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Population ecology is a sub-field of
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biol ...
that deals with the dynamics of
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
s and how these populations interact with the
environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
, such as
birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
and
death rate Mortality rate, or death rate, is a measure of the number of death Death is the permanent, irreversible cessation of all biological functions that sustain a living Living or The Living may refer to: Common meanings *Life, a conditi ...
s, and by
immigration Immigration is the international movement of people to a destination country A country is a distinct territorial body or political entity A polity is an identifiable political entity—any group of people who have a collective ident ...

immigration
and
emigration Emigration is the act of leaving a resident country or place of residence with the intent to settle elsewhere (to permanently leave a country). Conversely, immigration Immigration is the international movement of people to a destination ...
. The discipline is important in
conservation biology Conservation biology is the study of the conservation of nature and of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of contine ...
, especially in the development of population viability analysis which makes it possible to predict the long-term probability of a species persisting in a given patch of
habitat In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers at the ...

habitat
. Although population ecology is a subfield of
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, it provides interesting problems for
mathematicians A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry) ...
and
statisticians A statistician is a person who works with theoretical or applied statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, ...

statisticians
who work in
population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. ...
.


History

In the 1940s ecology was divided into autecology—the study of individual species in relation to the environment—and
synecology In ecology, a community is a group or association (ecology), association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community ...
—the study of groups of species in relation to the environment. The term autecology (from
Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
: αὐτο, ''aúto'', "self"; οίκος, ''oíkos'', "household"; and
λόγος ''Logos'' (, ; grc, wikt:λόγος, λόγος, lógos; from , , ) is a term in Western philosophy, psychology, rhetoric, and religion derived from a Greek word variously meaning "ground", "plea", "opinion", "expectation", "word", "speech", " ...
, ''lógos'', "knowledge"), refers to roughly the same field of study as concepts such as
life cycles Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...
and
behaviour Behavior (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American English ...
as adaptations to the environment by individual organisms.
Eugene Odum Eugene Pleasants Odum (September 17, 1913 – August 10, 2002) was an American biologist A biologist is a professional who has specialized knowledge in the field of biology, understanding the underlying mechanisms that govern the functioning ...
, writing in 1953, considered that synecology should be divided into population ecology,
community ecology In ecology, a community is a group or association of population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, classifi ...
and
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
ecology, renaming autecology as 'species ecology' (Odum regarded "autecology" as an archaic term), thus that there were four subdivisions of ecology.


Terminology

A population is defined as a group of interacting organisms of the same species. A demographic structure of a population is how populations are often quantified. The total number of individuals in a population is defined as a population size, and how dense these individuals are is defined as population density. There is also a population’s geographic range, which has limits that a species can tolerate (such as temperature). Population size can be influenced by the per capita population growth rate (rate at which the population size changes per individual in the population.) Births, deaths, emigration, and immigration rates all play a significant role in growth rate. The maximum per capita growth rate for a population is known as the intrinsic rate of increase. In a population, carrying capacity is known as the maximum population size of the species that the environment can sustain, which is determined by resources available. In many classic population models, r is represented as the intrinsic growth rate, where K is the carrying capacity, and N0 is the initial population size.


Population dynamics

The development of population ecology owes much to the mathematical models known as
population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. ...
, which were originally formulae derived from
demography Demography (from prefix ''demo-'' from Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period ...

demography
at the end of the 18th and beginning of 19th century. The beginning of population dynamics is widely regarded as the work of
Malthus Thomas Robert Malthus (; 13/14 February 1766 – 23 December 1834) was an English cleric Clergy are formal leaders within established religion Religion is a social system, social-cultural system of designated religious behaviour, beh ...

Malthus
,Malthus, Thomas Robert.
An Essay on the Principle of Population The book ''An Essay on the Principle of Population'' was first published anonymously in 1798, but the author was soon identified as Thomas Robert Malthus Thomas Robert Malthus (; 13/14 February 1766 – 23 December 1834) was an English cler ...

An Essay on the Principle of Population
: Library of Economics
formulated as the
Malthusian growth model A Malthusian growth model, sometimes called a simple exponential growth model, is essentially exponential growth Exponential growth is a process that increases quantity over time. It occurs when the instantaneous rate of change (that is, the deri ...
. According to Malthus, assuming that the conditions (the environment) remain constant (''
ceteris paribus ' or ' () is a Latin phrase meaning "other things equal"; English translations of the phrase include "all other things being equal" or "other things held constant" or "all else unchanged". A prediction or a statement about a ontic, causal, epist ...
''), a population will grow (or decline) . This principle provided the basis for the subsequent predictive theories, such as the
demographic Demography (from prefix ''demo-'' from Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period ...

demographic
studies such as the work of
Benjamin Gompertz Benjamin Gompertz (5 March 1779 – 14 July 1865) was a British self-educated Autodidacticism (also autodidactism) or self-education (also self-learning and self-teaching) is education Education is the process of facilitating le ...
and
Pierre François Verhulst 250px, Pierre François Verhulst Pierre François Verhulst (28 October 1804, Brussels – 15 February 1849, Brussels) was a Belgian mathematician and a doctor in number theory from the University of Ghent in 1825. He is best known for the logist ...
in the early 19th century, who refined and adjusted the Malthusian demographic model. A more general model formulation was proposed by F. J. Richards in 1959, further expanded by Simon Hopkins, in which the models of Gompertz, Verhulst and also
Ludwig von Bertalanffy Karl Ludwig von Bertalanffy (19 September 1901 – 12 June 1972) was an Austrian biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has specialized ...
are covered as special cases of the general formulation. The Lotka–Volterra predator-prey equations are another famous example, as well as the alternative Arditi–Ginzburg equations.


