90-480: Flatwoods , pineywoods , pine savannas and longleaf pine–wiregrass ecosystem are terms that refer to an ecological community in the southeastern coastal plain of North America . Flatwoods are an ecosystem maintained by wildfire or prescribed fire and are dominated by longleaf pine ( Pinus palustris ), and slash pine ( Pinus elliotii ) in the tree canopy and saw palmetto ( Serenoa repens ), gallberry ( Ilex glabra ) and other flammable evergreen shrubs in
180-507: A community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis , biotic community , biological community , ecological community , or life assemblage . The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization". Community ecology or synecology
270-468: A significant comparison, numbers must be altered for the size of the population that is under study. For example, the fertility rate is calculated as the ratio of the number of births to women of childbearing age to the total number of women in this age range. If these adjustments were not made, we would not know if a nation with a higher rate of births or deaths has a population with more women of childbearing age or more births per eligible woman. Within
360-478: A biological community with respect to ecological interactions is referred to as community structure. Species can compete with each other for finite resources . It is considered an important limiting factor of population size , biomass and species richness . Many types of competition have been described, but proving the existence of these interactions is a matter of debate. Direct competition has been observed between individuals, populations and species, but there
450-568: A community is defined by the interactions between the organisms in it. All species are interdependent, each playing a vital role in the working of the community. Due to this communities are repeatable and easy to identify, with similar abiotic factors controlling throughout. Frederic Clements developed the holistic (or organismic) concept of community, as if it were a superorganism or discrete unit, with sharp boundaries. Clements proposed this theory after noticing that certain plant species were regularly found together in habitats, he concluded that
540-430: A community, by creating physical changes to the environment itself. These species can occupy any trophic level, but tend to be producers. Red mangrove is a foundation species in marine communities. The mangrove's root provides nursery grounds for young fish, such as snappers . Whitebark pine ( Pinus albicaulis ) is a foundation species. Post fire disturbance the tree provides shade (due to its dense growth) enabling
630-416: A community. They cause physical changes to the habitat and alter the resources available to the other organisms present. Dam building beavers are ecological engineers. Through the cutting of trees to form dams they alter the flow of water in a community. These changes influence the vegetation on the riparian zone , studies show biodiversity is increased. Burrowing by the beavers creates channels, increasing
720-401: A country (or other entity) contains Population t persons at time t . What is the size of the population at time t + 1 ? Natural increase from time t to t + 1: Net migration from time t to t + 1: These basic equations can also be applied to subpopulations. For example, the population size of ethnic groups or nationalities within a given society or country is subject to
810-472: A filter to the type of species that are present in the community. Each population has the same adaptive value (competitive and dispersal abilities) and resources demand. Local and regional composition represent a balance between speciation or dispersal (which increase diversity), and random extinctions (which decrease diversity). Species interact in various ways: competition, predation , parasitism , mutualism , commensalism , etc. The organization of
900-550: A fine scale in terms of local processes constructing (or destructing) an assemblage of species, such as the way climate change is likely to affect the make-up of grass communities. Recently this local community focus has been criticized. Robert Ricklefs , a professor of biology at the University of Missouri and author of Disintegration of the Ecological Community , has argued that it is more useful to think of communities on
990-411: A group means bigger prey can be taken, however, the food source must be shared. Wolves are group predators, whilst tigers are solitary. Predation is density dependant , often leading to population cycles. When prey is abundant predator species increases, thus eating more prey species and causing the prey population to decline. Due to lack of food the predator population declines. Due to lack of predation
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#17328518656241080-460: A new product, and to analyze certain dynamics of a company's workforce. Choosing a new location for a branch of a bank, choosing the area in which to start a new supermarket, consulting a bank loan officer that a particular location would be a beneficial site to start a car wash, and determining what shopping area would be best to buy and be redeveloped in metropolis area are types of problems in which demographers can be called upon. Standardization
1170-498: A period of transition where in demography emerged from statistics as a separate field of interest. This period included a panoply of international 'great demographers' like Adolphe Quetelet (1796–1874), William Farr (1807–1883), Louis-Adolphe Bertillon (1821–1883) and his son Jacques (1851–1922), Joseph Körösi (1844–1906), Anders Nicolas Kaier (1838–1919), Richard Böckh (1824–1907), Émile Durkheim (1858–1917), Wilhelm Lexis (1837–1914), and Luigi Bodio (1840–1920) contributed to
