Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature reserves, it can be caused by immobile, travel restricted populations of native or non-native wild animals. However, "overgrazing" is a controversial concept, based on equilibrium system theory, it reduces the usefulness and biodiversity of the land and is one cause of desertification and erosion. Overgrazing is seen as a cause of the spread of invasive species of non-native plants and of weeds, it is caused by nomadic grazers in huge populations of travel herds, such as the American bison of the Great Plains, or migratory Wildebeests of the African savannas, or by holistic planned grazing. Sustainable grassland production is based on grass and grassland management, land management, animal management and livestock marketing. Grazing management, with sustainable agriculture and agroecology practices, is the foundation of grassland-based livestock production since it affects both animal and plant health and productivity.
There are several new grazing models and management systems that attempt to reduce or eliminate overgrazing like Holistic management and Permaculture One indicator of overgrazing is that the animals run short of pasture. In some regions of the United States under continuous grazing, overgrazed pastures are predominated by short-grass species such as bluegrass and will be less than 2-3 inches tall in the grazed areas. In other parts of the world, overgrazed pasture is taller than sustainably grazed pasture, with grass heights over 1 meter and dominated by unpalatable species such as Aristida or Imperata. In all cases, palatable tall grasses such as orchard grass are non-existent. In such cases of overgrazing, soil may be visible between plants in the stand, allowing erosion to occur, though in many circumstances overgrazed pastures have a greater sward cover than sustainably grazed pastures. Under rotational grazing, overgrazed plants do not have enough time to recover to the proper height between grazing events.
The animals resume grazing before the plants have restored carbohydrate reserves and grown back roots lost after the last defoliation. The result is the same as under continuous grazing: in some parts of the United States tall-growing species die and short-growing species that are more subject to drought injury predominate the pasture, while in most other parts of the world tall, drought tolerant, unpalatable species such as Imperata or Aristida come to dominate; as the sod thins, weeds encroach into the pasture in some parts of the United States, whereas in most other parts of the worlds overgrazing can promote thick swards of native unpalatable grasses that hamper the spread of weeds. Another indicator of overgrazing in some parts of North America is that livestock run out of pasture, hay needs to be fed early in the fall. In contrast, most areas of the world do not experience the same climatic regime as the continental United States and hay feeding is conducted. Overgrazing is indicated in livestock performance and condition.
Cows having inadequate pasture following their calf's weaning may have poor body condition the following season. This may reduce the vigor of cows and calves at calving. Cows in poor body condition do not cycle as soon after calving, which can result in delayed breeding; this can result in a long calving season. With good cow genetics, ideal seasons and controlled breeding 55% to 75% of the calves should come in the first 21 days of the calving season. Poor weaning weights of calves can be caused by insufficient pasture, when cows give less milk and the calves need pasture to maintain weight gain. Overgrazing increases soil erosion. Reduction in soil depth, soil organic matter and soil fertility impair the land's future natural and agricultural productivity. Soil fertility can sometimes be mitigated by applying organic fertilizers. However, the loss of soil depth and organic matter takes centuries to correct, their loss is critical in determining the soil's water-holding capacity and how well pasture plants do during dry weather.
Native plant grass species, both individual bunch grasses and in grasslands, are vulnerable. In the continental United States, to prevent overgrazing, match the forage supplement to the herd's requirement; this means. Another potential buffer is to plant warm-season perennial grasses such as switchgrass, which do not grow early in the season; this reduces the area that the livestock can use early in the season, making it easier for them to keep up with the cool-season grasses. The animals use the warm-season grasses during the heat of the summer, the cool-season grasses recover for fall grazing; the grazing guidelines in the table are for cool-season forages. When using continuous grazing, manage pasture height at one-half the recommended turn-in height for rotational grazing to optimize plant health; the growth habit of some forage species, such as alfalfa, does not permit their survival under continuous grazing. When managing for legumes in the stand, it is beneficial to use rotational grazing and graze the stand close and give adequate rest to stimulate the legumes' growth.
