Polycystic kidney disease (PKD or PCKD, also known as polycystic kidney syndrome) is a genetic disorder in which the renal tubules become structurally abnormal, resulting in the development and growth of multiple cysts within the kidney. These cysts may begin to develop in utero, in infancy, in childhood, or in adulthood.
Cysts are non-functioning tubules filled with fluid pumped into them,
which range in size from microscopic to enormous, crushing adjacent
normal tubules and eventually rendering them non-functional as well.
Diagnosis may be suspected from one, some, or all of the
following: new onset flank pain or red urine; a positive family history;
palpation of enlarged kidneys on physical exam; an incidental finding
on abdominal sonogram; or an incidental finding of abnormal kidney function on routine lab work (BUN, serum creatinine, or eGFR). Definitive diagnosis is made by abdominal CT exam.
Signs and symptoms include high blood pressure, headaches, abdominal pain, blood in the urine, and excessive urination. Other symptoms include pain in the back, and cyst formation (renal and other organs).
CT scan showing autosomal dominant polycystic kidney diseaseCartoon of autosomal dominant polycystic kidney disease with normal kidney inset to right of diagramCartoon of autosomal recessive polycystic kidney disease with normal kidney inset to right of diagram
Autosomal dominant polycystic kidney disease (ADPKD) is the most common of all the inherited cystic kidney diseaseswith an incidence of 1:500 live births. Studies show that 10% of end-stage kidney disease (ESKD) patients being treated with dialysis in Europe and the U.S. were initially diagnosed and treated for ADPKD.
Genetic mutations in any of the three genesPKD1, PKD2, and PKD3 have similar phenotypical presentations.
Gene PKD1 is located on chromosome 16
and codes for a protein involved in regulation of cell cycle and
intracellular calcium transport in epithelial cells, and is responsible
for 85% of the cases of ADPKD.
Gene PKD2 is identified, using genetic linkage study, on chromosome 4. A group of voltage-linked cation channels, with inward selectivity for K>Na>>Ca and outward selectivity for Ca2+ ≈ Ba2+ > Na+ ≈ K+, are coded for by PKD2 on chromosome 4.
PKD3 recently appeared in research papers as a postulated third gene. Fewer than 10% of cases of ADPKD appear in non-ADPKD families. Cyst formation begins in utero from any point along the nephron,
although fewer than 5% of nephrons are thought to be involved. As the
cysts accumulate fluid, they enlarge, separate entirely from the
nephron, compress the neighboring kidney parenchyma, and progressively compromise kidney function.
Autosomal recessive
Autosomal recessive polycystic kidney disease
(ARPKD) (OMIM #263200) is the less common of the two types of PKD, with
an incidence of 1:20,000 live births and is typically identified in the
first few weeks after birth. Unfortunately, the kidneys are often underdeveloped resulting in a 30% death rate in newborns with ARPKD. PKHD1 is involved.
Mechanism
PKD1 and PKD2
Both autosomal dominant and autosomal recessive polycystic kidney disease cyst formation are tied to abnormal cilia-mediated signaling. The polycystin-1 and polycystin-2
proteins appear to be involved in both autosomal dominant and recessive
polycystic kidney disease due to defects in both proteins.
Both proteins have communication with calcium channel proteins, and
causes reduction in resting (intracellular) calcium and endoplasmic
reticulum storage of calcium.
The disease is characterized by a ‘second hit’ phenomenon, in
which a mutated dominant allele is inherited from a parent, with cyst
formation occurring only after the normal, wild-type gene sustains a
subsequent second genetic ‘hit’, resulting in renal tubular cyst
formation and disease progression.
PKD results from defects in the primary cilium,
an immotile, hair-like cellular organelle present on the surface of
most cells in the body, anchored in the cell body by the basal body.
In the kidney, primary cilia have been found to be present on most
cells of the nephron, projecting from the apical surface of the renal
epithelium into the tubule lumen. The cilia were believed to bend in the
urine flow, leading to changes in signalling, however this has since
been shown to be an experimental error (the bending of cilia was an
artifact of focal plane compensation, and also the actual effect on
micturition by severe hypertension and cardiac arrest) and that bending
of cilia does not contribute to alterations in Ca flux. While it is not
known how defects in the primary cilium lead to cyst development, it is
thought to possibly be related to disruption of one of the many
signaling pathways regulated by the primary cilium, including
intracellular calcium, Wnt/β-catenin, cyclic adenosine monophosphate
(cAMP), or planar cell polarity (PCP). Function of the primary cilium is
impaired, resulting in disruption of a number of intracellular
signaling cascades which produce differentiation of cystic epithelium,
increased cell division, increased apoptosis, and loss of resorptive
capacity.
Diagnosis
Polycystic kidney disease can be ascertained via a CT scan of abdomen, as well as, an MRI and ultrasound of the same area. A physical exam/test can reveal enlarged liver, heart murmurs and elevated blood pressure
Natural history
Most cases progress to bilateral disease in adulthood.
Treatment
Chr 11 FISH-mapped BACs from CGAP
In 2018, Jynarque (Tolvaptan) was introduced
as the first FDA-approved treatment for PKD. In a recent long-term
study, patients using Tolvaptan had a 6.4% higher kidney function after 5
years compared to standard of care. In 2019, a team of researchers at UCSB found that a ketogenic diet might be able to halt, or even reverse progression in mice, and the results of a first human case series study are showing potential benefit. The results of a 3-month randomized, prospective dietary intervention clinical trial are pending. In addition, recent research indicates that mild to moderate calorie restriction or time-restricted feeding slow the progression of autosomal dominant polycystic kidney disease (ADPKD) in mice.
Patient communities have been combining both ketogenic diets and time-restricted feeding with a low-oxalate diet to prevent the formation of stones and early reports show an average of 17% increase in kidney function
after approximately one year on a ketogenic, time-restricted dietary
regimen. If and when the disease progresses enough in a given case, the nephrologist or other practitioner and the patient will have to decide what form of renal replacement therapy will be used to treat end-stage kidney disease (kidney failure, typically stage 4 or 5 of chronic kidney disease).
That will either be some form of dialysis,
which can be done at least two different ways at varying frequencies
and durations (whether it is done at home or in the clinic depends on
the method used and the patient's stability and training) and
eventually, if they are eligible because of the nature and severity of
their condition and if a suitable match can be found, unilateral or
bilateral kidney transplantation.
A Cochrane Review study of autosomal dominant polycystic kidney
disease made note of the fact that it is important at all times, while
avoiding antibiotic resistance,
to control infections of the cysts in the kidneys, and if affected, the
liver, when needed for a certain duration to combat infection, by
using, "bacteriostatic and bacteriocidal drugs".