1260-405: A regional scale, drawing on evolutionary taxonomy and biogeography , where some species or clades evolve and others go extinct. Today, community ecology focuses on experiments and mathematical models, however, it used to focus primarily on patterns of organisms. For example, taxonomic subdivisions of communities are called populations , while functional partitions are called guilds . Within
1350-472: A role in the trophic pyramid. They provide energy source and nutrients to the plant species in the community. Decomposers such as fungi and bacteria recycle energy back to the base of the food web by feeding on dead organisms from all trophic levels. A guild is a group of species in the community that utilize the same resources in a similar way. Organisms in the same guild experience competition due to their shared resource. Closely related species are often in
1440-475: A species changing gradually along complex environmental gradients. Each species changes independently in relation to other species present along the gradient. Association of species is random and due to coincidence. Varying environmental conditions and each species' probability of arriving and becoming established along the gradient influence the community composition. Individualistic theory proposes that communities can exist as continuous entities, in addition to
1530-426: A standardized mortality rate (SMR) or standardized incidence rate (SIR). Population change is analyzed by measuring the change between one population size to another. Global population continues to rise, which makes population change an essential component to demographics. This is calculated by taking one population size minus the population size in an earlier census . The best way of measuring population change
1620-418: A vampire bat feeding on a cow. Parasitism can however lead to death of the host organism over time. Another example is the feeding on plants of herbivores , for example, a cow grazing. Herbivory is a type of predation in which a plant (the prey in this example) will attempt to dissuade the predator from eating the plant by pumping a toxin to the plant leaves. This may cause the predator to consume other areas of
1710-405: A variety of areas where researchers want to know how populations of social actors can change across time through processes of birth, death, and migration . In the context of human biological populations, demographic analysis uses administrative records to develop an independent estimate of the population. Demographic analysis estimates are often considered a reliable standard for judging
1800-499: A variety of demographic methods for modelling population processes. They include models of mortality (including the life table , Gompertz models , hazards models , Cox proportional hazards models , multiple decrement life tables , Brass relational logits), fertility (Hermes model, Coale -Trussell models, parity progression ratios ), marriage (Singulate Mean at Marriage, Page model), disability ( Sullivan's method , multistate life tables), population projections ( Lee-Carter model ,
1890-446: Is a result of a parasitic relationship between a female Anopheles mosquito and Plasmodium . Mosquitos get the parasite by feeding on an infected vertebrate. Inside the mosquito the plasmodium develops in the midgut's wall. Once developed to a zygote the parasite moves to the salivary glands where it can be passed on to a vertebrate species, for example humans. The mosquito acts as a vector for Malaria. The parasite tends to reduce
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#17328518656241980-458: Is an interaction between species in which both species benefit. An example is Rhizobium bacteria growing in nodules on the roots of legumes. This relationship between plant and bacteria is endosymbiotic , the bacteria living on the roots of the legume. The plant provides compounds made during photosynthesis to the bacteria, that can be used as an energy source. Whilst Rhizobium is a nitrogen fixing bacteria, providing amino acids or ammonium to
2070-417: Is because developed countries have proportionally more older people, who are more likely to die in a given year, so that the overall mortality rate can be higher even if the mortality rate at any given age is lower. A more complete picture of mortality is given by a life table , which summarizes mortality separately at each age. A life table is necessary to give a good estimate of life expectancy. Suppose that
2160-431: Is likely to continue to rise. Populations can change through three processes: fertility, mortality, and migration. Fertility involves the number of children that women have and is to be contrasted with fecundity (a woman's childbearing potential). Mortality is the study of the causes, consequences, and measurement of processes affecting death to members of the population. Demographers most commonly study mortality using
2250-460: Is little evidence that competition has been the driving force in the evolution of large groups. Predation is hunting another species for food. This is a positive-negative interaction, the predator species benefits while the prey species is harmed. Some predators kill their prey before eating them, also known as kill and consume. For example, a hawk catching and killing a mouse. Other predators are parasites that feed on prey while alive, for example,
2340-589: Is moisture. Overall, wet pine savannas have more species than pine savannas, and the distribution of each species within a savanna is intimately connected with soil moisture regimes. Temporary ponds, and seepage areas, are therefore a critical control on plant species composition . Orchids and pitcher plants , for example, are associated with wetter locations. But even these wetter locations burn during dry periods, allowing regeneration of species of pitcher plant and sundew . Pineywoods are characterized by low basal area and large widely spaced mature pine. Historically,
