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______________________________________________________________________________ Part C: The Biosphere Unit 1: The dynamics of Biological Populations Biological Populations All organisms, including humans are organized into populations, which are themselves grouped into communities and form ecosystems. In sociology and biology, a population is the collection people or organisms of a particular species, living in a given geographic area (or space), at a given time. These are usually measured by a census. In biology, an isolated population denotes a breeding group whose members breed mostly or solely among themselves, usually as a result of physical isolation from other populations. However, they could biologically breed with any members of the species. A metapopulation is a group of sub-populations in a given area, where the individuals of the various sub-populations are able to cross uninhabitable areas of the region. A population is a group of organisms of a particular species living at a particular place at a particular time. For example the population of frogs living in the Maltese Islands is divided into metapopulations for example, the subpopulation at Chadwick Lakes, at Wied il-Ghasel, etc. Populations of different species are grouped together to form a community. Therefore a community is a group of populations of different species living in the same place at the same time, and which interact with each other. Thus for example, a community of organisms living in a freshwater rockpool would consist of several populations of plants and animals such as the frog, the wild mint, the water beetle. Individuals within a community interact with each other and the physical characteristics of the surrounding environment to form an ecosystem. Ecosystems are organized together into biomes, which together form the biosphere, that is the place on earth were living organisms are found. The biosphere is the part of a planet's outer shell, including air, land, surface rocks and water, within which life occurs, and which biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, hydrosphere, and atmosphere. This biosphere is generally thought to have evolved, beginning through a process of biogenesis, at least some 3.5 billion years ago. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 1 ______________________________________________________________________________ 1. Factors affecting population size Populations are affected by a variety of factors which either input individuals into a system or else remove them. These factors are births, deaths and migration. 1.1 Birth Rate Birth is the process by which an offspring is produced from the body of the mother thus increasing the size of a population. Birth is a result of either sexual reproduction as in animals and plants, or asexual reproduction (such as binary fission) such as occurs in protists and other unicellular organisms (e.g. Paramecium). Sexual reproduction is rather more complex and results in offspring developing within or outside the female after fertilization by a male. This takes three forms that is ovipary (lays eggs, offspring hatch from eggs), vivipary (young born directly from mother) or ovovivipary (eggs hatch in mother which gives birth to live young. The birth rate (natality) is often used to describe populations and countries, that is the number offspring born per unit time (usually a year). It is given the following formula: N n offspring t time( year ) In demography (the study of populations), the crude birth rate (CBR) of a population is the number of childbirths per 1000 persons per year. It can be mathematically represented by: where n is the number of childbirths in that year, and p is the current population. In general, the natality and crude natality are an indicator of the state of a natural population since they give an indication of the fertility (the ability of females to bear offspring). When high, recruitment is occurring, thus showing that a population is healthy, whereas when low, the population may be unhealthy. To note, however, is that natality changes with seasons since animals are not fertile throughout the year. Other indicators of fertility are frequently used such as the total fertility rate that is the average number of offspring born to each female over the course of her life. In general, the total fertility rate is a better indicator of (current) fertility rates because unlike the crude birth rate it is not affected by the age distribution of the population (that is the different ages of the population, especially females). Yet other methods of measuring birth rate are applicable to humans, such ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 2 ______________________________________________________________________________ as the General fertility rate (GFR), which measures the number of births per 1000 women aged 15 to 45 (the child bearing age). Human populations are different than biological populations. In fact, natality is rather constant throughout the year since women are fertile throughout the year, although events may cause changes such as booms in offspring. Also, fertility rates tend to be higher in less economically developed countries and lower in more economically developed countries. 