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Ecology

Ecology (from Greek: οἶκος, "house"; -λογία, "study of") is the scientific study of the relation of living organisms to each other and their surroundings. Ecosystems are defined by a web, community, or network of individuals that arrange into a self-organized and complex hierarchy of pattern and process. Ecosystems create a biophysical feedback between living (biotic) and nonliving (abiotic) components of an environment that generates and regulates the biogeochemical cycles of the planet. Ecosystems provide goods and services that sustain human societies and general well-being. Ecosystems are sustained by biodiversity within them.[1][2] Biodiversity is the full-scale of life and its processes, including genes, species and ecosystems forming lineages that integrate into a complex and regenerative spatial arrangement of types, forms, and interactions.[3]

Ecology is a sub-discipline of biology, the study of life. The word "ecology" ("oekologie") was coined in 1866 by the German scientist Ernst Haeckel (1834–1919). Haeckel was a zoologist, artist, writer, and later in life a professor of comparative anatomy.[4][5] Ancient philosophers of Greece, including Hippocrates and Aristotle, were among the earliest to record notes and observations on the natural history of plants and animals; the early rudiments of modern ecology. Modern ecology mostly branched out of natural history science that flourished in the late 19th century. Charles Darwin's evolutionary treatise and the concept of adaptation as it was introduced in 1859 is a pivotal cornerstone in modern ecological theory.[1][2][6]

Ecology is not synonymous with environment, environmentalism, natural history or environmental science.[1][7] Ecology is closely related to the biological disciplines of physiology, evolution, genetics and behavior.[8][9] An understanding of how biodiversity affects ecological function is an important focus area in ecological studies. Ecosystems sustain every life-supporting function on the planet, including climate regulation, water filtration, soil formation (pedogenesis), food, fibe
r

s, medicines, erosion control, and many other natural features of historical, spiritual or scientific value.[10][11]

Ecologists seek to explain:

* life processes and adaptations
* distribution and abundance of organisms
* the movement of materials and energy through living communities
* the successional development of ecosystems,[8] and
* the abundance and distribution of biodiversity in context of the environment.[1][7][8]

There are many practical applications of ecology in conservation biology, wetland management, natural resource management (agriculture, forestry, fisheries), city planning (urban ecology), community health, economics, basic & applied science and it provides a conceptual framework for understanding and researching human social interaction (human ecology).[8][12][13][14]
Scale and complexity

Ecosystems are forever confronted with a range of natural environmental fluctuations that vary transiently in magnitude through space and time.[16] It can take thousands of years for ecological processes to mature; the life-span of a tree, for example, can encompass different successional stages. The ecological process is extended even further through time as trees die, decay and provide habitat as nurse logs or coarse woody debris. The area of an ecosystem can vary greatly from tiny to vast. A single tree is of little consequence to the classification of a forest ecosystem, but critically relevant to the smaller organisms living in and on it.[17] 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.[18] Fine scale structure of aphid populations can be constrained by top-down influences of tree growth that is related to site specific variables, such as soil type, moisture content, slope of the land, and forest canopy closure. Likewise, finer scale dynamics operating in the aphid populations can impart bottom-up influence on tree growth rates.[19] The scale of ecological dynamics can operate as a closed island with respect to local site variables, such as aphids migrating on a tree, while at the same time remain open with regard to broader scale influences, such as atmosphere or climate. Hence, ecologists have devised means of hierarchically classifying ecosystems by analyzing data collected from finer scale units, such as vegetation associations, climate, and soil types, and integrate this information to identify larger emergent patterns of uniform organization and processes that operate on regional, local, and chronological scales.[20][21]

There are different views on complexity and how it relates to ecology. One perspective lumps things that we do not understand into this category by virtue of the computational effort it would require to piece together the numerous interacting parts. Alternatively, complexity in life sciences can be viewed as emergent self-organized systems with multiple possible outcomes directed by random accidents of history; an extension of the first perspective.[22] Global patterns of biological diversity are complex. This biocomplexity stems from the interplay among ecological processes that operate and influence patterns that grade into each other, such as transitional areas or ecotones that stretch across different scales.[23] "Complexity in ecology is of at least six distinct types: spatial, temporal, structural, process, behavioral, and geometric."[24]:3 Small scale patterns do not necessarily explain large scale phenomena, otherwise captured in the expression 'the sum is greater than the parts'.[25][26] Ecologists have identified emergent and self-organizing phenomena that operate at different environmental scales of influence, ranging from molecular to planetary, and these require different sets of scientific explanation.[27][28] Long-term ecological studies provide important track records to better understand the complexity of ecosystems over longer temporal and broader spatial scales. The International Long Term Ecological Network[29] manages and exchanges scientific information among research sites. The longest experiment in existence is the Park Grass Experiment that was initiated in 1856.[30] Another example includes the Hubbard Brook study in operation since 1960.[31]

To structure the study of ecology into a manageable framework of understanding, the biological world is conceptually organized as a nested hierarchy of organization, ranging in scale from genes, to cells, to tissues, to organs, to organisms, to species and up to the level of the biosphere.[32] Together these hierarchical scales of life form a panarchy.[22] Ecosystems are primarily researched at three key levels of organization—organisms, populations, and communities. Ecologists study ecosystems by sampling a certain number of individuals that are representative of a population. Ecosystems consist of communities interacting with each other and the environment. In ecology, communities are created by the interaction of the populations of different species in an area.[1][33]

Biodiversity is the variety of life and its processes. It includes the variety of living organisms, the genetic differences among them, the communities and ecosystems in which they occur, and the ecological and evolutionary processes that keep them functioning, yet ever changing and adapting.[34]:5