2430-407: Is passed up through trophic levels. Energy is lost at each level, due to ecological inefficiencies . The trophic level of an organism can change based on the other species present. For example, tuna can be an apex predator eating the smaller fish, such as mackerel. However, in a community where a shark species is present the shark becomes the apex predator, feeding on the tuna. Decomposers play
2520-632: Is recurring fire. Without fire, the habitat is eventually invaded by other species of woody plants. A number of rare and endangered animals are typical of this habitat including red-cockaded woodpeckers ( Picoides borealis ), gopher tortoises ( Gopherus polyphemus ), frosted flatwoods salamanders ( Ambystoma cingulatum ), and striped newts ( Notophthalmus perstriatus ). Many rare and usual herbaceous plants are found here, particularly orchids (e.g. Calopogon species, Pogonia ophioglossoides ), sedges (e.g. Rhynchospora species) and carnivorous plants (e.g. Sarracenia species). A second key factor
2610-545: Is seen as the intellectual father of ideas of overpopulation and the limits to growth. Later, more sophisticated and realistic models were presented by Benjamin Gompertz and Verhulst . In 1855, a Belgian scholar Achille Guillard defined demography as the natural and social history of human species or the mathematical knowledge of populations, of their general changes, and of their physical, civil, intellectual, and moral condition. The period 1860–1910 can be characterized as
2700-554: Is the case in much of the developing world, and most of historical demography . One of these techniques in contemporary demography is the sister method, where survey researchers ask women how many of their sisters have died or had children and at what age. With these surveys, researchers can then indirectly estimate birth or death rates for the entire population. Other indirect methods in contemporary demography include asking people about siblings, parents, and children. Other indirect methods are necessary in historical demography. There are
2790-663: Is the study of the interactions between species in communities on many spatial and temporal scales, including the distribution, structure, abundance, demography , and interactions of coexisting populations. The primary focus of community ecology is on the interactions between populations as determined by specific genotypic and phenotypic characteristics. It is important to understand the origin, maintenance, and consequences of species diversity when evaluating community ecology. Community ecology also takes into account abiotic factors that influence species distributions or interactions (e.g. annual temperature or soil pH ). For example,
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2880-432: Is today widely taught in many universities across the world, attracting students with initial training in social sciences, statistics or health studies. Being at the crossroads of several disciplines such as sociology , economics , epidemiology , geography , anthropology and history , demography offers tools to approach a large range of population issues by combining a more technical quantitative approach that represents
2970-510: Is used in a wide variety of contexts. For example, it is often used in business plans, to describe the population connected to the geographic location of the business. Demographic analysis is usually abbreviated as DA. For the 2010 U.S. Census, The U.S. Census Bureau has expanded its DA categories. Also as part of the 2010 U.S. Census, DA now also includes comparative analysis between independent housing estimates, and census address lists at different key time points. Patient demographics form
3060-421: Is using the intercensal percentage change. The intercensal percentage change is the absolute change in population between the censuses divided by the population size in the earlier census. Next, multiply this a hundredfold to receive a percentage . When this statistic is achieved, the population growth between two or more nations that differ in size, can be accurately measured and examined. For there to be
3150-763: The International Union for the Scientific Study of Population , or a national association such as the Population Association of America in the United States, or affiliates of the Federation of Canadian Demographers in Canada . Population composition is the description of population defined by characteristics such as age, race , sex or marital status . These descriptions can be necessary for understanding
3240-649: The Leslie Matrix ), and population momentum ( Keyfitz ). The United Kingdom has a series of four national birth cohort studies, the first three spaced apart by 12 years: the 1946 National Survey of Health and Development , the 1958 National Child Development Study , the 1970 British Cohort Study , and the Millennium Cohort Study , begun much more recently in 2000. These have followed the lives of samples of people (typically beginning with around 17,000 in each study) for many years, and are still continuing. As
3330-505: The Middle Ages , Christian thinkers devoted much time in refuting the Classical ideas on demography. Important contributors to the field were William of Conches , Bartholomew of Lucca , William of Auvergne , William of Pagula , and Muslim sociologists like Ibn Khaldun . One of the earliest demographic studies in the modern period was Natural and Political Observations Made upon
3420-565: The United States and much of Europe ), registry statistics are the best method for estimating the number of births and deaths. A census is the other common direct method of collecting demographic data. A census is usually conducted by a national government and attempts to enumerate every person in a country. In contrast to vital statistics data, which are typically collected continuously and summarized on an annual basis, censuses typically occur only every 10 years or so, and thus are not usually