1.1.2 Factors affecting Birth Rate Several factors affect biological populations namely the level of resources (be they nutrients, females, light, prey) and the climatic conditions of the particular time. Once again, human populations are affected differently since they are more complex. Therefore humans are affected by the following: - Pro-natalist policies and anti-natalist policies from government; - Existing age-sex structure; - Social and religious beliefs - especially relation to contraception; - Female literacy levels; - Economic prosperity (although in theory when the economy is doing well families can afford to have more children in practice the higher the economic prosperity the lower the birth rate); - Poverty levels – children can be seen as an economic resource in developing countries as they can earn money; and - Infant Mortality Rate – a family may have more children if a country's IMR is high as it is likely some of those children will die. 1.2 Death Rate Death is the full cessation of vital functions in the biological life. It is expressed as the death rate (mortality) and is calculated through the following formula: M n deaths t time( year ) Mortality is the number of deaths (from a disease or in general) per 1000 individuals and is typically reported on an annual basis in humans. It is distinct from morbidity rate, which refers to the number of individuals who have a disease compared to the total number of individuals in a population. Several other definitions occur: - The crude death rate: the total number of deaths per 1000 people. The formula is: CDR n 1000 P ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 3 ______________________________________________________________________________ - The perinatal mortality rate: the sum of neonatal deaths and fetal deaths (stillbirths) per 1,000 births; The maternal mortality rate: the number of maternal deaths due to childbearing per 100,000 live births.; The infant mortality rate: the number of deaths of children less than 1 year old per 1000 live births; and The standardised mortality rate (SMR) or age-specific mortality rate (ASMR): This refers to the total number of deaths per 1000 people of a given age (e.g. 16-65 or 65+); The mortality is often indicative of the state of the population. A high death rate means the population is in decline while if the death rate is low, the population is probably sound. However, the crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the death rate differs between the three life groups of humans and thus one would rather use ASMR. The ASMR reveals that in underdeveloped (developing) countries, infant mortality rate is high compared to developed countries. Also, for example the number of deaths per 1000 people can be higher for developed nations than in less-developed countries, despite standards of health being better in developed countries. This is because developed countries have relatively more older people, who are more likely to die in a given year, so that the overall mortality rate can be higher even if the mortality rate at any given age is lower. A more complete picture of mortality is given by a life table which summarises mortality separately at each age. A life table is necessary to give a good estimate of life expectancy. A life table (also called a mortality table or actuarial table) is a table which shows, for a person at each age, what the probability is that they die before their next birthday. From this starting point, a number of statistics can be derived and thus also included in the table: - the probability of surviving any particular year of age - remaining life expectancy for people at different ages - the proportion of the original birth cohort still alive. Life tables are usually constructed separately for men and for women because of their substantially different mortality rates. Other characteristics can also be used to distinguish different risks, such as smoking-status, occupation, socio-economic class, and others. An example of a life table is given on the following page. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 4 ______________________________________________________________________________ 1.2.1 Infant mortality The infant/ offspring mortality is often an indication of the vulnerability of a species. For example, oysters produce 100million eggs, only 1 of which survives to adulthood. Larger organisms such as elephants produce 6 offspring of which approximately 2 survive. In terms of human populations, infant mortality is a clear indication of the how much a population is ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 5 ______________________________________________________________________________ developed. The international levels of infant mortality, depicted as the number of deaths in a thousand births. The ten countries with the highest infant mortality rate are: 1. Angola 192.50 2. Afghanistan 165.96 3. Sierra Leone 145.24 4. Mozambique 137.08 5. Liberia 130.51 6. Niger 122.66 7. Somalia 118.52 8. Mali 117.99 9. Tajikistan 112.10 10. Guinea-Bissau 108.72 1.2.2 Causes of Death In biological populations, causes of death are natural including age, predation, parasitism and disease. On the other hand, in human populations natural and induced deaths are distinguished. Natural deaths are death due to age but induced to deaths are due to diseases. For example the 10 leading causes of death in the United States in 2002 were: 1. 