Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is invisible to the human eye. Humidity indicates the likelihood for dew, or fog to be present; the amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will reach the saturation point without adding or losing water mass; the amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C. Three primary measurements of humidity are employed: absolute and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.
Specific humidity is the ratio of water vapor mass to total moist air parcel mass. Humidity plays an important role for surface life. For animal life dependent on perspiration to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces; this effect can be calculated using a heat index table known as a humidex. Absolute humidity is the total mass of water vapor present in mass of air, it does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to 30 grams per cubic metre when the air is saturated at 30 °C. Absolute humidity is the mass of the water vapor, divided by the volume of the air and water vapor mixture, which can be expressed as: A H = m H 2 O V n e t; the absolute humidity changes as air pressure changes, if the volume is not fixed. This makes it unsuitable for chemical engineering calculations, e.g. in drying, where temperature can vary considerably.
As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air known as the humidity ratio or mass mixing ratio, better suited for heat and mass balance calculations. Mass of water per unit volume as in the equation above is defined as volumetric humidity; because of the potential confusion, British Standard BS 1339 suggests avoiding the term "absolute humidity". Units should always be checked. Many humidity charts are given in g/kg or kg/kg; the field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named psychrometrics. The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the equilibrium vapor pressure of water over a flat surface of pure water at a given temperature: ϕ = p H 2 O p H 2 O ∗ Relative humidity is expressed as a percentage. Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog.
In hot summer weather, a rise in relative humidity increases the apparent temperature to humans by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at air temperature of 80.0 °F would feel like 83.6 °F ±1.3 °F. Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air parcel. Specific humidity is equal to the mixing ratio, defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel; as temperature decreases, the amount of water vapor needed to reach saturation decreases. As the temperature of a parcel of air becomes lower it will reach the point of saturation without adding or losing water mass. A device used to measure humidity is called a hygrometer. A humidistat is a humidity-triggered switch used to control a dehumidifier. There are various devices used to regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, electrolytic hygrometer.
The gravimetric method, while the most accurate, is cumbersome. For fast and accurate measurement the chilled mirror method is effective. For process on-line measurements, the most used sensors nowadays are based on capacitance measurements to measure relative humidity with internal conversions to d
Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology encompasses the biology of regeneration, asexual reproduction and the growth and differentiation of stem cells in the adult organism. In the late 20th century, the discipline transformed into evolutionary developmental biology; the main processes involved in the embryonic development of animals are: regional specification, cell differentiation and the overall control of timing explored in evolutionary developmental biology: Regional specification refers to the processes that create spatial pattern in a ball or sheet of similar cells. This involves the action of cytoplasmic determinants, located within parts of the fertilized egg, of inductive signals emitted from signaling centers in the embryo; the early stages of regional specification do not generate functional differentiated cells, but cell populations committed to develop to a specific region or part of the organism. These are defined by the expression of specific combinations of transcription factors.
Morphogenesis relates to the formation of three-dimensional shape. It involves the orchestrated movements of cell sheets and of individual cells. Morphogenesis is important for creating the three germ layers of the early embryo and for building up complex structures during organ development. Cell differentiation relates to the formation of functional cell types such as nerve, secretory epithelia etc. Differentiated cells contain large amounts of specific proteins associated with the cell function. Growth involves both an overall increase in size, the differential growth of parts which contributes to morphogenesis. Growth occurs through cell division but through changes of cell size and the deposition of extracellular materials; the control of timing of events and the integration of the various processes with one another is the least well understood area of the subject. It remains unclear; the development of plants involves similar processes to that of animals. However plant cells are immotile so morphogenesis is achieved by differential growth, without cell movements.
The inductive signals and the genes involved are different from those that control animal development. Cell differentiation is the process. For example, muscle fibers and hepatocytes are well known types of differentiated cell. Differentiated cells produce large amounts of a few proteins that are required for their specific function and this gives them the characteristic appearance that enables them to be recognized under the light microscope; the genes encoding these proteins are active. Their chromatin structure is open, allowing access for the transcription enzymes, specific transcription factors bind to regulatory sequences in the DNA in order to activate gene expression. For example, NeuroD is a key transcription factor for neuronal differentiation, myogenin for muscle differentiation, HNF4 for hepatocyte differentiation. Cell differentiation is the final stage of development, preceded by several states of commitment which are not visibly differentiated. A single tissue, formed from a single type of progenitor cell or stem cell consists of several differentiated cell types.