Prognosis
ADPKD individuals might have a normal life; conversely, ARPKD can cause kidney dysfunction and can lead to kidney failure by the age of 40–60. ADPKD1 and ADPKD2 are very different, in that ADPKD2 is much milder.
Currently, there are no therapies proven effective to prevent the progression of ADPKD.
Epidemiology
PKD
is one of the most common hereditary diseases in the United States,
affecting more than 600,000 people. It is the cause of nearly 10% of all
end-stage renal disease. It equally affects men, women, and all races. PKD occurs in some animals as well as humans.
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which it is performed: cold forging (a type of cold working), warm forging, or hot forging (a type of hot working). For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons. Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewellery.
Since the Industrial Revolution, forged parts are widely used in mechanisms and machines wherever a component requires high strength; such forgings usually require further processing (such as machining) to achieve a finished part. Today, forging is a major worldwide industry.
History
Forging is one of the oldest known metalworking processes. Traditionally, forging was performed by a smith using hammer and anvil, though introducing water power to the production and working of iron in the 12th century allowed the use of large trip hammers or power hammers that increased the amount and size of iron that could be produced and forged. The smithy or forge
has evolved over centuries to become a facility with engineered
processes, production equipment, tooling, raw materials and products to
meet the demands of modern industry.
In modern times, industrial forging is done either with presses
or with hammers powered by compressed air, electricity, hydraulics or
steam. These hammers may have reciprocating weights in the thousands of
pounds. Smaller power hammers,
500 lb (230 kg) or less reciprocating weight, and hydraulic presses are
common in art smithies as well. Some steam hammers remain in use, but
they became obsolete with the availability of the other, more
convenient, power sources.
Advantages and disadvantages
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain texture
deforms to follow the general shape of the part. As a result, the
texture variation is continuous throughout the part, giving rise to a
piece with improved strength characteristics.
Additionally, forgings can achieve a lower total cost than casting or
fabrication. Considering all the costs that are incurred in a product's
life cycle from procurement to lead time to rework, and factoring in the
costs of scrap, and downtime and other quality considerations, the
long-term benefits of forgings can outweigh the short-term cost savings
that castings or fabrications might offer.
Some metals may be forged cold, but iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forming,
which would increase the difficulty of performing secondary machining
operations on the piece. Also, while work hardening may be desirable in
some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable to precipitation hardening, such as most aluminium alloys and titanium, can be hot forged, followed by hardening.
Production forging involves significant capital expenditure for
machinery, tooling, facilities and personnel. In the case of hot
forging, a high-temperature furnace (sometimes referred to as the forge)
is required to heat ingots or billets.
Owing to the size of the massive forging hammers and presses and the
parts they can produce, as well as the dangers inherent in working with
hot metal, a special building is frequently required to house the
operation. In the case of drop forging operations, provisions must be
made to absorb the shock and vibration generated by the hammer. Most
forging operations use metal-forming dies, which must be precisely
machined and carefully heat-treated to correctly shape the workpiece, as
well as to withstand the tremendous forces involved.
Processes
A cross-section of a forged connecting rod that has been etched to show the grain flow
There are many different kinds of forging processes available; however, they can be grouped into three main classes:
Squeezed in closed compression dies: produces multidirectional flow
Common forging processes include: roll forging, swaging, cogging,
open-die forging, impression-die forging (closed die forging), press
forging, cold forging, automatic hot forging and upsetting.
All of the following forging processes can be performed at various
temperatures; however, they are generally classified by whether the
metal temperature is above or below the recrystallization temperature.
If the temperature is above the material's recrystallization temperature
it is deemed hot forging; if the temperature is below the
material's recrystallization temperature but above 30% of the
recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 30% of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that it can be done more quickly and precisely, and as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.
Drop forging
Drop forging is a forging process where a hammer is raised and then
"dropped" into the workpiece to deform it according to the shape of the
die. There are two types of drop forging: open-die drop forging and
impression-die (or closed-die) drop forging. As the names imply, the
difference is in the shape of the die, with the former not fully
enclosing the workpiece, while the latter does.
Open-die drop forging
Open-die drop forging (with two dies) of an ingot to be further processed into a wheelA large 80 ton cylinder of hot steel in an open-die forging press, ready for the upsetting phase of forging
Open-die forging is also known as smith forging. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil.
Open-die forging gets its name from the fact that the dies (the
surfaces that are in contact with the workpiece) do not enclose the
workpiece, allowing it to flow except where contacted by the dies. The
operator therefore needs to orient and position the workpiece to get the
desired shape. The dies are usually flat in shape, but some have a
specially shaped surface for specialized operations. For example, a die
may have a round, concave, or convex surface or be a tool to form holes
or be a cut-off tool.
Open-die forgings can be worked into shapes which include discs, hubs,
blocks, shafts (including step shafts or with flanges), sleeves,
cylinders, flats, hexes, rounds, plate, and some custom shapes.
Open-die forging lends itself to short runs and is appropriate for art
smithing and custom work. In some cases, open-die forging may be
employed to rough-shape ingots
to prepare them for subsequent operations. Open-die forging may also
orient the grain to increase strength in the required direction.
Advantages of open-die forging
Reduced chance of voids
Better fatigue resistance
Improved microstructure
Continuous grain flow
Finer grain size
Greater strength
Better response to thermal treatment
Improvement of internal quality
Greater reliability of mechanical properties, ductility and impact resistance
"Cogging"
is the successive deformation of a bar along its length using an
open-die drop forge. It is commonly used to work a piece of raw material
to the proper thickness. Once the proper thickness is achieved the
proper width is achieved via "edging". "Edging"
is the process of concentrating material using a concave shaped
open-die. The process is called "edging" because it is usually carried
out on the ends of the workpiece. "Fullering"
is a similar process that thins out sections of the forging using a
convex shaped die. These processes prepare the workpieces for further
forging processes.
Edging
Fullering
Impression-die forging
Impression-die
forging is also called "closed-die forging". In impression-die forging,
the metal is placed in a die resembling a mold, which is attached to an
anvil. Usually, the hammer die is shaped as well. The hammer is then
dropped on the workpiece, causing the metal to flow and fill the die
cavities. The hammer is generally in contact with the workpiece on the
scale of milliseconds. Depending on the size and complexity of the part,
the hammer may be dropped multiple times in quick succession. Excess
metal is squeezed out of the die cavities, forming what is referred to
as "flash".
The flash cools more rapidly than the rest of the material; this cool
metal is stronger than the metal in the die, so it helps prevent more
flash from forming. This also forces the metal to completely fill the
die cavity. After forging, the flash is removed.
