Introduction to Population Ecology
Olalere Shittu
Dept. of Zoology
University of Ilorin, Nigeria
Introduction
Population ecology is the study of populations (especially
population abundance) and how they change over time.
Crucial to this study are the various interactions between a
population and its resources. A population can decline because it
lacks resources or it can decline because it is prey to another species
that is increasing in numbers.
Populations are limited by their resources in its capacity to grow;
the maximum population abundance (for a given species) an
environment can sustain is called the carrying capacity.
Introduction
As a population approaches its carrying capacity, overcrowding
signifies that there are less resources for the individuals in the
population
and
this
results
in
a
reduction in birth rate. A population with these features is said to be
density dependent.
Most populations are density dependent to some extent, but some
grow (almost) exponentially and these are, in effect, density
independent.
Ecological models that focus on a single species and the relevant
carrying capacity are single species models. Alternatively, multispecies or community models focus on the interactions of specific
species.
Characteristics of Population
Population characteristics are based on the individual characteristics
of the individuals within the population. Population is described in
terms of size, age, and structure. Along with the population’s physical
attributes, factors which affect the population includes mortality from
competition and predation.
Population dynamics is the study of changes in populations through
time.
Demography is the study of the characteristics of populations. It
provides a mathematical description of how those characteristics
change over time. Demographics can include any statistical factors
that influence population growth or decline, but several parameters
are particularly important: population size, density, age structure,
Population Size
oThis is defined as the number of individuals present in a subjectively
designated geographic range.
oThe most fundamental demographic parameter is the number of
individuals within a population. Despite the simplicity in its concept,
locating all individuals during a census (a full count of every
individual) is nearly impossible, so ecologists usually estimate
population size by counting individuals within a small sample area
and extrapolating that sample to the larger population. Regardless of
the challenges in measuring population size, it is an important
characteristic of a population with significant implications for the
dynamics of the population as a whole.
Population Size
Populations display distinctive behaviors based on their size, viz:
Small populations face a greater risk of extinction. Individuals in
these populations can have a hard time finding quality mates so,
fewer individuals mate and those that do risk inbreeding.
Additionally, individuals in small population are more susceptible to
random deaths. Events like fire, floods, and disease have a greater
chance of killing all individuals in the population.
Large populations experience their own problems. As they approach
the maximum sustainable population size (carrying capacity), large
populations show characteristic behavior.
Population Size
Populations nearing their carrying capacity experience the following
difficulties:
1) They face greater competition for resources,
2) Shifts in predator-prey relationships, and
3) Lowered fecundity. If the population grows too large, it may begin
to exceed the carrying capacity of the environment and degrade
available habitat (Figure 1).
Population Size
Fig. 1: Graph showing population growth over time
Exponential growth.: This means
that the population grows very
quickly over a short amount of
time, and on a graph looks like
the letter 'J'.
Logistic growth: Due to limited
resources,
the
environment
cannot continue to support EG,
therefore the growth of the
population begins to level out and
takes the shape of an 'S'.
Carrying
capacity:
When
population growth is restricted
and the size of the population
becomes stable, denoted by K .
Population Size
Fluctuations in population growth
In theory, populations grow quickly, reach K, and level out. In
reality, populations tend to fluctuate around K, and there are several
ways this might occur in nature.
Carrying capacity fluctuations can be chaotic. These are erratic
fluctuations around K, often due to environmental factors that have
an immediate impact on population size, e.g. disease and natural
disasters.
Fluctuations around K can also be cyclical (stable limit cycle). This
type of oscillation around K is different from chaotic because it is
regular and produces a normal pattern for the population.
Population Size
Damped oscillations are fluctuations of population size above and
below K that lessen with time. The population will eventually reach a
stable limit, and the fluctuations will become minimal.
Logistic Growth Model
The growth curve of a population, which is always limited by one or more
factors in the environment, is expressed by the logistic growth equation:
The growth rate of the population (dN/dt) equals its rate of increase
(r X N, the No. of individuals present at any one time), adjusted for the
amount of resources available.
The adjustment is made by multiplying rN by the fraction of K still
unused (K minus N, divided by K). As N increases (the population grows in
size), the fraction by which r is multiplied (the remaining resources)
becomes smaller and smaller, and the rate of increase of the population
declines.
Logistic Growth Model
In mathematical terms, as N approaches K, the rate of population
growth (dN/dt) begins to slow, reaching 0 when N = K (blue line in
the figure).
a
b
Density dependence in song sparrow: Reproductive success decreases (a)
and mortality rates increases (b) as population size increases.
Population Size
Population Density (Complete description of size)
The size of a population in relation to the amount of space that it
occupies. Density is usually expressed as the number of individuals
per unit area or volume. For example: the number of crows per square
kilometre or the number of plankton per litre. Like all population
properties, density is a dynamic characteristic that changes over time
as individuals are added to or removed from the population. Closely
related species of Gannet birds will maintain very different densities.
