Ecosystem ecology is the integrated study of living and non-living components of ecosystems and their interactions within an ecosystem framework. This science examines how ecosystems work and relates this to their components such as chemicals, soil and animals. Ecosystem ecology examines physical and biological structures and examines how these ecosystem characteristics interact with each other; this helps us understand how to maintain high quality water and economically viable commodity production. A major focus of ecosystem ecology is on functional processes, ecological mechanisms that maintain the structure and services produced by ecosystems; these include primary productivity and trophic interactions. Studies of ecosystem function have improved human understanding of sustainable production of forage, fiber and provision of water. Functional processes are mediated by regional-to-local level climate and management, thus ecosystem ecology provides a powerful framework for identifying ecological mechanisms that interact with global environmental problems global warming and degradation of surface water.
This example demonstrates several important aspects of ecosystems: Ecosystem boundaries are nebulous and may fluctuate in time Organisms within ecosystems are dependent on ecosystem level biological and physical processes Adjacent ecosystems interact and are interdependent for maintenance of community structure and functional processes that maintain productivity and biodiversityThese characteristics introduce practical problems into natural resource management. Who will manage which ecosystem? Will timber cutting in the forest degrade recreational fishing in the stream? These questions are difficult for land managers to address while the boundary between ecosystems remains unclear. We need better understanding of the interactions and interdependencies of these ecosystems and the processes that maintain them before we can begin to address these questions. Ecosystem ecology is an inherently interdisciplinary field of study. An individual ecosystem is composed of populations of organisms, interacting within communities, contributing to the cycling of nutrients and the flow of energy.
The ecosystem is the principal unit of study in ecosystem ecology. Population and physiological ecology provide many of the underlying biological mechanisms influencing ecosystems and the processes they maintain. Flowing of energy and cycling of matter at the ecosystem level are examined in ecosystem ecology, but, as a whole, this science is defined more by subject matter than by scale. Ecosystem ecology approaches organisms and abiotic pools of energy and nutrients as an integrated system which distinguishes it from associated sciences such as biogeochemistry. Biogeochemistry and hydrology focus on several fundamental ecosystem processes such as biologically mediated chemical cycling of nutrients and physical-biological cycling of water. Ecosystem ecology forms the mechanistic basis for regional or global processes encompassed by landscape-to-regional hydrology, global biogeochemistry, earth system science. Ecosystem ecology is philosophically and rooted in terrestrial ecology; the ecosystem concept has evolved during the last 100 years with important ideas developed by Frederic Clements, a botanist who argued for specific definitions of ecosystems and that physiological processes were responsible for their development and persistence.
Although most of Clements ecosystem definitions have been revised by Henry Gleason and Arthur Tansley, by contemporary ecologists, the idea that physiological processes are fundamental to ecosystem structure and function remains central to ecology. Work by Eugene Odum and Howard T. Odum quantified flows of energy and matter at the ecosystem level, thus documenting the general ideas proposed by Clements and his contemporary Charles Elton. In this model, energy flows through the whole system were dependent on biotic and abiotic interactions of each individual component. Work demonstrated that these interactions and flows applied to nutrient cycles, changed over the course of succession, held powerful controls over ecosystem productivity. Transfers of energy and nutrients are innate to ecological systems regardless of whether they are aquatic or terrestrial. Thus, ecosystem ecology has emerged from important biological studies of plants, terrestrial and marine ecosystems. Ecosystem services are ecologically mediated functional processes essential to sustaining healthy human societies.
Water provision and filtration, production of biomass in forestry and fisheries, removal of greenhouse gases such as carbon dioxide from the atmosphere are examples of ecosystem services essential to public health and economic opportunity. Nutrient cycling is a process fundamental to forest production. However, like most ecosystem processes, nutrient cycling is not an ecosystem characteristic which can be “dialed” to the most desirable level. Maximizing production in degraded systems is an overly simplistic solution to the complex problems of hunger and economic security. For instance, intensive fertilizer use in the midwestern United States has resulted in degraded fisheries in the Gulf of Mexico. Regrettably, a “Green Revolution” of intensive chemical fertilization has been recommended for agriculture in developed and developing countries; these strategies risk alteration of ecosystem processes that may be difficult to restore when applied at broad scales without adequate assessme
The biomass is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, the mass of one or more species, or to community biomass, the mass of all species in the community, it can include plants or animals. The mass can be expressed as the total mass in the community. How biomass is measured depends on why it is being measured. Sometimes, the biomass is regarded as the natural mass of organisms in situ. For example, in a salmon fishery, the salmon biomass might be regarded as the total wet weight the salmon would have if they were taken out of the water. In other contexts, biomass can be measured in terms of the dried organic mass, so only 30% of the actual weight might count, the rest being water. For other purposes, only biological tissues count, teeth and shells are excluded. In some applications, biomass is measured as the mass of organically bound carbon, present; the total live biomass on Earth is about 550–560 billion tonnes C, the total annual primary production of biomass is just over 100 billion tonnes C/yr.
The total live biomass of bacteria may be much less. The total number of DNA base pairs on Earth, as a possible approximation of global biodiversity, is estimated at ×1037, weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4×1012 tonnes of carbon. An ecological pyramid is a graphical representation that shows, for a given ecosystem, the relationship between biomass or biological productivity and trophic levels. A biomass pyramid shows the amount of biomass at each trophic level. A productivity pyramid shows the turn-over in biomass at each trophic level. An ecological pyramid provides a snapshot in time of an ecological community; the bottom of the pyramid represents the primary producers. The primary producers take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates; this mechanism is called primary production. The pyramid proceeds through the various trophic levels to the apex predators at the top.
When energy is transferred from one trophic level to the next only ten percent is used to build new biomass. The remaining ninety percent is dissipated as heat; this energy loss means that productivity pyramids are never inverted, limits food chains to about six levels. However, in oceans, biomass pyramids can be wholly or inverted, with more biomass at higher levels. Terrestrial biomass decreases markedly at each higher trophic level. Examples of terrestrial producers are grasses and shrubs; these have a much higher biomass than the animals that consume them, such as deer and insects. The level with the least biomass are the highest predators in the food chain, such as foxes and eagles. In a temperate grassland and other plants are the primary producers at the bottom of the pyramid. Come the primary consumers, such as grasshoppers and bison, followed by the secondary consumers, shrews and small cats; the tertiary consumers, large cats and wolves. The biomass pyramid decreases markedly at each higher level.
Ocean or marine biomass, in a reversal of terrestrial biomass, can increase at higher trophic levels. In the ocean, the food chain starts with phytoplankton, follows the course: Phytoplankton → zooplankton → predatory zooplankton → filter feeders → predatory fish Phytoplankton are the main primary producers at the bottom of the marine food chain. Phytoplankton use photosynthesis to convert inorganic carbon into protoplasm, they are consumed by microscopic animals called zooplankton. Zooplankton comprise the second level in the food chain, includes small crustaceans, such as copepods and krill, the larva of fish, squid and crabs. In turn, small zooplankton are consumed by both larger predatory zooplankters, such as krill, by forage fish, which are small, filter-feeding fish; this makes up the third level in the food chain. The fourth trophic level consists of predatory fish, marine mammals and seabirds that consume forage fish. Examples are swordfish and gannets. Apex predators, such as orcas, which can consume seals, shortfin mako sharks, which can consume swordfish, make up the fifth trophic level.
Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels. Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers is larger than the biomass of primary producers; this happens because the ocean's primary producers are tiny phytoplankton that grow and reproduce so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers reproduce slowly. There is an exception with cyanobacteria. Marine cyanobacteria are the smallest known photosynthetic organisms. Prochlorococcus is the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion individuals. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic regions of the oceans; the bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, forms part of the base of the ocean food chain.