In commercial impression-die forging, the workpiece is usually
moved through a series of cavities in a die to get from an ingot to the
final form. The first impression is used to distribute the metal into
the rough shape in accordance to the needs of later cavities; this
impression is called an "edging", "fullering", or "bending" impression.
The following cavities are called "blocking" cavities, in which the
piece is working into a shape that more closely resembles the final
product. These stages usually impart the workpiece with generous bends
and large fillets.
The final shape is forged in a "final" or "finisher" impression cavity.
If there is only a short run of parts to be done, then it may be more
economical for the die to lack a final impression cavity and instead
machine the final features.
Impression-die forging has been improved in recent years through
increased automation which includes induction heating, mechanical
feeding, positioning and manipulation, and the direct heat treatment of
parts after forging.
One variation of impression-die forging is called "flashless forging",
or "true closed-die forging". In this type of forging, the die cavities
are completely closed, which keeps the workpiece from forming flash. The
major advantage to this process is that less metal is lost to flash.
Flash can account for 20 to 45% of the starting material. The
disadvantages of this process include additional cost due to a more
complex die design and the need for better lubrication and workpiece
placement.
There are other variations of part formation that integrate
impression-die forging. One method incorporates casting a forging
preform from liquid metal. The casting is removed after it has
solidified, but while still hot. It is then finished in a single cavity
die. The flash is trimmed, then the part is quench hardened. Another
variation follows the same process as outlined above, except the preform
is produced by the spraying deposition of metal droplets into shaped
collectors (similar to the Osprey process).
Closed-die forging has a high initial cost due to the creation of
dies and required design work to make working die cavities. However, it
has low recurring costs for each part, thus forgings become more
economical with greater production volume. This is one of the major
reasons closed-die forgings are often used in the automotive and tool
industries. Another reason forgings are common in these industrial
sectors is that forgings generally have about a 20 percent higher
strength-to-weight ratio compared to cast or machined parts of the same
material.
Design of impression-die forgings and tooling
Forging dies are usually made of high-alloy or tool steel.
Dies must be impact- and wear-resistant, maintain strength at high
temperatures, and have the ability to withstand cycles of rapid heating
and cooling. In order to produce a better, more economical die the
following standards are maintained:
The dies part along a single, flat plane whenever possible. If not, the parting plane follows the contour of the part.
The parting surface is a plane through the center of the forging and not near an upper or lower edge.
Adequate draft is provided; usually at least 3° for aluminium and 5° to 7° for steel.
Generous fillets and radii are used.
Ribs are low and wide.
The various sections are balanced to avoid extreme difference in metal flow.
Full advantage is taken of fiber flow lines.
Dimensional tolerances are not closer than necessary.
Barrelling occurs when, due to friction between the work piece and the die or punch, the work piece bulges at its centre in such a way as to resemble a barrel. This leads to the central part of the work piece to come in contact with the sides of the die
sooner than if there were no friction present, creating a much greater
increase in the pressure required for the punch to finish the forging.
The dimensional tolerances of a steel part produced using the
impression-die forging method are outlined in the table below. The
dimensions across the parting plane are affected by the closure of the
dies, and are therefore dependent on die wear and the thickness of the
final flash. Dimensions that are completely contained within a single
die segment or half can be maintained at a significantly greater level
of accuracy.
Dimensional tolerances for impression-die forgings
Mass [kg (lb)]
Minus tolerance [mm (in)]
Plus tolerance [mm (in)]
0.45 (1)
0.15 (0.006)
0.46 (0.018)
0.91 (2)
0.20 (0.008)
0.61 (0.024)
2.27 (5)
0.25 (0.010)
0.76 (0.030)
4.54 (10)
0.28 (0.011)
0.84 (0.033)
9.07 (20)
0.33 (0.013)
0.99 (0.039)
22.68 (50)
0.48 (0.019)
1.45 (0.057)
45.36 (100)
0.74 (0.029)
2.21 (0.087)
A lubricant is used when forging to reduce friction and wear. It is
also used as a thermal barrier to restrict heat transfer from the
workpiece to the die. Finally, the lubricant acts as a parting compound
to prevent the part from sticking in the dies.
Press forging
Press
forging works by slowly applying a continuous pressure or force, which
differs from the near-instantaneous impact of drop-hammer forging. The
amount of time the dies are in contact with the workpiece is measured in
seconds (as compared to the milliseconds of drop-hammer forges). The
press forging operation can be done either cold or hot.
The main advantage of press forging, as compared to drop-hammer
forging, is its ability to deform the complete workpiece. Drop-hammer
forging usually only deforms the surfaces of the work piece in contact
with the hammer and anvil; the interior of the workpiece will stay
relatively undeformed. Another advantage to the process includes the
knowledge of the new part's strain rate. By controlling the compression
rate of the press forging operation, the internal strain can be
controlled.
There are a few disadvantages to this process, most stemming from
the workpiece being in contact with the dies for such an extended
period of time. The operation is a time-consuming process due to the
amount and length of steps. The workpiece will cool faster because the
dies are in contact with workpiece; the dies facilitate drastically more
heat transfer than the surrounding atmosphere. As the workpiece cools
it becomes stronger and less ductile, which may induce cracking if
deformation continues. Therefore, heated dies are usually used to reduce
heat loss, promote surface flow, and enable the production of finer
details and closer tolerances. The workpiece may also need to be
reheated.
When done in high productivity, press forging is more economical
than hammer forging. The operation also creates closer tolerances. In
hammer forging a lot of the work is absorbed by the machinery; when in
press forging, the greater percentage of work is used in the work piece.
Another advantage is that the operation can be used to create any size
part because there is no limit to the size of the press forging machine.
New press forging techniques have been able to create a higher degree
of mechanical and orientation integrity. By the constraint of oxidation
to the outer layers of the part, reduced levels of microcracking occur
in the finished part.
Press forging can be used to perform all types of forging,
including open-die and impression-die forging. Impression-die press
forging usually requires less draft than drop forging and has better
dimensional accuracy. Also, press forgings can often be done in one
closing of the dies, allowing for easy automation.
Upset forging increases the diameter of the workpiece by compressing its length. Based on number of pieces produced, this is the most widely used forging process.
A few examples of common parts produced using the upset forging process
are engine valves, couplings, bolts, screws, and other fasteners.
Upset forging is usually done in special high-speed machines called crank presses.
