Population DynamicsOutcomes:

- Describe the factors that cause a change in the diversity of a gene pool

- Describe the molecular basis of gene-pool change and the significance of these changes over time

- Describe the growth of populations in terms of population density

- Describe, & explain, quantitatively, factors that influence change in population size

- Explain different population growth patterns

- Describe the characteristics and reproductive strategies of r-selected and K-selected organisms

- Describe the basis of species interactions and symbiotic relationships and describe the influence of these interactions on population changes

- Explain the role of defence mechanisms in predation and competition

• Populations change when Hardy-Weinberg conditions are not met!

• Population has a small/ limited size • migration occurs • mutations occur • natural selection occurs • mating is non-random

• If even one of these events is occurring then the population is not in Hardy-Weinberg and the population will change over time

Changes to gene pools occur because:

• Small populations: chance fluctuation with change in allele frequencies

• Migration: removes alleles from the population as well as bring in alleles from other populations

• Mutations: add new alleles/ removes existing alleles

• Natural selection: individuals with certain alleles have more reproductive success removing alleles with less success

• Non-random mating: some mates are preferred over others based on desired traits (removing unsuccessful alleles)

Genetic Drift

• Change in the genetic makeup of a population resulting from chance

• In large populations chance events do not impactt allele frequencies but chance is a significant factor in small populations (ex. whooping cranes) causing a fixation of alleles (reducing genetic diversity)

• Founder effect: small number of individuals separate from the original population and find a new one - the allele frequencies in the new population are likely not the same and may deviate further as the new population expands (ex. Amish and Creveld syndrome)

• Bottleneck effect: severe events (weather, disease, etc) reduce the population drastically causing the allele frequency of the survivors to be very different from the original population

Gene Flow• Organisms MIGRATE altering the allele frequency

of the population they are leaving and of any population that they may enter

• Reduces differences between populations

Mutations• Randomly occurring event that alter the inheritable

genetic material of an individual (ex. wirehair cats)

• Source of NEW genetic diversity

• Variable: can change single base pairs in DNA or be large scale changes (deletions, insertions, inversions)

• They can be neutral, beneficial or harmful to the organism

• The mutation will only influence the population if it becomes common (beneficial to individuals or allowing the individual to remain viable to reproductive age

Natural Selection

• Natural selection acts on mutations of individuals and their phenotype.

• Alleles that allow individuals to be more successful will be passed on to future generations

• Harmful mutations are selected AGAINST and beneficial mutations are selected FOR

Non-random mating• Sexual selection favours any trait that influence the mating

success of the individual

• Sexual dimorphism: the difference in physical appearance of males and females

• Common forms of sexual selection are female mate choice and male vs. male competition

• Mates are chosen based on traits: colours, courtship displays, songs, etc

• Sometimes traits are detrimental (colours that attract mates also attract predators)

Population size and Density• Population size: estimated total number of organisms

• Population (DP): number of species within a defined area (considers dispersion within habitat)

• To calculate density use area or volume depending on the ecosystem/species

DP= N/A DP= N/V

• example: a population of 475 elk live in 600 hectare (ha) region in Elk Island National Park

DP= 475 elk/ 600 ha = 0.792 elk/ha

• Population size varies among species

• Smaller organisms = higher population densities

• Density can be deceiving because of unused or unusable space within a habitat

• Dispersion of groups of organisms within a population varies throughout the range (population dispersion)

• population dispersion occurs due to a variability of environmental conditions and suitable habitats throughout the population’s geographic range

• Clumped dispersion: most populations - organisms are densely grouped in areas of habitat that is favourable for survival

• Uniform dispersion: individuals are evenly distributed throughout the habitat (often results from competition for feeding, breeding or nesting - common in crops and some birds)

• Random dispersion: minimally influenced by interactions with other individuals, habitat conditions are virtually uniform and rare in nature (more common in forests)

Population growth/change

• Populations change constantly through natality (birth), mortality (deaths), immigration (individuals coming into the population), emmigration (individuals leaving the population

