Population Ecology
ZLY 101
Dr. Wasantha Weliange
POPULATION ECOLOGY
• branch of ecology
• We can study
1. structure & dynamics of
populations
2. the variables that determine the
abundance & distribution of a
population
a) in time & space
b) with regard to environment
Population
• is a group of organisms, of the same species, which roughly occupy
the same geographical area at the same time.
Different definitions
• In human demography
– a population is a set of humans
in a given area.
• In genetics
– a population is a group of
interbreeding individuals of the
same species, which is isolated
from other groups.
• In population ecology
– a population is a group of
individuals of the same species
inhabiting the same area.
Spatial scale population
• Local populations can occupy very small habitat patches like a puddle.
• A set of local populations connected by dispersing individuals is called
a meta-population.
Scale Level Population
• Regional level
– population of the region
• Island level
• – population of an island
• Continental level
– population of the African
continent
• Global level
– Even the entire species can
be viewed as a population
Continent level populations
Differ in their stability
• Some populations are stable for
thousands of years
• Some persist only because of
continuous immigration from
other areas
• Populations on small islands get
extinct, but then these islands
can be re-colonized.
Characteristics of a population
1.
2.
3.
4.
5.
6.
Population size
Population density
Patterns of dispersion
Demographics
Population growth
Limits on population growth
• Above characteristics of a population are shaped by the
interactions between individuals and their environments on
both ecological and evolutionary time scales, and natural
selection can modify these characteristics in a population.
Population size
• is the number of individuals
living in the defined area
• Grassland, lake, pond,
river, tundra
• increases and decreases over
time
– resource availability
– Competition
– Parasitism
– Predation
– climate
– Natality / motality
Population density
• The number of individuals of a
species in an unit area
Natality & mortality
• used to calculate the dynamics of a
population
• key factors determining size of a
population over time
• Natality
– is the greatest influence on a
population’s increase
• Crude birth rate
– number of births per 1000
population/year
Dispersion
• Dispersal is movement of individuals
or their offspring into or out of an area
• regulates population size
• mixing of genes between populations
is important for evolution
• Dispersal is accomplished through
– immigration (into a population)
– emigration (out of a population)
– Migration (frequent movement
into or out of a population area).
Dispersion
Patterns of dispersion
• Individual members of populations may be distributed over a
geographical area in a number of different ways including:
1. Clumped distribution (attraction) - AGGREGATED
2. Uniform distribution (repulsion) - REGULAR
3. Random distribution (minimal interaction/influence) RANDOM
Clumped distribution (attraction)
Uniform distribution (repulsion)
Random
distribution
(minimal
interaction/influen
ce)
Age structure = Population pyramid =
age pyramid
• graphical illustration shows the distribution of various age groups in
a population
• two back-to-back bar
graphs
• Population - X axis
• Age - Yaxis
• Males on the left
• Females on the right
POPULATION Growth rate
• Population growth occurs when birth rates exceed death rates or
immigration exceeds emigration.
• Population size regulates by
– physical factors
• Weather
• Water
• nutrient availability
– biological factors
• food availability
• Predators
• Parasitoids
• Competitors
• diseases
Factors affecting a population
1.
density-dependent
– Competition for resources
– Food
– Parasitism
– Predation
– Diseases
– Toxic wastes
1.
density-independent
1. Flood
2. Drought
3. Fire
4. climatic conditions
5. most pest control actions
Population Regulation in an Ecosystem
Density Dependant Factors
• Shortage of Food
– As food will become
limited when the
population reaches a
size, the death rate will
increase until the food
supply is sustainable for
the population to be
survive
Population Regulation in an Ecosystem
Density Dependant Factors
• Predators
– increase in population
size will result more
prey for the species'
predators
– therefore more chance
of that species'
members of being
killed
Population Regulation in an Ecosystem
Density Dependant Factors
• Diseases
– Diseases spread rapidly
across the densely
populated area
– especially across the
same members of a
species
Population Regulation in an Ecosystem
Density Dependant Factors
• Toxic waste
– important for organisms
living in a small-enclosed
ecosystem
– a large population
produce large amount of
waste
– increases the risk of
disease, and reduces
hygiene
Salton Sea in imminent danger of drying up,
becoming toxic waste dump
Population Regulation in an Ecosystem
Density Independent Factors
• occur regardless of a population
size
• environmental disasters
1. forest fire
2. tidal wave
3. Tsunamis
4. Land sides
Population Regulation in an Ecosystem Density Independent Factors
• Pollution
– Density independent
factor that is not
determined by a species'
population, but by its
external environment.
Population Regulation in an Ecosystem Density Independent Factors
• Human intervention also has increasingly played a part in
determining the population of species
– via domestication
– consequence from our actions – introductions