Population Ecology

What is a population?

a group of individuals from the same species that live in the same area at the same time

what is population ecology

study of how and why the number of individuals in a population changes over time

The number of individuals present in a population depends on what four processes?

birth, death, immigration, and emigration

populations grow due to

birth and immigration when individuals enter a population by moving from another population

populations decline due to

deaths and emigration which occurs when individuals leave a population to join another population

Demography

the study of factors that determine the size and structure of populations through time

If a population consists primarily of young individuals with a high survival rate and reproductive rate… the population size should?

increase over time

Life table

summarizes the probability that an individual will survive and reproduce in any time during its lifetime

survivorship

the probability of an individual in a population surviving to a specific age

The survivorship curve

plot of the logarithm of the number of survivors vs age

Type 1 survivorship curve

survivorship throughout life is high, and most individuals approach the maximum life span (humans)

Type 2 survivorship curve

most individuals experience relatively constant survivorship over time ex. songbirds

Type 3 survivorship curve

high death rates early in life, with high survivorship after maturity ex. plants

Fecundity

number of female offspring produced by each female in the population

Age-specific fecundity

average number of female offspring produced by a female in a given age class

what does data on survivorship and fecundity allow researchers to calculate?

the growth rate of a population

Fitness trade-off

occur because every individual has a restricted amount of time and energy at its disposal

a female can…

maximize fecundity, maximize survival, or balance between the two

life history

describes how an organism allocates it resources to growth, reproduction and activities related to survival

aspects of an organism’s life history

survivorship, age-specific fecundity, age at first reproduction, and growth rate

organisms with high fecundity tend to…

grow quickly, reach sexual maturity at a young age, produce many small eggs or seeds

organisms with high survivorship tend to

grow slowly and invest their energy and time into traits that reduce damage from enemies and increase their ability to compete for resources

A population’s growth rate is the…

change in the number of individuals in the population per unit of time

if no immigration or emigration is occurring

growth rate = N (number of individuals in a population) x r (per-capita rate of increase)

Per-capita rate of increase (r)

is the difference between the birthrate and death rate per individual r= b-d

If the per-capita birthrate is greater than the per-capita death rate then…

r is positive and the population is growing

intrinsic rate of increase

when birthrates per individual are as high as possible and death rates per individual are as low as possible r reaches this

intrinsic rate of increase equation

N/t = rmax x N

exponential population growth occurs when…

r does not change over time and is density independent (does not depend on the number of individuals in the population)

two circumstances of exponential growth

1. a few individuals found a new population in a new habitat

2. a population has been devastated and beings to recover, starting with a few surviving individuals

population density

the number of individuals per unit area

what happens when population density gets very high?

the population’s per-capital birthrate should decrease and the per-capita death rate increase, causing r to decline (density dependent)

Carrying capacity (K)

the maximum number of individuals in a population that can be supported in a particular habitat over a sustained period of time

carrying capacity of a habitat depends on what factors

food, space, water, soil quality, resting/nesting sites

If a population size of N is below carrying capacity K…

the population should continue to grow and a populations growth rate should be proportional to (K-N)/K

logistic growth equation

ΔN / Δt = r_max N((K-N)/K)

the logistic growth equation describes…

logistic population growth- population growth where the growth rate slows down as the population size approaches carrying capacity

logistic growth is density depended which means

as a population approaches a habitat’s carrying capacity, its growth rate should slow

three distinct stages of density-dependent growth

1. Initially, growth is exponential (r is constant)

2. Growth rate begins to decline (N increases) when competition for density-dependent factors begins to occur

3. Growth rate reaches 0 at the carry capacity ( N vs. t is flat)

Density dependence graph

density independent factors such as _____ are ____. They change _____

variation in weather patterns, abiotic, birthrates and death rates regardless of population size

density dependent factors such as _____ are usually ___ change ______

increased predation when a deer population increases, biotic, in intensity as a function of population size

density-dependent changes in survivorship and fecundity cause

logistic population growth

population cycles

regular fluctuations in size

hypotheses for population cycles

depends on density depended factors. Predation, disease, or food shortages intensify at high population density and cause population numbers to crash

age structure of a population tends to be…

uniformed in developed countries and bottom-heavy in developing countries

population momentum

increase survivorship and many young women still have children even though fecundity is lower

in 2021, the world population is estimated at

over 7.8 billion, and 79 million people are added each year

worldwide growth rate

1.2%

replacement rate

each woman producing exactly enough offspring to replace herself and her offspring’s father

when the replacement rate is sustained for a generation…

zero population growth

the future of the human population relies on

fertility rates and how many children each woman living decides to have