The machines are usually set up to work in the horizontal plane, to
facilitate the quick exchange of workpieces from one station to the
next, but upsetting can also be done in a vertical crank press or a
hydraulic press. The initial workpiece is usually wire or rod, but some
machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity
of over 1000 tons. The standard upsetting machine employs split dies
that contain multiple cavities. The dies open enough to allow the
workpiece to move from one cavity to the next; the dies then close and
the heading tool, or ram, then moves longitudinally against the bar,
upsetting it into the cavity. If all of the cavities are utilized on
every cycle, then a finished part will be produced with every cycle,
which makes this process advantageous for mass production.
These rules must be followed when designing parts to be upset forged:
The length of unsupported metal that can be upset in one blow
without injurious buckling should be limited to three times the diameter
of the bar.
Lengths of stock greater than three times the diameter may be upset
successfully, provided that the diameter of the upset is not more than
1.5 times the diameter of the stock.
In an upset requiring stock length greater than three times the
diameter of the stock, and where the diameter of the cavity is not more
than 1.5 times the diameter of the stock, the length of unsupported
metal beyond the face of the die must not exceed the diameter of the
bar.
Automatic hot forging
The
automatic hot forging process involves feeding mill-length steel bars
(typically 7 m (23 ft) long) into one end of the machine at room
temperature and hot forged products emerge from the other end. This all
occurs rapidly; small parts can be made at a rate of 180 parts per
minute (ppm) and larger can be made at a rate of 90 ppm. The parts can
be solid or hollow, round or symmetrical, up to 6 kg (13 lb), and up to
18 cm (7.1 in) in diameter. The main advantages to this process are its
high output rate and ability to accept low-cost materials. Little labor
is required to operate the machinery.
There is no flash produced so material savings are between 20 and
30% over conventional forging. The final product is a consistent
1,050 °C (1,920 °F) so air cooling will result in a part that is still
easily machinable (the advantage being the lack of annealing
required after forging). Tolerances are usually ±0.3 mm (0.012 in),
surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly
double that of conventional forging because contact times are on the
order of 0.06-second. The downside is that this process is only feasible
on smaller symmetric parts and cost; the initial investment can be over
$10 million, so large quantities are required to justify this process.
The process starts by heating the bar to 1,200 to 1,300 °C (2,190
to 2,370 °F) in less than 60 seconds using high-power induction coils.
It is then descaled with rollers, sheared into blanks, and transferred
through several successive forming stages, during which it is upset,
preformed, final forged, and pierced (if necessary). This process can
also be coupled with high-speed cold-forming operations. Generally, the
cold forming operation will do the finishing stage so that the
advantages of cold-working can be obtained, while maintaining the high
speed of automatic hot forging.
Examples of parts made by this process are: wheel hub unit
bearings, transmission gears, tapered roller bearing races, stainless
steel coupling flanges, and neck rings for liquid propane (LP) gas
cylinders.[22] Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.
Roll forging
Roll
forging is a process where round or flat bar stock is reduced in
thickness and increased in length. Roll forging is performed using two
cylindrical or semi-cylindrical rolls, each containing one or more
shaped grooves. A heated bar is inserted into the rolls and when it hits
a spot the rolls rotate and the bar is progressively shaped as it is
rolled through the machine. The piece is then transferred to the next
set of grooves or turned around and reinserted into the same grooves.
This continues until the desired shape and size is achieved. The
advantage of this process is there is no flash and it imparts a
favorable grain structure into the workpiece.
Examples of products produced using this method include axles, tapered levers and leaf springs.
This process is also known as precision forging. It was
developed to minimize cost and waste associated with post-forging
operations. Therefore, the final product from a precision forging needs
little or no final machining. Cost savings are gained from the use of
less material, and thus less scrap, the overall decrease in energy used,
and the reduction or elimination of machining. Precision forging also
requires less of a draft, 1° to 0°. The downside of this process is its
cost, therefore it is only implemented if significant cost reduction can
be achieved.
Cold forging
Near
net shape forging is most common when parts are forged without heating
the slug, bar or billet. Aluminum is a common material that can be cold
forged depending on final shape. Lubrication of the parts being formed
is critical to increase the life of the mating dies.
Unlike the above processes, induction forging is based on the type of
heating style used. Many of the above processes can be used in
conjunction with this heating method.
Multidirectional forging
Multidirectional
forging is forming of a work piece in a single step in several
directions. The multidirectional forming takes place through
constructive measures of the tool. The vertical movement of the press
ram is redirected using wedges which distributes and redirects the force
of the forging press in horizontal directions.
Isothermal forging
Isothermal forging is a process by which the materials and the die are heated to the same temperature (iso-
meaning "equal"). Adiabatic heating is used to assist in the
deformation of the material, meaning the strain rates are highly
controlled. This technique is commonly used for forging aluminium, which
has a lower forging temperature than steels. Forging temperatures for
aluminum are around 430 °C (806 °F), while steels and super alloys can
be 930 to 1,260 °C (1,710 to 2,300 °F).
Benefits:
Near net shapes which lead to lower machining requirements and therefore lower scrap rates
Reproducibility of the part
Due to the lower heat loss smaller machines can be used to make the forging
Disadvantages:
Higher die material costs to handle temperatures and pressures
Uniform heating systems are required
Protective atmospheres or vacuum to reduce oxidation of the dies and material
Low production rates
Materials and applications
Solid forged billets of steel (glowing incandescently) being loaded in a large industrial chamber furnace, for re-heating
Forging of steel
Depending on the forming temperature steel forging can be divided into:
Hot forging of steel
Forging temperatures above the recrystallization temperature between 950–1250 °C
Good formability
Low forming forces
Constant tensile strength of the workpieces
Warm forging of steel
Forging temperatures between 750–950 °C
Less or no scaling at the workpiece surface
Narrower tolerances achievable than in hot forging
Limited formability and higher forming forces than for hot forging
Lower forming forces than in cold forming
Cold forging of steel
Forging temperatures at room conditions, self-heating up to 150 °C due to the forming energy
Narrowest tolerances achievable
No scaling at workpiece surface
Increase of strength and decrease of ductility due to strain hardening
Low formability and high forming forces are necessary
For industrial processes steel alloys are primarily forged in hot
condition. Brass, bronze, copper, precious metals and their alloys are
manufactured by cold forging processes; each metal requires a different
forging temperature.
Forging of aluminium
Aluminium forging is performed at a temperature range between 350–550 °C
Forging temperatures above 550 °C are too close to the solidus
temperature of the alloys and lead in conjunction with varying effective
strains to unfavorable workpiece surfaces and potentially to a partial
melting as well as fold formation.
