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1. What is a population?
In Biology a population is a set of individuals of the same species living in a given place and in a given time.
Population Ecology - Image Diversity: world human population
2. What is population density?
Population density is the relation between the number of individuals of a population and the area or volume they occupy. For example, in 2001 the human population density of the United States (according to the World Bank) was 29.71 inhabitants per square kilometer and China had a population density of 135.41 humans per square kilometer.
3. What is population growth rate?
Population growth rate (PGR) is the percent variation between the number of individuals in a population at two different times. Therefore the population growth rate can be positive or negative.
4. How different are the concepts of migration, emigration and immigration?
Migration is the moving of individuals of a species from one place to another. Emigration is the migration seen as an exit of individuals from one region (to another where they will settle permanently or temporarily). Immigration is the migration seen as the settling in one region (permanently or temporarily) of individuals coming from another region. Therefore individuals emigrate "from" and immigrate "to".
5. What are the main factors that affect the growth of a population?
The main factors that make populations grow are births and immigration. The main factors that make populations decrease are deaths and emigration.
6. What are some examples of migratory animals?
Examples of migratory animals are: southern right whales from Antarctica, that procreate on the Brazilian coast; migratory salmons that are born in the river, go to the sea and return to the river to reproduce and die; migratory birds from cold regions that spend the winter in tropical regions, etc.
Population Ecology - Image Diversity: migratory animals
7. What is biotic potential?
Biotic potential is the capability of growth of a given population under hypothetical optimum conditions, i.e., in an environment without limiting factors to such growth. Under such conditions the population tends to grow indefinitely.
Population Ecology - Image Diversity: biotic potential curve
8. What is the typical shape of a population growth curve? How can the biotic potential be represented in the same way graphically?
A typical population growth curve (number of individuals x time, linear scale) has a sigmoidal shape. There is a short and slow initial growth followed by a fast and longer growth and again a decrease in growth preceding the stabilization or equilibrium stage.
The population growth according to the biotic potential curve however is not sigmoidal, it is only crescent-shaped and points up to the infinite value of the scale (there is neither a decreasing stage nor equilibrium).
Population Ecology - Image Diversity: population growth curve
9. What is environmental resistance?
Environmental resistance is the action of limiting abiotic and biotic factors that disallow the growth of a population as it would grow according to its biotic potential. Actually each ecosystem is able to sustain a limited number of individuals of a given species.
The environmental resistance is an important concept of population ecology.
10. What are the main limiting factors for the growth of a population?
The factors that limit the growth of a population can be divided into biotic factors and abiotic factors. The main abiotic limiting factors are: availability of water and light, availability of shelter. The main limiting biotic factors are: population density and inharmonious (negative) ecological interactions (competition, predatism, parasitism, ammensalism).
11. How do the availability of water and light and the climate affect the growth of a population?
The availability of water and light and the climate are abiotic factors that limit the growth of a population. Since the producers are responsible for the synthesis of organic material transferred along the food chains of an ecosystem, water and light affect the availability of food and a population cannot grow beyond the number of individuals the environment is able to feed. For example, in the desert, the biomass is relatively small and populations that live in this ecosystem are smaller (compared to the same species in environments with large available biomass). The climate, including the temperature, affects the population growth because excessive change in this factor, as the occurrence of droughts or floods, may cause significant population decline; small climatic changes can also alter the photosynthesis rate and reduce the availability of food in the ecosystem.
12. How do populations of predators and prey vary in predatism?
Whenever a predator population increases at the first moment the prey population tends to decrease. At a second moment the decrease of the prey population and the bigger population density of predators cause the predator population to decrease. The prey population then reverts the tendency to decrease and begins to grow.
If variations in the size of populations occur in an unexpected intensity (different from the usual intensity of the ecological interaction) for example, due to ecological accidents killing many prey, the prey-predator equilibrium is disturbed and both species can be harmed. The existence of the predator sometimes is fundamental for the survival of the prey population, since the absence of predatism favors the proliferation of the prey and, in some cases, when the excessive proliferation creates a population size over the sustenance capacity of the ecosystem, environmental degradation occurs and the entire prey population is destroyed.
Population Ecology - Image Diversity: predator x prey curve
13. What is the relationship between environmental resistance and the population growth according to the biotic potential curve and the real population growth curve?
The difference between the real population growth curve (number of individuals x time) and the population growth according to the biotic potential curve of a given population is a result of environmental resistance.
14. How different is the growth according to the biotic potential of a viral population from the growth according to the biotic potential of a bacterial population?
The growth curve according to the biotic potential of virus and bacteria both present a positive exponential pattern. The difference between them is that in each time period bacteria double their population while the viral population multiplies dozens or hundreds of times. The viral population growth curve thus has more intense growth. This happens because bacteria reproduce by binary division, each cell generating two daughter cells, while each virus replicates generating dozens or even hundreds of new viruses.
15. What are age pyramids?
Age pyramids are graphical representations in form of superposed rectangles each representing the number of individuals included in age ranges into which a population is divided. Generally the lower age ranges are represented more to the bottom of the pyramid, always below higher ranges, and the variable dimension that represents the number of individuals is the width (there are age pyramids however in which the variable dimension is the height).
Population Ecology - Image Diversity: age pyramids
16. What are the analyses provided by the study of human age pyramids?
The study of human age pyramids can provide the following analyses: proportion of individuals at an economically active age; proportion of elderly (indicating the quality of the pension and health systems); proportion of children and youth (indicating need for job generation and educational services); reproductive profile (shows the population growth tendency); postnatal survival rate (indicates quality of the health system, hygiene conditions, nutrition and poverty); longevity profile; etc.
It is possible to predict whether a population belongs to a rich and industrialized society or to a poor country since the patterns of the age pyramids differ according to these conditions.
17. What are the main characteristics of the age pyramids of developed countries?
In a stabilized human population the age pyramid has a narrower base since the reproduction rate is not so high. The adult age ranges are generally wider than the infantile ranges showing that in practice there is no population growth. There is a proportionally high number of older individuals meaning that the life quality is elevated and the population has access to health services and good nutrition. These are features of the age pyramids of developed countries.
Population Ecology - Image Diversity: France age pyramid
18. What is the typical conformation of the age pyramids of underdeveloped countries?
The age pyramids of peripheral countries or underdeveloped countries have characteristics related to the poverty of such populations, with a wider base and narrow apex. The base age range, if much wider than the other levels, indicates a high birth rate. The levels just above the base may present an impressive reduction in poorer populations due to infant mortality. Ranges that represent the youth are also wide showing future pressure on job and habitation needs. The widths of the rectangles diminish as age increases to the apex that represents the elderly, demonstrating difficult life conditions, precarious health services and low life expectancy.
Population Ecology - Image Diversity: kenya age pyramid
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