Population Dynamics

Population characteristics All populations show characteristics that help scientists predict their future dynamics Population size = the number of individual organisms present at a given time Numbers can increase, decrease, cycle or remain the same

Population characteristics Population density = the number of individuals in a population per unit area Large organisms usually have low densities They need many resources and a large area to survive High densities make it easier to find mates But increase competition and vulnerability to predation Increased transmission of diseases Low densities make it harder to find mates But individuals enjoy plentiful resources and space

Population characteristics Population distribution (dispersion) = spatial arrangement of organisms Random = haphazardly located individuals, with no pattern Uniform = individuals are evenly spaced Territoriality, competition Clumped = arranged according to availability of resources Most common in nature

Population characteristics Sex ratio = proportion of males to females In monogamous species, a 1:1 sex ratio maximizes population growth Age distribution (structure) = the relative numbers of organisms of each age in a population Age structure diagrams (pyramids) = show the age structure of populations In species that continue growing as they age Older individuals reproduce more (i.e. a tree) Experience makes older individuals better breeders

Birth and death rates Survivorship curves = the likelihood of death varies with age Type I: more deaths at older ages Type II: equal number of deaths at all ages Type III: more deaths at young ages

Four factors of population change Natality = births within the population Mortality = deaths within the population Immigration = arrival of individuals from outside the population Emigration = departure of individuals from the population Births and immigration add individuals Deaths and emigration remove individuals Crude birth (death) rates: number of births (deaths) per 1000 individuals per year

Population growth rate Natural rate of population growth = (Crude birth rate) – (crude death rate) Population change due to internal factors Population growth rate = (Crude birth rate + immigration rate) – (Crude death rate + emigration rate) Net changes in a population’s size/1000/year Growth rate as a percent = Population growth rate * 100% Populations of different sizes can be compared

Exponential population growth Exponential growth A population increases by a fixed percent Graphed as a J-shaped curve Exponential growth cannot be sustained indefinitely It occurs in nature with a: Small population Low competition Ideal conditions

Limiting factors restrain population growth Limiting factors = physical, chemical and biological attributes of the environment They restrain population growth Environmental resistance = all limiting factors taken together Stabilizes the population size Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators Aquatic systems: salinity, sunlight, temperature, etc.

Carrying capacity Carrying capacity = the maximum population size of a species that its environment can sustain Limiting factors slow and stop exponential growth An S-shaped logistic growth curve Many factors contribute to environmental resistance and influence a population’s growth rate and carrying capacity

Perfect logistic curves aren’t often found

Reproductive strategies vary among species Biotic potential = an organism’s capacity to produce offspring K-selected species = species with long gestation periods and few offspring Have a low biotic potential Stabilize at or near carrying capacity Good competitors r-selected species = species which reproduce quickly Have a high biotic potential Little parental care, populations fluctuate greatly

Our world at seven billion Populations continue to rise in most countries Particularly in poverty-stricken developing nations Although the rate of growth is slowing, we are still increasing in numbers It would take 30 years, counting once each second, to count to a billion! It would take 210 years to count to 7 billion!

The human population is growing rapidly Our population grows by over 80 million each year It took until 1800 to reach 1 billion In 1930 (130 years later) we reached 2 billion We added the most recent billion in 12 years Due to exponential growth, even if the growth rate remains steady, population will continue to grow

Rates of growth vary from region to region At today’s 1.2% global growth rate, the population will double in 58 years (70/1.2 = 58) If China’s rate had continued at 2.8%, it would have had 2 billion people in 2004

Is population growth a problem? Technology, sanitation, medication, and increased food increase population Death rates drop, but not birth rates Population growth was seen as good Support for elderly, a larger labor pool Thomas Malthus’ An Essay on the Principles of Population (1798) Humans will outstrip food supplies War, disease, starvation reduce populations

Paul Ehrlich’s Population Bomb (1968) Neo-Malthusians: population growth will increase faster than food production Population growth causes famine and conflict Civilization would end by the end of the 20th century Intensified food production fed more people