Biodiversity (an abbreviation of biological diversity) describes the diversity of life from genes to ecosystems and spans every level of biological organization. Biodiversity means different things to different people and there are many ways to index, measure, characterize, and represent its complex organization.[3][35] Biodiversity includes species diversity, ecosystem diversity, genetic diversity and the complex processes operating at and among these respective levels.[35][36][37] Biodiversity plays an important role in ecological health as much as it does for human health.[38][39] Preventing or prioritizing species extinctions is one way to preserve biodiversity, but populations, the genetic diversity within them and ecological processes, such as migration, are being threatened on global scales and disappearing rapidly as well. Conservation priorities and management techniques require different approaches and considerations to address the full ecological scope of biodiversity. Populations and species migration, for example, are more sensitive indicators of ecosystem services that sustain and contribute natural capital toward the well-being of humanity.[40][41][42][43] An understanding of biodiversity has practical application for ecosystem-based conservation planners as they make ecologically responsible decisions in management recommendations to consultant firms, governments and industry.[44]
Ecological niche and habitat
There are many definitions of the niche dating back to 1917,[47] but G. Evelyn Hutchinson made conceptual advances in 1957[48][49] and introduced the most widely accepted definition: "The niche is the set of biotic and abiotic conditions in which a species is able to persist and maintain stable population sizes."[47]:519 The ecological niche is a central concept in the ecology of organisms and is sub-divided into the fundamental and the realized niche. The fundamental niche is the set of environmental conditions under which a species is able to persist. The realized niche is the set of environmental plus ecological conditions under which a species persists.[1][47][49] The Hutchisonian niche is defined more technically as an "euclidean hyperspace whose dimensions are defined as environmental variables and whose size is a function of the number of values that the environmental values may assume for which an organism has positive fitness."[50]:71

The habitat of a species is a related but distinct concept that describes the environment over which a species is known to occur and the type of community that is formed as a result.[51] More specifically, "habitats can be defined as regions in environmental space that are composed of multiple dimensions, each representing a biotic or abiotic environmental variable; that is, any component or characteristic of the environment related directly (e.g. forage biomass and quality) or indirectly (e.g. elevation) to the use of a location by the animal."[52]:745 For example, the habitat might refer to an aquatic or terrestrial environment that can be further categorized as montane or alpine ecosystems.

Biogeographical patterns and range distributions are explained or predicted through knowledge and understanding of a species traits and niche requirements.[53] Species have functional traits that are uniquely adapted to the ecological niche. A trait is a measurable property, phenotype, or characteristic of an organism that influences its performance. Genes play an important role in the development and expression of traits.[54] Resident species evolve traits that are fitted to their local environment. This tends to afford them a competitive advantage and discourages similarly adapted species from having an overlapping geographic range. The competitive exclusion principle suggests that two species cannot coexist indefinitely by living off the same limiting resource. When similarly adapted species are found to overlap geographically, closer inspection reveals subtle ecological differences in their habitat or dietary requirements.[55]
Niche construction

Organisms are subject to environmental pressures, but they are also modifiers of their habitats. The regulatory feedback between organisms and their environment can modify conditions from local (e.g., a beaver pond) to global scales (e.g., Gaia), over time and even after death, such as decaying logs or silica skeleton deposits from marine organisms.[57] The process and concept of ecosystem engineering has also been called niche construction. Ecosystem engineers are defined as: "...organisms that directly or indirectly modulate the availability of resources to other species, by causing physical state changes in biotic or abiotic materials. In so doing they modify, maintain and create habitats."[58]:373

The ecosystem engineering concept has stimulated a new appreciation for the degree of influence that organisms have on the ecosystem and evolutionary process. The terms niche construction are more often used in reference to the under appreciated feedback mechanism of natural selection imparting forces on the abiotic niche.[45][59] An example of natural selection through ecosystem engineering occurs in the nests of social insects, including ants, bees, wasps, and termites. There is an emergent homeostasis or homeorhesis in the structure of the nest that regulates, maintains and defends the physiology of the entire colony. Termite mounds, for example, maintain
a constant internal temperature through the design of air-conditioning chimneys. The structure of the nests themselves are subject to the forces of natural selection. Moreover, the nest can survive over successive generations, which means that ancestors inherit both genetic material and a legacy niche that was constructed before their time.[2][45][46][60]
Population ecology

The population is the unit of analysis in population ecology. A population consists of individuals of the same species that live, interact and migrate through the same niche and habitat.[61] A primary law of population ecology is the Malthusian growth model.[62] This law states that:

"...a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."[62]:18

This Malthusian premise provides the basis for formulating predictive theories and tests that follow. Simplified population models usually start with four variables including death, birth, immigration, and emigration. Mathematical models are used to calculate changes in population demographics using a null model. A null model is used as a null hypothesis for statistical testing. The null hypothesis states that random processes create observed patterns. Alternatively the patterns differ significantly from the random model and require further explanation. Models can be mathematically complex where "...several competing hypotheses are simultaneously confronted with the data."[63] An example of an introductory population model describes a closed population, such as on an island, where immigration and emigration does not take place. In these island models the rate of population change is described by:

\frac{dN}{dT} = B - D = bN - dN = (b - d)N = rN,

where N is the total number of individuals in the population, B is the number of births, D is the number of deaths, b and d are the per capita rates of birth and death respectively, and r is the per capita rate of population change. This formula can be read out as the rate of change in the population (dN/dT) is equal to births minus deaths (B – D).[62][64]

Using these modelling techniques, Malthus' population principle of growth was later transformed into a model known as the logistic equation:

\frac{dN}{dT} = aN(1-\frac{N}{K}),

where N is the number of individuals measured as biomass density, a is the maximum per-capita rate of change, and K is the carrying capacity of the population. The formula can be read as follows: the rate of change in the population (dN/dT) is equal to growth (aN) that is limited by carrying capacity (1 – N/K). The discipline of population ecology builds upon these introductory models to further understand demographic processes in real study populations and conduct statistical tests. The field of population ecology often uses data on life history and matrix algebra to develop projection matrices on fecundity and survivorship. This information is used for managing wildlife stocks and setting harvest quotas.[64][65]
Metapopulation ecology