Forging temperatures below 350 °C reduce formability by increasing
the yield stress, which can lead to unfilled dies, cracking at the
workpiece surface and increased die forces
Due to the narrow temperature range and high thermal conductivity,
aluminium forging can only be realized in a particular process window.
To provide good forming conditions a homogeneous temperature
distribution in the entire workpiece is necessary. Therefore, the
control of the tool temperature has a major influence to the process.
For example, by optimizing the preform geometries the local effective
strains can be influenced to reduce local overheating for a more
homogeneous temperature distribution.
Application of aluminium forged parts
High-strength
aluminium alloys have the tensile strength of medium strong steel
alloys while providing significant weight advantages. Therefore,
aluminium forged parts are mainly used in aerospace, automotive industry
and many other fields of engineering especially in those fields, where
highest safety standards against failure by abuse, by shock or vibratory
stresses are needed. Such parts are for example pistons, chassis parts, steering components and brake parts. Commonly used alloys are AlSi1MgMn (EN AW-6082) and AlZnMgCu1,5 (EN AW-7075).
About 80% of all aluminium forged parts are made of AlSi1MgMn. The
high-strength alloy AlZnMgCu1,5 is mainly used for aerospace
applications.
Forging of magnesium
Magnesium forging occurs at a temperature range between 290–450 °C
Magnesium alloys are more difficult to forge due to their low
plasticity, low sensitivity to strain rates and narrow forming
temperature.
Using semi-open die hot forging with a three-slide forging press (TSFP)
has become a newly developed forging method for Mg-Al alloy AZ31,
commonly used in forming aircraft brackets. This forging method has shown to improve tensile properties but lacks uniform grain size.
Even though the application of magnesium alloys increases by 15–20%
each year in the aerospace and automotive industry, forging magnesium
alloys with specialized dies is expensive and an unfeasible method to
produce parts for a mass market. Instead, most magnesium alloy parts for
industry are produced by casting methods.
Equipment
Hydraulic drop-hammer(a) Material flow of a conventionally forged disc; (b) Material flow of an impactor forged disc
The most common type of forging equipment is the hammer and anvil.
Principles behind the hammer and anvil are still used today in drop-hammer
equipment. The principle behind the machine is simple: raise the hammer
and drop it or propel it into the workpiece, which rests on the anvil.
The main variations between drop-hammers are in the way the hammer is
powered; the most common being air and steam hammers. Drop-hammers
usually operate in a vertical position. The main reason for this is
excess energy (energy that is not used to deform the workpiece) that is
not released as heat or sound needs to be transmitted to the foundation.
Moreover, a large machine base is needed to absorb the impacts.
To overcome some shortcomings of the drop-hammer, the counterblow machine or impactor
is used. In a counterblow machine both the hammer and anvil move and
the workpiece is held between them. Here excess energy becomes recoil.
This allows the machine to work horizontally and have a smaller base.
Other advantages include less noise, heat and vibration. It also
produces a distinctly different flow pattern. Both of these machines can
be used for open-die or closed-die forging.
Forging presses
A forging press,
often just called a press, is used for press forging. There are two
main types: mechanical and hydraulic presses. Mechanical presses
function by using cams, cranks and/or toggles to produce a preset (a
predetermined force at a certain location in the stroke) and
reproducible stroke. Due to the nature of this type of system, different
forces are available at different stroke positions. Mechanical presses
are faster than their hydraulic counterparts (up to 50 strokes per
minute). Their capacities range from 3 to 160 MN (300 to 18,000 short
tons-force). Hydraulic presses use fluid pressure and a piston to
generate force. The advantages of a hydraulic press over a mechanical
press are its flexibility and greater capacity. The disadvantages
include a slower, larger, and costlier machine to operate.
The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.
The 10th millennium BC spanned the years 10,000 BC to 9001 BC (c. 12 ka to c. 11 ka). It marks the beginning of the transition from the Palaeolithic to the Neolithic via the interim Mesolithic (Northern Europe and Western Europe) and Epipaleolithic (Levant and Near East) periods, which together form the first part of the Holocene
epoch that is generally believed to have begun c. 9700 BC (c. 11.7 ka)
and is the current geological epoch. It is impossible to precisely date
events that happened around the time of this millennium, and all dates
mentioned here are estimates mostly based on geological analysis,
anthropological analysis, and radiometric dating.
Holocene epoch
The main characteristic of the Holocene has been the worldwide abundance of Homo sapiens sapiens (humankind). The epoch began in the wake of the Würm glaciation, generally known as the Last Ice Age, which began 109 ka and ended 14 ka when Homo sapiens sapiens was in the Palaeolithic (Old Stone) Age. Following the Late Glacial Interstadial from 14 ka to 12.9 ka, during which global temperatures rose significantly, the Younger Dryas
began. This was a temporary reversal of climatic warming to glacial
conditions in the Northern Hemisphere and coincided with the end of the
Upper Palaeolithic. The Younger Dryas ceased c. 9700 BC, marking the
cutover from Pleistocene to Holocene.
In the Holocene's first millennium, the Palaeolithic began to be superseded by the Neolithic (New Stone) Age which lasted about 6,000 years, depending on location. The gradual transition period is sometimes termed Mesolithic (northern and western Europe) or Epipalaeolithic (Levant and Near East). The glaciers retreated as the world climate became warmer and that inspired an agricultural revolution,
though at first, the dog was probably the only domesticated animal.
This was accompanied by a social revolution in that humans gained from
agriculture the impetus to settle. Settlement is the key precursor to
civilisation, which cannot be achieved by a nomadic lifestyle.
The world population,
c. 10,000 BC, is believed to have been more or less stable. It has been
estimated that there were some five million people at the time of the Last Glacial Maximum,
growing to forty million by 5000 BC and 100 million by 1600 BC, which
is an average growth rate of 0.027% p.a. from the Neolithic to the
Middle Bronze Age. Around 10,000 BC, most people lived in hunter-gatherer communities scattered across all continents except Antarctica and Zealandia. As the Würm/Wisconsin ended, settlement of northern regions was again possible.
Beginnings of agriculture
Agriculture
developed in different parts of the world at different times. In many
places, people learned how to cultivate without outside help; elsewhere,
as in western Europe, the skills were imported.
A decrease in human height accompanied the rise of agriculture near the
start of the Holocene period (10,000 BC) and was later correlated with
urban population density.
The Natufian culture prevailed in the Levant
through the 10th millennium and was unusual in that it supported a
sedentary or semi-sedentary population even before the introduction of
agriculture. An early example is 'Ain Mallaha, which may have been the first village in which people were wholly sedentary. The Natufian people are believed to have founded another early settlement on the site of Jericho (Tell es-Sultan) where there is evidence of building between 9600 BC and 8200 BC. Dates for the Natufian are indeterminate and range broadly from c. 13,050 BC to c. 7550 BC. It is possible that the early cultivation of figs began in the Jordan River valley sometime after the middle of the 10th millennium.