3510-542: The biodiversity of the community. A species' trophic level is their position in the food chain or web. At the bottom of the food web are autotrophs , also known as primary producer . Producers provide their own energy through photosynthesis or chemosynthesis , plants are primary producers. The next level is herbivores (primary consumers), these species feed on vegetation for their energy source. Herbivores are consumed by omnivores or carnivores . These species are secondary and tertiary consumers. Additional levels to
3600-666: The life table , a statistical device that provides information about the mortality conditions (most notably the life expectancy) in the population. Migration refers to the movement of persons from a locality of origin to a destination place across some predefined, political boundary. Migration researchers do not designate movements 'migrations' unless they are somewhat permanent. Thus, demographers do not consider tourists and travellers to be migrating. While demographers who study migration typically do so through census data on place of residence, indirect sources of data including tax forms and labour force surveys are also important. Demography
3690-433: The social dynamics from historical and comparative research. This data is often compared using a population pyramid . Population composition is also a very important part of historical research. Information ranging back hundreds of years is not always worthwhile, because the numbers of people for which data are available may not provide the information that is important (such as population size ). Lack of information on
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3780-730: The Application of Probabilities to Life Contingencies (1838). In 1755, Benjamin Franklin published his essay Observations Concerning the Increase of Mankind, Peopling of Countries, etc. , projecting exponential growth in British colonies . His work influenced Thomas Robert Malthus , who, writing at the end of the 18th century, feared that, if unchecked, population growth would tend to outstrip growth in food production, leading to ever-increasing famine and poverty (see Malthusian catastrophe ). Malthus
3870-556: The Bills of Mortality (1662) by John Graunt , which contains a primitive form of life table . Among the study's findings were that one-third of the children in London died before their sixteenth birthday. Mathematicians, such as Edmond Halley , developed the life table as the basis for life insurance mathematics. Richard Price was credited with the first textbook on life contingencies published in 1771, followed later by Augustus De Morgan , On
3960-787: The Southeast have contributed to the decline of this community type. However, with the restoration of fire, and natural flooding regimes, it is possible to restore small areas of habitat. Some private organisations are engaged in such projects. With concerted effort, several large wilderness areas could still be restored east of the Mississippi River. Some of the largest remaining areas of this habitat type are found in De Soto National Forest , Eglin Air Force Base, and Apalachicola National Forest . Community (ecology) In ecology,
4050-425: The accuracy of the census information gathered at any time. In the labor force , demographic analysis is used to estimate sizes and flows of populations of workers; in population ecology the focus is on the birth, death, migration and immigration of individuals in a population of living organisms, alternatively, in social human sciences could involve movement of firms and institutional forms. Demographic analysis
4140-467: The basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made. The figure in this section shows the latest (2004) UN ( United Nations ) WHO projections of world population out to
4230-424: The bee. The plant benefits through fertilisation, whilst the bee is provided with food. Commensalism is a type of relationship among organisms in which one organism benefits while the other organism is neither benefited nor harmed. The organism that benefited is called the commensal while the other organism that is neither benefited nor harmed is called the host . For example, an epiphytic orchid attached to
4320-696: The best source of data on births and deaths. Analyses are conducted after a census to estimate how much over or undercounting took place. These compare the sex ratios from the census data to those estimated from natural values and mortality data. Censuses do more than just count people. They typically collect information about families or households in addition to individual characteristics such as age, sex, marital status, literacy/education, employment status, and occupation, and geographical location. They may also collect data on migration (or place of birth or of previous residence), language, religion, nationality (or ethnicity or race), and citizenship. In countries in which
4410-518: The category of standardization , there are two major approaches: direct standardization and indirect standardization. A stable population does not necessarily remain fixed in size. It can be expanding or shrinking. The crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the number of deaths per 1,000 people can be higher in developed nations than in less-developed countries, despite standards of health being better in developed countries. This
4500-475: The community, each species occupies a niche . A species' niche determines how it interacts with the environment around it and its role within the community. By having different niches species are able to coexist. This is known as niche partitioning. For example, the time of day a species hunts or the prey it hunts. Niche partitioning reduces competition between species such that species are able to coexist because they suppress their own growth more than they limit