Populations are also studied and modeled according to the metapopulation concept. The metapopulation concept was introduced in 1969:[67] "as a population of populations which go extinct locally and recolonize."[68]:105 Metapopulation ecology is another statistical approach that is often used in conservation research.[69] Metapopulation research simplifies the landscape into patches of varying levels of quality.[70]

In metapopulation terminology there are emigrants (individuals that leave a patch), immigrants (individuals that move into a patch) and sites are classed either as sources or sinks. A site is a generic term that refers to places where ecologists sample populations, such as ponds or defined sampling areas in a forest. Source patches are productive sites that generate a seasonal supply of juveniles that migrate to other patch locations. Sink patches are unproductive sites that only receive migrants and will go extinct unless rescued by an adjacent source patch or environmental conditions become more favorable. Metapopulation models examine patch dynamics over time to answer questions about spatial and demographic ecology. The ecology of metapopulations is a dynamic process of extinction and colonization. Small patches of lower quality (i.e., sinks) are maintained or rescued by a seasonal influx of new immigrants. A dynamic metapopulation structure evolves from year to year, where some patches are sinks in dry years and become sources when conditions are more favorable. Ecologists use a mixture of computer models and field studies to
explain metapopulation structure.[71][72]
Community ecology
Community ecology is the study of the interactions among a collection of interdependent species that cohabitate the same geographic area. An example of a study in community ecology might measure primary production in a wetland in relation to decomposition and consumption rates. This requires an understanding of the community connections between plants (i.e., primary producers) and the decomposers (e.g., fungi and bacteria).[74] or the analysis of predator-prey dynamics affecting amphibian biomass.[75] Food webs and trophic levels are two widely employed conceptual models used to explain the linkages among species.[76][77]
Food webs

A food web is the archetypal ecological network. They are a type of concept map that illustrate pathways of energy flows in an ecological community, usually starting with solar energy being used by plants during photosynthesis. As plants grow, they accumulate carbohydrates and are eaten by grazing herbivores. Step by step lines or relations are drawn until a web of life is illustrated.[78][79][80][81]
Freshwater aquatic and terrestrial food-webs
Generalized food web of waterbirds from Chesapeake Bay

There are different ecological dimensions that can be mapped to create more complicated food webs, including: species composition (type of species), richness (number of species), biomass (the dry weight of plants and animals), productivity (rates of conversion of energy and nutrients into growth), and stability (food webs over time). A food web diagram illustrating species composition shows how change in a single species can directly and indirectly influence many others. Microcosm studies are used to simplify food web research into semi-isolated units such as small springs, decaying logs, and laboratory experiments using organisms that reproduce quickly, such as daphnia feeding on algae grown under controlled environments in jars of water.[82][83]

Principles gleaned from food web microcosm studies are used to extrapolate smaller dynamic concepts to larger systems.[83] Food webs are limited because they are generally restricted to a specific habitat, such as a cave or a pond. The food web illustration (right) only shows a small part of the complexity connecting the aquatic system to the adjacent terrestrial land. Many of these species migrate into other habitats to distribute their effects on a larger scale. In other words, food webs are incomplete, but are nonetheless a valuable tool in understanding community ecosystems.[84]
Trophic dynamics

The Greek root of the word troph, τροφή, trophē, means food or feeding. Links in food-webs primarily connect feeding relations or trophism among species. Biodiversity within ecosystems can be organized into vertical and horizontal dimensions. The vertical dimension represents feeding relations that become further removed from the base of the food chain up toward top predators. The horizontal dimension represents the abundance or biomass at each level.[85] When the relative abundance or biomass of each functional feeding group is stacked into their respective trophic levels they naturally sort into a ‘pyramid of numbers’.[86] Functional groups are broadly categorized as autotrophs (or primary producers), heterotrophs (or consumers), and detrivores (or decomposers). Heterotrophs can be further sub-divided into different functional groups, including: primary consumers (strict herbivores), secondary consumers (predators that feed exclusively on herbivores) and tertiary consumers (predators that feed on a mix of herbivores and predators).[87] Omnivores do not fit neatly into a functional category because they eat both plant and animal tissues. It has been suggested that omnivores have a greater functional influence as predators because relative to herbivores they are comparatively inefficient at grazing.[88]

The decomposition of dead organic matter, such as leaves falling on the forest floor, turns into soils that feed plant production. The total sum of the planet's soil ecosystems is called the pedosphere where a very large proportion of the Earth's biodiversity sorts into other trophic levels. Invertebrates that feed and shred larger leaves, for example, create smaller bits for smaller organisms in the feeding chain. Collectively, these are the detrivores that regulate soil formation.[89][90] Tree roots, fungi, bacteria, worms, ants, beetles, centipedes, spiders, mammals, birds, reptiles, amphibians and other less familiar creatures all work to create the trophic web of life in soil ecosystems. As organisms feed and migrate through soils they physically displace materials, which is an important ecological process called bioturbation. Biomass of soil microorganisms are influenced by and feed back into the trophic dynamics of the exposed solar surface ecology. Paleoecological studies of soils places the origin for bioturbation to a time before the Cambrian period. Other events, such as the evolution of trees and amphibians moving into land in the Devonian period played a significant role in the development of soils and ecological trophism.[75][90][91]
Functional trophic groups sort out hierarchically into pyramidic trophic levels because it requires specialized adaptations to become a photosynthesizer or a predator, so few organisms have the adaptations needed to combine both abilities. This explains why functional adaptations to trophism (feeding) organizes different species into emergent functional groups.[88] Trophic levels are part of the holistic or complex systems view of ecosystems.[92][93] Each trophic level contains unrelated species that grouped together because they share common ecological functions. Grouping functionally similar species into a trophic system gives a macroscopic image of the larger functional design.[94]
Keystone species

