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Levels of Organization, Population Ecology. Chapter 3. We study ecology at several levels. Ecology : studies interactions among organisms And their environment Ecology and evolution are tightly intertwined Biosphere : the total living things on Earth And the areas they inhabit

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We study ecology at several levels
We study ecology at several levels

Ecology: studies interactions among organisms

And their environment

Ecology and evolution are tightly intertwined

Biosphere: the total living things on Earth

And the areas they inhabit

Ecologists: study relationships at higher levels

Levels of ecological studies
Levels of ecological studies

Organismal ecology examines relationships between individuals and their environment

Population ecology: investigates population changes

The distribution and abundance of individuals

Why some populations increase and others decrease

Community ecology: focuses on patterns of species diversity and interactions

Ecosystem ecology studies living and nonliving components of systems to reveal patterns

Nutrient and energy flows

Each organism has habitat needs
Each organism has habitat needs

Habitat: the environment where an organism lives

It includes living and nonliving elements

Habitat use: nonrandom patterns where organisms live

Habitat selection: the process by which organisms actively select habitats in which to live

Species use different criteria to select habitat

Soil, topography, vegetation, other species

Species have different habitat needs

Depending on body size, season, etc.

Species survival depends on having suitable habitats

The niche a multidimensional concept
The niche: a multidimensional concept

Niche: an organism’s use of resources

Along with its functional role in a community

Habitat use, food selection, role in energy and matter flow, interactions with other individuals

Specialists: have narrow niches and specific needs

Extremely good at what they do

But vulnerable when conditions change

Generalists: species with broad niches

They use a wide array of habitats and resources

Survive in many different places

Population size
Population size

All populations show characteristics that affect their future dynamics

  • Population size: the number of individuals present at a given time

    • Can increase, decrease, cycle, or remain the same

Humans drove passenger pigeons, North America’s most abundant bird, to extinction

Population density
Population density

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

Also increase transmission of diseases

Low densities make it harder to find mates

But individuals enjoy more space and resources

Population distribution
Population distribution

Population distribution (dispersion): spatial arrangement of organisms

Random: haphazardly located individuals, with no pattern

Resources are widespread

Uniform: evenly spaced individuals

Territoriality, competition

Clumped: most common in nature

Arranged according to resources

Sex ratios and age structure
Sex ratios and age structure

  • Sex ratio: proportion of males to females

    • In monogamous species, a 1:1 sex ratio maximizes population growth

  • Age structure (distribution): the relative numbers of organisms of each age in a population

    • Helps predict population growth or decline

  • In species that continue growing as they age

    • Older individuals reproduce more (e.g., a tree)

    • Experience makes older individuals better breeders

Four factors of population growth or decline
Four factors of population growth or decline

  • Natality: births within the population

  • Mortality: deaths within the population

  • Immigration: arrival of individuals from outside the population

    • Births and immigration add individuals to a population

  • Emigration: departure of individuals from the population

    • Deaths and emigration remove individuals

Population growth rate
Population growth rate

  • Growth rate: rate of change in a population’s size per unit time

    • Equals (birth rate + immigration rate) – (death rate + emigration rate)

    • Tells us the net changes in a population’s size per 1000 individuals per year

  • Growth rate is expressed as a percent:

    • Population growth rate * 100%

    • Populations of different sizes can be compared

Exponential population growth
Exponential population growth

  • Exponentialgrowth:a population increases by a fixed percent

    • Graphed as a J-shaped curve

  • It occurs in nature with:

    • Small populations

    • Low competition

    • Ideal conditions

Limiting factors restrain population growth
Limiting factors restrain population growth

  • Exponential growth rarely lasts

  • Limiting factors: physical, chemical, and biological attributes of the environment limiting population growth

  • Environmental resistance: all limiting factors together

    • Stabilizes the population size at its carrying capacity

    • Terrestrial animals: space, food, water, mates, shelter, breeding sites, temperature, disease, predators

    • Plants: sunlight, moisture, soil chemistry

    • Aquatic systems: salinity, sunlight, temperature, etc.

Carrying capacity
Carrying capacity

  • Carrying capacity: the maximum population size the environment can sustain

    • Determined by limiting factors

  • Limiting factors slow and stop exponential growth

    • Forms an S-shaped logistic growth curve

Population density affects limiting factors
Population density affects limiting factors

  • Density-dependent factors: limiting factors whose influence is affected by population density

    • Increased density increases the risk of predation, competition for mates, and disease

    • Results in the logistic growth curve

    • Environmental resistance has a stronger effect on larger populations

  • Density-independent factors: limiting factors whose influence is not affected by population density

    • Temperature extremes, floods, fires, and landslides

Carrying capacities can change
Carrying capacities can change

  • Environments are complex and ever-changing

    • The carrying capacity can change

  • Humans lower environmental resistance for ourselves

    • Increasing our carrying capacity

    • Technologies have overcome limiting factors

    • We have appropriated immense amounts of resources

  • But by increasing the carrying capacity for humans

    • We have reduced the carrying capacity for countless other organisms

    • Calling into question our own long-term survival

Reproductive strategies vary among species
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 (i.e., a low biotic potential)

    • Offspring have a high likelihood of survival

    • The population stabilizes at or near carrying capacity

    • Good competitors

  • r-selected species: species that reproduce quickly

    • Have a high biotic potential

    • Little parental care, populations fluctuate greatly

Population changes affect communities
Population changes affect communities

  • Scientists have noticed troubling changes in the environment

  • As Monteverde dried out, species have disappeared

    • Golden toads, harlequin frogs, and more had been pushed from their cloud-forest habitat into extinction

    • Species from lower, drier habitats moved into the cloud forest

    • Population sizes of cloud-forest bird species declined

  • Changing climate and disease are causing population fluctuations and changing the makeup of communities

Conserving biodiversity
Conserving biodiversity

  • Human development, resource use, and population pressure are changing populations and communities

  • Factors threatening biodiversity have complex social, economic, and political roots

    • We must understand these factors to solve problems

  • Millions of people are working to protect biodiversity and to safeguard ecological and evolutionary processes

Costa rica s protection is paying off
Costa Rica’s protection is paying off

Costa Rica was losing forests at the world’s fastest rate

Now, 25% of its area is under protection

Ecotourism: tourists visit protected areas

  • Ecotourism provides thousands of jobs and billions of dollars to Costa Rica’s economy


The fundamentals of evolution and population ecology are integral to environmental science

Natural selection, speciation, and extinction help determine Earth’s biodiversity

Understanding how ecological processes function at the population level is crucial to protecting biodiversity