How Do Populations Grow

How do populations grow?

What affects population size?

n Births (+)

n Deaths (-)

n Immigration (+)

n Emigration (-)

Intrinsic growth rate: r

n r = birth rate – death rate

n r is a per capita rate per unit time

Per capita rates

n Rates per individual

n Total number of events in a time interval divided by the number of individuals

n Per capita birth rate per month =

Number of births per month

Population size

Biotic potential

n Maximum rate of increase per individual under ideal conditions

n Varies between species

n In nature, biotic potential is rarely reached

Exponential growth model

dN/dt = rN

n dN/dt = population growth per unit time

n r = intrinsic rate of increase

n N = number of individuals in population

Factors that regulate population growth

n Intrinsic factors - Operate within or between individual organisms in the same species

n Extrinsic factors - Imposed from outside the population

n Biotic factors - Caused by living organisms

n Abiotic factors - Caused by non-living environmental components

Limits to population growth

n A population’s growth depends on the resources of its environment

n A limiting factor is any essential resource that is in short supply

n All limiting factors acting on a population dictate sustainable population size

Carrying capacity

n The maximum number of individuals that the environment can support based on limiting resources

Logistic growth

n Growth rates regulated by internal and external factors until coming into equilibrium with environmental resources

n Growth rate slows as population approaches carrying capacity

n S curve

Logistic growth model

dN/dt = rN

n Rate of population growth is highest at K/2

How do populations grow?

n Exponentially

n J-shaped

n Unlimited resources

n Logistically

n S-shaped

n Resources are limited

What factors affect population growth?

What limits the distribution and abundance of organisms?

n Abiotic environment

n Thermal limitations

n Ability to respond to change

n Interactions (intraspecific and interspecific)

n Characteristics inherent within the individual

n Not how an individual responds, but how an individual lives

n Life-history

Importance of density

n Some factors that regulate population sizes are directly related to density

n In other words, as density changes, the factor will change with density

n Density dependent factors

n Some factors that regulate populations are not related to density at all

n In other words, as density changes, the factor does not change

n Density independent factors

Density-dependent factors

n Higher proportion of population is affected as population density increases

n Tend to reduce population size by decreasing natality or increasing mortality

n Interspecific interactions

n Predator-Prey oscillations

n Intraspecific interactions

n Territoriality

n Stress and crowding

n Stress-related diseases

Allee effect

Density-independent factors

n Factors unaffected by population density

n Examples include drought, hurricanes and floods

Density-independent factor

n Decline in the population of one of Darwin's finches (Geospiza fortis) on Galapagos Islands due to drought

Community interactions

n Hare fluctuations due to both

n Food plants abundance

n Predator abundance

Why don’t all populations maximize their growth? i.e., why don’t all organisms reproduce like rabbits?

Cost of reproduction

n Due to limited resources, increased reproduction may decrease survival and chances of future production

n Therefore, there is a cost of reproduction

n Trade-offs

n Remember that natural selection will favor the life history that maximizes lifetime reproductive success

Life history

n A life history is the complete life cycle of an organism

n “Choices” in life history

n How much to invest in each child

n How often to reproduce

n When to reproduce

n All “choices” involve significant trade-offs!

n So, organisms must make choices that maximize their reproductive success overall given ecological conditions

Cost of reproduction

n Investment per offspring

n Key reproductive tradeoff concerns the amount of resources to invest in producing any single offspring.

n number of offspring versus size of each offspring

– In many species, offspring size critically affects chances of survival

Parental investment

n Tradeoff between number of offspring and size of offspring

Number of reproductive events

n Trade off between current and future reproductive success

Cost of reproduction

n Reproductive events per lifetime

n Semelparity - organisms focus all reproductive efforts on a single, large event

n Iteroparity - organisms produce offspring several times over many seasons

Cost of reproduction

n Age at first reproduction

n Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals

Life-history strategies

n All of these trade-offs – and more -occur simultaneously

n Can not maximize one trait without losing in another

n Results in some common strategies for dealing with life

n r vs K

Life history strategies

n K-selected species tend to produce relatively few, large offspring

n large investment in parental care

n r-selected species tend to produce many, small offspring

n small investment in parental care

r/K species

n Species at near exponential phase of life = r

n Rapid growth

n “r-selecting” habitats

n Species at more constant density = K

n Growth more limited by resources

n “K-selecting” habitats

r-selection vs. K-selection

Human population growth

n Until the Middle Ages, human populations were held in check by diseases, famines and wars

n Took all of human history to reach 1 billion

n 150 years to reach 3 billion

n 12 years to go from 5 to 6 billion

Human population growth

Human Population Density

Two demographic worlds

n First is poor, young, and rapidly growing

n Less-developed countries

n Africa, Asia, Latin America

n Contain 80% of world population, and will account for 90% of projected growth

n Second is wealthy, old, and mostly shrinking

n North America, Western Europe, Japan.

n Average age is about 40

n Populations expected to decline

Fertility and birth rates

n Total fertility rate - Number of children born to an average woman in a population during her life

Birth rates in the United States

TFR world-wide

Birth reduction pressures

n Higher education and personal freedom for women often result in decisions to limit childbearing

n When women have more opportunities to earn a salary, they are less likely to have children

n Education and socioeconomic status are usually inversely related to fertility

Life span and life expectancy

n Life expectancy - Average age a newborn can expect to attain in any given society

n Declining mortality is the primary cause of most population growth in last 300 years

Age structure

Emigration and immigration

n Emigration and immigration play a large role in human population dynamics

n Developed regions expect 2 million immigrants a year for next 50 years

n Immigration is a controversial issue

n “Guest workers” often perform dangerous or disagreeable work, while being paid low wages with few rights

n Locals complain immigrants take away jobs and overload social services

Demographic transition

n Model of falling death rates and birth rates due to improved living conditions accompanying economic development

n Pre-Modern Society - Poor conditions keep death rates high

n Birth rates high

n Economic development brings better conditions and standard of living

n Death rates fall

n Birth rates stay constant or even rise

Demographic transition

n Eventually, birth rates begin to fall.

n Populations grow rapidly in time between death rates and birth rates fall

n Developed Countries - Transition is complete and both death and birth rates are low and population is in equilibrium

Ecological footprint

Amount of productive land required to support an individual at the standard of living of a particular population through the course of his/her life