• Biotic potential: maximum reproductive rate achieved under ideal conditions (compare the biotic potential of an elephant to that of a rabbit)

Determining changes in population size

• Population growth quantified (see changes, mathematical models)

• Number of individuals in a population: N

• Change in the number of individuals: NΔ

[ ( ) ( )] [ ( ) ( )]N natality n immigration i mortality m emmigration eΔ = + − +

• Positive growth: number of births plus immigrants is higher than number of deaths plus emigrants

• Negative growth: number of deaths plus emigrants is higher than number of births plus immigrants

• Constant (0): number of births plus immigrants equals number of births plus immigrants

• Open population: immigration and emigration occurs

• Closed population: immigration and emigration do not occur (think of islands and other isolated areas)

• Often we are more interested in growth rate (gr) which is a change in population size per unit of time and is often measured in years

• Growth rate is often expressed as per capita growth rate (cgr) which is a change in the population size relative to the initial size and is useful when comparing populations of different sizes

Ngrt

Δ=Δ

NcgrNΔ

=

Limits to resources• Finite limit to biotic and abiotic resources at any

given time

• Increase in population size = decrease amount of available resources per individual

• Carrying capacity: maximum number of organisms that can be sustained by available resources over a period of time (it is dynamic)

Population growth models• Mathematical models, based on data from field and are a visual tool

to see population patterns in the past and predict future.

• Exponential growth: a population increases by a fixed rate over a fixed time period (variable assigned: r)

• Exponential growth created a J-shaped graph and natality > mortality

• For exponential growth there is an unlimited supply of biotic and abiotic resources (the only limit is biotic potential

• A smooth J-shaped curve demonstrates a population that reproduces continually throughout the year and a jagged curve represents a population that has rapid increases during breeding seasons.

• Resources are limited, so a population will eventually approach carrying capacity

• Environmental resistance (ER) is when a factor limits a population’s ability to realize its biotic potential as it nears the environment’s carrying capacity

• This will change the shape in the curve of the graph: an increase in ER, growth rate slows until natality and mortality are approximately equal and the population stabilizes

Logistic Growth Model• Logistic growth fits more accurately within nature,

the curve is S-shaped (has 3 phases)• Lag phase: population small, increasing

slowly • Log phase: exponential growth • Stationary phase: approach carrying

capacity (variable: K), growth slows, mortality increases

• If a population overshoots carrying capacity there is a dieback or death phase

• Logistic growth: works for populations growing under suitable conditions

• Few natural populations fit perfectly: no population exists by itself, there are many interactions among members

Density-Dependent Factors• A factor that affects a population only when it has

reached a particular density (ie. limiting population growth)

• Darwin recognized the struggle for resources within a population would limit population size

• Includes factors such as disease, competition, predation, and other biological effects (biotic factors).

• Predation: the consumption of prey by predators

• If prey have large, dense populations, intense competition for limited resources results in individuals with poor health, making them easier for predators to catch.

• Disease: In dense, overcrowded populations, pathogens pass easily from hist to host (population declines)

• Interspecific competition: competition BETWEEN species

• Intraspecific competition: competition WITHIN a species

• Minimum viable population size: the smallest number of individuals a population can have and still persist/ survive for a certain amount of time

Density-Independent Factors

• Limit population growth through changes in environmental conditions (abiotic factors)

• Impact on organisms is felt regardless of population density (ex. climate, pesticides, disasters)

Limiting factors and population size

• Resources in shortest supply is the limiting factor

• Determines how large a population can grow

• The limiting factor determines the carrying capacity of a population

Environmental stability and population change

• Change in an ecosystem affects population growth

• K is the number of individuals in a population at carrying capacity

• K-selected organisms: have traits that adapt them to living in a population at or near the carrying capacity

• Found in stable environments • Known for reproductive strategies (produce few offspring

but devote a lot of time to ensure their survival) - low biotic potential

• Are generally large • Have long lifespans, are slow growing and need lots of

parental care

• r- represents the rate of increase of a population experiencing exponential growth, rapid increase in size and are found in unstable environments