4590-406: The connections between habitats. This aids the movement of other organisms in the community such as frogs. Community structure is the composition of the community. It is often measured through biological networks , such as food webs . Food webs are a map showing species networks and the energy that links the species together through trophic interactions. Holistic theory refers to the idea that
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#17328518656244680-553: The core of the data for any medical institution, such as patient and emergency contact information and patient medical record data. They allow for the identification of a patient and their categorization into categories for the purpose of statistical analysis. Patient demographics include: date of birth , gender , date of death , postal code, ethnicity, blood type , emergency contact information, family doctor, insurance provider data, allergies , major diagnoses and major medical history. Formal demography limits its object of study to
4770-494: The core of the discipline with many other methods borrowed from social or other sciences. Demographic research is conducted in universities, in research institutes, as well as in statistical departments and in several international agencies. Population institutions are part of the CICRED (International Committee for Coordination of Demographic Research) network while most individual scientists engaged in demographic research are members of
4860-528: The development of demography and to the toolkit of methods and techniques of demographic analysis. Demography is the statistical and mathematical study of the size, composition, and spatial distribution of human populations and how these features change over time. Data are obtained from a census of the population and from registries: records of events like birth , deaths , migrations, marriages, divorces , diseases, and employment . To do this, there needs to be an understanding of how they are calculated and
4950-405: The dimensions and dynamics of populations; it can cover whole societies or groups defined by criteria such as education , nationality , religion , and ethnicity . Educational institutions usually treat demography as a field of sociology , though there are a number of independent demography departments. These methods have primarily been developed to study human populations, but are extended to
5040-418: The discrete groups referred to in the holistic theory. Stephen P. Hubbell introduced the neutral theory of ecology (not to be confused with the neutral theory of molecular evolution ). Within the community (or metacommunity ), species are functionally equivalent, and the abundance of a population of a species changes by stochastic demographic processes (i.e., random births and deaths). Equivalence of
5130-644: The distribution and abundance of organisms. As it relates to organizations and demography, organizations go through various liabilities to their continued survival. Hospitals, like all other large and complex organizations are impacted in the environment they work. For example, a study was done on the closure of acute care hospitals in Florida between a particular time. The study examined effect size, age, and niche density of these particular hospitals. A population theory says that organizational outcomes are mostly determined by environmental factors . Among several factors of
5220-408: The family. The causes of turnover can be split into two separate factors, one linked with the culture of the organization, and the other relating to all other factors. People who do not fully accept a culture might leave voluntarily. Or, some individuals might leave because they fail to fit in and fail to change within a particular organization. A basic definition of population ecology is a study of
5310-437: The flatwoods were dominated by longleaf pine, which can live to be 500 years old. Large scale overharvesting in conjunction with detrimental silvicultural practices like replacement with faster growing loblolly pine has drastically reduced the range of the longleaf pine ecosystem. Longleaf requires frequent fires, ideally every 1–3 years, which prevent invasion of the habitat by other tree species. Decades of fire exclusion in
5400-405: The growth of other species (i.e., the competition within a species is greater than the competition between species, or intraspecific competition is greater than interspecific). The number of niches present in a community determines the number of species present. If two species have the same niche (e.g., the same food demands) then one species outcompetes the other. The more niches filled, the higher
5490-411: The host to prepare an environment suitable for life. For example, kelp has a root like system, called a holdfast , that attaches it to the seabed. Once rooted it provides molluscs , such as sea snails, with a home that protects them from predation. The opposite of commensalism is amensalism , an interspecific relationship in which a product of one organism has a negative effect on another organism but
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#17328518656245580-465: The loss of the wolf population through overhunting resulted in the loss of biodiversity in the community. The wolves had controlled the number of elks in the park, through predation. Without the wolves the elk population drastically increased, resulting in overgrazing . This negatively affected the other organisms in the park; the increased grazing from the elks removed food sources from other animals present. Wolves have since been reintroduced to return
5670-418: The measurement of population processes, while the broader field of social demography or population studies also analyses the relationships between economic, social, institutional, cultural, and biological processes influencing a population. Demographic thoughts traced back to antiquity, and were present in many civilisations and cultures, like Ancient Greece , Ancient Rome , China and India . Made up of