Control of their formation involves a process of lateral inhibition, based on the properties of the Notch signaling pathway. For example, in the neural plate of the embryo this system operates to generate a population of neuronal precursor cells in which NeuroD is expressed. Regeneration indicates the ability to regrow a missing part; this is prevalent amongst plants, which show continuous growth, among colonial animals such as hydroids and ascidians. But most interest by developmental biologists has been shown in the regeneration of parts in free living animals. In particular four models have been the subject of much investigation. Two of these have the ability to regenerate whole bodies: Hydra, which can regenerate any part of the polyp from a small fragment, planarian worms, which can regenerate both heads and tails. Both of these examples have continuous cell turnover fed by stem cells and, at least in planaria, at least some of the stem cells have been shown to be pluripotent; the other two models show only distal regeneration of appendages.
These are the insect appendages the legs of hemimetabolous insects such as the cricket, the limbs of urodele amphibians. Considerable information is now available about amphibian limb regeneration and it is known that each cell type regenerates itself, except for connective tissues where there is considerable interconversion between cartilage and tendons. In terms of the pattern of structures, this is controlled by a re-activation of signals active in the embryo. There is still debate about the old question of whether regeneration is a "pristine" or an "adaptive" property. If the former is the case, with improved knowledge, we might expect to be able to improve regenerative ability in humans. If the latter each instance of regeneration is presumed to have arisen by natural selection in circumstances particular to the species, so no general rules would be expected; the sperm and egg fuse in the process of fertilization to form a fertilized egg, or zygote. This undergoes a period of divisions to form a ball or sheet of similar cells called a blastula or blastoderm.
These cell divisions are rapid with no growth so the daughter cells are half the size of the mother cell and the whole embryo stays ab
The trophic level of an organism is the position it occupies in a food chain. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves; the trophic level of an organism is the number of steps. A food chain starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, finish with apex predators at level 4 or 5; the path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths; the word trophic derives from the Greek τροφή referring to nourishment. The concept of trophic level was developed by Raymond Lindeman, based on the terminology of August Thienemann: "producers", "consumers" and "reducers"; the three basic ways in which organisms get food are as producers and decomposers. Producers are plants or algae. Plants and algae do not eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis.
For this reason, they are called primary producers. In this way, it is energy from the sun that powers the base of the food chain. An exception occurs in deep-sea hydrothermal ecosystems. Here primary producers manufacture food through a process called chemosynthesis. Consumers are species that can not need to consume other organisms. Animals that eat primary producers are called herbivores. Animals that eat other animals are called carnivores, animals that eat both plant and other animals are called omnivores. Decomposers break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as bacteria and fungi, feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again. Trophic levels can be represented starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.
Level 1: Plants and algae make their own food and are called producers. Level 2: Herbivores eat plants and are called primary consumers. Level 3: Carnivores that eat herbivores are called secondary consumers. Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators by definition are at the top of their food chains. In real world ecosystems, there is more than one food chain for most organisms, since most organisms eat more than one kind of food or are eaten by more than one type of predator. A diagram that sets out the intricate network of intersecting and overlapping food chains for an ecosystem is called its food web. Decomposers are left off food webs, but if included, they mark the end of a food chain, thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level; the trophic level of a species may vary.
All plants and phytoplankton are purely phototrophic and are at level 1.0. Many worms are at around 2.1. A 2013 study estimates the average trophic level of human beings at 2.21, similar to pigs or anchovies. This is only an average, plainly both modern and ancient human eating habits are complex and vary greatly. For example, a traditional Eskimo living on a diet consisting of seals would have a trophic level of nearly 5. In general, each trophic level relates to the one below it by absorbing some of the energy it consumes, in this way can be regarded as resting on, or supported by, the next lower trophic level. Food chains can be diagrammed to illustrate the amount of energy that moves from one feeding level to the next in a food chain; this is called an energy pyramid. The energy transferred between levels can be thought of as approximating to a transfer in biomass, so energy pyramids can be viewed as biomass pyramids, picturing the amount of biomass that results at higher levels from biomass consumed at lower levels.