• r-selected organisms produce many offspring but devote little parental resources to offspring• generally small in size • Have short lifespans and high biotic potential • Competition is usually not a factor

K-selected species r- selected species

Live in predictable, stable environments Exploit rapidly changing environments

Long-lived Short-lived

Population size stable Population size highly variable

Density-dependent mortality Density-independent mortality

Competition intense Competition low

Multiple reproductive events beginning later in life Single reproductive event at young age

Prolonged parental care of young Little or no parental car of young

Modest numbers of offspring Very high numbers of offspring

S-shaped population growth curve J-shaped population curve

Large body size Small body size

Interactions and Communities

• Populations are constantly interacting within a larger community

• Within each community, each organism occupies its own ecological niche

• Symbiosis: includes a variety of interactions in which two species live together in close, usually physical, association

• parasitism: beneficial for one organism and harmful (but not fatal) to the other - social parasites manipulate another species to complete their own life cycles

• mutualism: beneficial for both • commensalism: beneficial to one organism and

the other individual is unaffected

Symbiosis

Mutualism Commensalism

Parasitism Social Parasitism

• Interspecific competition: occurs between individuals of different species and restricts population growth. There are two types:

1) Interference competition: actual fighting over resources

2) Exploitative competition: consumption or use of shared resources

• The strongest competition occurs between populations of species with overlapping niches (the greater the overlap, the greater the competition)

• Gause’s Principle - competitive exclusion: if the resources are limited, no 2 species can remain in competition for the exact same niche indefinitely

• Severe competition can be avoided by resource partitioning: different species with similar niches use resources in different ways (ex. owls and hawks)

Results of interspecific competition:1. Population size of the weaker competitor could

decline 2. One species could change its behaviour so that

it is able to survive using different resources 3. Individuals in one population could migrate to

another habitat where resources are more plentiful

Predation• Population density of predators increase while population

density of prey decreases

• If prey populations increase there is more food for predators but if they decrease there is less food for predators

Defence Mechanisms• Predator-Prey interactions have caused the

evolution of various defence mechanisms in plant and animal species:• Passive mechanisms: hiding

• Active mechanisms: fleeing

• Mimicry: looking like another animal (used by both predators and prey)

• Toxins: poisonous, often provide warnings through bright coloration

Succession• Gradual changes in the vegetation of an area as it

develops towards a final, stable community called a climax community

• Stages are often referred to as seral stages

• Primary succession: occurs in areas where no community existed previously

• Secondary succession: community is partially or completely destroyed and its dominant plant species have been eliminated

Primary Succession• Occurs after a volcanic eruption or when bare rock or

mineral soil is exposed by human activity or from beneath a retreating glacier

• Lichens, then mosses, are established first (they release chemicals to break down rock and create soil

• Soil and organic matter develop to support small plants

• Plants support a community of diversifying organisms

Secondary Succession• Occurs after fire, flood, landslides and/ or human disturbance

• Soil is already formed

• A pioneer community forms (first plants and then animals), they thrive in the direct sunlight, empty space and varying soil and moisture conditions

• Larger plants then begin to grow (provide shade)

• Decay process increases the fertility of the soil

• Small woody shrubs begin to grow, and eventually trees

• As plant communities change, niches change and more animals are able to survive in these niches

• Climax community is eventually reached

• Succession leading to a stable climax community is rare

• Successional changes are highly variable and influenced by disturbances

• Example: Once they are established, grasslands are maintained by routine fire disturbance (preventing succession to a forest community)

• Succession is triggered by disturbances that provide new habitats and removal of previously dominant plant/ animal species

• Gradually follows a pattern in which smaller pioneer species are replaces by larger species over time

• The number of species increases dramatically during the early stages, levels off in the intermediary phase and declines as the climax community is reached

https://www.youtube.com/watch?v=Gtqb41CjQfc