696,447 Heart disease; 2. 557,197 Malignant Neoplasms (i.e. cancer) 3. 162,555 Cerebrovascular disease 4. 124,777 Chronic low. respiratory disease 5. 105,796 Unintentional injury 6. 73,248 Diabetes mellitus 7. 65,418 Influenza & pneumonia 8. 58,866 Alzheimer's disease 9. 40,801 Nephritis 10. 33,569 Septicemia The factors affecting a country's death rate are: - Nutrition levels; - Standards of diet and housing; - Access to clean drinking water; - Hygiene levels; - Levels of infectious diseases; - Large scale events and natural disasters. 1.3 Migration Migration occurs when living things move from one biome to another. Immigration is movement into a biome, and emigration is movement out from a biome. Therefore, immigration increases ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 6 ______________________________________________________________________________ the size of a population, whilst emigration decreases it. The species that periodically migrate are called migratory, those that do not are called resident (or sedentary). A typical example is bird migration which is very common. The longest known migration of a bird is that of the Arctic Tern, which migrates from the Arctic to the Antarctic and back each year. Flyways are routes that certain bird species take to migrate. Whales and other animals, such as gnus, butterflies, moths, salmon, eels, and lemmings are also known to migrate. The periodic migration of plagues of locusts is a phenomenon recorded since Biblical times. 1.3.1 Causes of Migration Several causes of migration occur. Organisms migrate in order to avoid local shortages of food, usually caused by winter. Animals may also migrate to a certain location to breed, as is the case with some fish. Therefore, organisms move from an unfavourable environment into a more favorable one to survive. In human history, migration was very common. Colonization of Europe by early hominids was by migration from Africa, as was colonization of America. The main migration routes of humans are given below. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 7 ______________________________________________________________________________ 1.4 Population Growth Rates and Doubling Time 1.4.1 How a Population Grows When a population is placed in a new habitat (such as Paramecium in a newly formed pond) and all necessary nutrients are being supplied, the population goes through several changes depicted through the following sigmoid graph (two examples are given): The graph is composed of four parts. Phase 1 (A-B): Lag phase. The population is introduced into a new habitat. The introduced individuals must adapt themselves(acclimatize) to the new habitat. In doing so they do not reproduce and their numbers might decrease. Phase 2 (B-C): Log or exponential phase. Unlimited population growth occurs after the initial acclimatization period. Abundant food is present, no disease, no predators etc. Therefore natality and immigration are high whilst mortality and emigration are low. Phase 3 (C-D): Decline or transitional phase. Limiting factors slowing population growth. Due to increased density, resources such as food start to decrease. Therefore mortality and emigration start to increase and natality and immigration decrease. Phase 4 (D-E): Plateau or stationary phase. No growth. The limiting factors balance the population’s capacity to increase. The population reaches the Carrying Capacity (K) of the environment. At this point, recruitment is approximately equal to mortality and iemigration. 1.4.2 Population Growth and Doubling Time The rate of growth is expressed as a percentage for each population, commonly between about 0.1% and 3% annually for countries. You'll find two percentages associated with population natural growth and overall growth. Natural growth represents the births and deaths in a country's population and does not take into account migration. The overall growth rate takes migration into account. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 8 ______________________________________________________________________________ For example, Canada's natural growth rate is 0.3% while its overall growth rate is 0.9%, due to Canada's open immigration policies. In the U.S., the natural growth rate is 0.6% and overall growth is 0.9%. For most purposes, the overall growth rate is the more frequently utilized. The growth rate can be used to determine a country or region or even the planet's doubling time, which tells us how long it will take for a country's current population to double assuming a constant rate of natural increase (RNI). The doubling time of a given population in a certain area is thus the time in years it takes for that population to double its size given its current population growth. This time can be found either from a graph or from a simple equation: In this equation, often referred to as the law of 70, k is the fractional growth rate (the annual growth rate per unit of time as given under exponential growth), P is the percent growth rate (100 times the fractional growth rate. This is the percent by which a quantity grows in a set time period), and TD is the doubling time. 70 is obtained through the natural logarithm (ln) of 2. For example, given