Besides the fig trees, the people may have begun cultivation of wild
plants such as barley and pistachio; and they possibly began herding
goats, pigs and cattle.
Agriculture began to be developed by the various communities of the Fertile Crescent,
which included the Levant, but it would not be widely practised for
another 2,000 years by which time Neolithic culture was becoming well
established in many parts of the Near East. Among the earliest cultivated plants were forms of millet and rice grown in the Middle East, possibly in this millennium but more likely after 9000 BC. By about 9500 BC, people in south-eastern Anatolia were harvesting wild grasses and grains. The earliest evidence of sheep herding has been found in northern Iraq, dated before 9000 BC.
Pottery
Prehistoric chronology is almost entirely reliant upon the dating of material objects of which pottery
is by far the most widespread and the most resistant to decay. All
locations and generations developed their own shapes, sizes and styles
of pottery, including methods and styles of decoration, but there was
consistency among stratified deposits and even shards can be classified
by time and place.
Pottery is believed to have been discovered independently in various
places, beginning with China c. 18,000 BC, and was probably created
accidentally by fires lit on clay soil. The main discovery of pottery dated to the 10th millennium has been at Bosumpra Cave (early tenth-millennium cal. BC) on the Kwahu Plateau in southeastern Ghana and Ounjougou
(c.9400 BC) in Central Mali, providing evidence of an independent
invention of pottery in Sub-Saharan Africa in different climatic zones.
The first chronological pottery system was the Early, Middle and Late Minoan framework devised in the early 20th century by Sir Arthur Evans for his findings at Knossos. This covered the Bronze Age
in twelve phases from c. 2800 BC to c. 1050 BC and the principle was
later extended to mainland Greece (Helladic) and the Aegean islands
(Cycladic). Dame Kathleen Kenyon was the principal archaeologist at Tell es-Sultan (ancient Jericho) and she discovered that there was no pottery there. The potter's wheel had not yet been invented and, where pottery as such was made, it was still hand-built, often by means of coiling, and pit fired.
Kenyon discovered vessels such as bowls, cups, and plates at
Jericho which were made from stone. She reasonably surmised that others
made from wood or vegetable fibres would have long since decayed. Using Evans' system as a benchmark, Kenyon divided the Near East Neolithic into phases called Pre-Pottery Neolithic A (PPNA), from c. 10,000 BC to c. 8800 BC; Pre-Pottery Neolithic B (PPNB), from c. 8800 BC to c. 6500 BC; and then Pottery Neolithic (PN), which had varied start-points from c. 6500 BC until the beginnings of the Bronze Age towards the end of the 4th millennium.
In the 10th millennium, the Natufian culture co-existed with the PPNA
which prevailed in the Levantine and upper Mesopotamian areas of the
Fertile Crescent.
In North Africa, Saharan rock art engravings in what is known as the Bubalus (Large Wild Fauna) period have been dated to between 10,000 BC and 7000 BC. Wall paintings found in Ethiopia and Eritrea depict human activity; some of the older paintings are thought to date back to around 10,000 BC. In Prehistoric Egypt, a culture of hunter-gatherers replaced a grain grinding culture in 10,000 BC. The Abu Maditel mounds in the Sinai Peninsula have been dated c. 9660 to c. 9180 BC.
Americas
The Clovis culture
was widely distributed throughout North America. The people were
hunter-gatherers and the culture's duration is believed to have been
from c.9050 BC to c.8800 BC. There is evidence of increasing use of Clovis point tool technology for hunting.
Elsewhere in North America, the Petroglyphs at Winnemucca Lake, in what is today northwest Nevada, were carved by this time, possibly as early as 12.8 ka or as late as 10 ka.
Eurasia
The sites at Göbekli Tepe, which is home to megalithic structures, Hallan Çemi Tepesi, both in south-eastern Anatolia, and at Tell Qaramel, in north-west Syria, may have been occupied during this millennium. It was found out that gastronomy first emerged in Göbekli Tepe in this millennium. This most important discovery shedding light on the beginning of gastronomy in Anatolia
consists of religious places in which feasts were held in this
millennium that were found in the archaeological excavations conducted
in Göbekli Tepe in Urfa province which is called the zero point of time. As a result of the investigations carried out in Göbekli Tepe
which is dated to this millennium, it was determined that the people
created cult structures and had a culture for religious purposes during
the early Neolithic period and that they held feasts in which they offered foods while performing these cultures. The Sassi di Matera in southern Italy is believed to have been the site of human settlement since the Paleolithic period in this millennium, making it one of the oldest continuously inhabited settlements in history. At the Hasankeyf Mound in Turkey, Europe, almost all archaeological data date to this millennium. In Great Britain, which was not then an island, the Star Carr site in North Yorkshire is believed to have been inhabited by Maglemosian peoples for about 800 years from c. 9335 BC to c. 8525 BC.
Environmental changes
In the southern hemisphere, rising sea levels had gradually formed Bass Strait, separating Tasmania from mainland Australia.
This process is believed to have been complete by about the beginning
of the 10th millennium. Bass Strait had been a plain populated by
indigenous people who are thought to have first arrived around 40,000
years ago.
The ongoing Quaternary System/Period represents the last 2.58 million years since the end of the Neogene and is officially divided into the Pleistocene and Holocene
Series/Epochs. The Holocene has been assigned an age of 11,700 calendar
years before 2000 CE which means it began c. 9700 BC in the 10th
millennium. It is preceded in the geological time scale by the Late Pleistocene sub-epoch, also known as the Tarantian Stage/Age, which awaits formal ratification by the International Union of Geological Sciences (IUGS) and tentatively spans the time from c. 126,000 BC to c. 9700 BC. Preceding the Late Pleistocene is the Middle Pleistocene
sub-epoch, or Chibanian Stage/Age, which also awaits ratification and
tentatively spans the time from c. 773,000 BC to c. 126,000 BC. The Early Pleistocene from c. 2,580,000 BC until c. 773,000 is sub-divided into two Stages/Ages which have been officially defined: the Gelasian (until c. 1,800,000 BC) and the Calabrian.
The Holocene calendar, devised by Cesare Emiliani in 1993, places its epoch
at 10,000 BC (with the year 2023 being rendered as 12023 HE). The Human
Era calendar attempts to simplify the calculation of time spans across the BC-CE divide by designating a more universally relevant epoch date: the start of human settlements (instead of the birth of Jesus Christ). CE dates can be converted by adding 10,000 years; converting BC dates requires subtraction from 10,001 (since the Gregorian calendar lacks a year zero).