Overgrazing is used as an example in the economic concept now known as the Tragedy of the Commons devised in a 1968 paper by Garrett Hardin. This cited the work of a Victorian economist who used the over-grazing of common land as an example of behaviour. Hardin's example could only apply to unregulated use of land regarded as
Junipers are coniferous plants in the genus Juniperus of the cypress family Cupressaceae. Depending on taxonomic viewpoint, between 50 and 67 species of junipers are distributed throughout the Northern Hemisphere, from the Arctic, south to tropical Africa, from Ziarat, east to eastern Tibet in the Old World, in the mountains of Central America; the highest-known juniper forest occurs at an altitude of 16,000 ft in southeastern Tibet and the northern Himalayas, creating one of the highest tree-lines on earth. Junipers vary in size and shape from tall trees, 20–40 m tall, to columnar or low-spreading shrubs with long, trailing branches, they are evergreen with needle-like and/or scale-like leaves. They can be either dioecious; the female seed cones are distinctive, with fleshy, fruit-like coalescing scales which fuse together to form a "berry"-like structure, 4–27 mm long, with one to 12 unwinged, hard-shelled seeds. In some species, these "berries" are red-brown or orange; the seed maturation time varies between species from 6 to 18 months after pollination.
The male cones are similar to those of other Cupressaceae, with six to 20 scales. In zones 7 through 10, junipers can release pollen several times each year. A few species of junipers bloom in autumn, while most species pollinate from early winter until late spring. Many junipers have two types of leaves; when juvenile foliage occurs on mature plants, it is most found on shaded shoots, with adult foliage in full sunlight. Leaves on fast-growing'whip' shoots are intermediate between juvenile and adult. In some species, all the foliage is with no scale leaves. In some of these, the needles are jointed at the base, in others, the needles merge smoothly with the stem, not jointed; the needle-leaves of junipers are hard and sharp, making the juvenile foliage prickly to handle. This can be a valuable identification feature in seedlings, as the otherwise similar juvenile foliage of cypresses and other related genera is soft and not prickly. Juniper is the exclusive food plant of the larvae of some Lepidoptera species, including Bucculatrix inusitata and juniper carpet, is eaten by the larvae of other Lepidoptera species such as Chionodes electella, Chionodes viduella, juniper pug, pine beauty.
Junipers are gymnosperms, which means they have no flowers or fruits. Depending on the species, the seeds they produce take 1 -- 3 years; the impermeable coat of the seed keeps water from getting in and protects the embryo when being dispersed. It can result in a long dormancy, broken by physically damaging the seed coat. Dispersal can occur from being swallowed whole by mammals; the resistance of the seed coat allows it to be passed down through the digestive system and out without being destroyed along the way. These seeds last a long time, as they can be dispersed long distances over the course of a few years; the number of juniper species is in dispute, with two recent studies giving different totals, Farjon accepting 52 species, Adams accepting 67 species. The junipers are divided into several sections, though which species belong to which sections is still far from clear, with research still on-going; the section Juniperus is an obvious monophyletic group though. Juniperus sect. Juniperus: Needle-leaf junipers.
The adult leaves are needle-like, in whorls of three, jointed at the base. Juniperus sect. Juniperus subsect. Juniperus: Cones with 3 separate seeds. Juniperus communis – Common juniper Juniperus communis subsp. Alpina – Alpine juniper Juniperus conferta – Shore juniper Juniperus rigida – Temple juniper or needle juniper Juniperus sect. Juniperus subsect. Oxycedrus: Cones with 3 separate seeds. Juniperus brevifolia – Azores juniper Juniperus cedrus – Canary Islands juniper Juniperus deltoides – Eastern prickly juniper Juniperus formosana – Chinese prickly juniper Juniperus lutchuensis – Ryukyu juniper Juniperus navicularis – Portuguese prickly juniper Juniperus oxycedrus – Western prickly juniper or cade juniper Juniperus macrocarpa – Large-berry juniper Juniperus sect. Juniperus subsect. Caryocedrus: Cones with 3 seeds fused together. Juniperus drupacea – Syrian juniperJuniperus sect. Sabina: Scale-leaf junipers; the adult leaves are scale-like, similar to those of Cupressus species, in opposite pairs or whorls of three, the juvenile needle-like leaves are not jointed at the base.
