Construcción de cepas recombinantes de P pastoris portadoras

75 regulation. This controversial construct although questioned by contemporary social scientists, provided a framework for investigating Darwin’s theory of natural selection. The cumulative effects of the early works of Malthus (1798) and Darwin (1858) led to a better understanding of the factors and processes that influence human population growth and regulation including a delineation of a defined discipline of population study. Such studies indirectly provided a framework for establishing the relationship between education and population and hence the role of education in population growth.

SELF ASSESSMENT EXERCISE

Using the graphs 1 2 3, explain the relationship between Resources, population and environment.

76 Planetary Growth

( Source: Ehindero 2006 p.111)

.. the planet is passing through a period of dramatic growth and Fundamental change. Our human world of 5 billion must make room in a finite environment for another human world. The population could stabilize at between 8 billion and 14 billion sometime in the 21^st Century, according to UN Projects. One of the world’s environmental dilemmas is the rate and forms in which natural resources are being consumed and wastes are being produced even as developing countries are visioning the same developmental patterns that have led the world to its present environmental trap. In the words of the French oceanographer Jacques-Yves Coustean, when asked to comment on the dangers that threaten the earth today… “The resources of our planet are finite.., there is a limit that should not be exceeded, a habitability threshold that must not be crossed.” Over-population is our planet’s number one problem, of the 5.7 billion people on earth, less than 2 billion live in decent condition, The UNESCO Courier 1991.

It is contentious whether religion is a prominent factor in current population trend, especially in developing societies. According Cousteau (1991), religion has nothing to do with it. Italy is the world’s most Catholic country and yet it has the world lowest birth rate. Spain, which is also Catholic, is in a similar position, its birth rate is dropping vertiginously. In Indonesia, the world’s biggest Islamic country, a birth control campaign within the last twenty years has reduced the Fig 9

77 birth rate by almost 50 percent. So, religion is little involved. Fear of the future (i.e. insecurity) e.g. insurance against old age when retirement benefits cannot adequately take care of basic needs, coupled with the ingrained cultural belief and the importance attached to male children seem, influence the high birth rate served in many developing societies. These, in addition to poverty and illiteracy especially among women in rural communities, compound the population problem faced by the world.

Population density is defined as the size of a particular population in relation to a specific unit space, and expressed with reference 1 the number of species per unit area or volume. Hence, human population density is described in terms of 100 persons per square kilometer; plants in 300 trees per hectare and for bacteria or diatoms 6 million bacteria per cubic meter of water. The density organism on an area varies, changing with weather conditions, season, and nutrients.

Smith (1966) stated that there is an upper limit to the density of a population within a unit area imposed by size and tropic level. Generally, the smaller the organism, the greater is the abundance per unit area. The larger the size of the organisms, the lesser is its numerical density.

The lower the tropic level of the species, the greater is its density.

Population density is determined by three related terms - natality, mortality, and dispersal. In other words, population density is determined by the number of organisms added to the population through reproduction and the number leaving it through death. Natality is the production of new individuals in the population. It is a comprehensive term which covers all forms of production; fission, germination, hatching and birth. Technically, natality refers to the number of offspring’s-produced per female per unit time and for humans, the term birth rate is used rather than natality. Mortality refers to death of organisms in a given population leading to a decrease in number. It is expressed as the number of individuals dying in a given period. For human beings, death rate (rather than mortality) is used and it denotes the number of deaths in a give time interval divided by the average population. Mortality vanes with age and it is better expressed in terms of life expectancy because the number of individuals who survive is of more significance in a population than those who die (UNESCO: 1986), identifies two forms of mortality, physiologic or maximum longevity and ecological longevity. According to it, physiological longevity refers to the average life span of members of a population living under optimum conditions and natural deaths due to old age or senescence. It is constant for a given population Ecological longevity, on the other hand, depends on and varies with environmental conditions.

The movement of organisms and other agents of reproductive structures (e.g. seeds, spores, larvae, nymphs, tadpoles etc) in or out of the population is referred to as dispersal. For human beings it is called immigration (one way movement into a population to increase that population) and emigration (a one way outward movement leading to a decrease in population) Migration refers to the periodic departure and return of members of the population. Other

78 characteristics of population are age structure, growth form and the regulation of population size.

The age structure of population affects its natality and mortality because the potentials and actual reproductive capacity of organisms in that population is restricted to certain age bracket while mortality is also limited to certain ages. Generally, there are three different age categories of a given population: pre-reproductive, reproductive and post- reproductive. Each of this is characterized by known reproductive and non-reproductive capability. The number, ratio and changes in the ratio of organisms in these age-groups will be manifested in the natality and mortality which will eventually change the age characteristic of the population. A UNESCO (1986) publication states that a large proportion of the pre-reproductive individuals characterize a rapidly expanding population. A more even distribution of different age groups leads to a more stationary population. A large proportion of post-reproductive individuals indicate a declining population.

Fig 10

Fig 10: Different types of population as a result of different age group ratio; A: An expanding population. B: A stable population and C: A declining population (Kormondy 1976).

The other population characteristic is its growth pattern which is ideally towards establishing equilibrium with its environment. Under favorable environmental conditions, a newly established population takes on a growth pattern which is geometric-doubling s number rather quickly. Such a growth pattern continues until it reaches its maximum, sustainable by available resources when it s said to have attained its carrying capacity: After this, decline sets in as a result of the detrimental effects (e.g. unavailability of resources) of increased density on the carrying capacity.

Earlier in this Module, we saw forms of population growth curves as we graphed population number against time. We identified the arithmetic growth curve, the geometric growth curve the J-shaped growth curve which illustrates an exponential increase in density, then an abrupt stop followed by a plunge due to a sudden input of a limiting effect. The other type is the S-shaped or Sigmoid Growth Curve which shows a slow increase at first then becomes rapid and slows down

Post-reproductive Reproductive Pre reproductive

79 again with increase in environmental resistance. The equilibrium level may be maintained for sometimes before a gradual decrease.

Fig 11 & 12

Finally, all the above we know that a population is a group of organism, (plant or animal) of the specie occupying, a particular space at a particular point in time. The population, in addition, is an organizational unit (level) through which energy flows and nutrients are cycled. A population is a self-regulating system that helps to keep the ecosystem (or community) in equilibrium (dynamically stable over time.) Populations also have birth rates, death rates, age structures, densities, growth patterns, and dispersion in both time and space (both dispersion within the ecosystem and to different ecosystems).

SELF ASSESSMENT EXERCISE

What do you understand by population?