A keystone species is a species that is disproportionately connected to more species in the food-web. Keystone species have lower levels of biomass in the trophic pyramid relative to the importance of their role. The many connections that a keystone species holds means that it maintains the organization and structure of entire communities. The loss of a keystone species results in a range of dramatic cascading effects that alters trophic dynamics, other food-web connections and can cause the extinction of other species in the community.[95][96]

Sea otters (Enhydra lutris) are commonly cited as an example of a keystone species because they limit the density of sea urchins that feed on kelp. If sea otters are removed from the system, the urchins graze until the kelp beds disappear and this has a dramatic effect on community structure.[97] Hunting of sea otters, for example, is thought to have indirectly led to the extinction of the Steller's Sea Cow (Hydrodamalis gigas).[98] While the keystone species concept has been used extensively as a conservation tool, it has been criticized for being poorly defined from an operational stance. It is very difficult to experimentally determine in each different ecosystem what species may hold a keystone role. Furthermore, food-web theory suggests that keystone species may not be all that common. It is therefore unclear how generally the keystone species model can be applied.[97][99]
Ecosystem ecology
The concept of the ecosystem was first introduced in 1935 to describe habitats within biomes that form an integrated whole and a dynamically responsive system having both physical and biological complexes. Within an ecosystem there are inseparable ties that link organisms to the physical and biological components of their environment to which they are adapted.[100] Ecosystems are complex adaptive systems where the interaction of life processes form self-organizing patterns across different scales of time and space.[101] This section introduces key areas of ecosystem ecology that are used to inquire, understand and explain observed patterns of biodiversity and ecosystem function across different scales of organization.
Biome
Ecological units of organization are defined through reference to any magnitude of space and time on the planet. Communities of organisms, for example, are somewhat arbitrarily defined, but the processes of life integrate at different levels and organize into more complex wholes. Biomes, for example, are a larger unit of organization that categorize regions of the Earth's ecosystems mainly according to the structure and composition of vegetation.[102] Different researchers have applied different methods to define continental boundaries of biomes dominated by different functional types of vegetative communities that are limited in distribution by climate, precipitation, weather and other environmental variables. Examples of biome names include: tropical rainforest, temperate broadleaf and mixed forests, temperate deciduous forest, taiga, tundra, hot desert, and polar desert.[103] Other researchers have recently started to categorize other types of biomes, such as the human and oceanic microbiomes. To a microbe, the human body is a habitat and a landscape.[104] The microbiome has been largely discovered through advances in molecular genetics that have revealed a hidden richness of microbial diversity on the planet. The oceanic microbiome plays a significant role in the ecological biogeochemistry of the planet's oceans.[105]
Biosphere
Ecological theory has been used to explain self-emergent regulatory phenomena at the planetary scale. The largest scale of ecological organization is the biosphere: the total sum of ecosystems on the planet. Ecological relations regulate the flux of energy, nutrients, and climate all the way up to the planetary scale. For example, the dynamic history of the planetary CO2 and O2 composition of the atmosphere has been largely determined by the biogenic flux of gases coming from respiration and photosynthesis, with levels fluctuating over time and in relation to the ecology and evolution of plants and animals.[106] When sub-component parts are organized into a whole there are oftentimes emergent properties that describe the nature of the system. This the Gaia hypothesis,[28] and is an example of holism applied in ecological theory.[107] The ecology of the planet acts as a single regulatory or holistic unit called Gaia. The Gaia hypothesis states that there is an emergent feedback loop generated by the metabolism of living organisms that maintains the temperature of the Earth and atmospheric conditions within a narrow self-regulating range of tolerance.[28]
Relation to evolution
Ecology and evolution are considered sister disciplines of the life sciences. Natural selection, life history, development, adaptation, populations, and inheritance are examples of concepts that thread equally into ecological and evolutionary theory. Morphological, behavioral and/or genetic traits, for example, can be mapped onto evolutionary trees to study the historical development of a species in relation to their functions and roles in different ecological circumstances. In this framework, the analytical tools of ecologists and evolutionists overlap as they organize, classify and investigate life through common systematic principals, such as phylogenetics or the Linnaean system of taxonomy.[108] The two disciplines often appear together, such as in the title of the journal Trends in Ecology and Evolution.[109] There is no sharp boundary separating ecology from evolution and they differ more in their areas of applied focus. Both disciplines discover and explain emergent and unique properties and processes operating across different spatial or temporal scales of organization.[28][110][111] While the boundary between ecology and evolution is not always clear, it is understood that ecologists study the abiotic and biotic factors that influence the evolutionary process.[7][112]
Behavioral ecology

All organisms are motile to some extent. Even plants express complex behavior, including memory and communication.[114] Behavioral ecology is the study of ethology and its ecological and evolutionary implications. Ethology is the study of observable movement or behavior in nature. This could include investigations of motile sperm of plants, mobile phytoplankton, zooplankton swimming toward the female egg, the cultivation of fungi by weevils, the mating dance of a salamander, or social gatherings of amoeba.[115][116][117][118][119]

Adaptation is the central unifying concept in behavioral ecology."International Society for Behavioral Ecology". http://www.behavecol.com/pages/society/welcome.html. Behaviors can be recorded as traits and inherited in much the same way that eye and hair color can. Behaviors evolve and become adapted to the ecosystem because they are subject to the forces of natural selection.[36] Hence, behaviors can be adaptive, meaning that they evolve functional utilities that increases reproductive success for the individuals that inherit such traits.[120] This is also the technical definition for fitness in biology, which is a measure of reproductive success over successive generations.[36]

Predator-prey interactions are an introductory concept into food-web studies as well as behavioral ecology.[121] Prey species can exhibit different kinds of behavioral adaptations to predators, such as avoid, flee or defend. Many prey species are faced with multiple predators that differ in the degree of danger posed. To be adapted to their environment and face predatory threats, organisms must balance their energy budgets as they invest in different aspects of their life history, such as growth, feeding, mating, socializing, or modifying their habitat. Hypotheses posited in behavioral ecology are generally based on adaptive principals of conservation, optimization or efficiency.[1][7][122] For example,