5760-514: The mosquito's lifespan and inhibits the production of offspring. Demography Demography (from Ancient Greek δῆμος ( dêmos ) 'people, society' and -γραφία ( -graphía ) 'writing, drawing, description') is the statistical study of human populations : their size, composition (e.g., ethnic group, age), and how they change through the interplay of fertility (births), mortality (deaths), and migration. Demographic analysis examines and measures
5850-413: The nation will become a nation of immigrants. This influx is projected to rise over the next century as new immigrants and their children will account for over half the U.S. population. These demographic shifts could ignite major adjustments in the economy, more specifically, in labor markets. People decide to exit organizations for many reasons, such as, better jobs, dissatisfaction, and concerns within
5940-458: The original data-collection procedures may prevent accurate evaluation of data quality. The demographic analysis of labor markets can be used to show slow population growth, population aging , and the increased importance of immigration. The U.S. Census Bureau projects that in the next 100 years, the United States will face some dramatic demographic changes. The population is expected to grow more slowly and age more rapidly than ever before and
6030-425: The original organism is unaffected. An example is an interaction between tadpoles of the common frog and a freshwater snail . The tadpoles consume large amounts of micro-algae. Making algae less abundant for the snail, the algae available for the snail is also of lower quality. The tadpole, therefore, has a negative effect on the snail without a gaining noticeable advantage from the snail. The tadpoles would obtain
6120-487: The park community to optimal functioning. See Wolf reintroduction and History of wolves in Yellowstone for more details on this case study. A marine example of a keystone species is Pisaster ochraceus . This starfish controls the abundance of Mytilus californianus , allowing enough resources for the other species in the community. An ecosystem engineer is a species that maintains, modifies and creates aspects of
6210-416: The plant communities inhabiting deserts are very different from those found in tropical rainforests due to differences in annual precipitation. Humans can also affect community structure through habitat disturbance , such as the introduction of invasive species . On a deeper level the meaning and value of the community concept in ecology is up for debate. Communities have traditionally been understood on
6300-441: The plant or not consume the plant at all. Predation may affect the population size of predators and prey and the number of species coexisting in a community. Predation can be specialist, for example the least weasel predates solely on the field vole. Or generalist, e.g. polar bear primarily eats seals but can switch diet to birds when seal population is low. Species can be solitary or group predators. The advantage of hunting in
6390-462: The plant. Insects pollinating the flowers of angiosperms , is another example. Many plants are dependent on pollination from a pollinator. A pollinator transfers pollen from the male flower to the female's stigma . This fertilises the flower and enables the plant to reproduce. Bees, such as honeybees , are the most commonly known pollinators. Bees get nectar from the plant that they use as an energy source. Un-transferred pollen provides protein for
6480-512: The prefix demo- and the suffix -graphy , the term demography refers to the overall study of population. In ancient Greece, this can be found in the writings of Herodotus , Thucydides , Hippocrates , Epicurus , Protagoras , Polus , Plato and Aristotle . In Rome, writers and philosophers like Cicero , Seneca , Pliny the Elder , Marcus Aurelius , Epictetus , Cato , and Columella also expressed important ideas on this ground. In
6570-447: The prey population increases. See Lotka–Volterra equations for more details on this. A well-known example of this is lynx - hare population cycles seen in the north. Predation can result in coevolution – evolutionary arms race , prey adapts to avoid predator, predator evolves. For example, a prey species develops a toxin that kills its predator and the predator evolves resistance to the toxin making it no longer lethal. Mutualism
6660-566: The questions they answer which are included in these four concepts: population change , standardization of population numbers, the demographic bookkeeping equation, and population composition. There are two types of data collection —direct and indirect—with several methods of each type. Direct data comes from vital statistics registries that track all births and deaths as well as certain changes in legal status such as marriage, divorce, and migration (registration of place of residence). In developed countries with good registration systems (such as
6750-527: The regrowth of other plant species in the community, This growth prompts the return of invertebrates and microbes needed for decomposition. Whitebark pine seeds provide food for grizzly bears. Keystone species have a disproportionate influence on the community than most species. Keystone species tend to be at the higher trophic levels, often being the apex predator. Removal of the keystone species causes top-down trophic cascades . Wolves are keystone species, being an apex predator. In Yellowstone National Park
6840-426: The same amount of food with or without the presence of the snail. An older, taller tree can inhibit the growth of smaller trees. A new sapling growing in the shade of a mature tree struggles to get light for photosynthesis. The mature tree also has a well-developed root system, helping it outcompete the sapling for nutrients. Growth of the sapling is therefore impeded, often resulting in death. The relationship between