Source–sink dynamics is a theoretical model used by ecologists to describe how variation in habitat quality may affect the population growth or decline of organisms.
Since quality is likely to vary among patches of habitat, it is
important to consider how a low quality patch might affect a population.
In this model, organisms occupy two patches of habitat. One patch,
the source, is a high quality habitat that on average allows the
population to increase. The second patch, the sink, is a very low
quality habitat that, on its own, would not be able to support a
population. However, if the excess of individuals
produced in the source frequently moves to the sink, the sink
population can persist indefinitely. Organisms are generally assumed to
be able to distinguish between high and low quality habitat, and to
prefer high quality habitat. However, ecological trap theory describes the reasons why organisms may actually prefer sink patches over source patches.
Finally, the source–sink model implies that some habitat patches may
be more important to the long-term survival of the population, and
considering the presence of source–sink dynamics will help inform conservation decisions.
Theory development
Although the seeds of a source–sink model had been planted earlier, Pulliam
is often recognized as the first to present a fully developed
source–sink model. He defined source and sink patches in terms of their
demographic parameters, or BIDE rates (birth, immigration, death, and emigration
rates). In the source patch, birth rates were greater than death
rates, causing the population to grow. The excess individuals were
expected to leave the patch, so that emigration rates were greater than
immigration rates. In other words, sources were a net exporter of
individuals. In contrast, in a sink patch, death rates were greater
than birth rates, resulting in a population decline toward extinction
unless enough individuals emigrated from the source patch. Immigration
rates were expected to be greater than emigration rates, so that sinks
were a net importer of individuals. As a result, there would be a net
flow of individuals from the source to the sink (see Table 1).
Pulliam's work was followed by many others who developed and tested the source–sink model. Watkinson and Sutherland
presented a phenomenon in which high immigration rates could cause a
patch to appear to be a sink by raising the patch's population above its
carrying capacity
(the number of individuals it can support). However, in the absence of
immigration, the patches are able to support a smaller population.
Since true sinks cannot support any population, the authors called these
patches "pseudo-sinks". Definitively distinguishing between true sinks
and pseudo-sinks requires cutting off immigration to the patch in
question and determining whether the patch is still able to maintain a
population. Thomas et al.
were able to do just that, taking advantage of an unseasonable frost
that killed off the host plants for a source population of Edith's checkerspot butterfly (Euphydryas editha).
Without the host plants, the supply of immigrants to other nearby
patches was cut off. Although these patches had appeared to be sinks,
they did not become extinct without the constant supply of immigrants.
They were capable of sustaining a smaller population, suggesting that
they were in fact pseudo-sinks.
Watkinson and Sutherland's caution about identifying pseudo-sinks was followed by Dias,
who argued that differentiating between sources and sinks themselves
may be difficult. She asserted that a long-term study of the
demographic parameters of the populations in each patch is necessary.
Otherwise, temporary variations in those parameters, perhaps due to
climate fluctuations or natural disasters, may result in a
misclassification of the patches. For example, Johnson described periodic flooding of a river in Costa Rica which completely inundated patches of the host plant for a rolled-leaf beetle (Cephaloleia fenestrata).
During the floods, these patches became sinks, but at other times they
were no different from other patches. If researchers had not
considered what happened during the floods, they would not have
understood the full complexity of the system.
Dias
also argued that an inversion between source and sink habitat is
possible so that the sinks may actually become the sources. Because
reproduction in source patches is much higher than in sink patches,
natural selection is generally expected to favor adaptations to the
source habitat. However, if the proportion of source to sink habitat
changes so that sink habitat becomes much more available, organisms may
begin to adapt to it instead. Once adapted, the sink may become a
source habitat. This is believed to have occurred for the blue tit (Parus caeruleus) 7500 years ago as forest composition on Corsica changed, but few modern examples are known. Boughton described a source—pseudo-sink inversion in butterfly populations of E. editha.
Following the frost, the butterflies had difficulty recolonizing the
former source patches. Boughton found that the host plants in the
former sources senesced much earlier than in the former pseudo-sink
patches. As a result, immigrants regularly arrived too late to
successfully reproduce. He found that the former pseudo-sinks had
become sources, and the former sources had become true sinks.
One of the most recent additions to the source–sink literature is by Tittler et al., who examined wood thrush (Hylocichla mustelina)
survey data for evidence of source and sink populations on a large
scale. The authors reasoned that emigrants from sources would likely be
the juveniles produced in one year dispersing to reproduce in sinks in
the next year, producing a one-year time lag between population changes
in the source and in the sink. Using data from the Breeding Bird Survey,
an annual survey of North American birds, they looked for relationships
between survey sites showing such a one-year time lag. They found
several pairs of sites showing significant relationships 60–80 km apart.
Several appeared to be sources to more than one sink, and several
sinks appeared to receive individuals from more than one source. In
addition, some sites appeared to be a sink to one site and a source to
another (see Figure 1). The authors concluded that source–sink dynamics
may occur on continental scales.
One of the more confusing issues involves identifying sources and sinks in the field. Runge et al.
point out that in general researchers need to estimate per capita
reproduction, probability of survival, and probability of emigration to
differentiate source and sink habitats. If emigration is ignored, then
individuals that emigrate may be treated as mortalities, thus causing
sources to be classified as sinks. This issue is important if the
source–sink concept is viewed in terms of habitat quality (as it is in
Table 1) because classifying high-quality habitat as low-quality may
lead to mistakes in ecological management. Runge et al. showed how to integrate the theory of source–sink dynamics with population projection matrices and ecological statistics in order to differentiate sources and sinks.
Table 1. Summary characteristics of variations on the source–sink dynamics model.
Source–sink
Source–pseudosink
Ecological trap
Source patch (high quality habitat)
Stable or growing Attractive Net exporter
Stable or growing Attractive Net exporter
Stable or growing Avoided (or equal) Net exporter
Sink, pseudo-sink, or trap patch (low quality habitat)
Declines to extinction Avoided Net importer
Declines to stable size Either Net importer
Declines to extinction Attractive (or equal) Net importer
Habitat patches are represented in terms of their (1) inherent
abilities to maintain a population (in the absence of immigration), (2)
their attractiveness to organisms that are actively dispersing and
choosing habitat patches, and (3) whether they are net exporters or
importers of dispersing individuals. Note that in all of these systems,
source patches are capable of supporting stable or growing populations
and are net exporters of individuals. The major difference between them
is that in the ecological trap model, the source patch is avoided (or
at least not preferred to the low quality trap patch). All of the low
quality patches (whether sinks, pseudo-sinks, or traps) are net
importers of dispersing individuals, and in the absence of dispersal,
would show a population decline. However, pseudo-sinks would not
decline to extinction as they are capable of supporting a smaller
population. The other major difference between these low quality patch
types is in their attractiveness; sink populations are avoided while
trap patches are preferred (or at least not avoided).
