What do demographers measure

Although a population's age structure is not always pyramidal in shape, most populations have younger cohorts that are larger than older cohorts. Figure 4: A communal nest of skink eggs displays the fecundity of several individuals. All rights reserved. While maximum fecundity is a constant for populations, realized fecundity varies over time based on the size, density, and age structure of the population. External conditions, such as food and habitat availability, can also influence fecundity.

Density-dependent regulation provides a negative feedback if the population grows too large, by reducing birth rates and halting population growth through a host of mechanisms Lebreton et al. In white-footed mice, for example, populations regulate their reproductive rate via a stress hormone. As population densities increase, so do aggressive interactions between individuals even when food and shelter are unlimited.

High population densities lead to frequent aggressive encounters, triggering a stress syndrome in which hormonal changes delay sexual maturation , cause reproductive organs to shrink, and depress the immune system Krohne To visualize mortality and fecundity within a population, ecologists create life tables to display age-specific statistical summaries of a population's survival patterns.

First developed by Roman actuaries, life tables were used to estimate how long individuals of a particular age category were expected to live in order to value life insurance products Trenerry Raymond Pearl first introduced the life table to biology when he applied it to laboratory studies of the fruit fly, Drosophila. Life tables are particularly useful for species with discrete developmental stages and mortality rates that vary widely from one stage to the next Figure 5.

Nezara viridula assumes different body forms through metamorphosis between different discrete life stages. Photo courtesy of Jovo26 via Wikimedia Commons. Interestingly, sex ratio is not always random but can be manipulated at birth by environmental or physiological mechanisms. All crocodiles and many reptiles utilize a strategy called environmental sex determination, wherein incubation temperature determines the sex of each individual Delmas et al.

For example, low temperatures will produce males and high temperatures will produce females. In times of limited resources or high population densities, females can manipulate the sex ratios of their clutch by spending more or less time incubating their eggs Girondot et al.

These values can change between different equations of similar form. Picked at the discretion of the researcher. Andren, H. Corvid density and nest predation in relation to forest fragmentation: A landscape perspective. Ecology 73, Bull, J. Evolution of environmental sex determination from genotypic sex determination. Heredity 47, Caughley, G. Directions in conservation biology. Journal of Animal Ecology 63, Delmas, V. A mechanistic model of temperature-dependent sex determination in a chelonian, the European pond turtle.

Functional Ecology 22, Dodge, Y. The Oxford Dictionary of Statistical Terms. The slow-growth-high-mortality hypothesis: A test using the cabbage butterfly. Ecology 78, Girondot, M.

Smithsonian Books, Hutchinson, G. Population studies: Animal ecology and demography. Bulletin of Mathematical Biology 53, Jiguet, F. Bird population trends are linearly affected by climate change along species thermal ranges. Krohne, D. An analysis of dispersal in an unmanipulated population of Peromyscus leucopus.

American Midland Naturalist , Lebreton, J-D. Modeling survival and testing biological hypotheses using marked animals: A unified approach with case studies. Ecological Monographs 62, Martin, T. Avian life history evolution in relation to nest site, nest predation, and food. Ecological Monographs 65, Pearl, R. Knopf, A tour of the globe to explore its population. Use this section to: - compare demographic indicators for different countries; - help prepare for a class or an oral presentation; - find simple answers to your questions; - reflect on complex issues; - learn the basics of demography; - extend your knowledge through play All about population in Figures: tables on the French and world population and access to several online databases.

The latest data on the population of metropolitan France structure and trends are given in a series of tables. More complete datasets dating back to earlier years can also be downloaded in CSV format.

INSEE includes the DOMs in its annual demographic overview and in most of the tables in its detailed annual study of the demographic situation in France. A provisional estimate is first issued and final figures are published at a later date.

Censuses are conducted in certain years to enumerate the population. The population changes from one year to the next. Natural change is the difference between births and deaths, and can be determined precisely from vital records. Net migration is the difference between the number of immigrants and the number of emigrants. It is estimated on the basis of available statistics.

