Embed Size (px)
Citation preview
POPULATION ECOLOGY
POPULATION ECOLOGY
Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size.
CHARACTERISTICS OF POPULATION
Population size Population
density Dispersion
patterns Demographics Survivorship
curves Population
growth
POPULATION SIZE
“In population genetics and population ecology, population size (usually denoted N) is the number of individual organisms in a population”.
Factors that Govern Population Size
1. Crude Birth Rate (CBR)2. Crude Death Rate (CDR)3. Immigration4. Emigration
Natality The birthrate, which is the ratio of total live
births to total population in a particular area over a specified period of time
Mortality The death rate, which is also the ratio of the
total number of deaths to the total population.
Immigration The number of organisms moving into area
occupied by the population is called immigration.
Emigration The number of organisms moving out of the
area occupied by the population is called emigration.
Factors that increase population size
1. Natality is recruitment to a population through reproduction.
2. Immigration from external populations e.g. Bird migration.
Factor reducing population size1. Mortality which is the death rate from
any source e.g. predation.2. Emigration, where individuals leave
the population for another habitat.
Population Change = (births + immigration) – (deaths + emigration)
Parameters that effect size or density of a population
POPULATION DENSITY
“Population density is a measurement of the number of people in an area. It is an average number”.
It is usually shown as the number of people per square kilometer.
Density = Population/ Area
NEED TO ESTIMATE POPULATION DENSITY
1. How to quantify nature – ecologist role
2. Estimates are allows for comparisons between different populations in terms of space and time measure.
2 attributes
Mobility
Based on movements of these organisms
Size
Small animals/plants are usually more abundant than large
animals/plants
MEASUREMENTS OF DENSITY
Density is measured by two methods.
1. Total count method2. Sampling method
TOTAL COUNT METHOD
Direct counting of populations. Possible for few animals. Breeding colonies can be photographed
then later counted.
Examples
Human pop census Trees in a given area
SAMPLING METHODS
Depends on the type of organism and its natural abundance and distribution.
Two broad categories:
1. Plot-based (quadrant) methods
2. Capture-based methods
QUADRAT SAMPLING METHOD
Widely used in plant studies
Total population = Average number per quadrat × Total area/Area of quadrat
MARK-RECAPTURE METHOD
Used for very mobile or elusive species
First used in the 1890s by C. G. Peterson to estimate fish abundance.
POPULATION DISPERSION The dispersion of a population is the
pattern of spacing among individuals within the geographic boundaries.
3 types uniformclumped
random
CLUMPED DISPERSION
Clumped is a pattern when individuals are aggregated in patches.
Most frequent pattern of distribution in a population
Reasons: Some area of habitat are more suitable than
others Heterogeneous environment with resources
concentrated in patches Tendency of offspring to remain with parents Mating or social behavior of the individuals
UNIFORM DISPERSION Evenly spaced distributions, in which
members of the population maintain a minimum distance from one another.
In plants due to competition for water, sunlight, or available nutrients
ExampleCreosote bushes in the Mojave desert
In animals due to strong territoriality
ExampleThe desert lizard Uta sp
RANDOM DISPERSION
It is a spacing pattern based on total unpredictability.
Least common pattern of distribution
Reasons
Members of a species do not frequently interact with one another
Not heavily influenced by the microenvironments within their habitat
FACTORS AFFECTING DISTRIBUTIONDensity Independent
Floods Hurricanes Unseasonable
Weather Fire Clear Cutting Pesticide Spraying
Density Dependant
Competition for Resources
Predation Parasitism Infectious
Disease
DEMOGRAPHY
Demography is the study of the vital statistics of a population and how they change over time
Two statistics that are of particular import are a population's age structure and a population's sex ratio.
SEX RATIO
It is the ratio of males to females in a population.
Primary sex ratio Secondary sex ratio Tertiary sex ratio
The human sex ratio is of particular interest to anthropologists and demographers.
AGE STRUCTURE
The age structure describes the number of individuals in each age class as a ratio of one class to another.
Age classes can be specific categories, such as people in the same age range.