Birth and immigration: The influx of new individuals from other areas
can increase a population's density, while death and emigration; the
movement of individuals out of a population to other areas can
decrease its density
Population Density
Similar to population size, population density displays distinctive
characteristics at both high and low values. Density-dependent
factors, including competition, predation, migration and disease,
intensify within populations as density increases. In contrast, densityindependent factors, such as weather, fire regimes, and flooding,
impact populations regardless of their specific densities
Population Density
Similar to population size, population density displays distinctive
characteristics at both high and low values. Density-dependent
factors, including competition, predation, migration and disease,
intensify within populations as density increases. In contrast, densityindependent factors, such as weather, fire regimes, and flooding,
impact populations regardless of their specific densities.
The unit of population density differ in different populations. It is
generally expressed as the number of individuals, or the population
biomass (weight basis) per unit area or volume. Larger organisms like
trees may be expressed as 600 trees per hectare, whereas smaller ones
like phytoplanktons as 3million cells per cubic metre of water. In
terms of weight, it may be 100 pounds of fish per hectare of water surface.
Population Density
Patterns of dispersion of organisms in nature differs, hence it becomes
important to distinguish between crude density and specific (ecological)
density.
1) Crude density is the density (number or biomass) per unit total space
2) Specific or ecological or economic density is the density (number or
biomass) per unit of habitat space i.e. available area or volume that can
actually be colonized by the population.
Broadly, population density is the total number of species within some natural
habitat. Mathematically:D = n/a / t .
Where D = density, n = number of individuals, a = area, t = the time unit.
Population Density
Fecundity: This is the potential reproductive capacity of a an organism
or population measured by the number of gametes or asexual
propagules
As age structure suggests, some individuals within a population have a
greater impact on population-level processes, such as growth. Fecundity
describes the number of offspring an individual or a population is able
to produce during a given period of time. In demographic studies,
fecundity is calculated in age-specific birth rates, which may be
expressed as the number of births per unit of time, the number of births
per female per unit of time, or the number of births per 1,000
individuals per unit of time. Maximum (or physiological) fecundity is the
theoretical maximum number of offspring produced in a population assuming
no ecological constraints.
Fecundity
However, since every ecosystem implements constraints on its
populations, ecologists prefer to measure realized (or ecological)
fecundity, which is the observed number of offspring produced in a
population under actual environmental conditions.
While maximum fecundity is a constant for populations, realized
fecundity varies over time based on the size, density, and age
structure of the population. External conditions, such as food and
habitat availability, can also influence fecundity. Density-dependent
regulation provides a negative feedback if the population grows too
large, by reducing birth rates and halting population growth through
a host of mechanisms
Fecundity
In white-footed mice, for example, populations regulate their
reproductive rate via a stress hormone. As population densities
increase, so do aggressive interactions between individuals (even
when food and shelter are unlimited). High population densities lead
to frequent aggressive encounters, triggering a stress syndrome in
which hormonal changes delay sexual maturation, cause reproductive
organs to shrink, and depress the immune system
Natality
This broadly covers the production of new individuals by any
organism. These new individuals are born, hatched, germinated, arise
by division etc. In human population, natality rate means birth rate.
Two types of natality rate, viz:
1. Maximum (absolute or potential or physiological) natality is the
theoretical maximum production of new individuals under ideal
conditions (i.e. no ecological limiting factors, reproduction being
limited only by physiological factors) and is a constant for a given
population.
2. Ecological or realized natality refers to population increase under
an actual, existing specific condition.
Natality
Natality can be expressed as
Nn/t = Absolute natality (B)
Nn/Nt = Specific natality rate (b) (natality rate per unit population)
Where,
N = the initial number of organisms
n = the new individuals in the population
t = the time.
Mortality (Death rate)
Population size may decrease as a result of emigration or death. It
may be expressed as percentage, numbers per thousand dying per
year. Mortality may take any of the following forms:-
1. Minimum mortality: It is also called specific or potential mortality,
and represents the theoretical minimum loss under ideal or nonlimiting conditions e.g. death as a result of old age or decline in
physiology
2. Ecological or realized mortality: this is the actual loss of an
individual under a given environmental condition. Mortality varies
with population and environmental conditions.
Mortality
Mortality can be expressed mathematically as deaths per time:
A birth ratio (100 X Births/deaths) = Vital index
The important consideration is the members who survives. Thus
survival rates are generally expressed by survival curves (loss rates)
3. Survivorship: It involves direct counting of individuals per unit
time or intervals. It is calculated as:
Survival rate = m1/ m0 X 100
Where m0 = number of marked individuals at the beginning and m1 =
number of surviving individuals at the time interval.