Adjustments are sometimes made to establish overall consistency between census population figures and inter-census estimates of population change. To estimate its relative scale, population change is often expressed in relation to the mean population of a given year. Rates of birth, death, natural increase and total variation are obtained in this way.

Registered births are recorded in statistical bulletins which provide a rich source of information. Completed cohort fertility is also calculated every year. For example, women born in were 34 years old in We know how many children they have had before age 34 and we can estimate how many children they will have during their reproductive life. Fertility can be controlled by means of contraception and induced abortion.

Information on contraceptive practice in France is obtained through surveys conducted regularly by INED since among the entire female population. The number of induced abortions is estimated on the basis of abortion notifications and hospital statistics. INED is responsible for publication of abortion statistics.

Marriage registers provide a rich source of information on marriage practices: number of marriages, previous marital status and nationality of spouses, etc. The first task in a population census is to enumerate dwellings. Persons living in the same dwelling form a household, so each household comprises all the persons living in a single dwelling.

According to the census definitions, a household may comprise one or more families. Death registers provide a means to measure variations in mortality. Deaths by sex and age are used to construct life tables which give life expectancy at birth, i.

The mortality of children under the age of one year is also specifically calculated. When a person dies, a doctor records the cause of death on a death certificate which is sent to INSERM where all causes of death are coded. Two statistical series drawing on different sources are presented. INSEE uses annual census surveys that provide an overview of net migration and entries and departures of non-immigrants and immigrants including undocumented foreigners. Population censuses provide an opportunity to count the number of inhabitants who were born outside France.

Among foreign-born inhabitants, a distinction is generally made between persons born with French nationality and immigrants, who are born with a different nationality. Inhabitants are distinguished by their nationality, i. French or foreign and, among French citizens, those who were born French and those who have been naturalized. The census provides information on current nationality and nationality at birth. Foreigners and immigrants form two different categories.

Immigrants "born abroad as a foreign national" may still be foreigners at the time of the census or may have become French. Foreigners, for their part, may have been born abroad in which case they are immigrants or in France in which case they are not immigrants. When analyzed together, fertility and mortality rates help researchers understand the overall growth occurring in a population. Another key element in studying populations is the movement of people into and out of an area.

Migration may take the form of immigration, which describes movement into an area to take up permanent residence, or emigration, which refers to movement out of an area to another place of permanent residence.

Children have always contributed to the total number of migrants crossing the southern border of the United States illegally, but in , a steady overall increase in unaccompanied minors from Central America reached crisis proportions when tens of thousands of children from El Salvador, Guatemala, and Honduras crossed the Rio Grande and overwhelmed border patrols and local infrastructure Dart Congress In some cases, these children are looking for relatives and can be placed with family while awaiting a hearing on their immigration status; in other cases they are held in processing centers until the Department of Health and Human Services makes other arrangements Popescu The surge placed such a strain on state resources that Texas began transferring the children to Immigration and Naturalization facilities in California and elsewhere, without incident for the most part.

Given the fact that these children are fleeing various kinds of violence and extreme poverty, how should the U. Should the government pass laws granting a general amnesty?

Or should it follow a zero-tolerance policy, automatically returning any and all unaccompanied minor migrants to their countries of origin so as to discourage additional immigration that will stress the already overwhelmed system?

A functional perspective theorist might focus on the dysfunctions caused by the sudden influx of underage asylum seekers, while a conflict perspective theorist might look at the way social stratification influences how the members of a developed country are treating the lower-status migrants from less-developed countries in Latin America.

An interactionist theorist might see significance in the attitude of the Murrietta protesters toward the migrant children. Which theoretical perspective makes the most sense to you? Changing fertility, mortality, and migration rates make up the total population composition , a snapshot of the demographic profile of a population.

This number can be measured for societies, nations, world regions, or other groups. The population composition includes the sex ratio , the number of men for every hundred women, as well as the population pyramid , a picture of population distribution by sex and age. Figure 1. This population pyramid shows the breakdown of the U.