Exponential vs. Logistic Growth

When describing growth models, there are two types of models that can be used: exponential and logistic. When the per capita of a rate of increase takes the same positive value regardless of population size, then it shows exponential growth. When the per capita rate of increase decreases as the population increases towards a maximum limit, then the graph shows logistic growth.


Fisheries and wildlife management

In
fisheries Fishery can mean either the enterprise of raising or harvesting fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical ...
and
wildlife management Wildlife management is the management process influencing interactions among and between wildlife Wildlife traditionally refers to undomesticated animal Animals (also called Metazoa) are multicellular A multicellular organism ...
, population is affected by three dynamic rate functions. *Natality or
birth rate The crude birth rate (CBR) in a period is the total number of live births per 1,000 population divided by the length of the period in years. The number of live births is normally taken from a universal registration system for births; population ...
, often recruitment, which means reaching a certain size or reproductive stage. Usually refers to the age a fish can be caught and counted in nets. *
Population growth rate Population growth is the increase in the number of individuals in a population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, ...
, which measures the growth of individuals in size and length. More important in fisheries, where population is often measured in biomass. *
Mortality Mortality is the state of being mortal, or susceptible to death (1906) Death is the permanent, Irreversible process, irreversible cessation of all biological process, biological functions that sustain a living organism. Brain death is some ...
, which includes harvest mortality and natural mortality. Natural mortality includes non-human predation, disease and old age. If ''N''1 is the number of individuals at time 1 then N_1 = N_0 + B - D + I - E where ''N''0 is the number of individuals at time 0, ''B'' is the number of individuals born, ''D'' the number that died, ''I'' the number that immigrated, and ''E'' the number that emigrated between time 0 and time 1. If we measure these rates over many time intervals, we can determine how a population's density changes over time. Immigration and emigration are present, but are usually not measured. All of these are measured to determine the harvestable surplus, which is the number of individuals that can be harvested from a population without affecting long-term population stability or average population size. The harvest within the harvestable surplus is termed "compensatory" mortality, where the harvest deaths are substituted for the deaths that would have occurred naturally. Harvest above that level is termed "additive" mortality, because it adds to the number of deaths that would have occurred naturally. These terms are not necessarily judged as "good" and "bad," respectively, in population management. For example, a fish & game agency might aim to reduce the size of a deer population through additive mortality. Bucks might be targeted to increase buck competition, or does might be targeted to reduce reproduction and thus overall population size. For the management of many fish and other wildlife populations, the goal is often to achieve the largest possible long-run sustainable harvest, also known as
maximum sustainable yieldIn population ecology Population ecology is a sub-field of ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their p ...
(or MSY). Given a population dynamic model, such as any of the ones above, it is possible to calculate the population size that produces the largest harvestable surplus at equilibrium. While the use of population dynamic models along with statistics and optimization to set harvest limits for fish and game is controversial among some scientists, it has been shown to be more effective than the use of human judgment in computer experiments where both incorrect models and natural resource management students competed to maximize yield in two hypothetical fisheries. To give an example of a non-intuitive result, fisheries produce more fish when there is a nearby
refuge Refuge is a place or state of safety. It may also refer to a more specific meaning: Safety * Area of refuge, a location in a building that may be used by occupants in the event of a fire * Mountain hut, a shelter for travelers in mountainous area ...
from human predation in the form of a
nature reserve A nature reserve (also known as a natural reserve, wildlife refuge, wildlife sanctuary, biosphere reserve or bioreserve, natural or nature preserve, or nature conservation area) is a protected area Protected areas or conservation areas a ...
, resulting in higher catches than if the whole area was open to fishing.


r/K selection

An important concept in population ecology is the r/K selection theory. For example, if an animal has the choice of producing one or a few offspring, or to put a lot of effort or little effort in offspring -- these are all examples of trade-offs. In order for species to thrive, they must choose what is best for them, leading to a clear distinction between r and K selected species. The first variable is ''r'' (the intrinsic rate of natural increase in population size, density independent) and the second variable is ''K'' (the carrying capacity of a population, density dependent). An ''r''-selected species (e.g., many kinds of insects, such as aphids) is one that has high rates of fecundity, low levels of parental investment in the young, and high rates of mortality before individuals reach maturity. Evolution favors productivity in r-selected species. In contrast, a ''K''-selected species (such as humans) has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Evolution in ''K''-selected species favors efficiency in the conversion of more
resources Resource refers to all the materials available in our environment which help us to satisfy our needs and wants. Resources can broadly be classified upon their availability — they are classified into renewable A renewable resource, also know ...
into fewer offspring. K-selected species generally experience stronger competition, where populations generally live near carrying capacity. These species have heavy investment in offspring, resulting in longer lived organisms, and longer period of maturation. Offspring of K-selected species generally have a higher probability of survival, due to heavy parental care and nurturing.