"The threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behavior according to current levels of risk."[123]

"The optimal flight initiation distance occurs where expected postencounter fitness is maximized, which depends on the prey’s initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk."[124]
Symbiosis: Leafhoppers (Eurymela fenestrata) are protected by ants (Iridomyrmex purpureus) in a symbiotic relationship. The ants protect the leafhoppers from predators and in return the leafhoppers feeding on plants exude honeydew from their anus that provides energy and nutrients to tending ants.[125]

Elaborate sexual displays and posturing are encountered in the behavioral ecology of animals. The birds of paradise, for example, display elaborate ornaments and song during courtship. These displays serve a dual purpose of signaling healthy or well-adapted individuals and desirable genes. The elaborate displays are driven by sexual selection as an advertisement of quality of traits among male suitors.[126]Social ecology
Social ecological behaviors are notable in the social insects, slime moulds, social spiders, human society, and naked mole rats where eusocialism has evolved. Social behaviors include reciprocally beneficial behaviors among kin and nest mates.[36][117][127] Social behaviors evolve from kin and group selection. Kin selection explains altruism through genetic relationships, whereby an altruistic behavior leading to death is rewarded by the survival of genetic copies distributed among surviving relatives. The social insects, including ants, bees and wasps are most famously studied for this type of relationship because the male drones are clones that share the same genetic make-up as every other male in the colony.[36] In contrast, group selectionists find examples of altruism among non-genetic relatives and explain this through selection acting on the group, whereby it becomes selectively advantageous for groups if their members express altruistic behaviors to one another. Groups that are predominantly altruists beat groups that are predominantly selfish.[36][128]
Coevolution

Ecological interactions can be divided into host and associate relationships. A host is any entity that harbors another that is called the associate.[129] Host and associate relationships among species that are mutually or reciprocally beneficial are called mutualisms. If the host and associate are physically connected, the relationship is called symbiosis. Approximately 60% of all plants, for example, have a symbiotic relationship with arbuscular mycorrhizal fungi. Symbiotic plants and fungi exchange carbohydrates for mineral nutrients.[130] Symbiosis differs from indirect mutualisms where the organisms live apart. For example, tropical rainforests regulate the Earth's atmosphere. Trees living in the equatorial regions of the planet supply oxygen into the atmosphere that sustains species living in distant polar regions of the planet. This relationship is called commensalism because many other host species receive the benefits of clean air at no cost or harm to the associate tree species supplying the oxygen.[131] The host and associate relationship is called parasitism if one species benefits while the other suffers. Competition among species or among members of the same species is defined as reciprocal antagonism, such as grasses competing for growth space.[132]
Parasites: A harvestman arachnid is parasitized by mites. This is parasitism because the harvestman is being consumed as its juices are slowly sucked out while the mites gain all the benefits traveling on and feeding off of their host.

Popular ecological study systems for mutualism include, fungus-growing ants employing agricultural symbiosis, bacteria living in the guts of insects and other organisms, the fig wasp and yucca moth pollination complex, lichens with fungi and photosynthetic algae, and corals with photosynthetic algae.[133][134]
Biogeography

The word biogeography is an amalgamation of biology and geography. Biogeography is the comparative study of the geographic distribution of organisms and the corresponding evolution of their traits in space and time.[135] The Journal of Biogeography was established in 1974.[136] Biogeography and ecology share many of their disciplinary roots. For example, the theory of island biogeography, published by the mathematician Robert MacArthur and ecologist Edward O. Wilson in 1967[137] is considered one of the fundamentals of ecological theory.[138]

Biogeography has a long history in the natural sciences where questions arise concerning the spatial distribution of plants and animals. Ecology and evolution provide the explanatory context for biogeographical studies.[135] Biogeographical patterns result from ecological processes that influence range distributions, such as migration and dispersal.[138] and from historical processes that split populations or species into different areas.[139] The biogeographic processes that result in the natural splitting of species explains much of the modern distribution of the Earth's biota. The splitting of lineages in a species is called vicariance biogeography and it is a sub-discipline of biogeography.[139][140][141] There are also practical applications in the field of biogeography concerning ecological systems and processes. For example, the range and distribution of biodiversity and invasive species responding to climate change is a serious concern and active area of research in context of global warming.[41][142]
Molecular ecology

The important relationship between ecology and genetic inheritance predates modern techniques for molecular analysis. Molecular ecological research became more feasible with the development of rapid and accessible genetic technologies, such as the polymerase chain reaction (PCR). The rise of molecular technologies and influx of research questions into this new ecological field resulted in the publication Molecular Ecology in 1992.[145] Molecular ecology uses various analytical techniques to study genes in an evolutionary and ecological context. In 1994, John Avise also played a leading role in this area of science with the publication of his book, Molecular Markers, Natural History and Evolution.[146] Newer technologies opened a wave of genetic analysis into organisms once difficult to study from an ecological or evolutionary standpoint, such as bacteria, fungi and nematodes.

Molecular ecology engendered a new research paradigm to investigate ecological questions considered otherwise intractable. Molecular investigations revealed previously obscured details in the tiny intricacies of nature and improved resolution into probing questions about behavioral and biogeographical ecology. For example, molecular ecology revealed promiscuous sexual behavior and multiple male partners in tree swallows previously thought to be socially monogamous.[147] In a biogeographical context, the marriage between genetics, ecology and evolution resulted in a new sub-discipline called phylogeography.[148]
Relation to the environment

The environment is dynamically interlinked, imposed upon and constrains organisms at any time throughout their life cycle.[149] Like the term ecology, environment has different conceptual meanings and to many these terms also overlap with the concept of nature. Environment "...includes the physical world, the social world of human relations and the built world of human creation."[150]:62 The environment in ecosystems includes both physical parameters and biotic attributes. The physical environment is external to the level of biological organization under investigation, including abiotic factors such as temperature, radiation, light, chemistry, climate and geology. The biotic environment includes genes, cells, organisms, members of the same species (conspecifics) and other species that share a habitat.[151] The laws of thermodynamics applies to ecology by means of its physical state. Armed with an understanding of metabolic and thermodynamic principles a complete accounting of energy and material flow can be traced through an ecosystem.[152]