Canada's overall growth of 0.9% in the year 2006, dividing 70 by 0.9 (from the 0.9%) will yield a value of 77.7 years. Thus, in 2083, if the current rate of growth remains constant, Canada's population will double from its current 33 million to 66 million. This is an accurate estimate because of recent trends. In natural populations, animals which have a high fertility rate (the average number of offspring each female will have during their offspring-bearing age) such as mice, have a low doubling time, whereas animals with a smaller fertility rate, or else their offspring have maternal ties (such as mammals) have a long doubling time. In humans the situation, as usual, is slightly different. The map on the following page gives the approximate doubling time for countries. It shows how long it will take for the population to double in size assuming it continues to grow at the current rate. The darker the colour, the longer the doubling time. For example, the United Kingdom at 433 years has a longer doubling time than Madagascar at 21 years. Green shading on the map indicates that the population is decreasing or that no information is Taking for example the African countries, doubling time is low, whereas European countries have a larger doubling time. This is linked to the generation of more offspring in less developed countries. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 9 ______________________________________________________________________________ The information below is taken from the Population Concern 1997 World Population Data Sheet. Countries are in order of Doubling Time (years): Rank Country 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sweden Greece Portugal Spain Austria Denmark Moldova Belgium Poland United Kingdom Slovakia Japan San Marino Yugoslavia Finland Switzerland Netherlands Doubling Time (years) 4077 years 1733 years 1733 years 1386 years 990 years 990 years 866 years 693 years 573 years 433 years 408 years 289 years 289 years 279 years 257 years 231 years 223 years 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Barbados France Georgia Norway Luxembourg Ireland Liechtenstein Malta Kazakhstan Canada Bosnia-Herzegovina United States Armenia Cuba Australia Cyprus Macedonia New Zealand 204 years 204 years 204 years 204 years 178 years 147 years 139 years 136 years 133 years 127 years 122 years 116 years 108 years 102 years 100 years 90 years 86 years 86 years Countries with high birth rates and low death rates have the fastest rates of growth and therefore the shortest doubling times. This is the number of years it will take for a population to double in size if the current rate of increase continues. However, alternatively, the doubling time for a countrywith low birth rate and high mortality rate is high. The doubling time can therefore be compared with the total fertility rate, that is the average number of children each woman in that country will have, assuming that the birth rate stays the same throughout her child-bearing years (15-49). Taking, for example the situation in West Africa, the following is observed: ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 10 ______________________________________________________________________________ Country Total Fertility Rate TFR Doubling Time DT Life Expectancy (no. of children) Rank (years) Rank (years) Benin 6.8 21 Burkina Faso 6.9 23 11 47 Cape Verde 3.8 16 36 1.5 65 Cote d'Ivoire 5.7 12.5 27 52 Gambia 5.9 28 45 Ghana 5.5 24 Guinea 5.7 GuineaBissau 12.5 54 9 56 29 45 5.8 34 43 Liberia 6.4 22 59 Mali 6.7 23 Mauritania 5.4 27 52 Niger 7.4 21 47 Nigeria 6.2 Senegal 6.0 Sierra Leone 6.5 36 1.5 34 Togo 6.9 20 16 57 1 23 9 11 11 26 46 54 49 ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 11 ______________________________________________________________________________ Exercise 1: Is there is a link between doubling time and total fertility? 1 Rank both sets of data in the correct column (longest doubling time = rank 1; highest total fertility = rank 1). 2 Plot the rank position for each country as a point on a scatter graph like the one on the right. Name each point as you go. 3 Add the labels 'Lowest Fertility Rate' and 'Highest Fertility Rate' at the correct ends of the side axis. Add 'Longest Doubling Time' and 'Shortest Doubling Time' to the bottom axis. The points should form a band from the top left to the bottom right. If the link between total fertility rate and doubling time was perfect, all the points would be on the diagonal line. In this case only one country, __________ is on the line, but the spread still shows a negative link or negative correlation. This means that as one thing increases, the other thing decreases. In this case, as ________________________ increases _______________ decreases. The fact that some countries are far from the diagonal line shows that there must be other factors affecting the situation, ie. that total fertility rate is quite a good predictor of doubling time, but that there must also be links with other factors. 4 Countries whose position is away from the line in segment A of the graph have doubling times which are quite long given their fertility rates, eg. Guinea. Countries in segment B, like Ghana, have short doubling times not in keeping with their modest fertility rates. Suggest some factors which might make the doubling time shorter than expected. 