Environmental studies

POPULATION ECOLOGY: the study of populations in relationship to their environment and the
social interactions amongst each other.
Population ecology includes the study of:
 Population size, factors that influence it, methods used to count populations and the two types of
growth curves displayed by different organisms
 The types of interactions and symbioses found in nature and how organisms specialise to eliminate
competition
 How different types of social organisations offer protection, enhance hunting, ensure the fittest
survive and allow individuals of the same species to specialise into serving unique functions
 Succession, looking at how communities develop and in so doing how they alter their abiotic and
biotic environments
 Human population growth forms in different age-gender pyramids
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Population size
 Species: a group of organisms capable of interbreeding, producing fertile offspring and share many
similar characteristics
 Population: a population is a group of the same species that occupies the same habitat at the same time
and are able to interbreed
 Community: Groups of different species populations that interact with each other within a specific
habitat ecosystem a biological community
 Ecosystem: a biological community of interacting organisms and their physical environment
 Population ecology: refers to the study of population change

Factors influencing population change

• Inherent or in-born factors: natality, mortality and dispersal
• Environmental changes: earthquakes, volcanoes, earth slides, tornados, droughts

Population size is influenced by both abiotic and biotic factors present in the environment.
Factors that influence population size include:
 natality
 mortality
 migration (including immigration and emigration)

Natality: percentage increase of the population for a given period of a time (new/initial times 100)

r-strategy species (fish and metamorphic k-strategy species (mammals and birds)
insects)
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Causes of fluctuations in natural populations

Density dependent and independent factors contribute to

fluctuations in population size. These factors increase
environmental resistance as indicated by arrows on the
graph below. Together, these factors limit population
growth.

Types of environmental resistance

Environmental resistance includes carrying capacity,
density dependent and density independent factors.
 Carrying capacity: Carrying capacity refers to the
number of individuals an environment can support in a
specific season.
If populations exceed their environmental carrying
capacity, there will not be enough resources to sustain the
population and so the population size will fall because of
increased environmental resistance.
Population size may fluctuate in natural populations
around the carrying capacity. This is known as a stable
population size.

If however the population size exceeds the carrying

capacity, resources will be exhausted and the population
will rapidly decrease in size. This type of population is
called an unstable population.
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 Density dependent and density independent factors

Determining population size

Depending on the type of population, the size of the population can be determined either directly or
indirectly

Direct counting techniques

Direct techniques determine the actual population sizes. Direct techniques are suitable for counting larger,
slow moving organisms or organisms that are sessile.

Direct counting techniques include:

 Aerial photography: By taking an aerial photo of a population, the actual number of individual members
who make up the population can be determined by counting.
 Census: A census is a direct method used to determine the actual number of humans in an area or a
country.

Indirect counting techniques

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Indirect counting techniques are used to estimate population size where it is difficult to determine the actual
number of organisms. Indirect counting techniques 240 are used to estimate the population size of smaller
organisms or fast-moving organisms.

They include:
Mark and recapture: used to estimate animal populations. It involves taking two samples of the population.
 The first sample of organisms is marked and then released. Adequate time must be allowed for the
population to mix after the initial marking. This period must not be long enough to allow natality, mortality,
immigration and emigration to have an effect.
 The second sample represents the recaptured sample. The number of marked individuals and unmarked
organisms must be determined. The following formula (Petersen Index) is used to determine the size of the
population

The following precautions must be taken to ensure reliable results:

 The sample must be large enough to truly represent a population. o The markings used must remain
for the entire period of the procedure.
 The markings must be suitable for the type of organism and must not harm or interfere with its
movements and behaviour.
 The marked organisms should be released into the environment close to the site of capture and be
given enough time to mix.
 Several secondary samples should be taken, and population sizes calculated.
 The average of each trial should be recorded to improve the reliability of the investigation.

Quadrats

Population growth forms

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Populations have characteristic patterns of growth. When population sizes are plotted over time, two
different growth patterns can be found in nature. They are referred to as geometric (or a J-shaped growth
curve) and logistic (or S-shaped growth curve) growth.