Environmental and ecological relations are studied through reference to conceptually manageable and isolated parts. Once the effective environmental components are understood they conceptually link back together as a holocoenotic[153] system. In other words, the organism and the environment form a dynamic whole (or umwelt).[154]:252 Change in one ecological or environmental factor can concurrently affect the dynamic state of an entire ecosystem.[155][156]

Ecological studies are necessarily holistic as opposed to reductionistic.[27][157] Holism has three scientific meanings or uses that identify with: 1) the mechanistic complexity of ecosystems, 2) the practical description of patterns in quantitative reductionist terms where correlations may be identified but nothing is understood about the causal relations without reference to the whole system, which leads to 3) a metaphysical hierarchy whereby the causal relations of larger systems are understood without reference to the smaller parts. An example of the metaphysical aspect to holism is the trend of increased exterior thickness in shells of different species. The reason for a thickness increase can be understood through reference to principals of natural selection via predation without any reference to the biomolecular properties of the exterior shells.[158]
Metabolism and the early atmosphere

The Earth formed approximately 4.5 billion years ago[160] and environmental conditions were too extreme for life to form for the first 500 million years. During this early Hadean period, the Earth started to cool, allowing a crust and oceans to form. Environmental conditions were unsuitable for the origins of life for the first billion years after the Earth formed. The Earth's atmosphere transformed from being dominated by hydrogen, to one composed mostly of methane and ammonia. Over the next billion years the metabolic activity of life transformed the atmosphere to higher concentrations of carbon dioxide, nitrogen, and water vapor. These gases changed the way that light from the sun hit the Earth's surface and greenhouse effects trapped heat. There were untapped sources of free energy within the mixture of reducing and oxidizing gasses that set the stage for primitive ecosystems to evolve and, in turn, the atmosphere also evolved.[161]
The leaf is the primary site of photosynthesis in most plants.

Throughout history, the Earth's atmosphere and biogeochemical cycles have been in a dynamic equilibrium with planetary ecosystems. The history is characterized by periods of significant transformation followed by millions of years of stability.[162] The evolution of the earliest organisms, likely anaerobic methanogen microbes, started the process by converting atmospheric hydrogen into methane (4H2 + CO2 → CH4 + 2H2O). Anoxygenic photosynthesis converting hydrogen sulfide into other sulfur compounds or water (2H2S + CO2 → hv → CH2O → H2O → + 2S or 2H2 + CO2 + hv → CH2O + H2O), as occurs in deep sea hydrothermal vents today, reduced hydrogen concentrations and increased atmospheric methane. Early forms of fermentation also increased levels of atmospheric methane. The transition to an oxygen dominant atmosphere (the Great Oxidation) did not begin until approximately 2.4-2.3 billion years ago, but photosynthetic processes started 0.3 to 1 billion years prior.[162][163]
Radiation: heat, temperature and light

The biology of life operates within a certain range of temperatures. Heat is a form of energy that regulates temperature. Heat affects growth rates, activity, behavior and primary production. Temperature is largely dependent on the incidence of solar radiation. The latitudinal and longitudinal spatial variation of temperature greatly affects climates and consequently the distribution of biodiversity and levels of primary production in different ecosystems or biomes across the planet. Heat and temperature relate importantly to metabolic activity. Poikilotherms, for example, have a body temperature that is largely regulated and dependent on the temperature of the external environment. In contrast, homeotherms regulate their internal body temperature by expending metabolic energy.[7][112][152]

There is a relationship between light, primary production, and ecological energy budgets. Sunlight is the primary input of energy into the planet's ecosystems. Light is composed of electromagnetic energy of different wavelengths. Radiant energy from the sun generates heat, provides photons of light measured as active energy in the chemical reactions of life, and also acts as a catalyst for genetic mutation.[7][112][152] Plants, algae, and some bacteria absorb light and assimilate the energy through photosynthesis. Organisms capable of assimilating energy by photosynthesis or through inorganic fixation of H2S are autotrophs. Autotrophs—responsible for primary production—assimilate light energy that becomes metabolically stored as potential energy in the form of biochemical enthalpic bonds.[7][112][152]
Physical environments
Water
The rate of diffusion of carbon dioxide and oxygen is approximately 10,000 times slower in water than it is in air. When soils become flooded, they quickly lose oxygen and transform into a low-concentration (hypoxic -with less than 2 mg O2l−1) environment and eventually become completly (anoxic) environment where anaerobic bacteria thrive among the roots. Water also influences the spectral composition and amount of light as it reflects off the water surface and submerged particles.[164] Aquatic plants exhibit a wide variety of morphological and physiological adaptations that allow them to survive, compete and diversify these environments. For example, the roots and stems develop large air spaces (Aerenchyma) that regulate the efficient transportation gases (for example, CO2 and O2) used in respiration and photosynthesis. In drained soil, microorganisms use oxygen during respiration. In aquatic environments, anaerobic soil microorganisms use nitrate, manganese ions, ferric ions, sulfate, carbon dioxide and some organic compounds. The activity of soil microorganisms and the chemistry of the water reduces the oxidation-reduction potentials of the water. Carbon dioxide, for example, is reduced to methane (CH4) by methanogenic bacteria. Salt water plants (or halophytes) have specialized physiological adaptations, such as the development of special organs for shedding salt and osmo-regulate their internal salt (NaCl) concentrations, to live in estuarine, brackish, or oceanic environments.[164] The physiology of fish is also specially adapted to deal with high levels of salt through osmoregulation. Their gills form electrochemical gradients that mediate salt excresion in saline environments and uptake in fresh water.[165]
Gravity
The shape and energy of the land is affected to a large degree by gravitational forces. On a larger scale, the distribution of gravitational forces on the earth are uneven and influence the shape and movement of tectonic plates as well as having an influence on geomorphic processes such as orogeny and erosion. These forces govern many of the geophysical properties and distributions of ecological biomes across the Earth. On a organism scale, gravitational forces provide directional cues for plant and fungal growth (gravitropism), orientation cues for animal migrations, and influence the biomechanics and size of animals.[7] Ecological traits, such as allocation of biomass in trees during growth are subject to mechanical failure as gravitational forces influence the position and structure of branches and leaves.[166] The cardiovascular systems of all animals are functionally adapted to overcome pressure and gravitational forces that change according to the features of organisms (e.g., height, size, shape), their behavior (e.g., diving, running, flying), and the habitat occupied (e.g., water, hot deserts, cold tundra).[167]
Pressure
Climatic and osmotic pressure places physiological constraints on organisms, such as flight and respiration at high altitudes, or diving to deep ocean depths. These constraints influence vertical limits of ecosystems in the biosphere as organisms are physiologically sensitive and adapted to atmospheric and osmotic water pressure differences.[7] Oxygen levels, for example, decrease with increasing pressure and are a limiting factor for life at higher altitudes.[168] Water transportation through trees is another important ecophysiological parameter where osmotic pressure gradients factor in.[169][170][171] Water pressure in the depths of oceans requires that organisms adapt to these conditions. For example, mammals, such as whales, dolphins and seals are specially adapted to deal with changes in sound due to water pressure differences.[172] Different species of hagfish provide another example of adaptation to deep-sea pressure through specialized protein adaptations.[173]
Wind and turbulence