5 Life expectancy is the number of years a person born in that country can expect to live. The shortest life expectancy in West Africa is just _____ years. The longer life expectancy in Liberia (_____ years) suggests that the standard of living and public health services are better than the average for the region. Many countries in the more developed world are noted for their excellent life expectancy - check out Sweden, Norway, Canada, the UK and Iceland using the World Population Data Sheet. Notice that women tend to live longer than men. See if your teacher can explain that! ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 12 ______________________________________________________________________________ 2. Population Growth Population growth is change in population over time, and can be quantified as the change in the number of individuals in a population per unit time. In demographics and ecology, Population Growth Rate (PGR) refers to the change in population over a specific time period expressed as a percentage of the number of individuals in the population at the beginning of that period. This can be written as the formula: 2.1 Factors Affecting Population Growth Over time, assuming that resources (food and space) are available, the intrinsic nature of any population is to grow! Surpluses and increases in food availability allow populations to grow at exponential rates. However, populations have a limit to the rate of growth and they can undergo and to the size of the population that can be attained. Several factors contribute to the size of a population namely environmental resistance. In naturally occurring systems, resources do not continually increase over time. Biological events (environmental resistance) occur that allow a population to stabilize and be in balance with resources. The environmental resistance is thus the limiting influences of environmental factors upon the increase in numbers of individuals in a population. Environmental resistance falls within three categories: - Abiotic (non-biological) factors that contribute to limiting a population size include examples such as: wind (gentle breezes vs. tornados), rain (drought vs. flood), rain pH, amount and quality of sunlight, climate, lightning, fire. - Biotic (biological) factors that contribute to limiting a population size include examples such as: disease, resource availability, predators, Competitors. - Social factors contribute to limiting the size of a population within species that have social systems. The social system of flocking birds is hierarchical. The relationship among birds is categorized as: dominant, sub-dominant and juvenile. A birds place in the "pecking order" will ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 13 ______________________________________________________________________________ determine whether it has a high, medium or low quality environment to exploit. The quality of the environment will determine whether the bird thrives or merely survive. In general these three types of resistance can be defined as being density-dependent (biotic, social) or else density- independent (abiotic, biotic, social). In the former, resistance increases with increasing density of the population and in the latter, density does not have a role. 2. 2 Density dependent and Density- Independent factors Density-Independent factors are factors acting randomly, with reference to the population density, to regulate population size. These do not depend on the size of a population and examples are: 1) Weather - unfavourable weather can limit breeding success or cause mortality during the non- breeding season (e.g., winter mortality). 2) Food Supply - abundance of food is often dependent on climatic factors, and in poor years food supply may limit population size. 3) Habitat Limitation - seasonal, annual, or anthropogenic changes can affect resource levels such as nesting sites or availability of nesting materials. (2) and (3) contribute to setting a limit on the Carrying Capacity of the habitat. Density-Dependent Factors regulate populations according to the population density. Therefore they increase with increasing population size and include: 1) Predation: Acts more strongly at high densities. Keeps populations below the theoretical carrying capacity in some cases. 2) Competition: if resources are limiting, only a given number of individuals may persist in a given habitat (the number that can persist is the carrying capacity). At low densities competition is not an important factor in regulating population size, but at high densities (near the carrying capacity) it becomes very important. 3) Parasites and Disease: have the greatest effect at high population densities. Can be important factors regulating population sizes in some cases. Example: Avian cholera can kill very large numbers of wintering or migrating waterfowl that are concentrated in relatively small areas. 2. 3 The Carrying Capacity The maximum population size that can be attained under specific conditions is termed the carrying capacity (K). This is the maximum population size that may be maintained indefinitely (for a long period of time) by the present resources of a certain environment. 2. 