populations

Ecology

Study of interactions between organisms and with their environment

Organism

An individual of a particular species (ex. one elephant)

Population

Members of the same species living in a certain area (ex. a herd of elephants)

Community

Members of all species living in a certain area (ex. elephants, lions, antelope, acacia trees, grass)

Ecosystem

All members of a community plus their physical habitat (soil, water, air, climate)

Biosphere

Zone of Earth's surface that can support life

Habitat

Physical location where a population normally lives and the resources they use (ex. ocean, desert, rainforest, another organism's body for parasites)

Population density

Number of individuals of a species per unit of area or volume

Why are population sizes measured?

To set hunting/fishing limits, judge the health of a species, and choose locations for construction/development

How are population sizes estimated?

Sampling, since exact counts are difficult (ex. counting in transects, mark-recapture)

Growth rate (r) formula

(birth rate + migration in) – (death rate + migration out)

Positive growth rate

The population is growing

Negative growth rate

The population is declining

Growth rate of zero

The population is stable

4 things that affect birth rate

Number of reproductive episodes in a lifetime, number of offspring per episode, age at first reproduction, and age structure of the population

Effect of more reproductive episodes per lifetime

More chances to reproduce = faster growth (ex. mice can have 10 litters/year; century plants reproduce only once in their lifetime)

Effect of more offspring per reproductive episode

Faster growth (ex. mice have up to 14 pups per litter; elephants have 1 calf at a time)

Effect of younger age at first reproduction

More chances to reproduce within a lifetime = faster growth (ex. mice at 6 weeks vs. humans as teenagers)

Age structure of a growing population

The majority of the population is younger (pyramid shape)

Age structure of a declining population

The majority of the population is older

Age structure of a stable population

Roughly equal numbers of individuals in all age groups

Exponential growth

Very rapid population growth that continues as long as resources are available; only limit is the growth rate; J-shaped curve (ex. bacterial outbreaks, insect infestations)

J-shaped curve

Graph shape of exponential growth

Logistic growth

Growth starts quickly, then slows as resources limit the species and levels off at carrying capacity; S-shaped curve; occurs in habitats with limited size or resources

S-shaped curve

Graph shape of logistic growth

Carrying capacity (K)

Maximum population size an ecosystem can support indefinitely; sets the upper limit on population size, and the population fluctuates around it

Does carrying capacity stay the same?

No, it varies with the environment (larger during wet season, smaller during drought)

Predator-prey cycle

Predators overhunt prey → predators decrease as prey decrease → fewer predators lets prey increase → predators increase again → repeat

Density-dependent factors

Factors whose effects change with population size/density; more severe at higher density; biotic (competition, disease, predation)

Biotic

Resulting from interactions with living organisms

Abiotic

Nonliving effects

Competition (as a density-dependent factor)

Larger population = more individuals using the same resources

Disease (as a density-dependent factor)

Denser population = faster transmission

Predation (as a density-dependent factor)

Larger populations attract the attention of more predators

Density-independent factors

Factors that affect populations the same way regardless of density; population size doesn't cause the problem; abiotic (natural disasters, pollution)

Examples of density-independent factors

Natural disasters (fires, earthquakes, floods, volcanic eruptions) and pollution (oil spills, toxic chemicals in air/water/soil)

Why can't you harvest every available individual?

No one would be left to make a new generation

Maximum sustainable yield

Amount that can be harvested without impairing population growth; typically around half of carrying capacity

Maximum sustainable yield is about what fraction of K?

About half of carrying capacity

Life history

All statistics about an organism's life from conception to death, especially adaptations that affect reproductive success

Reproduction investment

Amount of energy/resources spent on reproduction (attracting mates, gestating/incubating, feeding & protecting offspring, producing flowers/fruit)

Risk of too much reproductive investment

Can decrease the organism's own growth and health

Risk of putting off reproduction

Organism may die before producing offspring (decreases population growth)

r-selected species

Short lifespan, early first reproduction, many offspring per cycle, low reproductive investment, high mortality risk; exponential growth (ex. weeds, insects, rodents, invertebrates)

K-selected species

Long lifespan, late first reproduction, few offspring (sometimes 1), high reproductive investment, low mortality risk; logistic growth (ex. most birds, larger mammals, coconut trees)

Which strategy has exponential growth: r or K?

r-selected species

Which strategy has logistic growth: r or K?