Provisionally, all the other junipers are included here. Old World species Juniperus chinensis – Chinese juniper Juniperus convallium – Mekong juniper Juniperus excelsa – Greek juniper Juniperus excelsa polycarpos – Persian juniper Juniperus foetidissima – Stinking juniper Juniperus indica – Black juniper Juniperus komarovii – Komarov's juniper Juniperus phoenicea – Phoenicean juniper Juniperus pingii – Ping juniper Juniperus procera – East African juniper Juniperus procumbens – Ibuki juniper Juniperu
Outline of geography
The following outline is provided as an overview of and topical guide to geography: Geography – study of earth and its people. An academic discipline – a body of knowledge given to − or received by − a disciple. Modern geography is an all-encompassing discipline that seeks to understand the Earth and all of its human and natural complexities − not where objects are, but how they have changed and come to be. Geography has been called'the world discipline'. A field of science – recognized category of specialized expertise within science, embodies its own terminology and nomenclature; such a field will be represented by one or more scientific journals, where peer reviewed research is published. There are many geography-related scientific journals. A natural science – field of academic scholarship that explores aspects of natural environment. A social science – field of academic scholarship that explores aspects of human society. An interdisciplinary field – a field that crosses traditional boundaries between academic disciplines or schools of thought, as new needs and professions have emerged.
Many of the branches of physical geography are branches of Earth science. As "the bridge between the human and physical sciences," geography is divided into two main branches: human geography physical geographyOther branches include: integrated geography geomatics regional geographyAll the branches are further described below... Physical geography – examines the natural environment and how the climate, vegetation & life, soil and landforms are produced and interact. Geomorphology – study of landforms and the processes that them, more broadly, of the processes controlling the topography of any planet. Seeks to understand why landscapes look the way they do, to understand landform history and dynamics, to predict future changes through a combination of field observation, physical experiment, numerical modeling. Hydrology – study of the movement and quality of water throughout the Earth, including the hydrologic cycle, water resources and environmental watershed sustainability. Glaciology – study of glaciers, or more ice and natural phenomena that involve ice.
Oceanography – studies a wide range of topics pertaining to oceans, including marine organisms and ecosystem dynamics. Biogeography – study of the distribution of species spatially and temporally. Over areal ecological changes, it is tied to the concepts of species and their past, or present living'refugium', their survival locales, or their interim living sites, it aims to reveal where organisms live, at what abundance. Climatology – study of climate, scientifically defined as weather conditions averaged over a period of time. Meteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and short term forecasting. Pedology – study of soils in their natural environment that deals with pedogenesis, soil morphology, soil classification. Palaeogeography – study of what the geography was in times past, most concerning the physical landscape, but the human or cultural environment. Coastal geography – study of the dynamic interface between the ocean and the land, incorporating both the physical geography and the human geography of the coast.
It involves an understanding of coastal weathering processes wave action, sediment movement and weather, the ways in which humans interact with the coast. Quaternary science – focuses on the Quaternary period, which encompasses the last 2.6 million years, including the last ice age and the Holocene period. Landscape ecology – the relationship between spatial patterns of urban development and ecological processes on a multitude of landscape scales and organizational levels. Quantitative geography – Quantitative research tools and methods applied to geography. See the quantitative revolution. Systems approach – Human geography – one of the two main subfields of geography, it is the study of human use and understanding of the world and the processes which have affected it. Human geography broadly differs from physical geography in that it focuses on the built environment and how space is created and managed by humans as well as the influence humans have on the space they occupy. Cultural geography – study of cultural products and norms and their variations across and relations to spaces and places.