Top-Down and Bottom-Up Controls


Top-Down Controls

In some populations, organisms in lower trophic levels are controlled by organisms at the top. This is known as top-down control. For example, the presence of top carnivores keep herbivore populations in check. If there were no top carnivores in the ecosystem, then herbivore populations would rapidly increase, leading to all plants being eaten. This ecosystem would eventually collapse.


Bottom-Up Controls

Bottom-up controls, on the other hand, are driven by producers in the ecosystem. If plant populations change, then the population of all species would be impacted. For example, if plant populations decreased significantly, the herbivore populations would decrease, which would lead to a carnivore population decreasing too. Therefore, if all of the plants disappeared, then the ecosystem would collapse. Another example would be if there were too many plants available, then two herbivore populations may compete for the same food. The competition would lead to an eventual removal of one population.


Do all ecosystems have to be either top-down or bottom-up?

An ecosystem does not have to be either top-down or bottom-up. There are occasions where an ecosystem could be bottom-up sometimes, such as a marine ecosystem, but then have periods of top-down control due to fishing.


Survivorship curves

Survivorship curves show the distribution of populations according to age. Survivorship curves are important to be able to compare generations, populations, or even different species. Humans and most other mammals have a type I survivorship because death occurs in older years. Typically, Type I survivorship generally includes K-selected species. Type II survivorship shows that death at any age is equally probable. Type III curves indicate few surviving the younger years, but after a certain age, individuals are much more likely to survive. Type III survivorship typically includes r-selected species.


Metapopulation

Populations are also studied and conceptualized through the "
metapopulation A metapopulation consists of a group of spatially separated populations of the same species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiv ...

metapopulation
" concept. The metapopulation concept was introduced in 1969:
"as a population of populations which go extinct locally and recolonize."
Metapopulation ecology is a simplified model of the landscape into patches of varying levels of quality. Patches are either occupied or they are not. Migrants moving among the patches are structured into metapopulations either as sources or sinks. Source patches are productive sites that generate a seasonal supply of migrants to other patch locations. Sink patches are unproductive sites that only receive migrants. In metapopulation terminology there are emigrants (individuals that leave a patch) and immigrants (individuals that move into a patch). Metapopulation models examine patch dynamics over time to answer questions about spatial and demographic ecology. An important concept in metapopulation ecology is the
rescue effectThe rescue effect is a phenomenon which was first described by Brown & Kodric-Brown,Brown JH, Kodric-Brown A. 1977 Turnover rates in insular biogeography: effect of immigration on extinction. Ecology 58, 445– 449. (doi:10.2307/ 1935620) and is com ...
, where small patches of lower quality (i.e., sinks) are maintained by a seasonal influx of new immigrants. Metapopulation structure evolves from year to year, where some patches are sinks, such as dry years, and become sources when conditions are more favorable. Ecologists utilize a mixture of computer models and field studies to explain metapopulation structure.


Journals

The first journal publication of the Society of Population Ecology, titled ''Population Ecology'' (originally called ''Researches on Population Ecology'') was released in 1952. Scientific articles on population ecology can also be found in the ''
Journal of Animal Ecology The ''Journal of Animal Ecology'' is a peer-reviewed scientific journal publishing research in all areas of animal ecology. It began publication in 1932, and as such is the second oldest journal of the British Ecological Society (after the ''Journal ...
'', ''
Oikos The ancient Greek word ''oikos'' (ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simpl ...
'' and other journals.


See also

* Density-dependent inhibition *
Irruptive growth Irruptive growth is a growth pattern over time, defined by a sudden rapid growth in the population of an organism. Irruptive growth is studied in population ecology. Population cycles often display irruptive growth, but with a predictable pattern su ...
* Lists of organisms by population *
Overpopulation Overpopulation or overabundance occurs when a species' population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, classifica ...

Overpopulation
*
Population density Population density (in agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise tr ...

Population density
*
Population distribution Species distribution is the manner in which a biological taxon is spatially arranged. The geographic limits of a particular taxon's distribution is its range, often represented as shaded areas on a map. Patterns of distribution change depending ...

Population distribution
*
Population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. ...
*
Population dynamics of fisheries A fishery is an area with an associated fish or Aquatic animal, aquatic population which is harvested for its Commercial fishing, commercial or Recreational fishing, recreational value. Fisheries can be Wild fisheries of the world, wild or Fish farm ...
*
Population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredient in the of the . Its pri ...
*
Population growth Population growth is the increase in the number of people in a population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size ...
*
Theoretical ecology 300px, Life on Earth-Flow of Energy and Entropy Theoretical ecology is the scientific discipline devoted to the study of ecosystem, ecological systems using theoretical methods such as simple conceptual models, mathematical models, computer simulat ...


References


Bibliography

* {{DEFAULTSORT:Population Ecology Applied statistics
Ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biol ...