POPULATION PYRAMID
Age structure diagram Graphical illustration that shows the
distribution of various age groups & sex ratio in a population.
Three age categories:
1. Prereproductive (ages 0-14) 2. Reproductive (ages 15-44) 3. Postreproductive (ages 45 and up)
LIFE TABLE A life table (mortality table ) is a table
which shows, for each age, what the probability is that a person of that age will die before his or her next birthday.
From this starting point, a number of inferences can be derived.
1. The probability of surviving any particular year of age
2. Remaining life expectancy for people at different ages
Separately for men and for women because of their substantially different mortality rates.
SURVIVORSHIP CURVE Graph showing the number or
proportion of individuals surviving at each age for a given species or group (e.g. males/females).
Constructed for a given cohort (a group of individuals of roughly the same age) based on a life table.
Three types 1. Type I2. Type II 3. Type III
Type I survivorship curves are characterized by high survival in early and middle life, followed a rapid decline in survivorship in later life.
Example: Humans Type II curves are an intermediate
between Type I and III, where roughly constant mortality rate is experienced regardless of age.
Example: Some birds In Type III curves, the greatest mortality is
experienced early on in life, with relatively low rates of death for those surviving this bottleneck.
Example: Octopus
POPULATION GROWTH
Refers to how the number of individuals in a population increases (or decreases) with time.
Controlled by the rate at which new individuals are added to the population -- the birth rate, and the rate at which individuals leave the population -- the death rate.
2 types of pop growth
Exponential population growth
dN = rmaxNdt
Logistic population growth
dN = rmaxN (K-N)dt K
Population Growth Mathematically Defined
N=K/2
EXPONENTIAL GROWTH
If a population has a constant birth rate through time and is never limited by food or disease, it has what is known as exponential growth.
With exponential growth the birth rate
alone controls how fast (or slow) the population grows.
EXPONENTIAL GROWTH
Continuous population growth in an unlimited environment can be modeled exponentially.
dN / dt = rmax N
As population size (N) increases, rate of population increase (dN/dt) gets larger.
EXPONENTIAL GROWTH For an exponentially growing population,
size at any time can be calculated as:
Nt = Noert
Nt = number individuals at time t. N0 = initial number of individuals. e = base of natural logarithms. r (rmax ) = per capita rate of increase. t = number of time intervals.
LOGISTIC POPULATION GROWTH
As resources are depleted, population growth rate slows and eventually stops: logistic population growth.
Sigmoid (S-shaped) curve
Carrying capacity (K) is the number of individuals of a population the environment can support.
Finite amount of resources can only support a finite number of individuals.
LOGISTIC POPULATION GROWTH
dN/dt = rmaxN(1-N/K)
rmax = Maximum per capita rate of increase under ideal conditions.
When N nears K, the right side of the equation nears zero.
As population size increases, logistic growth rate becomes a small fraction of growth rate.
Highest when N=K/2
LO
GIS
TIC
GR
OW
TH
Logistic growth curve
Environmental limits result in logistic
growth
Carrying capacity
New or changed environment
No limits
SM
OO
TH
TR
AN
SIT
ION
LIMITS TO POPULATION GROWTH
Environment limits population growth by altering birth and death rates.
Density-dependent factors Disease, Parasites, Resource Competition
Populations do not show continuous geometric increase When density increases other organisms reduces the fertility
and longevity of the individuals in the population This reduces the rate of increase of the pop until eventually
the pop ceases to grow The growth curve is defined as the sigmoid curve, S – shaped K = carrying capacity (upper asymptote or maximum value) –
the maximum number of individuals that environment can support
Density-independent factors Natural disasters Climate
LIFE HISTORY STRATEGIES
K and r selection (MacArthur and Wilson 1967)
r-selected species•r refers to the per capita rate of increase•Selection favoring rapid growth•Should be favored in new or disturbed environments•Less competition
K-selected species•K refers to carrying capacity•More prominent in species that are typically at their carrying capacity •Favors more efficient use of resources•Live with competition
HUMAN GROWTH
The history of human population growth
Figure 35.8A