It focuses on describing and analyzing the ways language, economy and other cultural phenomena vary or remain constant, from one place to another and on explaining how humans function spatially. Children's geographies – study of places and spaces of children's lives, characterized experientially and ethically. Children's geographies rests on the idea that children as a social group share certain characteristics which are experientially and ethically significant and which are worthy of study; the pluralisation in the title is intended to imply that children's lives will be markedly different in differing times and places and in differing circumstances such as gender and class. The range of focii within children's geographies include: Children and the city Children and the countryside Children and technology Children and nature and globalization Methodologies of researching children's worlds Ethics of resear
The painted bunting is a species of bird in the cardinal family, native to North America. The bright plumage of the male only comes in the second year of life; the painted bunting was described by Carl Linnaeus in his eighteenth-century work Systema Naturae. There are two recognized subspecies of the painted bunting: P. c. ciris –: nominate, breeds in the southeastern United States P. c. pallidior – Mearns, 1911: breeds in south central US and northern Mexico The male painted bunting is described as the most beautiful bird in North America and as such has been nicknamed nonpareil, or "without equal". Its colors, dark blue head, green back, red rump, underparts, make it easy to identify, but it can still be difficult to spot since it skulks in foliage when it is singing; the plumage of female and juvenile painted buntings is green and yellow-green, serving as camouflage. Once seen, the adult female is still distinctive, since it is a brighter, truer green than other similar songbirds. Adult painted buntings can measure 12–14 cm in length, span 21–23 cm across the wings and weigh 13–19 g.
The juveniles have two inserted molts in their first autumn, each yielding plumage like an adult female. The first starts a few days after fledging, replacing the juvenile plumage with an auxiliary formative plumage; the painted bunting occupies typical habitat for a member of its family. It is found in thickets, woodland edges with riparian thickets and brushy areas. In the east, the species breeds in maritime hammocks and scrub communities. Today, it is found along roadsides and in suburban areas, in gardens with dense, shrubby vegetation; the wintering habitat is the shrubby edges along the border of tropical forests or densely vegetated savanna. The breeding range is divided into two geographically separate areas; these include southern Arizona, southern New Mexico and eastern Texas, Arkansas, northern Florida, coastal Georgia, the southern coast and inland waterways such as the Santee River of South Carolina and northern Mexico. They winter in South Florida, the Bahamas, along both coasts of Mexico and through much of Central America.
They may be vagrants further north, including to New York Pennsylvania and New Jersey. The bird is found every few years as far north as New Brunswick, Canada. Painted buntings are shy and difficult to observe for the human eye, though can be approachable where habituated to bird feeders. Males sing in spring from exposed perches to advertise their territories, they engage in visual displays including flying bouncingly like a butterfly or in an upright display, body-fluff display, bow display and wing-quiver display. These displays are used in agonistic conflicts with other males or in breeding displays for females, with females engaging in displays. Males may physically clash with each other and may kill each other in such conflicts; when their breeding season has concluded, buntings migrate by night over short to medium distances. Western birds molt in mid-migration. Painted buntings feed by hopping along the ground, cautiously stopping every few moments to look around; the painted bunting eats a large quantity of grass seeds, including.
Seeds are eaten exclusively during winter. While breeding, painted bunting and nestlings eat small invertebrates, including, they have been known to visit spider webs to pick off small insects caught in them. Painted buntings are monogamous and are solitary or in pairs during the breeding season, but sometimes exhibit polygyny; the breeding season begins in late April and lasts through to early August, with activity peaking mid-May through to mid-July. The male starts to establish a small territory; the nest is hidden in low, dense vegetation and is built by the females and woven into the surrounding vegetation for strength. Each brood contains three or four gray-white eggs spotted with brown, which are incubated for around 10 days until the altricial young are hatched; the female alone cares for the young. The hatchlings are brooded for 12 to 14 days and fledge at that time. About 30 days after the first eggs hatch, the female painted bunting lays a second brood. Nests are parasitized by cowbirds.