However, when primary producers grow and are consumed the biomass at any one moment may be low. The efficiency with which energy or biomass is transferred from one trophic level to the next is called the ecological efficiency. Consumers at each level convert on average only about 10% of the chemical energy in their food to their own organic tissue. For this reason, food chains extend for more than 5 or 6 levels. At the lowest trophic level, plants convert about 1% of the sunlight they receive into chemical energy, it follows from this that the total energy present in the incident sunlight, embodied in a tertiary consumer is about 0.001% Both the number of trophic levels and the complexity of relationships between them evolve as life diversifies through time, the exception being intermittent mass extinction events. Food webs define ecosystems, the trophic levels define the position of organisms within the webs, but these trophic levels are not always simple integers, because organisms feed at more than one trophic level.
For example, some carnivores eat plants, some plants are carnivores. A large carnivore may eat both smaller car
Competition is an interaction between organisms or species in which both the organisms or species are harmed. Limited supply of at least one resource used by both can be a factor. Competition both within and between species is an important topic in ecology community ecology. Competition is one of abiotic factors that affect community structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. Competition is not always straightforward, can occur in both a direct and indirect fashion. According to the competitive exclusion principle, species less suited to compete for resources should either adapt or die out, although competitive exclusion is found in natural ecosystems. According to evolutionary theory, this competition within and between species for resources is important in natural selection. However, competition may play less of a role than expansion among larger clades.
Competition occurs by various mechanisms, which can be divided into direct and indirect. These apply to intraspecific and interspecific competition. Biologists recognize two types of competition: interference and exploitative competition. During interference competition, organisms interact directly by fighting for scarce resources. For example, large aphids defend feeding sites on cottonwood leaves by ejecting smaller aphids from better sites. In contrast, during exploitative competition, organisms interact indirectly by consuming scarce resources. For example, plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. Plants that produce many roots reduce soil nitrogen to low levels killing neighboring plants. Interference competition occurs directly between individuals via aggression etc. when the individuals interfere with foraging, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat. An example of this can be seen between the ant Novomessor cockerelli and red harvester ants, where the former interferes with the ability of the latter to forage by plugging the entrances to their colonies with small rocks.
Exploitation competition occurs indirectly through a common limiting resource which acts as an intermediate. For example, use of resources depletes the amount available to others. Apparent competition occurs indirectly between two species which are both preyed upon by the same predator. For example, species A and species B are both prey of predator C; the increase of species A may cause the decrease of species B, because the increase of As may aid in the survival of predator Cs, which will increase the number of predator Cs, which in turn will hunt more of species B. Competition varies from complete symmetric to size symmetric to size-asymmetric; the degree of size asymmetry has major effects on the structure and diversity of ecological communities, e.g. in plant communities size-asymmetric competition for light has stronger effects on diversity compared with competition for soil resources. Competition can occur between individuals of the same species, called intraspecific competition, or between different species, called interspecific competition.
Studies show. This occurs. Since individuals within a population require the same resources, crowding causes resources to become more limited; some individuals do not acquire enough resources and die or do not reproduce. This slows population growth. Species interact with other species that require the same resources. Interspecific competition can alter the sizes of many species' populations at the same time. Experiments demonstrate that when species compete for a limited resource, one species drives the populations of other species extinct; these experiments suggest that competing species cannot coexist because the best competitor will exclude all other competing species. Intraspecific competition occurs when members of the same species compete for the same resources in an ecosystem. Interspecific competition may occur when individuals of two separate species share a limiting resource in the same area. If the resource cannot support both populations lowered fecundity, growth, or survival may result in at least one species.
Interspecific competition has the potential to alter populations and the evolution of interacting species. An example among animals could be the case of lions. In fact, lions sometimes steal. Potential competitors can kill each other, in so-called'intraguild predation'. For example, in southern California coyotes kill and eat gray foxes and bobcats, all three carnivores sharing the same stable prey. An example among protozoa involves Paramecium caudatum. Russian ecologist, Georgy Gause, studied the competition between the
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species, he identified sexual selection as a mechanism, but found it problematic. There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric and sympatric. Speciation may be induced artificially, through animal husbandry, agriculture, or laboratory experiments. Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion. Rapid sympatric speciation can take place through polyploidy, such as by doubling of chromosome number. New species can be created through hybridisation followed, if the hybrid is favoured by natural selection, by reproductive isolation.
In addressing the question of the origin of species, there are two key issues: what are the evolutionary mechanisms of speciation, what accounts for the separateness and individuality of species in the biota? Since Charles Darwin's time, efforts to understand the nature of species have focused on the first aspect, it is now agreed that the critical factor behind the origin of new species is reproductive isolation. Next we focus on the second aspect of the origin of species. In On the Origin of Species, Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the rarity of transitional varieties in habitat space.
Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That defined species do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species. It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity; the cost of rarity arises. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a narrow band on that gradient, each species will, of necessity, consist of few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species’ population size happens, by chance, to increase, this will make it easier for its members to find sexual partners.
The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, therefore form pairs less than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species driving them to extinction. Only a few species remain, each distinctly different from the other; the cost of rarity not only involves the costs of failure to find a mate, but indirect costs such as the cost of communication in seeking out a partner at low population densities. Rarity brings with it other costs. Rare and unusual features are seldom advantageous. In most instances, they indicate a mutation, certain to be deleterious, it therefore behooves sexual creatures to avoid mates sporting unusual features. Sexual populations therefore shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta.
This uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, reptiles and many other taxa, in which a species can be described with a single illustration. Once a population has become as homogeneous in appearance as is typical of most species, its members will avoid mating with members of other populations that look different from themselves. Thus, the avoidance of mates displaying rare and unusual phenotypic features leads to reproductive isolation, one of the hallmarks of speciation. In the contrasting case of organisms that reproduce asexually, there is no cost of rarity. Thus, asexual organisms frequently show the continuous variation in form that Darwin expected evolution to produce, making their classification into "species" difficult. All forms of natural speciation have taken place over the course of evolution.
C4 carbon fixation
C4 carbon fixation or the Hatch–Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules, it is one of three known processes for carbon fixation. "C4" refers to the four-carbon molecule, the first product of this type of carbon fixation. C4 fixation is an elaboration of the more common C3 carbon fixation and is believed to have evolved more recently. C4 overcomes the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration; this is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and little oxygen. CO2 is shuttled via aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells CO2 is released by decarboxylation of the malate. C4 plants use. PEP binds to CO2 to make oxaloacetic acid. OAA makes malate. Malate enters bundle sheath cells and releases the CO2.
These additional steps, require more energy in the form of ATP. Using this extra energy, C4 plants are able to more efficiently fix carbon in drought, high temperatures, limitations of nitrogen or CO2. Since the more common C3 pathway does not require this extra energy, it is more efficient in the other conditions; the naming Hatch–Slack pathway is in honor of Marshall Davidson Hatch and C. R. Slack, who elucidated it in Australia in 1966; the first experiments indicating that some plants do not use C3 carbon fixation but instead produce malate and aspartate in the first step of carbon fixation were done in the 1950s and early 1960s by Hugo P. Kortschak and Yuri Karpilov; the C4 pathway was elucidated by Marshall Davidson Hatch and C. R. Slack, in Australia, in 1966. In C3 plants, the first step in the light-independent reactions of photosynthesis involves the fixation of CO2 by the enzyme RuBisCO into 3-phosphoglycerate. However, due to the dual carboxylase and oxygenase activity of RuBisCo, some part of the substrate is oxidized rather than carboxylated, resulting in loss of substrate and consumption of energy, in what is known as photorespiration.
In order to bypass the photorespiration pathway, C4 plants have developed a mechanism to efficiently deliver CO2 to the RuBisCO enzyme. They use their specific leaf anatomy where chloroplasts exist not only in the mesophyll cells in the outer part of their leaves but in the bundle sheath cells as well. Instead of direct fixation to RuBisCO in the Calvin cycle, CO2 is incorporated into a four-carbon organic acid, which has the ability to regenerate CO2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can use this CO2 to generate carbohydrates by the conventional C3 pathway; the first step in the pathway is the conversion of pyruvate to phosphoenolpyruvate, by the enzyme pyruvate orthophosphate dikinase. This reaction requires inorganic phosphate and ATP plus pyruvate, producing phosphoenolpyruvate, AMP, inorganic pyrophosphate; the next step is the fixation of CO2 into oxaloacetate by the enzyme PEP carboxylase. Both of these steps occur in the mesophyll cells: pyruvate + Pi + ATP → PEP + AMP + PPi PEP + CO2 → oxaloacetatePEP carboxylase has a lower Km for HCO−3 — and, higher affinity — than RuBisCO.
Furthermore, O2 is a poor substrate for this enzyme. Thus, at low concentrations of CO2, most CO2 will be fixed by this pathway; the product is converted to malate, a simple organic compound, transported to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated to produce pyruvate; the CO2 now enters the pyruvate is transported back to the mesophyll cell. Since every CO2 molecule has to be fixed twice, first by four-carbon organic acid and second by RuBisCO, the C4 pathway uses more energy than the C3 pathway; the C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss. There are several variants of this pathway: The four-carbon acid transported from mesophyll cells may be malate, as above, or aspartate.