4 The Biotic Potential Biotic potential is the maximum reproductive capacity of an organism under optimum environmental conditions. Full expression of the biotic potential of an organism is restricted by ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 14 ______________________________________________________________________________ environmental resistance. A species reaching its biotic potential would exhibit exponential population growth and be said to have a high fertility. Biotic Potential is a fundamental species characteristic, defined as "the inherent power of organisms to reproduce and survive". It is the sum of the number of young produced at each reproduction, number of reproductions over a period of time, sex ratio of the species, and their general ability to survive under given physical conditions. Therefore, the components of Biotic Potential are the Reproductive potential (potential natality; It is the upper limit to biotic potential, that is in the absence of mortality) and the survival potential ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 15 ______________________________________________________________________________ 3. Models of Population Growth Populations grow if the natality and immigration exceed the mortality and emigration. Populations decrease in nature if the opposite occurs. Populations in nature may grow in several different ways, described by mathematical models. Four common models are described below. These are the Linear growth, Irruptive growth models, Exponential (density independent) growth and Logistic (Sigmoid) growth). The latter two are called the Lotka-Volterra Models. 3.1 Linear growth (Density independent) A population starts off with a small number of members. Since numbers are small, the population increases in size very slowly. The population increases in a slow manner, increasing in a linear fashion, that is, the increase in individuals is linear over time. Therefore, birth rate and immigration are higher than mortality and emigration in a constant manner. 3.2 Exponential growth (Density independent) In this type of growth, a population starts off with a small number of members. Since the numbers are small, the population increases in size very slowly. When the population increases above a certain level, it quite suddenly starts to grow very rapidly, possibly at its maximum rate of growth (recruitment). This is exponential growth, and goes on for as long as there is enough room and enough food for all the members of this rapidly expanding population. 3.3 Logistic/ Sigmoid growth (Density dependent population) The population grows roughly exponentially to begin with, but then rise more slowly towards a “carrying capacity”. Thus, when the population increases to a certain level space and food start running out. The individuals in the population become more crowded, food is scarce and ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 16 ______________________________________________________________________________ disease spreads rapidly. Therefore, the natality and immigration decrease and mortality and emigration decrease. Eventually these two become equal and the population levels off. The rate of population growth slows down and stabilizes at a certain level called the carrying capacity. 3.4 Irruptive (Malthusian growth) Some populations do not follow this pattern of growth and stabilisation. Some populations start off small, start to grow exponentially after a while and keep on growing. They may grow so rapidly and become so large that they may exceed the carrying capacity (the maximum amount that can be maintained by the environment). This is called overshoot. When a population overshoots, resources used by members of the population run out very quickly. This is therefore followed by a dieback, where high rates of mortality occur. This means that the population would fall back to its previous levels. This is called irruptive growth or Malthusian growth. Irruptive growth, is thus a growth pattern defined by population explosions and subsequent sharp population crashes, or diebacks. Populations which exhibit irruptive growth do not stabilize around their carrying capacity but are subject to continual change. An example is given by locusts whereby populations may be low but suddenly increase rapidly forming swarms, which eventually die. This is also seen in some mammals such as lemmings. 3.5 Other Models 3.5.1 Two-species competition Each population grows logistically in the absence of competition, with its growth rate reduced as its population gets bigger. When a competitor is present, then the growth of each population is further reduced by the other. The outcome depends on the relative strength of interspecific competition The graph shows that both species initially grow at similar rates, but then species 1 escapes and suppresses species 2. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 17 ______________________________________________________________________________ 3.5.2 Predator – prey The prey grows exponentially in the absence of predation. It is predated at a rate proportional to the number of predators. The growth rate of the predator population is proportional to prey consumption, and predator mortality is proportional to its population. ________________________________________________________________________________________________________ J. Henwood The Biosphere V 1.0 18