Three main survivor curves
1. Convex curves (Type 1): Minimal mortality recorded at the beginning
i.e. young and adult stages. Individuals that have a high probability of
surviving through early and middle life but have a rapid decline in the
number of individuals surviving into late life.
2. Diagonal curves (Type II): Shows a roughly constant mortality rate for
the species through its entire life. This means that the individual's
chance of dying is independent of their age. Type II survivorship curves
are plotted as a diagonal line going downward on a graph.
3. Concave curves (Type III): It depicts species where few individuals will
live to adulthood and die as they get older because the greatest
mortality for these individuals is experienced early in life. This type of
survivorship curve is drawn as a concave curve on a graph.
Types of Survival curves
Life span and population
Information about life span and age are important factors in
characterizing a population and predicting its course. How many
people are below, within or beyond the reproductive age? How long is
the life span? Which age group are most vulnerable to die?
Concept of r-selected
Organisms adapted to survive in unstable environments are referred
to as r-selected. r-selected organisms’ live in settings where
population levels are well below the maximum number that the
environment can support i.e. the carrying capacity, there numbers
are growing exponentially at the maximum rate at which the
population can increase if resources are unlimited. Organisms that
are r-selected tend to be small, short lived, and opportunistic, and to
grow through irregular boom-and-burst population cycles. Examples
include insects, annual plants, bacteria, frogs and rats etc. Species
considered pests typically are r-selected organisms that are capable of
rapid growth when environmental conditions are favorable.
K-selected Organisms
Organisms adapted to survive in stable environments are referred to
as K-selected. This is because they live in environments in which the
number of individuals is at or near the environment's carrying
capacity (often abbreviated as K). K-selected species are typically
larger, grow more slowly, have fewer offspring and spend more time
parenting them. Examples include large mammals, birds, and longlived plants such as redwood trees. K-selected species are more prone
to extinction than r-selected species because they mature later in life
and have fewer offspring with longer gestation times.
Biotic Potential
Each population has the inherent power to grow. When the
environment is unlimited (Space, food, other organisms not exerting a
limiting effect), the specific growth rate (i.e. the population growth
rate par individual) becomes constant and maximum for the existing
conditions. The value of the growth rate under these favourable
conditions is maximum, is characteristic of a particular population
age structure, and is a single index of the inherent power of a
population to grow
Index r is the difference between the instantaneous specific natality
rate (rate per time per individual) and the instantaneous specific
death rate (r = b – d).
Age Structure
Not all individuals contribute equally to a population. Occasionally,
researchers find it useful to characterize the different contributions
made by different individuals. First, individuals are sorted into agespecific categories called cohorts, such as "juveniles" or "sub-adults".
Researchers then create a profile of the size and age structures of the
cohorts to determine the reproductive potential of that population, in
order to estimate current and future growth. Usually, a rapidly
expanding population will have larger reproductive cohorts, stable
populations show a more even distribution of age classes, and rapidly
declining populations have large older cohorts.
Spatial Distribution (Dispersion)
The spatial pattern in which individuals are dispersed within a given
area is that population’s distribution, which may vary with time and
available resources.
• There are three major types of spatial distributions:
– Clumped
– Uniform
– Random
Spatial Distribution (Dispersion)
Clumped distribution – includes family and social groups
• Examples: elephant herds, wolf packs, prides of lions, flocks of birds,
and schools of fish.
• Advantages:
– Provides many eyes to can search for local food sources.
– Confuses predators with sheer numbers.
– Cooperation for hunting more effectively.
Spatial Distribution (Dispersion)
Clumped distribution – includes family and social groups
• Examples: elephant herds, wolf packs, prides of lions, flocks of birds,
and schools of fish.
• Advantages:
– Provides many eyes to can search for local food sources.
– Confuses predators with sheer numbers.
– Cooperation for hunting more effectively.
Spatial Distribution (Dispersion)
Uniform distribution – constant distance maintained between
individuals; common among territorial animals defending scarce
resources or defending breeding territories.
• Examples: iguanas, shorebirds, tawny owls
• Advantage: a uniform distribution helps ensure adequate resources
for each individual.
Random distribution - Rare, exhibited by individuals that do not form
social groups; occurs when resources are not scarce enough to
require territorial spacing or cooperative behavior.
• Examples: Trees and other plants in rain forests.
Spatial Distribution (Dispersion)
Uniform distribution – constant distance maintained between
individuals; common among territorial animals defending scarce
resources or defending breeding territories.
• Examples: iguanas, shorebirds, tawny owls
• Advantage: a uniform distribution helps ensure adequate resources
for each individual.
Random distribution - Rare, exhibited by individuals that do not form
social groups; occurs when resources are not scarce enough to
require territorial spacing or cooperative behavior.
• Examples: Trees and other plants in rain forests.