Turbulent forces in air and water have significant effects on the environment and ecosystem distribution, form and dynamics. On a planetary scale, ecosystems are affected by circulation patterns in the global trade winds. Wind power and the turbulent forces it creates can influence heat, nutrient, and biochemical profiles of ecosystems.[7] For example, wind running over the surface of a lake creates turbulence, mixing the water column and influencing the environmental profile to create thermally layered zones, partially governing how fish, algae, and other parts of the aquatic ecology are structured.[176][177] Wind speed and turbulence also exert influence on rates of evapotranspiration rates and energy budgets in plants and animals.[164][178] Wind speed, temperature and moisture content can vary as winds travel across different landfeatures and elevations. The westerlies, for example, come into contact with the coastal and interior mountains of western North America to produce a rain shadow on the leeward side of the mountain. The air expands and moisture condenses as the winds move up in elevation which can cause precipitation; this is called orographic lift. This environmental process produces spatial divisions in biodiversity, as species adapted to wetter conditions are range-restricted to the coastal mountain valleys and unable to migrate across the xeric ecosystems of the Columbia Basin to intermix with sister lineages that are segregated to the interior mountain systems.[179][180]
Fire

Plants convert carbon dioxide into biomass and emit oxygen into the atmosphere.[181] Approximately 350 million years ago (near the Devonian period) the photosynthetic process brought the concentration of atmospheric oxygen above 17%, which allowed combustion to occur.[182] Fire releases CO2 and converts fuel into ash and tar. Fire is a significant ecological parameter that raises many issues pertaining to its control and suppression in management.[183] While the issue of fire in relation to ecology and plants has been recognized for a long time,[184] Charles Cooper brought attention to the issue of forest fires in relation to the ecology of forest fire suppression and management in the 1960s.[185][186]

Fire creates environmental mosaics and a patchiness to ecosystem age and canopy structure. Native North Americans were among the first to influence fire regimes by controlling their spread near their homes or by lighting fires to stimulate the production of herbaceous foods and basketry materials.[187] The altered state of soil nutrient supply and cleared canopy structure also opens new ecological niches for seedling establishment.[188][189] Most ecosystem are adapted to natural fire cycles. Plants, for example, are equipped with a variety of adaptations to deal with forest fires. Some species (e.g., Pinus halepensis) cannot germinate until after their seeds have lived through a fire. This environmental trigger for seedlings is called serotiny.[190] Some compounds from smoke also promote seed germination.[191] Fire plays a major role in the persistence and resilience of ecosystems.[192]
Biogeochemistry

Ecologists study and measure nutrient budgets to understand how these materials are regulated and flow through the environment.[7][112][152] This research has led to an understanding that there is a global feedback between ecosystems and the physical parameters of this planet including minerals, soil, pH, ions, water and atmospheric gases. There are six major elements, including H (hydrogen), C (carbon), N (nitrogen), O (oxygen), S (sulfur), and P (phosphorus) that form the constitution of all biological macromolecules and feed into the Earth's geochemical processes. From the smallest scale of biology the combined effect of billions upon billions of ecological processes amplify and ultimately regulate the biogeochemical cycles of the Earth. Understanding the relations and cycles mediated between these elements and their ecological pathways has significant bearing toward understanding global biogeochemistry.[193]

The ecology of global carbon budgets gives one example of the linkage between biodiversity and biogeochemistry. For starters, the Earth's oceans are estimated to hold 40,000 gigatonnes (Gt) carbon, vegetation and soil is estimated to hold 2070 Gt carbon, and fossil fuel emissions are estimated to emit an annual flux of 6.3 Gt carbon.[194] At different times in the Earth's history there has been major restructuring in these global carbon budgets that was regulated to a large extent by the ecology of the land. For example, through the early-mid Eocene volcanic outgassing, the oxidation of methane stored in wetlands, and seafloor gases increased atmospheric CO2 concentrations to levels as high as 3500 ppm.[195] In the Oligocene, from 25 to 32 million years ago, there was another significant restructuring in the global carbon cycle as grasses evolved a special type of C4 photosynthesis and expanded their ranges. This new photosynthetic pathway evolved in response to the drop in atmospheric CO2 concentrations below 550 ppm.[196] Ecosystem functions such as these feed back significantly into global atmospheric models for carbon cycling. Loss in the abundance and distribution of biodiversity causes global carbon cycle feedbacks that are expected to increase rates of global warming in the next century.[197] The effect of global warming melting large sections of permafrost creates a new mosaic of flooded areas where decomposition results in the emission of methane (CH4). Hence, there is a relationship between global warming, decomposition and respiration in soils and wetlands producing significant climate feedbacks and altered global biogeochemical cycles.[198][199] There is concern over increases in atmospheric methane in the context of the global carbon cycle, because methane is also a greenhouse gas that is 23 times more effective at absorbing long-wave radiation than CO2 on a 100 year time scale.[200]
Human ecology
Ecosystem services

