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POPULATION ECOLOGY
ECOLOGYEcology is a scientific discipline in which the relationships among living organisms and the interaction the organisms have with their environments are studied. An ecosystem is a biological community and all of the abiotic factors that affect it.
Population ecology is the field of ecology that focuses on the factors that affect the population size of a given organism, population growth rate, and spatial dispersion of individuals with populations.
A population is all of the individuals of the same species within an ecological community. Ecologists are interested in the growth of a population, fluctuations in population size, the spread of the population, and any other interactions within the population or between it and other populations.
Ecologists may also study different groups of populations that are not located in the same area but interact at certain times throughout the year. If this group of populations are the same species and can still interbreed, they are a metapopulation. Individuals within a metapopulation may migrate from one population to the other, which can help stabilize the size of the overall population.
Population Factors
Ecologists describe the organisms of populations in several different ways. The distribution of a population is the total area that population covers. The abundance of a population is the number of individuals within that population. Ecologists may also define the number of individuals within a certain space, which is the density of the population.
Ecologists also identify the age structure or sex ratio of a population. The age structure describes the number of individuals in different age classes, while the sex ratio describes the proportion of males to females in that population.
A biome is a large group of ecosystems that share the same climate and have similar types of communities. Biotic Factors : Living factors in an organism’s environment
Abiotic Factors : Nonliving factors in an organism’s environment. Organisms adapt to survive in the abiotic factors present in their natural environment.
Ecosystem Interactions
A habitat is an area where an organism lives.
A niche is the role or position that an organism has in its environment. Levels of Organization :
Levels increase in complexity as the numbers and interactions between organisms increase. Organism
Population
Biological communities Ecosystem
Biome Biosphere
Organisms and Their Relationships
The lowest level of organization is the individual organism itself.
Population - a group of organisms of the same species which have the potential to interbreed – or a population is a group of organisms of the same species occupying a particular place at a particular time
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Populations have a number of properties which are not possessed by individual organisms - this is because a population is the sum of many organisms interacting.
A biological community is a group of interacting populations that occupy the same geographic area at the same time
Community Interactions
Competition : Occurs when more than one organism uses a resource at the same time Predation: Many species get their food by eating other organisms.
Symbiotic Relationships: The close relationship that exists when two or more species live together
o Mutualism
o Commensalism
o Parasitism Energy in an Ecosystem
Autotrophs : Organism that collects energy from sunlight or inorganic substances to produce food Heterotrophs: Organism that gets it energy requirements by other organisms.
Properties of a population
Distribution - over what area is the population found. This includes:
o Abundance – how many total individuals
o Density - how many individuals in the same area at the same time
Properties dealing with changes in population size
Natality - may think of this as births, but includes more than just birth - hatching, germination, fission. Natality includes idea of fecundity - number of offspring produced per unit time - we are most concerned with realized fecundity - actual number of survivors
Mortality - death rate - its converse is survivorship - mortality looks at how many die per unit time, survivorship at how many don't die per unit time
Longevity examines life-span of individuals - again we are most interested in realized longevity, not potential longevity
Immigration - individuals moving into a population Emigration - individuals leaving a population What is an individual?
unitary organism - individuals are highly determinate in form and while growing pass through predictable (innately determined) sequences of life history stages
modular organisms - zygote develops into unit, or module, which produces more modules thus producing an organism with a variable number of modules, whose development is unpredictable and strongly influenced by environmental factors
3 Population Growth
Within any population, individuals are born and individuals die. If there are more individuals being born than dying, the population grows in size, while if more individuals are dying than being born the reverse is true. Individuals may also enter or leave a population, which is referred to as immigration and emigration.
To better understand population growth, ecologists have created models to study how birth, death, immigration, and emigration affect population size. The simplest model is called the exponential growth model. It says that the change in population size is exponential, or growing at an increasing rate. This is not a very realistic model because most populations do not continue to grow without slowing down.
Another model is the logistic growth model. This is a much more realistic way to represent population growth. This model takes into account how the available resources in the environment, such as food, water, and shelter, affect population size. The logistic growth model describes a point at which the population growth rate levels off, called the carrying capacity (K). The carrying capacity is the maximum sustainable population size given the available resources in the environment.
The logistic growth model is also more realistic because it takes into account the density of the population. While a population may grow in size, the resources available do not necessarily increase to meet this increased demand. When resources become scarcer, deaths increase because of competition for resources. The spread of diseases also increases, as does predation and starvation.
Growth rate: It answers the following questions. How many individuals are gained (birth and immigration) minus how many are lost (death and emigration)? What is happening to the net population size?
Age structure: how many individuals in different age categories, usually related to reproduction Population Dispersal
Spatial dispersion: Individuals in a population may be spread across the environment in different patterns (clumped dispersion, even dispersion, or random dispersion). Interaction with both the abiotic and biotic environments can influence patterns of spatial dispersion. For example, clumped dispersions may arise when individuals are limited to living in patchy with the appropriate abiotic environmental conditions and even dispersions may arise as the result of intraspecific competition. Species dispersal contributes to the distribution of organisms. Dispersal refers to the process of distribution of individuals within geographic population boundaries.
Question: Is the distribution of a species limited by dispersal, i.e. by movement of the organisms?
Answer can be obtained by transplant experiments. If the transplant is successful, then the organisms just haven’t reached the target area.
If the transplant is not successful, then other factors limit the distribution of the organisms, such as competitors, lack of a food source, etc.
4 Population Dynamics
It is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes. Population dynamics deals with the way populations are affected by birth and death rates, and by immigration and emigration, and studies topics such as ageing populations or population decline.
One common mathematical model for population dynamics is the exponential growth model. With the exponential model, the rate of change of any given population is proportional to the already existing population.
Demography: Demography is the subset of population ecology that studies statistics related to any populations. Ecologists study many different aspects of ecosystems. One aspect that is of particular importance is
population ecology. This field of study is concerned with populations and how they interact with their environment.
Demographics covers the basic statistical information about a population: age structure, density, births, deaths, growth, and reproduction.
Demography is the study of processes that influence population size - it is the way we study changes brought about by births, deaths and dispersal
The Fundamental Equation of Ecology – Harper 1977
Δ N = B – D + I – E
C
hange in
N
umber =
B
irths –
D
eaths +
I
mmigration –
E
migration
r/K Selection In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that tradeoff between quantity and quality of offspring. The focus upon either increased quantity of offspring at the expense of individual parental investment, or reduced quantity of offspring with a corresponding increased parental investment, varies widely, seemingly to promote success in particular environments.
The etymology is from an equation where r comes from rate and K comes from carrying capacity, the German word for a constant. r-selection makes a species prone to numerous reproduction at low cost per individual offspring, while K-selected species expend high cost in reproduction for a low number of more difficult to produce offspring. Neither mode of propagation is intrinsically superior, nor in fact they can coexist in the same habitat, as in rodents and elephants. r/K selection theory is also useful in studying the evolution of ecological and life history differences between subspecies such as African honey bee, A. m. scutellata, and the Italian bee, A. m. ligustica.
The theory was popular in the 1970s and 1980s, when it was used as a heuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies. A life-history paradigm has replaced the r/K selection paradigm and continues to incorporate many of its important themes.
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At its most elementary level, interspecific competition involves two species utilizing a similar resource. It rapidly gets more complicated, but stripping the phenomenon of all its complications, this is the basic principle: two consumers consuming the same resource.
An important concept in population ecology is the r/K selection theory. 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 into fewer offspring.
In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection.[5] These terms, r and K, are drawn from standard ecological algebra as illustrated in the simplified
Verhulst model of population dynamics:
where r is the maximum growth rate of the population (N), K is the carrying capacity of its local environmental setting, and the notation dN/dt stands for the derivative of N with respect to t (time). Thus, the equation relates the rate of change of the population N to the current population size and
expresses the effect of the two parameters. r-selection
As the name implies, r-selected species are those that place an emphasis on a high growth rate, and, typically exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K).
In unstable or unpredictable environments, r-selection predominates as the ability to reproduce quickly is crucial. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Among the traits that are thought to characterize r-selection are high fecundity, small body size, early maturity onset, short generation time, and the ability to
disperse offspring widely.
Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various
semelparouscephalopods and mammals, particularly small rodents.
K-selection
By contrast, K-selected species display traits associated with living at densities close to carrying capacity, and typically are strong competitors in such crowded niches that invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low r, high K). In scientific literature, r-selected species are occasionally referred to as "opportunistic" whereas K-r-selected species are described as "equilibrium".[9]
In stable or predictable environments, K-selection predominates as the ability to compete successfully for limited resources is crucial and populations of K-selected organisms typically are very constant in number and close to the maximum that the environment can bear (unlike r-selected populations, where population sizes can change much more rapidly).
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Traits that are thought to be characteristic of K-selection include large body size, long life expectancy, and the production of fewer offspring, which often require extensive parental care until they mature. Organisms whose life history is subject to K-selection are often referred to as K-strategists or K-selected.[10] Organisms with K-selected traits include large organisms such as elephants, primates and whales, but also smaller, long-lived organisms such as Arctic Terns.
Continuous spectrum
Although some organisms are identified as primarily r- or K-strategists, the majority of organisms do not follow this pattern. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.[12]
Similarly, reptiles such as sea turtles display both r- and K-traits: although sea turtles are large organisms with long lifespans (provided they reach adulthood), they produce large numbers of unnurtured offspring. Mammalian males tend to be r-type reproducers, whereas females tend to have K characteristics.[12]
The r/K dichotomy can be re-expressed as a continuous spectrum using the economic concept of discounted future returns, with r-selection corresponding to large discount rates and K-selection corresponding to small discount rates.
In certain species, the r/K dichotomy likewise manifests itself upon population-specific examination relative to the range of expressions of a given trait observed across that species. For example, among humans, populations having low socioeconomic status disproportionately exhibit r-type traits such as early pregnancies, large numbers of children, male dispersal of offspring across multiple households, and low parental investment (in part because of a lack of resources to invest), in keeping with their disproportionate occupation of niches featuring high levels of instability and existential risk to individual members, whereas populations having high socioeconomic status disproportionately exhibit K-type traits such as delayed pregnancies, small numbers of children, concentration of offspring within a single household, and high parental investment (in part because of the presence of more resources to invest), in keeping with their disproportionate occupation of niches featuring relative stability and lower levels of existential risk to individual members.