Common predators at the nest of eggs and brooding females are large snakes, including coachwhip snakes, eastern kingsnakes, eastern racers and black rat snakes. Bird-hunting raptors, including short-tailed hawks, Accipiter hawks, the small passerine loggerhead shrike, may hunt painted buntings, including the conspicuous breeding-plumaged male; the painted bunting can live to over 10 years of age, though most wild buntings live half that long. The male painted bunting was once a popular caged bird, but its capture and holding is illegal. Trapping for overseas sale may still occur in Central America. Populations are declining due to habitat being lost to development in coastal swamp thickets and woodland edges in the east and riparian habitats in migration and winter in the Southeastern United States and Mexico, they are categorized as near threatened by the IUCN and are protected by the U. S. Migratory Bird Act. L
An oak is a tree or shrub in the genus Quercus of the beech family, Fagaceae. There are 600 extant species of oaks; the common name "oak" appears in the names of species in related genera, notably Lithocarpus, as well as in those of unrelated species such as Grevillea robusta and the Casuarinaceae. The genus Quercus is native to the Northern Hemisphere, includes deciduous and evergreen species extending from cool temperate to tropical latitudes in the Americas, Asia and North Africa. North America contains the largest number of oak species, with 90 occurring in the United States, while Mexico has 160 species of which 109 are endemic; the second greatest center of oak diversity is China, which contains 100 species. Oaks have spirally arranged leaves, with lobate margins in many species. Many deciduous species are marcescent. In spring, a single oak tree produces small female flowers; the fruit is a nut called an oak nut borne in a cup-like structure known as a cupule. The acorns and leaves contain tannic acid, which helps to guard from insects.
The live oaks are distinguished for being evergreen, but are not a distinct group and instead are dispersed across the genus. The oak tree is a flowering plant. Oaks may be divided into two genera and a number of sections: The genus Quercus is divided into the following sections: Sect. Quercus, the white oaks of Europe and North America. Styles are short; the leaves lack a bristle on their lobe tips, which are rounded. The type species is Quercus robur. Sect. Mesobalanus, Hungarian oak and its relatives of Europe and Asia. Styles long; the section Mesobalanus is related to section Quercus and sometimes included in it. Sect. Cerris, the Turkey oak and its relatives of Europe and Asia. Styles long; the inside of the acorn's shell is hairless. Its leaves have sharp lobe tips, with bristles at the lobe tip. Sect. Protobalanus, the canyon live oak and its relatives, in southwest United States and northwest Mexico. Styles short, acorns mature in 18 months and taste bitter; the inside of the acorn shell appears woolly.
Leaves have sharp lobe tips, with bristles at the lobe tip. Sect. Lobatae, the red oaks of North America, Central America and northern South America. Styles long; the inside of the acorn shell appears woolly. The actual nut is encased in a thin, papery skin. Leaves have sharp lobe tips, with spiny bristles at the lobe; the ring-cupped oaks of eastern and southeastern Asia. Evergreen trees growing 10–40 m tall, they are distinct from subgenus Quercus in that they have acorns with distinctive cups bearing concrescent rings of scales. IUCN, ITIS, Encyclopedia of Life and Flora of China treats Cyclobalanopsis as a distinct genus, but some taxonomists consider it a subgenus of Quercus, it contains about 150 species. Species of Cyclobalanopsis are common in the evergreen subtropical laurel forests which extend from southern Japan, southern Korea, Taiwan across southern China and northern Indochina to the eastern Himalayas, in association with trees of genus Castanopsis and the laurel family. Interspecific hybridization is quite common among oaks but between species within the same section only and most common in the white oak group.
Inter-section hybrids, except between species of sections Mesobalanus, are unknown. Recent systematic studies appear to confirm a high tendency of Quercus species to hybridize because of a combination of factors. White oaks are unable to discriminate against pollination by other species in the same section; because they are wind pollinated and they have weak internal barriers to hybridization, hybridization produces functional seeds and fertile hybrid offspring. Ecological stresses near habitat margins, can cause a breakdown of mate recognition as well as a reduction of male function in one parent species. Frequent hybridization among oaks has consequences for oak populations around the world. Frequent hybridization and high levels of introgression have caused different species in the same populations to share up to 50% of their genetic information. Having high rates of hybridization and introgression produces genetic data that does not differentiate between two morphologically distinct species, but instead differentiates populations.