There are 50 million bacterial cells in
A food chain is a linear network of links in a food web starting from producer organisms and ending at apex predator species, detritivores, or decomposer species. A food chain shows how the organisms are related with each other by the food they eat; each level of a food chain represents a different trophic level. A food chain differs from a food web, because the complex network of different animals' feeding relations are aggregated and the chain only follows a direct, linear pathway of one animal at a time. Natural interconnections between food chains make it a food web. A common metric used to the quantify food web trophic structure is food chain length. In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web and the mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web. Food chains were first introduced by the Arab scientist and philosopher Al-Jahiz in the 9th century and popularized in a book published in 1927 by Charles Elton, which introduced the food web concept.
The food chain's length is a continuous variable that provides a measure of the passage of energy and an index of ecological structure that increases in value counting progressively through the linkages in a linear fashion from the lowest to the highest trophic levels. Food chains are used in ecological modeling, they are simplified abstractions of real food webs, but complex in their dynamics and mathematical implications. Ecologists have formulated and tested hypotheses regarding the nature of ecological patterns associated with food chain length, such as increasing length increasing with ecosystem size, reduction of energy at each successive level, or the proposition that long food chain lengths are unstable. Food chain studies have an important role in ecotoxicology studies tracing the pathways and biomagnification of environmental contaminants. Producers, such as plants, are organisms. All food chains must start with a producer. In the deep sea, food chains centered on hydrothermal vents and cold seeps exist in the absence of sunlight.
Chemosynthetic bacteria and archaea use hydrogen sulfide and methane from hydrothermal vents and cold seeps as an energy source to produce carbohydrates. Consumers are organisms. All organisms in a food chain, except the first organism, are consumers. In a food chain, there is reliable energy transfer through each stage. However, all the energy at one stage of the chain is not absorbed by the organism at the next stage; the amount of energy from one stage to another decreases. Heterotroph Lithotroph Trophic pyramid Predator-prey interaction
In biology, adipose tissue, body fat, or fat is a loose connective tissue composed of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction of cells including preadipocytes, vascular endothelial cells and a variety of immune cells such as adipose tissue macrophages. Adipose tissue is derived from preadipocytes, its main role is to store energy in the form of lipids, although it cushions and insulates the body. Far from being hormonally inert, adipose tissue has, in recent years, been recognized as a major endocrine organ, as it produces hormones such as leptin, estrogen and the cytokine TNFα; the two types of adipose tissue are white adipose tissue, which stores energy, brown adipose tissue, which generates body heat. The formation of adipose tissue appears to be controlled in part by the adipose gene. Adipose tissue – more brown adipose tissue – was first identified by the Swiss naturalist Conrad Gessner in 1551. In humans, adipose tissue is located: beneath the skin, around internal organs, in bone marrow, intermuscular and in the breast tissue.
Adipose tissue is found in specific locations, which are referred to as adipose depots. Apart from adipocytes, which comprise the highest percentage of cells within adipose tissue, other cell types are present, collectively termed stromal vascular fraction of cells. SVF includes preadipocytes, adipose tissue macrophages, endothelial cells. Adipose tissue contains many small blood vessels. In the integumentary system, which includes the skin, it accumulates in the deepest level, the subcutaneous layer, providing insulation from heat and cold. Around organs, it provides protective padding. However, its main function is to be a reserve of lipids, which can be oxidised to meet the energy needs of the body and to protect it from excess glucose by storing triglycerides produced by the liver from sugars, although some evidence suggests that most lipid synthesis from carbohydrates occurs in the adipose tissue itself. Adipose depots in different parts of the body have different biochemical profiles. Under normal conditions, it provides feedback for hunger and diet to the brain.
Mice have eight major adipose depots, four of which are within the abdominal cavity. The paired gonadal depots are attached to the uterus and ovaries in females and the epididymis and testes in males; the mesenteric depot forms a glue-like web that supports the intestines and the omental depot and - when massive - extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots, respectively; the two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is covered by a "frosting" of white adipose tissue; the inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial, which surrounds the heart, the paired popliteal depots, between the major muscles behind the knees, each containing one large lymph node.
Of all the depots in the mouse, the gonadal depots are the largest and the most dissected, comprising about 30% of dissectible fat. In an obese person, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus. A panniculus complicates surgery of the morbidly obese individual, it may remain as a literal "apron of skin" if a obese person loses large amounts of fat. This condition cannot be corrected through diet and exercise alone, as the panniculus consists of adipocytes and other supporting cell types shrunken to their minimum volume and diameter. Reconstructive surgery is one method of treatment. Visceral fat or abdominal fat is located inside the abdominal cavity, packed between the organs. Visceral fat is different from subcutaneous fat underneath the skin, intramuscular fat interspersed in skeletal muscles. Fat in the lower body, as in thighs and buttocks, is subcutaneous and is not spaced tissue, whereas fat in the abdomen is visceral and semi-fluid. Visceral fat is composed of several adipose depots, including mesenteric, epididymal white adipose tissue, perirenal depots.
Visceral fat is expressed in terms of its area in cm2. An excess of visceral fat is known as central obesity, or "belly fat", in which the abdomen protrudes excessively. New developments such as the Body Volume Index are designed to measure abdominal volume and abdominal fat. Excess visceral fat is linked to type 2 diabetes, insulin resistance, inflammatory diseases, other obesity-related diseases; the accumulation of neck fat has been shown to be associated with mortality. Several studies have suggested that visceral fat can be predicted from simple anthropometric measures, predicts mortality more than body mass index or waist circumference. Men are more to have fat stored in the abdomen due to sex hormone differences. Female sex hor
An autotroph or primary producer, is an organism that produces complex organic compounds from simple substances present in its surroundings using energy from light or inorganic chemical reactions. They are the producers such as plants on land or algae in water, they do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide; some autotrophs, such as green plants and algae, are phototrophs, meaning that they convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon. Autotrophs can be chemoautotrophs. Phototrophs use light as an energy source, while chemotrophs use electron donors as a source of energy, whether from organic or inorganic sources; such chemotrophs are lithotrophs. Lithotrophs use inorganic compounds, such as hydrogen sulfide, elemental sulfur and ferrous iron, as reducing agents for biosynthesis and chemical energy storage.
Photoautotrophs and lithoautotrophs use a portion of the ATP produced during photosynthesis or the oxidation of inorganic compounds to reduce NADP+ to NADPH to form organic compounds. The Greek term autotroph was coined by the German botanist Albert Bernhard Frank in 1892, it stems from the ancient Greek word τροφή, meaning "nourishment" or "food". Some organisms rely on organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy; such organisms are not defined rather as heterotrophic. An organism that obtains carbon from organic compounds but obtains energy from light is called a photoheterotroph, while an organism that obtains carbon from organic compounds but obtains energy from the oxidation of inorganic compounds is termed a chemoheterotroph, chemolithoheterotroph, or lithoheterotroph. Evidence suggests that some fungi may obtain energy from radiation; such radiotrophic fungi were found growing inside a reactor of the Chernobyl nuclear power plant.
Autotrophs are fundamental to the food chains of all ecosystems in the world. They take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates; this mechanism is called primary production. Other organisms, called heterotrophs, take in autotrophs as food to carry out functions necessary for their life. Thus, heterotrophs — all animals all fungi, as well as most bacteria and protozoa — depend on autotrophs, or primary producers, for the energy and raw materials they need. Heterotrophs obtain energy by breaking down organic molecules obtained in food. Carnivorous organisms rely on autotrophs indirectly, as the nutrients obtained from their heterotroph prey come from autotrophs they have consumed. Most ecosystems are supported by the autotrophic primary production of plants that capture photons released by the sun. Plants can only use a fraction of this energy for photosynthesis 1% is used by autotrophs; the process of photosynthesis splits a water molecule, releasing oxygen into the atmosphere, reducing carbon dioxide to release the hydrogen atoms that fuel the metabolic process of primary production.
Plants convert and store the energy of the photon into the chemical bonds of simple sugars during photosynthesis. These plant sugars are polymerized for storage as long-chain carbohydrates, including other sugars and cellulose; when autotrophs are eaten by heterotrophs, i.e. consumers such as animals, the carbohydrates and proteins contained in them become energy sources for the heterotrophs. Proteins can be made using nitrates and phosphates in the soil. Electrolithoautotroph Organotroph Electrotroph Primary nutritional groups Heterotrophic nutrition
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. Such species are described as playing a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be different or cease to exist altogether; some keystone species, such as the wolf, are apex predators. The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch. While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. An ecosystem may experience a dramatic shift if a keystone species is removed though that species was a small part of the ecosystem by measures of biomass or productivity.
It became a popular concept alongside flagship and umbrella species. Although the concept is valued as a descriptor for strong inter-species interactions, it has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems; the concept of the keystone species was introduced in 1969 by the zoologist Robert T. Paine. Paine developed the concept to explain his observations and experiments on the relationships between marine invertebrates of the intertidal zone, including starfish and mussels, he removed the starfish from an area, documented the effects on the ecosystem. In his 1966 paper, Food Web Complexity and Species Diversity, Paine had described such a system in Makah Bay in Washington. In his 1969 paper, Paine proposed the keystone species concept, using Pisaster ochraceus, a species of starfish, Mytilus californianus, a species of mussel, as a primary example; the concept became popular in conservation, was deployed in a range of contexts and mobilized to engender support for conservation where human activities had damaged ecosystems, such as by removing keystone predators.
A keystone species was defined by Paine as a species that has a disproportionately large effect on its environment relative to its abundance. It has been defined operationally by R. D. Davic in 2003 as "a interacting species whose top-down effect on species diversity and competition is large relative to its biomass dominance within a functional group."A classic keystone species is a predator that prevents a particular herbivorous species from eliminating dominant plant species. If prey numbers are low, keystone predators can be less abundant and still be effective, yet without the predators, the herbivorous prey would explode in numbers, wipe out the dominant plants, alter the character of the ecosystem. The exact scenario changes in each example, but the central idea remains that through a chain of interactions, a non-abundant species has an outsized impact on ecosystem functions. For example, the herbivorous weevil Euhrychiopsis lecontei is thought to have keystone effects on aquatic plant diversity by foraging on nuisance Eurasian watermilfoil in North American waters.
The wasp species Agelaia vicina has been labeled a keystone species for its unparalleled nest size, colony size, high rate of brood production. The diversity of its prey and the quantity necessary to sustain its high rate of growth have a direct impact on other species around it; the keystone concept is defined by its ecological effects, these in turn make it important for conservation. In this it overlaps with several other species conservation concepts such as flagship species, indicator species, umbrella species. For example, the jaguar is a charismatic big cat which meets all of these definitions: The jaguar is an umbrella species, flagship species, wilderness quality indicator, it promotes the goals of carnivore recovery and restoring connectivity through Madrean woodland and riparian areas, protecting and restoring riparian areas.... A reserve system that protects jaguars is an umbrella for many other species.... The jaguar a keystone in subtropical and tropical America... Sea otters protect kelp forests from damage by sea urchins.
When the sea otters of the North American west coast were hunted commercially for their fur, their numbers fell to such low levels – fewer than 1000 in the north Pacific ocean – that they were unable to control the sea urchin population. The urchins in turn grazed the holdfasts of kelp so that the kelp forests disappeared, along with all the species that depended on them. Reintroducing the sea otters has enabled the kelp ecosystem to be restored. For example, in Southeast Alaska some 400 sea otters were released, they have bred to form a population approaching 25,000. Keystone predators may increase the biodiversity of communities by preventing a single species from becoming dominant, they can have a profound influence on the balance of organisms in a particular ecosystem. Introduction or removal of this predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over.
The elimination of the gray wolf from Yellowstone National Park had profound impacts on the trophic pyramid. Without predation, herbivores began to over-graze many woody browse species, affecting the area's plant populations. In addition, wolves kept animals from grazing in riparian areas, which protected beavers from having their food s
The Hereford is a British breed of beef cattle that originated in the county of Herefordshire, in the West Midlands of England. It has been exported to many countries, there are more than five million purebred Hereford cattle in over fifty nations worldwide; the Hereford cattle export trade began from United Kingdom in 1817, starting in Kentucky, United States, spreading across the United States and Canada through Mexico to the great beef-raising countries of South America. Today, Hereford cattle dominate the world scene from Australasia to the Russian steppes, they can be found in Israel and throughout continental Europe and Scandinavia, in the temperate parts of Australia, the United States and Russia, in the centre and east of Argentina, in Uruguay, in Chile and New Zealand, where they make up the largest proportion of registered cattle. They are found all around Brazil and they are found in some Southern African countries, they found great popularity among ranchers of the American Southwest, testament to the hardiness of the breed.
The World Hereford Council is based in the United Kingdom. There are 17 member countries with 20 Hereford societies and 10 nonmember countries, with a total of eight societies. In the United States, the official Hereford organization, breed registry, is the American Hereford Association, it is the second-largest society of its kind in the country. Until the 18th century, the cattle of the Herefordshire area were similar to other cattle of southern England, being wholly red with a white switch, similar to the modern North Devon and Sussex breeds. During the 18th and early 19th centuries, other cattle were used to create a new type of draught and beef cattle which at first varied in colour, different herds ranging from yellow to grey and light brown, with varying amounts of white. However, by the end of the 18th century the white face characteristic of the modern breed was well established, the modern colour was established during the 19th century; the Hereford is still seen in the Herefordshire countryside today and featured prominently at agricultural shows.
The first imports of Herefords to the United States were around 1817 by the politician Henry Clay, with larger importation of the breed beginning in the 1840s. The Polled Hereford is a hornless variant of the Hereford with the polled gene, a natural genetic mutation, selected into a separate breed beginning in 1889. Iowa cattle rancher Warren Gammon capitalised on the idea of breeding Polled Herefords and started the Polled Hereford registry with 11 polled cattle; the American Polled Hereford Association was formed in 1910. The American Polled Hereford and American Hereford breeds have been combined since 1995, under the same American Hereford Association name. Many strains of Hereford have used other cattle breeds to import desired characteristics, this has led to changes in the breed as a whole. However, some strains have been kept separate, these have retained characteristics of the earlier breed, such as hardiness and thriftiness; the Traditional Hereford is now treated as a minority breed of value for genetic conservation.
Eye cancer occurs in Herefords in particular in countries with continued bright sunlight and those that prefer traits of low levels of red pigmentation around the eye. Studies have been made into eye cancer in Hereford cattle in the US and Canada, lid and corneoscleral pigment were found to be heritable and to decrease the risk of cancer. Vaginal prolapse is considered a heritable problem in Hereford cattle, but it may be influenced by nutrition. Another problem is exposed skin on the udder being of light pigmentation and therefore vulnerable to sun burn. Dwarfism is known to be prevalent in Hereford cattle and has been determined to be caused by an autosomal recessive gene. Due to equal occurrence in heifers and bulls, dwarfism is not considered a sex-linked characteristic. Black Hereford Hereford pig List of cattle breeds World Hereford Council American Hereford Association Australian Hereford Society Canadian Hereford Association Irish Hereford Breed Society New Zealand Hereford Association List of US State/National Hereford Associations List of Other International Hereford Associations Polled Hereford Breed Information - Cattle.com The Origin and Growth of Polled Herefords - Oklahoma State University Romanian Hereford cattle Society