Population growth will affect quality of life Population growth has caused famine, disease, conflict Prosperity, education, gender equality reduce birth rates Cornucopians (e.g., economists) say new resources will replace depleted ones But some resources (e.g., species) are irreplaceable Quality of life will suffer with unchecked growth Less space, food, wealth per person

Population growth affects the environment The IPAT model: I = P × A × T × S Total impact (I) on the environment results from: Population (P) = individuals need space and resources Affluence (A) = greater per capita resource use Technology (T) = increased exploitation of resources Sensitivity (S) = how sensitive an area is to human pressure Further model refinements include the effects of education, laws, and ethics on the formula

Population growth: causes and consequences

Demography Demography: the application of population ecology to the study of change in human populations All population principles apply to humans Environmental factors limit population growth Humans raise the environment’s carrying capacity through technology How many humans can the world sustain? 1–33 billion: prosperity to abject poverty Population growth can’t continue forever

Demography Demographers study: Population size Density and distribution Age structure Sex ratio Birth, death, immigration, and emigration rates

Population size and density The UN predicts 9 billion by 2050 Increased density impacts the environment But relieves pressure in less-populated areas Highest density: temperate, subtropical, tropical biomes Cities Lowest density: away from water

Population distribution Humans are unevenly distributed around the globe Unpopulated areas tend to be environmentally sensitive (high S value in the IPAT equation) Vulnerable to humans (e.g., deserts, arid grasslands)

Age structure affects population size Age structure diagrams (population pyramids) show age structure Wide base = many young: High reproduction Rapid population growth Even age distribution: Remains stable Births = deaths

Age structures: Canada vs. Madagascar Canada’s age structure is balanced Madagascar’s age structure is heavily weighted toward the young

Changing age structures pose challenges China’s age structure is changing In 1970, the median age was 20 By 2050, it will be 45 By 2050, over 300 million will be over 65 Fewer people will be working to support social programs

Many populations are aging Many populations are getting older They will need care and financial assistance Taxes will increase for Social Security and Medicare But fewer dependent children means lower crime rates The elderly can remain productive

Sex ratios Human sex ratios at birth slightly favor males For every 100 females born, 106 males are born Chinese females are selectively aborted 120 boys were reported for 100 girls Cultural gender preferences The government’s one-child policy The undesirable social consequences? Many single Chinese men Teenage girls are kidnapped and sold as brides

Factors in population change Whether a population grows, shrinks, or remains stable depends on rates of birth, death, and migration Birth and immigration add individuals Death and emigration remove individuals Technological advances caused decreased deaths The increased gap between birth and death rates resulted in population expansion Natural rate of population change = due to birth and death rates alone

Immigration and emigration War, civil strife, and environmental degradation cause people to flee their homes Each year, 25 million refugees escape poor environmental conditions This movement causes environmental problems No incentives to conserve resources

Falling growth rates do not mean fewer people Slower rates of growth do not mean a decreasing population—population size continues to increase

Factors affecting total fertility rate Total fertility rate (TFR) = the average number of children born to each female Replacement fertility = the TFR that keeps the size of a population stable (about 2.1) Causes of decreasing TFR: Medical care reduces infant mortality Urbanization increases childcare costs Children go to school instead of working Social Security supports the elderly Educated women enter the labor force

Life expectancy is increasing In countries with good sanitation, health care, and food, people live longer Life expectancy = average number of years that an individual is likely to continue to live Increases with reduced rates of infant mortality Urbanization, industrialization, and personal wealth reduce infant mortality rates Demographic transition = a model of economic and cultural change Explains the declining death and birth rates in industrializing nations

The demographic transition A stable pre-industrial state of high birth and death rates changes to a stable post-industrial state of low birth and death rates As mortality decreases, there is less need for large families Parents invest in quality of life Death rates fall before birth rates Resulting in population growth

The demographic transition’s four stages Population growth is seen as a temporary phenomenon