The ecosystems of planet Earth are coupled to human environments. Ecosystems regulate the global geophysical cycles of energy, climate, soil nutrients, and water that in turn support and grow natural capital (including the environmental, physiological, cognitive, cultural, and spiritual dimensions of life). Ultimately, every manufactured product in human environments comes from natural systems.[223] Ecosystems are considered common-pool resources because ecosystems do not exclude beneficiaries and they can be depleted or degraded.[228] For example, green space within communities provides sustainable health services that reduces mortality and regulates the spread of vector borne disease.[229] Research shows that people who are more engaged with regular access to natural areas have lower rates of diabetes, heart disease and psychological disorders.[230] These ecological health services are regularly depleted through urban development projects that do not factor in the common-pool value of ecosystems.[231][232]

The ecological commons delivers a diverse supply of community services that sustains the well-being of human society.[233][234] The Millennium Ecosystem Assessment, an international UN initiative involving more than 1,360 experts worldwide, identifies four main ecosystem service types having 30 sub-categories stemming from natural capital. The ecological commons includes provisioning (e.g., food, raw materials, medicine, water supplies), regulating (e.g., climate, water, soil retention, flood retention), cultural (e.g., science and education, artistic, spiritual), and supporting (e.g., soil formation, nutrient cycling, water cycling) services.[10]
Sixth mass extinction

Global assessments of biodiversity indicate that the current epoch, the Holocene (or Anthropocene)[236] is a sixth mass extinction.[237] Species loss is accelerating at 100–1000 times faster than average background rates in the fossil record.[238][239] The field of conservation biology involves ecologists that are researching, confronting, and searching for solutions to sustain the planet's ecosystems for future generations.[240]

"Human activities are associated directly or indirectly with nearly every aspect of the current extinction spasm."[238]:11472

Nature is a resilient system. Ecosystems regenerate, withstand, and are forever adapting to fluctuating environments. Ecological resilience is an important conceptual framework in conservation management and it is defined as the preservation of biological relations in ecosystems that persevere and regenerate in response to disturbance over time. Disturbances, such as fire, are both cause and product of natural fluctuations in death rates, species assemblages, and biomass densities within an ecological community. These disturbances create places of renewal where new directions emerge out of the patchwork of natural experimentation and opportunity.[192][241] [242] However, persistent, systematic, large and nonrandom disturbance caused by the niche constructing behavior of human beings, habitat conversion and land development, has pushed many of the Earth's ecosystems to the extent of their resilient thresholds. Three planetary thresholds have already been crossed, including 1) biodiversity loss, 2) climate change, and 3) nitrogen cycles. These biophysical systems are ecologically interrelated and naturally resilient, but human civilization has transitioned the planet to the Anthropocene epoch, where the threshold for planetary scale resilience has been crossed and the ecological state of the Earth is deteriorating rapidly to the detriment of humanity.[243] The world's fisheries and oceans, for example, are facing dire challenges as the threat of global collapse appears imminent, with serious ramifications for the well-being of humanity.[244] The ecology of the planet is further threatened by global warming, but investments in nature conservation can provide a regulatory feedback to store and regulate carbon and other greenhouse gases.[245][246]
Ecological footprint
In 1992, William Rees developed the ecological footprint concept. The ecological footprint and its close analog the water footprint has become a popular way of accounting for the level of impact that human society is imparting on the Earth's ecosystems.[247][248] All indications are that the human enterprise is unsustainable as the footprint of society is placing too much stress on the ecology of the planet.[249] The WWF 2008 living planet report and other researchers report that human civilization has exceeded the bio-regenerative capacity of the planet.[249][250] This means that the footprint of human consumption is extracting more natural resources than can be replenished by ecosystems around the world.
Ecological economics
Ecological economics is an economic science that extends its methods of valuation onto nature in an effort to address the inequity between market growth and biodiversity loss.[11] Natural capital is the stock of materials or information stored in biodiversity that generates services that can enhance the welfare of communities.[251] Population losses are the more sensitive indicator of natural capital than are species extinction in the accounting of ecosystem services. The prospect for recovery in the economic crisis of nature is grim. Populations, such as local ponds and patches of forest are being cleared away and lost at rates that exceed species extinctions.[42] The mainstream growth-based economic system adopted by governments worldwide does not include a price or markets for natural capital. This type of economic system places further ecological debt onto future generations.[252][253]

Human societies are increasingly being placed under stress as the ecological commons is diminished through an accounting system that has incorrectly assumed "... that nature is a fixed, indestructible capital asset."[254]:44 The current wave of threats, including massive extinction rates and concurrent loss of natural capital to the detriment of human society, is happening rapidly. This is called a biodiversity crisis, because 50% of the worlds species are predicted to go extinct within the next 50 years.[255][256] Conventional monetary analyses are unable to detect or deal with these sorts of ecological problems.[257] Multiple global ecological economic initiatives are being promoted to solve this problem. For example, governments of the G8 met in 2007 and set forth The Economics of Ecosystems and Biodiversity (TEEB) initiative:

In a global study we will initiate the process of analyzing the global economic benefit of biological diversity, the costs of the loss of biodiversity and the failure to take protective measures versus the costs of effective conservation.[258]
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