Numerous hypotheses have been proposed to explain how oak species are able to remain morphologically and ecologically distinct with such high levels of gene flow, but the phenomenon is still a mystery to botanists. The Fagaceae, or beech family, to which the oaks belong, is a slow evolving clade compared to other angiosperms, the patterns of hybridization and introgression in Quercus pose a gre
The Harris's sparrow is a large sparrow. Their breeding habitat is the north part of central Canada. In fact, this bird is Canada's only endemic breeder. In the winter they migrate to the Great Plains states of the United States, from lower South Dakota to upper Texas; the common name of this species commemorates the American amateur ornithologist Edward Harris. This species is the largest of the "sparrows" in the family Passerellidae, though other superficially dissimilar species in the family may exceed them in size, they weigh from 26 to 49 g. Among standard measurements, the wing chord is 7.7 to 9.2 cm, the tail is 7.6 to 8.8 cm, the bill is 1.1 to 1.4 cm and the tarsus is 2.2 to 2.5 cm. This is a distinctive looking species. Breeding plumage birds have conspicuous pink bills and black on the crown, face and upper breast, contrasting with grey on the sides of the head and neck; the back is brown, overlaid with heavy black streaking. There are two white wing bars. Breeding birds have white lower underparts with some black mottling on flanks.
Non-breeding adults are more buffy than grey and brown, with reduced or absent black markings and have whitish scalloping on the head and throat. Immatures have less black than all adult plumages marked with a white chin and throat, a black malar stripe and a broad smudgy black breast-band. Juveniles have a brownish crown streaked with black and fine dark streaks and some broader black markings on the underside. Birds moult from July to September. Confusion of wintering birds is possible with the winter Lapland longspur but that species is much shorter-tailed with white outer rectrices, has rich rufous in greater coverts, is terrestrial on open ground; the song of the Harris's sparrow is delivered from a high perch. The song consists of a series of one or more clear high wavering whistles followed by another series in higher or lower pitch, they have been known to call out a strong, metallic chink, as well as some variable musical twittering. The Harris's sparrow breeds in stunted coniferous forests and adjacent scrubs areas of the grand boreal forests where stands of spruce abut mossy bogs.
They nest near the northern limit of tree growth in the forest-tundra ecotone. Typical plant life in the breeding grounds consist of white and black spruce and American larch, dwarf birch-willow, wet sedge meadow and shrubby tundra with dwarf ericad/lichen plant life; the species migrates through tallgrass prairies to winter in open woodlands, woodland edges and clearings, dense riparian thickets and around brush piles. The Harris's sparrow occurs at feeders in suburban and rural gardens during the wintertime, they avoid dry shortgrass prairies and dense woods during winter. Migrating sparrows may be attracted to marsh giant ragweed. Spring migration commences around late February with birds arriving on breeding grounds by May. Sparrows arrive on their wintering grounds as early as late October, though during November and early December. Records from Banff National Park suggest. Fall migration is less strict than spring migration as Harris's sparrows wander about prairie providences in Canada until harsh weather forces them south.
The Harris's sparrow feeds on the ground, scratching vigorously in the leaves and soil for food. Observed foraging during nesting was 85% ground-gleaning. During the breeding season individuals forage alone or with a mate. Primary in the diet are seeds of Carex sedges and Scirpus bulrush. Important in the breeding season diet are fruits black crowberry, mountain bearberry and various Vaccinium species. Pine needles, flower buds and blossoms are eaten supplementally. Larval invertebrates are important from June to September in the diet, as available prey becomes more common at this point; the principal preferred animal prey includes beetles, bees, moths, spiders, cicadas and leafhoppers. The diet of wintering Harris's sparrow has not been quantitatively studied. Males of this species group together to sing at dusk. In winter flocks, Harris's sparrows maintain linear dominance hierarchies that determine access to food and roost sites; the most dominant birds are the oldest males which also have the largest bibs.
"Jump fights" between males occur, consisting of birds facing off leaping at each other, pecking and beating each other with wings. Testosterone alone does not affect the dominance status of winter birds. If first winter birds have their feathers dyed black, creating an artificially large bib, they rise in the dominance hierarchy past females and other young males, it is found in flocks up to several dozen in winter, individuals wandering outside of the "normal" range and turning up in flocks of other sparrow species. Breeding pairs establish a territory of about 200 to 300 km2. Nests are constructed in early to mid-June. Eggs are laid by late June to July, depending on when snow in their habitat melts. In this species, nests are placed in a well-hidden spot on the ground underneath a thick bush or small
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated