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Photo (a) shows penguins, which maintain a defined territory and therefore have a uniform distribution. Photo (b) shows a field of dandelions whose seeds are dispersed by wind, resulting in a random distribution patter. Photo (c) shows elephants, which travel in herds resulting in a clumped distribution pattern.
Species may have uniform, random, or clumped distribution. Territorial birds such as penguins tend to have uniform distribution. Plants such as dandelions with wind-dispersed seeds tend to be randomly distributed. Animals such as elephants that travel in groups exhibit clumped distribution. (credit a: modification of work by Ben Tubby; credit b: modification of work by Rosendahl; credit c: modification of work by Rebecca Wood)


While population size and density describe a population at one particular point in time, scientists must use demography to study the dynamics of a population. Demography is the statistical study of population changes over time: birth rates, death rates, and life expectancies. Each of these measures, especially birth rates, may be affected by the population characteristics described above. For example, a large population size results in a higher birth rate because more potentially reproductive individuals are present. In contrast, a large population size can also result in a higher death rate because of competition, disease, and the accumulation of waste. Similarly, a higher population density or a clumped dispersion pattern results in more potential reproductive encounters between individuals, which can increase birth rate. Lastly, a female-biased sex ratio (the ratio of males to females) or age structure (the proportion of population members at specific age ranges) composed of many individuals of reproductive age can increase birth rates.

In addition, the demographic characteristics of a population can influence how the population grows or declines over time. If birth and death rates are equal, the population remains stable. However, the population size will increase if birth rates exceed death rates; the population will decrease if birth rates are less than death rates. Life expectancy is another important factor; the length of time individuals remain in the population impacts local resources, reproduction, and the overall health of the population. These demographic characteristics are often displayed in the form of a life table.

Survivorship curves

Another tool used by population ecologists is a survivorship curve    , which is a graph of the number of individuals surviving at each age interval plotted versus time (usually with data compiled from a life table). These curves allow us to compare the life histories of different populations ( [link] ). Humans and most primates exhibit a Type I survivorship curve because a high percentage of offspring survive their early and middle years—death occurs predominantly in older individuals. These types of species usually have small numbers of offspring at one time, and they give a high amount of parental care to them to ensure their survival. Birds are an example of an intermediate or Type II survivorship curve because birds die more or less equally at each age interval. These organisms also may have relatively few offspring and provide significant parental care. Trees, marine invertebrates, and most fishes exhibit a Type III survivorship curve because very few of these organisms survive their younger years; however, those that make it to an old age are more likely to survive for a relatively long period of time. Organisms in this category usually have a very large number of offspring, but once they are born, little parental care is provided. Thus these offspring are “on their own” and vulnerable to predation, but their sheer numbers assure the survival of enough individuals to perpetuate the species.

Graph plots the log of number of individuals surviving versus time. Three curves are shown, representing Type I, Type II, and Type III survivorship patterns. Birds exhibit a Type II survivorship curve, which decreases linearly with time. Humans show a Type I survivorship curve, which starts with a gentle slope that becomes increasingly steep with time. Trees show a Type III survivorship pattern, which starts with a steep slope that becomes less steep with time.
Survivorship curves show the distribution of individuals in a population according to age. Humans and most mammals have a Type I survivorship curve because death primarily occurs in the older years. Birds have a Type II survivorship curve, as death at any age is equally probable. Trees have a Type III survivorship curve because very few survive the younger years, but after a certain age, individuals are much more likely to survive.

Section summary

Populations are individuals of a species that live in a particular habitat. Ecologists measure characteristics of populations: size, density, dispersion pattern, age structure, and sex ratio. Survivorship curves show the number of individuals surviving at each age interval plotted versus time.

Questions & Answers

what is a market demand schedule
Blessed Reply
What is demand curve
mohamed Reply
Types of demand curve
demand curve is a showing the aggregate of demand whether falling from right to the left in the table above
demand curve is the graphical representation of various quantities of a commodity bought at various prices
What is the formula for calculating elasticity
Hafsat Reply
what is demand
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demand is the quantity of commodity that consumers wil be able and wiling to buy at a given price at a given time
what is international trade?
Friday Reply
it is the exchange of goods and services across international borders.
what is taxation
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the definition of economic by Adam Smith
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Adam Smith define economic as the science of wealth
What is enconomics
what is meant by pisp in economics?
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Joy Reply
Demand refers to the consumers' desire to purchase goods at given prices. Demand can mean either demand for a specific good or aggregate demand for the total of all goods in an economy.
What is economc
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What is economc
What is Economic
Vicky Reply
What's the price index of consumption?
Pun Reply
what population
Hackman Reply
I think it's the sum total of people in a goegraphical area at a given time
it refers to the number of people living in a particular area over a given period of time
decribe law of demand
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the higher the price , the higher the quantity demanded
what is economic growth
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what is an economic system
Economic system is the mechanism which deals with the production, distribution and consumption of good and services in a particular society
of course
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What is a weight
in a comparison of the stages of meiosis to the stage of mitosis, which stages are unique to meiosis and which stages have the same event in botg meiosis and mitosis
Leah Reply
Which of the following is best at showing the life expandency of an individual within a a population
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perianth is present in which gymnosperms ?
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perianth is present in which gymnos4perms ?
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