Is the demographic transition universal? It has occurred in Europe, the U.S., Canada, Japan, and other nations over the past 200–300 years But it may or may not apply to developing nations The transition could fail in cultures that: Place greater value on childbirth or Grant women fewer freedoms For people to attain the material standard of living of North Americans, we would need the natural resources of four and a half more Earths

Birth control: key to controlling growth The greatest single factor slowing population growth Birth control = controlling the number of children born Reducing the frequency of pregnancy Contraception = deliberate prevention of pregnancy through a variety of methods Family planning = affects the number and spacing of children Clinics offer advice, information, and contraceptives Hindered by religious and cultural influences Rates range from 10% (Africa) to 90% (China)

Contraceptive methods for birth control

Empowering women reduces growth rates Fertility rates drop when women gain access to contraceptives, family planning programs, and educational opportunities Women with little power have unintended pregnancies Two-thirds of the world’s illiterate are women Educating women reduces fertility rates, delays childbirth, and gives them a voice in reproductive decisions

We are a long way from gender equity Over 60% of the world’s poor are women Violence against women remains shockingly common Many men resist women’s decision making The gap is obvious at high levels of government We are a long way from achieving gender equality The U.S. lags behind the world in proportion of women representatives

Policies and family planning work Many countries provide incentives, education, contraception, and reproductive health care Funding and policies that encourage family planning lower population growth rates in all nations Thailand’s educational-based approach to family planning reduced its growth rate from 2.3% to 0.6% Brazil, Mexico, Iran, Cuba, and other developing countries have active programs 1994’s UN population conference in Cairo, Egypt called for universal access to reproductive health care

Family planning reduces fertility rates Blue = with family planning Red = without family planning

Poverty and population growth are correlated Poorer societies have higher population growth rates Consistent with the demographic transition theory They have higher fertility and growth rates, with lower contraceptive use 99% of the next billion people added will be born in poor, less developed regions that are least able to support them

Correlation of poverty and population Poverty exacerbates population growth Population growth exacerbates poverty In 1960, 70% of all people lived in developing nations As of 2010, 82% live in these nations 99% of the next billion will be born in these nations

Poverty causes environmental degradation Population growth in poor nations increases environmental degradation Farming degrades soil in arid areas (Africa, China) Poor people cut forests, deplete biodiversity, and hunt endangered species (e.g., great apes) Africa’s Sahel and western China are turning to desert

Wealth also impacts the environment The population problem is not only in poor countries Affluent societies have enormous resource consumption and waste production People use resources from other areas, as well as from their own Ecological footprints are huge We are living beyond our means One American has as much environmental impact as 4.5 Chinese or 10 Indians or 19 Afghans

The Earth can’t support our consumption Biocapacity = the amount of biologically productive land and sea available to us Ecological deficit = ecological footprint > biocapacity Ecological reserve = ecological footprint < biocapacity We are running a global ecological deficit Humanity’s global ecological footprint surpassed Earth’s capacity in 1987

The wealth gap and population growth cause conflict The contrast between rich and poor societies causes social and environmental stress The richest 20% use 86% of the world’s resources Increasing tensions between “haves” and “have-nots”

HIV/AIDS impacts African populations The AIDS epidemic is having the greatest impact since the Black Death in the 14th century Of 33 million infected, two-thirds live in sub-Saharan Africa; 3,800 die/day Low rates of contraceptive use spread the disease HIV is established and spreading quickly around the world

AIDS has severe effects AIDS undermines the ability of poor nations to develop Millions of orphans are created Fewer teachers and workers to fill jobs Families and communities break down Income and food production decline Debt and medical costs skyrocket

Demographic fatigue Demographic fatigue = governments face overwhelming challenges related to population growth With the added of stress of HIV/AIDS, governments are stretched beyond their capabilities Problems grow worse and citizens lose faith Good news: HIV transmission has slowed recently Decreased AIDS deaths are due to policy, collaboration, research, NGOs, and grassroots efforts