Geometric growth curve

Geometric growth is characteristic of many micro-organisms e.g. bacteria and protists.
These species are able to reproduce quickly and double their numbers after every reproductive division in
favourable conditions. Eventually their resources become limited and/or waste products accumulate and
adversely affect the population. This causes death or an extinction event. Three distinct phases can be seen
in geometric growth curves:

Logistic growth form

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Logistic S-shaped growth is normally found in higher-order organisms (e.g. mammals) which typically
reproduce slower. Populations tend not to exceed the carrying capacity and, if they do, they show higher
levels of resilience to environmental resistance. An S-shaped graph forms typically consists of five phases

Predation
A predator is an organism that actively hunts, kills and consumes its prey to meet its energy needs.

Predators play an important role in regulating the number of other species. The relationship between the
number of predators and their prey, can be represented by a predator-prey graph

Interpreting a predator-prey graph

 The number of prey increases first
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o An environment with food, space and a lack of predators will allow the prey population to increase
exponentially.

 The number of predators only rises after the prey numbers have increase or are high. There is a delayed
reaction to the rise in prey.
o Predators are still in the initial lag phase. There are few sexually mature individuals reproducing, and
gestation periods are quite long.
o If new to an environment, predators might also need to acclimatise first.
o There is competition for prey

 Prey numbers starts to decrease.

o Due to higher environmental resistance (lack of food, water, space, etc. and increased predation levels).

 Predator numbers start to decrease once prey numbers are low or are busy decreasing rapidly.
o As prey decreases, food becomes a limiting factor for the predator. Increased competition between
predators for food causes higher mortality rates amongst predators.
o Predators may also emigrate out of the area to where more food may be found.

 Stationary (stable) phases are extremely short, and fluctuations of predator and prey numbers tend to be
high.
o Environmental resistance increases on prey populations as predators have access to a large supply of
prey. Prey numbers decrease as predator numbers increase. When predator numbers grow too large and
there is not enough prey anymore, their numbers decrease rapidly.

Competition
Competition is the interaction between individuals over scare resources.

Competitive exclusion
The competitive exclusion principle states that when two species competing for the same resources, one
species will out-compete the other.

Resource partitioning
Resource partitioning refers to the sharing of resources so that different species can co-exist in the same
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area.

Examples of resource partitioning include:

 Herbivores grazing in the same area – taller animals such as giraffes feed on the leaves at the top of trees
while larger buck feed from the middle and smaller buck from the lower branches
 Partitioning of light in a forest (stratification) – Plants grow to a certain height depending on their light
requirements. Plants which require less light grow in the shade of taller plants

Symbiotic relationships
Symbiosis is a close, long-term biological relationship between individuals of two or more species. There
are 3 types of symbiotic relationships:  mutualism  commensalism  parasitism

Human population

.
Reasons for exponential growth
 Agricultural improvements Through agricultural improvements, man has been able to increase and secure
food availability.
 Medicinal improvements. Our medical technology has improved drastically in the space of a few hundred
years. We can counter the effects of pathogenic diseases and their subsequent spread
 Technological improvements Technology has made our daily lives easier and more convenient. Carrying
capacity was further increased. Through constantly increasing our carrying capacity and decreasing our
environmental resistance, our population growth rate has exploded
Age-gender population pyramids
Data regarding a population can be gathered through conducting a census and then ordered according to the
number of individuals of a certain age category and of a certain gender. The results are then presented as a
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population pyramid.

Three types of population pyramids can be observed according to whether or not the population size is
growing or declining, namely:
 expanding or growing population pyramid
 stable population pyramid
 declining population pyramid
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Apart from the above, human populations are also regulated by the prevailing global environment at the
time, e.g. wars, famines, economic recessions, incurable superbugs that are easily transmitted through
international travel.
Governments can also regulate their population sizes through legislation. Some countries choose a more
direct method to regulate their population whereas other would want to decrease it through legislation and
policy.

Possible reasons for the longer life expectancy globally

 better health and sanitation
 social welfare improvements
 increased primary health care service