BIO2129 topic 9
extinctions are usually
rare in nature, because populations tend to persist over long periods
What is population density
The number of individuals of a species living in a specific area or volume
population density usually fluctuates around stable values because
populations don't just keep increasing or decreasing
What causes population growth patterns
Patterns depend on the characteristics of the environment, physical (abiotic) and biological (biotic) factors
what influences population growth (how fast a pop. increases or decreases) is influenced by
1. birth and death rates 2. environmental factors (climate, food, habitat, predation) 3. species characteristics (lifespan, reproduction rate)
populations fluctuate over time rather than
staying perfectly stable
extinction happens but the rate is
historically low
The population growth rate is affected
by population density
As population density increases, growth rate decreases because
resources (food, space, mates) become limited.
Examples of how the growth rate is affected by population density
Increased competition
Higher disease transmission
Predation pressure
Density-dependent factors cause
birth rates to fall and/or death rates to rise as density increases.
What is Density-dependent regulation
Population control factors that get stronger as population density increases (e.g., competition, disease).
What is Density-independent regulation
Population control factors that affect populations regardless of size or density (e.g., natural disasters, weather).
the example of clutch size is an example of
density-dependent regulation. the size of the clutch decreases as population density increases, since fewer resources are available, individuals invest less in reproduction. Therefore, fewer offspring are produced.
density dependent factors are like
Feedback mechanisms that help maintain equilibrium
Density-dependent =
biotic factors (living causes, like other organisms)
Density-independent =
abiotic factors (non-living causes, like weather, fire, drought)
As density increases, growth rate
slows and population size stabilizes near the carrying capacity (K)
What is Carrying capacity (K)
the maximum number of individuals that an environment can support over time without running out of resources
in the short the trout example explains that
When there are too many trout, competition for resources like food or space increases.
Result → death rates rise and birth rates drop, keeping population stable.
Populations tend to stabilize around K,
where birth = death.
population growth rates are proportional to
population density
Population growth rate (r = b - d) is a function of population density.
Growth depends on how many individuals are already in the population.
What are the Mechanisms of Density Dependence:
Resource competition: food, space, or nesting sites become limited.
Disease: spreads more easily in dense populations (e.g., COVID — more people = higher transmission).
Interference competition: direct interactions like aggression or territorial fights.
Predation: predators may find prey more easily at higher densities.
with density dependence, the population density tends to approach a stable value (K), this is associated with
logistics growth
What are Density-Independent Factors
Factors that kill the same proportion of the population regardless of its density. They cause random mortality and don’t depend on how crowded the population is.
Example of density-Independent Factors
Fires, extreme weather, droughts, floods, accidents.
Environmental stochasticity (random environmental variation).
Changes in resource availability.
wat are some key points for Density-Independent Factors
These factors don’t “pull” the population toward stability.
There’s no relationship between the magnitude of population loss and density.
Effects occur regardless of carrying capacity (K).
Acts as random mortality, hitting populations equally at any size.
Example of Population Density of Thrips
The abundance of thrips changes seasonally and yearly.
These variations are mostly predictable by weather, showing density-independent regulation.
Population size follows environmental factors (e.g., rainfall, temperature).
Changes aren’t caused by density but by external environmental conditions.
What is the Allee effect
Occur at very small or low-density populations.
Individuals can’t find mates or cooperate effectively → reproductive failure.
Population growth rate becomes negative at low densities (inverse density dependence).
In the Allee effect, small populations
can crash toward zero instead of growing toward K.
Mortality rates depend on
Population density (density-dependent mortality)
Age of individuals (juveniles vs adults)
Climate and environmental conditions
what is the goal of Estimating Population Dynamics
Understand how populations change in size and what affects their growth or decline. from mark–release–recapture (MRR)
Birth Rates Depend on:
Age, when reproduction starts.
Population density, higher density = fewer births.
Year/environmental conditions, vary with climate and resource availability.
Predicting Population Size (abundance) Requires
Initial abundance – how many individuals there are to start with.
Age structure – number of individuals in each age group.
Mortality – depends on age, density, and environment.
Fecundity (birth rates) – varies with age, density, and environmental factors.
Survival rates of young – strongly affect future abundance.
age structure is very important in predictin abundance!
Without age structure → predicted population sizes don’t match real (observed) data.
With age structure → predictions closely follow observed patterns