For instance, at low density, a drop in reproductive rate takes place as some females may go unmated because they were not found by males or because of an unbalanced sex ratio. Density-dependent factors are the population’s own response to density. 4.35). The J-shaped growth curve occurs in case of density-independent population whose growth slows down or stops. Subsequently, if growth persists at a constant rate a stable age distribution is quickly established. Depending upon the proportion of the three age-groups, populations can be said to be growing, mature or stable, and diminishing In other words, the ratio of various age groups in a population determines the reproductive status of the population. In physically stressed ecosystem, density-independent environmental factors play a greater role. For example, survivorship curves for two mule deer (Odocoileus hemi-onus) populations living in the chaparral of California shows (Fig. show this type of curves. They are generally more stable and less likely to suffer high mortality. These concepts of sources, sinks and meta-populations are important as they serve as a framework for studying many of our threatened and endangered species. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. This pyramid shows a high percentage of young individuals and an exponential growth of population due to high birth rate, as for example in yeast, housefly, Paramecium (Fig. In case of humans, during recent times, the three age categories are relatively equal with a third of the human life falling in each class. Due to limited resources, populations may be evenly distributed to minimize competition,  as is found in forests, where competition for sunlight produces an even distribution of trees. In such cases, the surplus offspring may disperse to other areas. It is known that mean water clarity (using a Secchi disk) is normally distributed with a population standard deviation of σ = 15.4 in. Such a case occurs for aquatic organisms when a new lake is formed or when some deer manage to reach an island where there was no deer earlier. For example, in ‘constant’ tropical environments, where populations fluctuate little, populations remain near the limit imposed by resources (K). The number aged one year has a life expectation of slightly less than one year (0.94). This is due to deer living in the managed area where food supply is high, have a shorter life expectancy than deer living in unmanaged area. For example, SMTP:firstname.lastname@example.org. Many techniques and methodology for population sampling has been tried. The growth rate of the population is determined by biotic potential and the size of the population is modified by the environmental resistance (by all the factors that control crowding). When the data from column lx (survivorship) are plotted against the x (age) column the resulting curve thus formed is called a survivorship curve. Thus, all the members born at the same time live out the full physiological life span characteristic of the species and all die at about the same time. The maximum mortality occurs at the egg, larval, seedling and old age. Such a situation would happen if environmental factors were unimportant and most of the organisms lived out their full physiological longevity. Individuals living under conditions where death results due to predation, accident, poor nutrition and infection are not factors. This is called population fluctuation. The movement of individuals may also influence the pattern of dispersion. The birth of new individuals is referred to as natality. The individuals from birth (born at approximately the same time) to the end of the life cycle, form a group known as a cohort and their investigation is turned as cohort analysis. 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