POB II - Lectures 22 & 23

K-selected organisms

Invest more in few offspring so each has a high chance of survival

• Maintain high reproductive rate at high population density

• Competitive species with stable populations

• Live near carrying capacity in predictable environment

• Tend to be slow developing, long-lived, & large-bodied

r-selected organism

Produce many offspring each with a low chance of survival

• Maximum reproductive success in uncrowded environment (low density)

• Populations are below carrying capacity—little competition

• Common in unpredictable, disturbed, unstable environments

• Tends to be short-lived, rapidly developing & small-bodied

Factors that determine population growth

Births, deaths, immigrants, emigrants

Exponential growth

• Growth in ideal & unrestricted conditions

• Population increases from the starting population (N) size at each time point by a constant proportion (r)

• Simplest model of population growth

• J-shaped growth curve

𝚫𝑵/𝚫𝒕 = 𝑩 − 𝑫

Intrinsic rate of increase

• r variable in population growth

• The per capita rate at which an exponentially growing population increases in size at each instant in time

• Determines how fast the population grows

• Ex. higher r with the same N means more individuals produced per generation

Exponential growth in biology

• Introduced species when unchecked (ex. invasive species)

• Species when recovering from a catastrophe (ex. protected species)

Why have humans not reached a carrying capacity?

• Can alter environment with the purpose of increasing carrying capacity

• Overcome density-dependent growth regulation

• Intelligence, society, communication (ex. domestication, agriculture)

• Migration and public health, sanitation, antibiotics

Demographic transition

• Humans specifically

• Shift from high birth & death rates to low birth & death rates

• Causes: health care, sanitation, education, social change, agriculture

Carrying capacity (K)

Maximum population size that can be supported

Determined by limited resources:

• Energy

• Shelter, refuge from threats

• Nutrient availability, water

• Suitable nesting sites

• Disease, waste accumulation

Why? Increase size, fewer access to resources

Logistic growth

The per capita rate of population growth approaches zero (levels off) as the population size near K

As populations become larger/denser:

• Individuals need sufficient resources to reproduce or per capita birth rate will decrease

• Starvation/disease may increase causing per capita death rate to increase

Birth rate must decrease OR death rate must increase (or both)

When population approaches carrying capacity (hint: limiting population)

Density dependent factors

• Population regulation

• Factors that alter birth or death rates as population increase

• Birth & death rates increase/decrease with density

• As a population grows: resources available per individual decrease, risks of predation, disease, parasites increase

• Only factors that can consistently cause population to decrease/increase

Density independent factors

• Population regulation

• Factors that alter population size regardless of density

• Birth and death rates do not change with density

• All individuals of a population equally likely to be affected

• Outcome is the same regardless of population size or density

• Lead to unpredictable & abrupt population changes

• Most often abiotic

Competition

• Density-dependent regulation

• More individuals means more competition for resources

• Fewer resources lowers growth, survival, fecundity

Intrinsic factors

• Density-dependent regulation

• More individuals = physiological changes

• Even when resources are abundant, high density conditions increase stress

• Hormonal changes suppress growth & reproduction

• Stress suppresses immune function & increases disease vulnerability

• (ex. small mammals)

Territoriality

• Density-dependent regulation

• Individuals divide & defend resources

• Secures sole access to resources

• Not enough territories = decrease in population

• Smaller territory size, smaller population density

Toxic wastes

• Density-dependent regulation

• More individuals, more waste accumulated

• Can kill individuals

• (ex. yeast produces toxic ethanol)

Disease

• Density-dependent regulation

• More individuals, increased likelihood disease transmission

Predation

• Density-dependent regulation

• More individuals, easier food for predators

Predator-prey cycle

• Predator & prey are density-dependent regulators of each other

• No competition: predator exponential growth

• No predator: prey exponential growth

• Signature cyclical graph: one increases, other decreases, one high, one low

Competition

The density-dependent factor that regulates predators

Predation

The density-dependent factor that regulates prey

Population dynamics

Spatial & temporal variation in population size as a result of abiotic & biotic conditions

Influenced in immigration & emigration

Emigration

• Population-level response to ecological pressures

• Allows escape from density-dependent regulation: competition, aggression, resource scarcity

• Ex. density determines wolf population at regional level, pack size is determined by food availability

Dispersal

• Population-level response to ecological pressure

• Animals move and separate due to pressures

Active dispersal

• Mobile animals are stimulated to move

• Ability to move far depends on species & type of barrier

• Factors: crowding, temperature changes, food availability/quality, photoperiod

Passive dispersal

• Non mobile organisms disperse

• Involves gravity, wind, water, or animals

• Dispersal distance depends on the dispersal agent

• Animals can assist with passive dispersal

• Ex. seeds and dogs

Migration

The intentional & directional movement of animals between two regions or habitats

How migration occurs

• Position tracking relative to the sun & stars

• Sensing of magnetic fields

Why migration occurs

• Better resources

• Milder climate

• Safer for raising offspring

When migration occurs

• Daily, seasonal

• Metapopulation dispersal

• Foraging

• Ex. zooplankton migrating from surface at night to depths in the daytime to avoid predators, birds leaving for the winter for food & nesting sites

Species-area curve

Biodiversity pattern that shows that the larger the geographic area of a community is, the more species it has

Island biogeography

Attempts to explain species-area relationship, explanation can apply to ANY HABITAT

Island equilibrium model

• MacArthur & Wilson

• Balance between immigration rate & extinction rate determines equilibrium number of species

• As number of species increases, immigration decreases (fewer potential colonizers from the source & limited niche space)

• As number of species increase, extinction increases (not enough resources to sustain everyone!)

• Q - equilibrium number of species

• Composition can change even when it stabilizes at Q

Island distance

The factor that primarily impacts immigration

Close islands

• Easier to get to, more immigrants can colonize

• Will support more species at equilibrium

• Higher colonization

Island size

This factor primarily impacts extinction

Larger islands

• More resources & habitat diversity, more species can coexist

• Lower extinction

Equilibrium number of species (Q)

The stable number of species that an island ecosystem can support reached when rate of immigration equals rate of extinction

Population connectivity

• Movement between populations maintains gene flow

• Populations can be connected in complex patterns depending on dispersal ability & landscape

Metapopulation

A population broken into subpopulations connected by dispersal across an inhospitable matrix

• Sub-populations occupy discrete patches of suitable habitat in a sea of unsuitable habitat

• Patches vary in size, quality, & isolation (determines if a location can support sub-populations & human individuals move between locations)

Regional extinction

The thing dispersal among patches prevents because individuals are able to find places to survive

Mainland-island dynamics

• One-way metapopulation

• Dispersal establishes new self-sustaining populations

• Colonization happens in one direction: from the source

Size & isolation

Other factors that determine species richness (think of other habitats rather than islands)

Source-sink

• Type of metapopulation

• Dispersing population in not self-sustaining

• Population dies out when occupying suitable habitats

Landscape connectivity

How much the landscape facilitates or impedes movement

Population connectivity

• Connected populations = gene flow

• Genetic diversity can tell us how recently & how much subpopulations are interacting

• Populations that interact more often share more genetic diversity

• Populations that do not interact do not share genetic diversity

• Important to maintain healthy ecosystems & resilient species (reduce inbreeding, stability of population through immigration and extinction, adaptations)