Modes of dispersal
Why
would individuals ever leave high quality source habitat for a low
quality sink habitat? This question is central to source–sink theory.
Ultimately, it depends on the organisms and the way they move and
distribute themselves between habitat patches. For example, plants
disperse passively, relying on other agents such as wind or water
currents to move seeds to another patch. Passive dispersal can result in
source–sink dynamics whenever the seeds land in a patch that cannot
support the plant's growth or reproduction. Winds may continually
deposit seeds there, maintaining a population even though the plants
themselves do not successfully reproduce.
Another good example for this case are soil protists. Soil protists
also disperse passively, relying mainly on wind to colonize other sites.
As a result, source–sink dynamics can arise simply because external
agents dispersed protist propagules (e.g., cysts, spores), forcing
individuals to grow in a poor habitat.
In contrast, many organisms that disperse actively should have no reason to remain in a sink patch, provided the organisms are able to recognize it as a poor quality patch (see discussion of ecological traps). The reasoning behind this argument is that organisms are often expected to behave according to the "ideal free distribution",
which describes a population in which individuals distribute themselves
evenly among habitat patches according to how many individuals the
patch can support. When there are patches of varying quality available, the ideal free distribution predicts a pattern of "balanced dispersal". In this model, when the preferred habitat patch becomes crowded enough that the average fitness
(survival rate or reproductive success) of the individuals in the patch
drops below the average fitness in a second, lower quality patch,
individuals are expected to move to the second patch. However, as soon
as the second patch becomes sufficiently crowded, individuals are
expected to move back to the first patch. Eventually, the patches
should become balanced so that the average fitness of the individuals in
each patch and the rates of dispersal between the two patches are even.
In this balanced dispersal model, the probability of leaving a patch
is inversely proportional to the carrying capacity of the patch.
In this case, individuals should not remain in sink habitat for very
long, where the carrying capacity is zero and the probability of leaving
is therefore very high.
An alternative to the ideal free distribution and balanced
dispersal models is when fitness can vary among potential breeding sites
within habitat patches and individuals must select the best available
site. This alternative has been called the "ideal preemptive
distribution", because a breeding site can be preempted if it has
already been occupied.
For example, the dominant, older individuals in a population may
occupy all of the best territories in the source so that the next best
territory available may be in the sink. As the subordinate, younger
individuals age, they may be able to take over territories in the
source, but new subordinate juveniles from the source will have to move
to the sink. Pulliam
argued that such a pattern of dispersal can maintain a large sink
population indefinitely. Furthermore, if good breeding sites in the
source are rare and poor breeding sites in the sink are common, it is
even possible that the majority of the population resides in the sink.
Importance in ecology
The source–sink model of population dynamics has made contributions to many areas in ecology. For example, a species' niche
was originally described as the environmental factors required by a
species to carry out its life history, and a species was expected to be
found only in areas that met these niche requirements.
This concept of a niche was later termed the "fundamental niche", and
described as all of the places a species could successfully occupy. In
contrast, the "realized niche", was described as all of the places a
species actually did occupy, and was expected to be less than the extent
of the fundamental niche as a result of competition with other species.
However, the source–sink model demonstrated that the majority of a
population could occupy a sink which, by definition, did not meet the
niche requirements of the species,
and was therefore outside the fundamental niche (see Figure 2). In
this case, the realized niche was actually larger than the fundamental
niche, and ideas about how to define a species' niche had to change.
Source–sink dynamics has also been incorporated into studies of metapopulations, a group of populations residing in patches of habitat.
Though some patches may go extinct, the regional persistence of the
metapopulation depends on the ability of patches to be re-colonized. As
long as there are source patches present for successful reproduction,
sink patches may allow the total number of individuals in the
metapopulation to grow beyond what the source could support, providing a
reserve of individuals available for re-colonization.
Source–sink dynamics also has implications for studies of the
coexistence of species within habitat patches. Because a patch that is a
source for one species may be a sink for another, coexistence may
actually depend on immigration from a second patch rather than the
interactions between the two species. Similarly, source–sink dynamics may influence the regional coexistence and demographics of species within a metacommunity, a group of communities connected by the dispersal of potentially interacting species. Finally, the source–sink model has greatly influenced ecological trap theory, a model in which organisms prefer sink habitat over source habitat.
Besides being ecological trap sink habitat may vary in their response i
major disturbance and colonization of sink habitat may allow species
survival even if population in source habitat extinct due to some
catastrophic event which may substantially increase metapopulational stability.
Conservation
Land
managers and conservationists have become increasingly interested in
preserving and restoring high quality habitat, particularly where rare,
threatened, or endangered species are concerned. As a result, it is
important to understand how to identify or create high quality habitat,
and how populations respond to habitat loss or change. Because a large
proportion of a species' population could exist in sink habitat,
conservation efforts may misinterpret the species' habitat
requirements. Similarly, without considering the presence of a trap,
conservationists might mistakenly preserve trap habitat under the
assumption that an organism's preferred habitat was also good quality
habitat. Simultaneously, source habitat may be ignored or even
destroyed if only a small proportion of the population resides there.
Degradation or destruction of the source habitat will, in turn, impact
the sink or trap populations, potentially over large distances.
Finally, efforts to restore degraded habitat may unintentionally
create an ecological trap by giving a site the appearance of quality
habitat, but which has not yet developed all of the functional elements
necessary for an organism's survival and reproduction. For an already
threatened species, such mistakes might result in a rapid population
decline toward extinction.
In considering where to place reserves,
protecting source habitat is often assumed to be the goal, although if
the cause of a sink is human activity, simply designating an area as a
reserve has the potential to convert current sink patches to source
patches (e.g. no-take zones). Either way, determining which areas are sources or sinks for any one species may be very difficult,
and an area that is a source for one species may be unimportant to
others. Finally, areas that are sources or sinks currently may not be
in the future as habitats are continually altered by human activity or
climate change. Few areas can be expected to be universal sources, or
universal sinks.
While the presence of source, sink, or trap patches must be considered
for short-term population survival, especially for very small
populations, long-term survival may depend on the creation of networks
of reserves that incorporate a variety of habitats and allow populations
to interact.