K-selected species

Do all species fit neatly into r or K?

No, many fall between the extremes and shift with the environment (ex. fewer offspring during a famine)

Trade-off of larger offspring

Cost more resources, limit number of offspring, harder on parent's body, but have higher chances of survival

Trade-off of higher parental investment

Parent sacrifices food/safety/chances to reproduce again, but offspring have a higher chance of survival

Common causes of death in organisms

Old age/health, accidents, disease, predation, scarce resources

Life table

Calculation of the likelihood of an individual surviving to a certain age in a population

Survivorship curve

Graph of the proportion of surviving individuals at each age

Type I survivorship

High survival in youth, most deaths in old age; high parental care (ex. humans, large mammals)

Type II survivorship

Equal chance of death at any age; some parental care but constant threats (ex. birds, mice, small prey animals)

Type III survivorship

Most deaths occur in youth; survivors to adulthood have lower death risk; many offspring, little parental care (ex. invertebrates, fish, many plants)

Why has human growth been exponential?

Since the Industrial Revolution, plus antibiotics, humans have controlled food supply and disease

Human population milestones

7.5 billion in 2017; 8 billion in 2022

Urban vs. rural population density

Urban areas are densely populated; rural areas are sparsely populated

Recent trend in global birth rate

It has slowed in recent years; individual countries' growth rates vary significantly

Earth's carrying capacity for humans

Unknown; some speculate 50-100 billion if resource use is strictly moderated

What happens if a population outgrows its resources?

A sharp crash (abrupt die-off)

More-developed countries (MDCs)

Highly industrialized, slow growth (low birth rates from contraceptives, low death rates from medical technology), high standard of living, high GDP; mostly North America, Europe, Japan

Less-developed countries (LDCs)

Less industrialized, rapid growth (high birth rates offset high death rates), lower standard of living, majority in poverty; mostly Latin America, Asia, Africa, Eastern Europe

Why do MDCs have low birth rates?

Access to contraceptives

Why do MDCs have low death rates?

Access to medical technology

Why do LDCs have high birth rates?

To offset high death rates

Demographic transition

Shift from high birth & death rates → falling death rate with high birth rate (rapid growth) → falling birth rate (slow growth)

Demographic transition – Stage 1

Low population growth because both birth and death rates are high (most of human history)

Demographic transition – Stage 2

Improved living conditions and disease control lower the death rate while birth rate stays high; rapid growth (typical of LDCs)

Demographic transition – Stage 3

Birth rates fall (career opportunities, birth control access) to balance death rates; slow growth or even decline (typical of MDCs)

Example of a stable population (age structure)

United States

Example of an increasing population (age structure)

Uganda

Example of a decreasing population (age structure)

Japan

Global human life expectancy

73 years (80+ in a few developed countries)

Top causes of death today

Heart disease, stroke, dementia, lung cancer

Top cause of death for most of human history

Infectious disease

Why are infectious disease deaths lower now?

Sanitation, antibiotics, and vaccines

Leading causes of death in developing countries

Respiratory infections, waterborne intestinal diseases (ex. cholera), HIV/AIDS

Ecological footprint

Amount of land needed to support a person's or country's overall lifestyle

What is included in an ecological footprint?

Energy consumption, land for agriculture (crops, cattle, timber, fiber), and physical space for buildings, streets, and landfills

Which countries have the largest ecological footprints?

The wealthiest and most populous countries

Who is responsible for the most resource consumption: MDCs or LDCs?

MDCs, even though LDCs have larger populations

Share of ecological footprint from energy consumption

About 50%; less than 20% of the world's population uses more than half of this energy

Effects of fossil fuel use

Air pollution, acid rain, and global climate change

Effect of demand for food production

Natural habitats are converted to farmland

Fresh water and the ecological footprint

Fresh water is NOT included in the footprint and is in decline; under 50% of people in poor countries have safe water

If everyone lived like people in MDCs…

We would need another half an Earth to provide enough resources