The three-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine. The enzyme that catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme; the C4 plants possess a characteristic leaf anatomy called kranz anatomy, from the German word for wreath. Their vascular bundles are surrounded by two rings of cells. Hence, the chloroplasts are called dimorphic; the primary function of kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby avoiding photorespiration. In order to maintain a higher CO2 concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to CO2, a property that may be enhanced by the presence of suberin; the carbon concentration mechanism in C4 plants distinguishes their isotopic signature from other photosynthetic organisms. Although most C4 plants exhibit kranz anatomy, there are, however, a few species that operate a limited C4 cycle without any distinct bundle sheath ti
Ecology is the branch of biology which studies the interactions among organisms and their environment. Objects of study include interactions of organisms that include biotic and abiotic components of their environment. Topics of interest include the biodiversity, distribution and populations of organisms, as well as cooperation and competition within and between species. Ecosystems are dynamically interacting systems of organisms, the communities they make up, the non-living components of their environment. Ecosystem processes, such as primary production, nutrient cycling, niche construction, regulate the flux of energy and matter through an environment; these processes are sustained by organisms with specific life history traits. Biodiversity means the varieties of species and ecosystems, enhances certain ecosystem services. Ecology is not synonymous with natural history, or environmental science, it overlaps with the related sciences of evolutionary biology and ethology. An important focus for ecologists is to improve the understanding of how biodiversity affects ecological function.
Ecologists seek to explain: Life processes and adaptations The movement of materials and energy through living communities The successional development of ecosystems The abundance and distribution of organisms and biodiversity in the context of the environment. Ecology has practical applications in conservation biology, wetland management, natural resource management, city planning, community health, economics and applied science, human social interaction. For example, the Circles of Sustainability approach treats ecology as more than the environment'out there', it is not treated as separate from humans. Organisms and resources compose ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living and non-living components of the planet. Ecosystems sustain life-supporting functions and produce natural capital like biomass production, the regulation of climate, global biogeochemical cycles, water filtration, soil formation, erosion control, flood protection, many other natural features of scientific, economic, or intrinsic value.
The word "ecology" was coined in 1866 by the German scientist Ernst Haeckel. Ecological thought is derivative of established currents in philosophy from ethics and politics. Ancient Greek philosophers such as Hippocrates and Aristotle laid the foundations of ecology in their studies on natural history. Modern ecology became a much more rigorous science in the late 19th century. Evolutionary concepts relating to adaptation and natural selection became the cornerstones of modern ecological theory; the scope of ecology contains a wide array of interacting levels of organization spanning micro-level to a planetary scale phenomena. Ecosystems, for example, contain interacting life forms. Ecosystems are dynamic, they do not always follow a linear successional path, but they are always changing and sometimes so that it can take thousands of years for ecological processes to bring about certain successional stages of a forest. An ecosystem's area can vary from tiny to vast. A single tree is of little consequence to the classification of a forest ecosystem, but critically relevant to organisms living in and on it.
Several generations of an aphid population can exist over the lifespan of a single leaf. Each of those aphids, in turn, support diverse bacterial communities; the nature of connections in ecological communities cannot be explained by knowing the details of each species in isolation, because the emergent pattern is neither revealed nor predicted until the ecosystem is studied as an integrated whole. Some ecological principles, however, do exhibit collective properties where the sum of the components explain the properties of the whole, such as birth rates of a population being equal to the sum of individual births over a designated time frame; the main subdisciplines of ecology, population ecology and ecosystem ecology, exhibit a difference not only of scale, but of two contrasting paradigms in the field. The former focus on organisms distribution and abundance, while the focus on materials and energy fluxes; the scale of ecological dynamics can operate like a closed system, such as aphids migrating on a single tree, while at the same time remain open with regard to broader scale influences, such as atmosphere or climate.
Hence, ecologists classify ecosystems hierarchically by analyzing data collected from finer scale units, such as vegetation associations and soil types, integrate this information to identify emergent patterns of uniform organization and processes that operate on local to regional and chronological scales. To structure the study of ecology into a conceptually manageable framework, the biological world is organized into a nested hierarchy, ranging in scale from genes, to cells, to tissues, to organs, to organisms, to species, to populations, to communities, to ecosystems, to biomes, up to the level of the biosphere; this framework exhibits non-linear behaviors.