6930-403: The same guild, due to traits inherited through common descent from their common ancestor . However, guilds are not exclusively composed of closely related species. Carnivores, omnivores and herbivores are all basic examples of guilds. A more precise guild would be vertebrates that forage for ground dwelling arthropods , this would contain certain birds and mammals. Flowering plants that have
7020-405: The same pollinator also form a guild. Certain species have a greater influence on the community through their direct and indirect interactions with other species. The population of influential species are affected by abiotic and biotic disturbances. These species are important in identifying communities of ecology. The loss of these species results in large changes to the community, often reducing
7110-417: The same sources of change. When dealing with ethnic groups, however, "net migration" might have to be subdivided into physical migration and ethnic reidentification ( assimilation ). Individuals who change their ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another. More generally, while
7200-447: The samples have been drawn in a nationally representative way, inferences can be drawn from these studies about the differences between four distinct generations of British people in terms of their health, education, attitudes, childbearing and employment patterns. Indirect standardization is used when a population is small enough that the number of events (births, deaths, etc.) are also small. In this case, methods must be used to produce
7290-523: The species in the community leads to ecological drift. Ecological drift leads to species' populations randomly fluctuating, whilst the overall number of individuals in the community remains constant. When an individual dies, there is an equal chance of each species colonising that plot. Stochastic changes can cause species within the community to go extinct, however, this can take a long time if there are many individuals of that species. Species can coexist because they are similar, resources and conditions apply
7380-400: The species were dependent on each other. Formation of communities is non-random and involves coevolution . The Holistic theory stems from the greater thinking of Holism —which refers to a system with many parts, all required for the system to function. Henry Gleason developed the individualistic (also known as open or continuum) concept of community, with the abundance of a population of
7470-413: The stability of the community. Climate change and the introduction of invasive species can affect the functioning of key species and thus have knock-on effects on the community processes. Industrialization and the introduction of chemical pollutants into environments have forever altered communities and even entire ecosystems. Foundation species largely influence the population, dynamics and processes of
7560-404: The theory, there are four that apply to the hospital closure example: size, age, density of niches in which organizations operate, and density of niches in which organizations are established. Problems in which demographers may be called upon to assist business organizations are when determining the best prospective location in an area of a branch store or service outlet, predicting the demand for
7650-435: The tree for support benefits the orchid but neither harms nor benefits the tree. This type of commensalism is called inquilinism , the orchid permanently lives on the tree. Phoresy is another type of commensalism, the commensal uses the host solely for transport. Many mite species rely on another organism, such as birds or mammals, for dispersal. Metabiosis is the final form of commensalism. The commensal relies on
7740-420: The trophic scale come when smaller omnivores or carnivores are eaten by larger ones. At the top of the food web is the apex predator , this animal species is not consumed by any other in the community. Herbivores, omnivores and carnivores are all heterotrophs . A basic example of a food chain is; grass → rabbit → fox. Food chains become more complex when more species are present, often being food webs. Energy
7830-453: The two trees is amensalism, the mature tree is unaffected by the presence of the smaller one. Parasitism is an interaction in which one organism, the host, is harmed while the other, the parasite, benefits. Parasitism is a symbiosis , a long-term bond in which the parasite feeds on the host or takes resources from the host. Parasites can live within the body such as a tapeworm . Or on the body's surface, for example head-lice . Malaria
7920-490: The understory, along with a high diversity of herb species. It was once one of the dominant ecosystem types of southeastern North America. Although grasses and pines are characteristic of this system, the precise composition changes from west to east, that is, from Texas to Florida . In Louisiana , savannas even differ between the east and west side of the Mississippi River . The key factor maintaining this habitat type
8010-474: The vital registration system may be incomplete, the censuses are also used as a direct source of information about fertility and mortality; for example, the censuses of the People's Republic of China gather information on births and deaths that occurred in the 18 months immediately preceding the census. Indirect methods of collecting data are required in countries and periods where full data are not available, such as
8100-518: The year 2150 (red = high, orange = medium, green = low). The UN "medium" projection shows world population reaching an approximate equilibrium at 9 billion by 2075. Working independently, demographers at the International Institute for Applied Systems Analysis in Austria expect world population to peak at 9 billion by 2070. Throughout the 21st century, the average age of the population
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