Columbus's voyages between the two – specifically, the global diffusion of crops, seeds, and plants from the New World to the Old, which radically transformed agriculture in both regions. His research made a lasting contribution to the way scholars understand the variety of contemporary ecosystems that arose due to these transfers. The term has become popular among historians and journalists and has since been enhanced with Crosby's later book in three editions, Ecological Imperialism: The Biological Expansion of Europe, 900–1900. Charles C. Mann, in his book 1493 further expands and updates Crosby's original research.

Background

The weight of scientific evidence is that humans first came to the New World from Siberia thousands of years ago. There is little additional evidence of contacts between the peoples of the Old World and those of the New World, although the literature speculating on pre-Columbian trans-oceanic journeys is extensive. The first inhabitants of the New World brought with them domestic dogs and, possibly, a container, the calabash, both of which persisted in their new home. The medieval explorations, visits, and brief residence of the Norsemen in Greenland, Newfoundland, and Vinland in the late 10th century and 11th century had no known impact on the Americas. Many scientists accept that possible contact between Polynesians and coastal peoples in South America around the year 1200 resulted in genetic similarities and the adoption by Polynesians of an American crop, the sweet potato. However, it was only with the first voyage of the Italian explorer Christopher Columbus and his crew to the Americas in 1492 that the Columbian exchange began, resulting in major transformations in the cultures and livelihoods of the peoples in both hemispheres.

Diseases

Further information: Native American disease and epidemics, Influx of disease in the Caribbean, Virgin soil epidemic, and Cocoliztli epidemics

Sixteenth-century Aztec drawings of victims of smallpox

The first manifestation of the Columbian exchange may have been the spread of syphilis from the native people of the Caribbean Sea to Europe. The history of syphilis has been well-studied, but the origin of the disease remains a subject of debate. There are two primary hypotheses: one proposes that syphilis was carried to Europe from the Americas by the crew of Christopher Columbus in the early 1490s, while the other proposes that syphilis previously existed in Europe but went unrecognized. The first written descriptions of the disease in the Old World came in 1493. The first large outbreak of syphilis in Europe occurred in 1494–1495 among the army of Charles VIII during its invasion of Naples. Many of the crew members who had served with Columbus had joined this army. After the victory, Charles's largely mercenary army returned to their respective homes, thereby spreading "the Great Pox" across Europe and killing up to five million people.

The Columbian exchange of diseases in the other direction was by far deadlier. The peoples of the Americas had had no contact to European and African diseases and little or no immunity. An epidemic of swine influenza beginning in 1493 killed many of the Taino people inhabiting Caribbean islands. The pre-contact population of the island of Hispaniola was probably at least 500,000, but by 1526, fewer than 500 were still alive. Spanish exploitation was part of the cause of the near-extinction of the native people. In 1518, smallpox was first recorded in the Americas and became the deadliest imported European disease. Forty percent of the 200,000 people living in the Aztec capital of Tenochtitlan, later Mexico City, are estimated to have died of smallpox in 1520 during the war of the Aztecs with conquistador Hernán Cortés. Epidemics, possibly of smallpox and spread from Central America, decimated the population of the Inca Empire a few years before the arrival of the Spanish. The ravages of European diseases and Spanish exploitation reduced the Mexican population from an estimated 20 million to barely more than a million in the 16th century. The indigenous population of Peru decreased from about 9 million in the pre-Columbian era to 600,000 in 1620. Scholars Nunn and Qian estimate that 80–95 percent of the Native American population died in epidemics within the first 100–150 years following 1492. Nunn and Qian also refer to the calculations of the scientist David Cook, in which in some cases no one survived due to diseases. The deadliest Old World diseases in the Americas were smallpox, measles, whooping cough, chicken pox, bubonic plague, typhus, and malaria.

Enslavement of Africans

Depiction of slaves working at a plantation in Virginia, 1670

The Atlantic slave trade consisted of the involuntary immigration of 11.7 million Africans, primarily from West Africa, to the Americas between the 16th and 19th centuries, far outnumbering the about 3.4 million Europeans who migrated, most voluntarily, to the New World between 1492 and 1840. The prevalence of African slaves in the New World was related to the demographic decline of New World peoples and the need of European colonists for labor. According to Nunn and Qian, another reason for the demand for slaves was the cultivation of special crops (for example, sugar cane), which were suitable for the climatic conditions of the new lands and were also very popular. The Africans had greater immunities to Old World diseases than the New World peoples, and were less likely to die from disease. The journey of enslaved Africans from Africa to America is commonly known as the "middle passage".nslaved Africans helped shape an emerging African-American culture in the New World. They participated in both skilled and unskilled labor. For example, according to the work of James L. Watson, slaves were involved in handicraft production. However, they could also work not only as ordinary workers, but also as managers of small enterprises in the commercial or industrial sphere. Their descendants gradually developed an ethnicity that drew from the numerous African tribes as well as European nationalities. The descendants of African slaves make up a majority of the population in some Caribbean countries, notably Haiti and Jamaica, and a sizeable minority in most American countries.

According to the scholar Jean MacGregor, in the New World there was denial and rebellion against slavery. The rebels went into the rainforests, fought the colonizers, refusing to stop until the signing of the agreement, or gathered in settlements. Conscious preservation of ethnic characteristics was also considered an unspoken manifestation of resistance. This applied to both men and women, but women had a greater influence on the transmission of ethnic culture to future generations as they were engaged in the upbringing of children. 

A movement for the abolition of slavery, known as abolitionism, developed in Europe and the Americas during the 18th century. The efforts of abolitionists eventually led to the abolition of slavery (the British Empire in 1833, the United States in 1865, and Brazil in 1888).

Silver

A figurine featuring the New World's independently invented wheel. Among the places where wheeled toys were found, Mesoamerica is the only one where the wheel was never put to practical use before the 16th century.

The New World produced 80 percent or more of the world's silver in the 16th and 17th centuries, most of it at Potosí in Bolivia, but also in Mexico. The founding of the city of Manila in the Philippines in 1571 for the purpose of facilitating trade in New World silver with China for silk, porcelain, and other luxury products has been called by scholars the "origin of world trade." China was the world's largest economy and in the 1570s adopted silver (which it did not produce in any quantity) as its medium of exchange. China had little interest in buying foreign products so trade consisted of large quantities of silver coming into China to pay for the Chinese products that foreign countries desired. Silver made it to Manila either through Europe and by ship around the Cape of Good Hope or across the Pacific Ocean in Spanish galleons from the Mexican port of Acapulco. From Manila, the silver was transported onward to China on Portuguese and later Dutch ships. Silver was also smuggled from Potosi to Buenos Aires, Argentina to pay slavers for African slaves imported into the New World.

The enormous quantities of silver imported into Spain and China created vast wealth but also caused inflation and the value of silver to decline. In 16th century China, six ounces of silver was equal to the value of one ounce of gold. In 1635, it took 13 ounces of silver to equal in value one ounce of gold. Taxes in both countries were assessed in the weight of silver, not its value. The shortage of revenue due to the decline in the value of silver may have contributed indirectly to the fall of the Ming dynasty in 1644. Likewise, silver from the Americas financed Spain's attempt to conquer other countries in Europe, and the decline in the value of silver left Spain faltering in the maintenance of its world-wide empire and retreating from its aggressive policies in Europe after 1650.

The wheel

Although large-scale use of wheels did not occur in the Americas prior to European contact, numerous small wheeled artifacts, identified as children's toys, have been found in Mexican archeological sites, some dating to approximately 1500 BC. Some argue that the primary obstacle to large-scale development of the wheel in the Americas was the absence of domesticated large animals that could be used to pull wheeled carriages. The closest relative of cattle present in Americas in pre-Columbian times, the American bison, is difficult to domesticate and was never domesticated by Native Americans; several horse species existed until about 12,000 years ago, but ultimately became extinct. The only large animal that was domesticated in the Western hemisphere, the llama, a pack animal, was not physically suited to use as a draft animal to pull wheeled vehicles, and use of the llama did not spread far beyond the Andes by the time of the arrival of Europeans.

On the other hand, Mesoamericans never developed the wheelbarrow, the potter's wheel, nor any other practical object with a wheel or wheels. Although present in a number of toys, very similar to those found throughout the world and still made for children today ("pull toys"), the wheel was never put into practical use in Mesoamerica before the 16th century. Possibly the closest New World civilizations came to the utilitarian wheel is the spindle whorl, and some scholars believe that the Mayan toys were originally made with spindle whorls and spindle sticks as "wheels" and "axes".

Effects

Crops

Inca-era terraces on Taquile are used to grow traditional Andean staples such as quinoa and potatoes, alongside wheat—a European introduction.

Because of the new trading resulting from the Columbian exchange, several plants native to the Americas have spread around the world, including potatoes, maize, tomatoes, and tobacco. Before 1500, potatoes were not grown outside of South America. By the 18th century, they were cultivated and consumed widely in Europe and had become important crops in both India and North America. Potatoes, a highly caloric crop, eventually became an important staple of the diet in much of Europe, contributing to an estimated 25% of the population growth in Afro-Eurasia between 1700 and 1900. Many European rulers, including Frederick the Great of Prussia and Catherine the Great of Russia, encouraged the cultivation of the potato. According to scholars Nunn and Qian, despite the absence of some vitamins (D, A), potatoes were in the diet of many people around the world. They also claim that the introduction of potatoes has a positive effect on the level of population and the level of urbanization, thereby explaining 47 percent of the increase in urbanization and 12 percent of the population growth.

Maize and cassava, introduced by the Portuguese from South America in the 16th century, gradually replaced sorghum and millet as Africa's most important food crops. Spanish colonizers of the 16th-century introduced new staple crops to Asia from the Americas, including maize and sweet potatoes, and thereby contributed to population growth in Asia. On a larger scale, the introduction of potatoes and maize to the Old World "resulted in caloric and nutritional improvements over previously existing staples" throughout the Eurasian landmass, enabling more varied and abundant food production. Nunn and Qian mention that despite the popularity of maize, cassava was in demand in the Old World more. These crops also have their negative consequences when overused (pellagra and konzo), but this does not reduce the importance of maize and cassava for human nutrition.

Tomatoes, which came to Europe from the New World via Spain, were initially prized in Italy mainly for their ornamental value. But starting in the 19th century, tomato sauces became typical of Neapolitan cuisine and, ultimately, Italian cuisine in general. According to scientists Nunn & Qian, the benefits of the discovery of America brought the Old World not only new unexplored plant species, but also a favorable soil for growing their highly valued crops (sugarcane and coffee). Sugar was very important for people, as it is high-calorie and inexpensive. In addition, it was actively used in cooking and, according to Hersh and Voth (2009), increased the well-being of residents by 8 percent. Coffee (introduced in the Americas circa 1720) from Africa and the Middle East and sugarcane (introduced from the Indian subcontinent) from the Spanish West Indies became the main export commodity crops of extensive Latin American plantations. Introduced to India by the Portuguese, chili and potatoes from South America have become an integral part of their cuisine. Scientists Nunn and Qian also note that the discovery and spread of chili peppers contributed to the creation of many famous dishes: Korean kimchi, Indian curry and etc. Speaking about chili pepper, scientists have written about the properties of capsicum, which contains many useful vitamins and minerals. Capsaicin contained in capsicum helps better digestion and is currently used in medicine.

Because crops traveled but often their endemic fungi did not, for a limited time yields were higher in their new lands. Dark & Gent 2001 term this the "Yield honeymoon". However, as globalization has continued the Columbian Exchange of pathogens has continued and crops have declined back toward their endemic yields – the honeymoon is ending.

Nunn and Qian also mention chocolate and vanilla in their work. The Spaniards were the first to grow cacao in 1590. In addition to the fact that cacao was consumed in the form of sweets, and it was expensive at first, cacao helped to cope with fatigue and add strength and energy. As for vanilla, the pods of the plant after chemical treatment acquired an aroma, which was then used both in cooking and in perfumery.

Rice

See also: Rice production in the United States

Rice was another crop that became widely cultivated during the Columbian exchange. As the demand in the New World grew, so did the knowledge of how to cultivate it. The two primary species used were Oryza glaberrima and Oryza sativa, originating from West Africa and Southeast Asia, respectively. European planters in the New World relied upon the skills of African slaves to cultivate both species. Georgia, South Carolina, Cuba and Puerto Rico were major centers of rice production during the colonial era. Enslaved Africans brought their knowledge of water control, milling, winnowing, and other agrarian practices to the fields. This widespread knowledge among African slaves eventually led to rice becoming a staple dietary item in the New World.

Fruits

Citrus fruits and grapes were brought to the Americas from the Mediterranean. At first planters struggled to adapt these crops to the climates in the New World, but by the late 19th century they were cultivated more consistently.

Bananas were introduced into the Americas in the 16th century by Portuguese sailors who came across the fruits in West Africa, while engaged in commercial ventures and the slave trade. Bananas were consumed in minimal amounts in the Americas as late as the 1880s. The U.S. did not see major increases in banana consumption until large plantations were established in the Caribbean.

Tomatoes

It took three centuries after their introduction in Europe for tomatoes to become a widely accepted food item. Nunn & Qian claim that, according to experts, long before the arrival of the Spaniards, wild tomatoes came from the north to South America, thereby initiating the cultivation of tomatoes in different parts of America. Tobacco, potatoes, chili peppers, tomatillos, and tomatoes are all members of the nightshade family. Similar to some European nightshade varieties, tomatoes and potatoes can be harmful or even lethal if the wrong part of the plant is consumed in excess. Physicians in the 16th century had good reason to suspect that this native Mexican fruit was poisonous; they suspected it of generating "melancholic humours".

In 1544, Pietro Andrea Mattioli, a Tuscan physician and botanist, suggested that tomatoes might be edible, but no record exists of anyone consuming them at this time. However, in 1592 the head gardener at the botanical garden of Aranjuez near Madrid, under the patronage of Philip II of Spain, wrote, "it is said [tomatoes] are good for sauces". In spite of these comments, tomatoes remained exotic plants grown for ornamental purposes, but rarely for culinary use. On October 31, 1548, the tomato was given its first name anywhere in Europe when a house steward of Cosimo I de' Medici, Duke of Florence, wrote to the Medici's private secretary that the basket of pomi d'oro "had arrived safely". At this time, the label pomi d'oro was also used to refer to figs, melons, and citrus fruits in treatises by scientists. In the early years, tomatoes were mainly grown as ornamentals in Italy. For example, the Florentine aristocrat Giovan Vettorio Soderini wrote that they "were to be sought only for their beauty" and were grown only in gardens or flower beds. Tomatoes were grown in elite town and country gardens in the fifty years or so following their arrival in Europe, and were only occasionally depicted in works of art. According to scholars Nunn and Qian, tomatoes are very useful, as they contain important vitamins (C and A). Even despite the small number of calories, tomatoes are a regular product among Westerners. The first Italian cookbook to include tomato sauce, Lo Scalco alla Moderna ('The Modern Steward'), was written by Italian chef Antonio Latini and was published in two volumes in 1692 and 1694. The use of tomato sauce with pasta appeared for the first time in 1790 in the Italian cookbook L'Apicio Moderno ('The Modern Apicius'), by chef Francesco Leonardi. Today around 32,000 acres (13,000 ha) of tomatoes are cultivated in Italy.

Nunn and Qian mention that the most problematic thing about eating tomatoes was their storage conditions. In hot weather, tomatoes deteriorated in just a few days. Such a process as canning solved this problem by allowing the product to be stored for months. Although canning was expensive in the beginning, after the introduction of mechanical labor, tomatoes became available to everyone.

Nunn and Qian also point to medical studies, according to which canned tomatoes contain a useful antioxidant (lycopene), which reduces the risk of developing serious diseases.

Livestock

Further information: Plains Indians § Horses

Native Americans learned to use horses to chase bison, dramatically expanding their hunting range.

Initially at least, the Columbian exchange of animals largely went in one direction, from Europe to the New World, as the Eurasian regions had domesticated many more animals. Horses, donkeys, mules, pigs, cattle, sheep, goats, chickens, large dogs, cats, and bees were rapidly adopted by native peoples for transport, food, and other uses. One of the first European exports to the Americas, the horse, changed the lives of many Native American tribes. The mountain tribes shifted to a nomadic lifestyle, based on hunting bison on horseback. They largely gave up settled agriculture. Horse culture was adopted gradually by Great Plains Indians. The existing Plains tribes expanded their territories with horses, and the animals were considered so valuable that horse herds became a measure of wealth. (This transfer reintroduced horses to the Americas, as the species had died out there prior to the development of the modern horse in Eurasia.)

While mesoamerican peoples (Mayas in particular) already practiced apiculture, producing wax and honey from a variety of bees (such as Melipona or Trigona), European bees (Apis mellifera)—more productive, delivering a honey with less water content and allowing for an easier extraction from beehives—were introduced in New Spain, becoming an important part of farming production.

The effects of the introduction of European livestock on the environments and peoples of the New World were not always positive. In the Caribbean, the proliferation of European animals consumed native fauna and undergrowth, changing habitat. If free ranging, the animals often damaged conucos, plots managed by indigenous peoples for subsistence.

The Mapuche of Araucanía were fast to adopt the horse from the Spanish, and improve their military capabilities as they fought the Arauco War against Spanish colonizers. Until the arrival of the Spanish, the Mapuches had largely maintained chilihueques (llamas) as livestock. The Spanish introduction of sheep caused some competition between the two domesticated species. Anecdotal evidence of the mid-17th century show that by then both species coexisted but that the sheep far outnumbered the llamas. The decline of llamas reached a point in the late 18th century when only the Mapuche from Mariquina and Huequén next to Angol raised the animal. In the Chiloé Archipelago the introduction of pigs by the Spanish proved a success. They could feed on the abundant shellfish and algae exposed by the large tides.

In the other direction, the turkey, guinea pig, and Muscovy duck were New World animals that were transferred to Europe.

Medicines

European exploration of tropical areas was aided by the New World discovery of quinine, the first effective treatment for malaria. According to scientists Nunn and Qian, cinchona trees from the Andes were processed and quinine was obtained from their bark. Europeans suffered from this disease, but some indigenous populations had developed at least partial resistance to it. In Africa, resistance to malaria has been associated with other genetic changes among sub-Saharan Africans and their descendants, which can cause sickle-cell disease. The resistance of sub-Saharan Africans to malaria in the southern United States and the Caribbean contributed greatly to the specific character of the Africa-sourced slavery in those regions.

Similarly, yellow fever is thought to have been brought to the Americas from Africa via the Atlantic slave trade. Because it was endemic in Africa, many people there had acquired immunity. Europeans suffered higher rates of death than did African-descended persons when exposed to yellow fever in Africa and the Americas, where numerous epidemics swept the colonies beginning in the 17th century and continuing into the late 19th century. The disease caused widespread fatalities in the Caribbean during the heyday of slave-based sugar plantation. The replacement of native forests by sugar plantations and factories facilitated its spread in the tropical area by reducing the number of potential natural mosquito predators. The means of yellow fever transmission was unknown until 1881, when Carlos Finlay suggested that the disease was transmitted through mosquitoes, now known to be female mosquitoes of the species Aedes aegypti.

Cultural exchanges

The evangelization of Mexico

One of the results of the movement of people between New and Old Worlds were cultural exchanges. For example, in the article "The Myth of Early Globalization: The Atlantic Economy, 1500–1800", Pieter Emmer makes the point that "from 1500 onward, a 'clash of cultures' had begun in the Atlantic". This clash of culture involved the transfer of European values to indigenous cultures. As an example, the emergence of the concept of private property in regions where property was often viewed as communal, concepts of monogamy (although many indigenous peoples were already monogamous), the role of women and children in the social system, and different concepts of labor, including slavery, although slavery was already a practice among many indigenous peoples and was widely practiced or introduced by Europeans into the Americas. Another example included the European abhorrence of human sacrifice, a religious practice among some indigenous populations.

During the initial stages of European colonization of the Americas, Europeans encountered fence-less lands. They believed that the land was unimproved and available for their taking, as they sought economic opportunity and homesteads. However, when European settlers arrived in Virginia, they encountered a fully established indigenous people, the Powhatan. The Powhatan farmers in Virginia scattered their farm plots within larger cleared areas. These larger cleared areas were a communal place for growing useful plants. As the Europeans viewed fences as hallmarks of civilization, they set about transforming "the land into something more suitable for themselves".

Tobacco was a New World agricultural product, originally a luxury good spread as part of the Columbian exchange. As is discussed in regard to the trans-Atlantic slave trade, the tobacco trade increased demand for free labor and spread tobacco worldwide. In discussing the widespread uses of tobacco, the Spanish physician Nicolas Monardes (1493–1588) noted that "The black people that have gone from these parts to the Indies, have taken up the same manner and use of tobacco that the Indians have". As Europeans traveled to other parts of the world, they took with them the practices related to tobacco. Demand for tobacco grew in the course of these cultural exchanges among peoples. Nunn and Qian mention that tobacco was used in the Old World as medicine and currency, although in the New World tobacco was the subject of religious customs. Tobacco was in great demand until 1950, until the disastrous effect of tobacco on the human body was discovered. Even now, according to calculations, people continue to die regularly from excessive tobacco consumption.

One of the most clearly notable areas of cultural clash and exchange was that of religion, often the lead point of cultural conversion. In the Spanish and Portuguese dominions, the spread of Catholicism, steeped in a European values system, was a major objective of colonization. Europeans often pursued it via explicit policies of suppression of indigenous languages, cultures and religions. In British America, Protestant missionaries converted many members of indigenous tribes to Protestantism. The French colonies had a more outright religious mandate, as some of the early explorers, such as Jacques Marquette, were also Catholic priests. In time, and given the European technological and immunological superiority which aided and secured their dominance, indigenous religions declined in the centuries following the European settlement of the Americas.

While the Mapuche did adopt the horse, sheep, and wheat, the over-all scant adoption of Spanish technology by Mapuche has been characterized as a means of cultural resistance.

According to Caroline Dodds Pennock, in Atlantic history indigenous people are often seen as static recipients of transatlantic encounters. But thousands of Native Americans crossed the ocean during the sixteenth century, some by choice.

Organism examples

See also: New World crops, Agriculture in Mesoamerica, Muisca agriculture, and List of food origins

Post-Columbian transfers of native organisms with close ties to humans. 

Late 15th to 20th century

Type of organism	Afro-Eurasia to the Americas	Americas to Afro-Eurasia
Domesticated animals	Barbary dove cat (domestic – several wild species already present) camel (18th–20th century) (technically reintroduced after extinction in North America) cattle (Would have been used for meat, dairy, and for pulling a plow or wagon.) chicken donkey duck (Old World domestic ducks are descended from the wild mallard, unlike the North American Muscovy duck) goat (the goats of the Old World, genus Capra, are different from the mountain goat of the New World, genus Oreamnos; would have become a source of meat and milk in Caribbean especially) goose (species of New World geese existed, but farmyards also would have wanted geese for laying eggs in addition to meat) guineafowl honey bee (European honey bee – other wild and domesticated species already present) golden hamster horse (technically reintroduced after extinction in North America) Mule koi ostrich pig pigeon quail (European and Japanese species) rabbit (domestic) sheep (Domestic only. Wild bighorn sheep do not live east of the Mississippi River and would not be discovered until after most of the interchange was complete.) water buffalo yak zebu	alpaca American mink guinea pig llama long-tailed chinchilla New World parrots (Kept as pets) Muscovy duck turkey (Mexican subspecies Meleagris gallopavo gallopavo.)
Other Animals	ring-necked pheasant mute swan peacock (Florida)	Canada goose (escaped from menageries of wealthy) ruddy duck
Cultivated plants	adzuki bean almond aloe vera anise apple apricot asparagus baobab banana (including cooking banana) barley basil beetroot black-eyed pea Brassica oleracea-derived vegetables broccoli Brussels sprouts cabbage cauliflower collard greens kale kohlrabi rapeseed breadfruit (brought from Asia to Caribbean) broad bean Cannabis (including hemp) cantaloupe carrot celery cherry chickpea cinnamon (not just species from Ceylon, but also Chinese and Southeast Asia) citrus (orange, lemon, etc.) coconut (brought from Asia to Caribbean) coffee common fig (other species of Ficus genus abundant, but earliest explorers did not know of existence or edibility until later.) coriander (also known as cilantro) cucumber cumin eggplant (aubergine) Ellis (oil palm) fennel finger millet foxtail millet flax garlic ginger goji grape (wild species present in North America, but not wine grapes) hazelnut hop jackfruit (brought from Asia to Caribbean) kiwifruit kola nut leek lentil lettuce mace mango mangosteen melon (watermelon, cantaloupe, honeydew, etc.) millet mint Momordica charantia (bitter melon) mung bean nutmeg oat okra olive onion opium poppy oregano parsnip pea peach pear pearl millet persimmon (Asian species only) pistachio plum (Cultivated food species from Asia only) pomegranate proso millet radish raspberry rice rosemary rye sage sesame sorghum soybean spinach sugarcane and sugar beet tamarind taro turmeric turnip walnut (commercial varieties) wheat yam (African yam; sweet potatoes are sometimes called "yam" in the U.S.)	açai American persimmon (ornamental) Annona glabra (alligator apple) Annona reticulata (custard apple) agave allspice amaranth (as grain) annatto arracacha arrowroot or Maranta arundinacea avocado black cherry black walnut (used for lumber and for ornamental purposes) blueberry (commercial varieties) Brazil nut Calathea allouia (leren) Canna indica (achira) capsicum (bell pepper and chili pepper) cashew cassava (manioc, tapioca, yuca) chayote cherimoya chia coca leaf cocoa bean cotton (long-staple species) cranberry (large cranberry, or bearberry species) cucurbits (many squashes and gourds) butternut squash pumpkin Hubbard squash acorn squash pattypan squash zucchini (courgette) Eryngium foetidum (culantro, Mexican coriander) Feijoa sellowiana (feijoa, pineapple guava, Brazilian guava, guavasteen) guarana guava (common) Helianthus (sunflower) Jerusalem artichoke jicama maize (corn) Magnolia, several species (ornamental purposes) Manilkara zapota (sapodilla) mashua Opuntia ficus-indica (prickly pear) Oxalis tuberosa (New Zealand yam) papaya Pawpaw passionfruit, fruit and flowers for gardens; multiple species. peanut pecan Phaseolus vulgaris (beans: pinto, lima, kidney, etc.) physalis (cape gooseberry) pineapple pitaya (dragon fruit) potato quinoa red oak (lumber and ornamental) rubber sassafras soursop stevia strawberry (commercial varieties) sugar-apple sugar maple sweet potato tamarillo tobacco tomato tomatillo ulluco vanilla wild rice (Texas, annual and northern species) yerba mate yucca
Cultivated fungi	Agaricus bisporus (button mushrooms, chestnut mushrooms, portobello mushrooms) cloud ear fungus enoki mushroom oyster mushroom (some varieties) Rhizopus oligosporus (tempeh) shiitake mushroom snow ear fungus truffle	huitlacoche (corn smut) oyster mushroom (some varieties)
Infectious diseases	bubonic plague chickenpox cholera diphtheria gonorrhea influenza leprosy malaria measles mumps rubella scarlet fever smallpox tuberculosis typhoid fever typhus whooping cough yaws yellow fever monkey pox	Chagas disease pinta syphilis (disputed)

Later history

Further information: Introduced species, Invasive species, and Lists of invasive species

Plants that arrived by land, sea, or air in the times before 1492 are called archaeophytes, and plants introduced to Europe after those times are called neophytes. Invasive species of plants and pathogens also were introduced by chance, including such weeds as tumbleweeds (Salsola spp.) and wild oats (Avena fatua). Some plants introduced intentionally, such as the kudzu vine introduced in 1894 from Japan to the United States to help control soil erosion, have since been found to be invasive pests in the new environment.

Fungi have also been transported, such as the one responsible for Dutch elm disease, killing American elms in North American forests and cities, where many had been planted as street trees. Some of the invasive species have become serious ecosystem and economic problems after establishing in the New World environments. A beneficial, although probably unintentional, introduction is Saccharomyces eubayanus, the yeast responsible for lager beer now thought to have originated in Patagonia. Others have crossed the Atlantic to Europe and have changed the course of history. In the 1840s, Phytophthora infestans crossed the oceans, damaging the potato crop in several European nations. In Ireland, the potato crop was totally destroyed; the Great Famine of Ireland caused millions to starve to death or emigrate.

In addition to these, many animals were introduced to new habitats on the other side of the world either accidentally or incidentally. These include such animals as brown rats and zebra mussels, which arrived on ships. Escaped and feral populations of non-indigenous animals have thrived in both the Old and New Worlds, often negatively impacting or displacing native species. In the New World, populations of feral European cats, pigs, horses, and cattle are common, and the Burmese python and green iguana are considered problematic in Florida. In the Old World, the Eastern gray squirrel has been particularly successful in colonising Great Britain, and populations of raccoons can now be found in some regions of Germany, the Caucasus, and Japan. Fur farm escapees such as coypu and American mink have extensive populations.