Transcript Population Factors

Factors Affecting
Population Change
Are you Dense…ity-Dependent?
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There are a variety of factors that can cause a
population to change in some manner.
These factors can be grouped into two main
categories:
1. Density-dependent factors – factors that influence
a population more and more as the population
increases.
2. Density-independent factors – factors that
influence a population regardless of population
density.
Density-Dependent Factors
• Factors that depend upon the population size
may include…
– Intraspecific competition – the fight for resources
between you and members of your own species.
– Predation – more predators means less prey.
– Disease – Communicable diseases spread more
rapidly through more densely populated regions.
– Allee effect – your numbers become so small that
finding a mate becomes next to impossible and your
species may not survive in the ecosystem. The Allee
effect is experienced when a population dips below
the minimum viable population size.
Density-Independent Factors
• Factors that affect the population
regardless of its size may include…
– Climate – rainfall and temperature – these
are going to be plentiful or not – high or low –
regardless of population numbers.
– Pesticides/Toxins – man applies these and
they will affect all organisms in the area –
especially if it is a broad-spectrum pesticide.
– Limiting factors – essential resources
required to live – if they are not there in large
enough quantities, you will suffer.
Interactions Within
Communities
Know Your Role!!!
• A community is comprised of all the populations in a
given ecosystem at a given time.
• With many different species living within the same
community – it is important to recognize that there are a
number of different ecological niches present and it is
the differences between these niches that allow all of the
species to live together in harmony.
• An ecological niche is an organism’s biological role
within the ecosystem. This includes the organisms usage
of both abiotic and biotic resources.
• A fundamental niche is the characteristics of and
organism and the resources it would use under ideal
conditions.
• The realized niche of an organism is the characteristics
and resources used under the prevailing environmental
conditions.
Interactions Between Species
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Competition – competing for resources required to
survive – may be between two different species or within
the same species.
Predation – A predator species hunts and eats a prey
species.
Symbiosis – two species maintain a close association
with each other – there are three types of symbiotic
relationships.
1. Parasitism – One gets a benefit and the other
experiences a negative. (+/–)
2. Mutualism – Both benefit from the interaction. (+/+)
3. Commensalism – One gets a benefit while the other
is unaffected. (+/o)
Symbiotic Relationships
• Parasitism
(+/–)
• Mutualism
(+/+)
• Commensalism
(+/o)
Interspecific Competition
• Interspecific competition occurs when members of
different species compete for a common resource.
• There are two forms of interspecific competition:
– Interference competition – The two species go head-tohead and fight for control of the resource. An example
may be two species of birds competing for space in a
tree.
– Exploitive competition – The two species consume the
same resource and the consumption of that resource by
one species limits the availability of the resource to the
other species. An example may be two species of
carnivore that both compete for antelope in Africa – as
one eats more deer, there is less for the other.
• The amount of competition increases as the amount of
niche overlap increases. A limit in the amount of a desired
resource also increases the level of competition.
Effects of Competition
• There are several possibilities resulting from interspecific
competition:
– The weaker species declines, or is eradicated, from the area.
– One species may have to change its behaviour or switch to a
different resource.
– One species has to move to a new area.
• Resource partitioning is an option that can reduce
competition. In this, the competing species occupy different
parts of the same resource. There may be several birds
needing resources from the same tree but they may occupy
different parts of the tree – trunk, understory and canopy.
• Another result of interspecific competition may be the
increased rate of adaptation and evolutionary change
within a species. You are pushed to survive and the traits that
allow you to do so become more prevalent in the species.
Resource Partitioning
Predation
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Predation is an interspecific interaction involving a
predator species (hunter) and a prey species
(hunted).
The two populations are dependent upon one
another.
A. As the predator numbers increase – the prey
numbers go into decline. (Too many killers!)
B. When prey go into decline – the predators begin to
go into decline. (No food to eat!)
C. When predators decline – the prey increase their
numbers. (Few killers killing!)
D. Prey numbers increase so predators increase too!
(Lots of food to eat!) Now the process repeats!
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This phenomena between the predator and prey
populations is known as predator-prey cycling.
Predator-Prey Cycling
Defence Mechanisms
• A variety of defence mechanisms have evolved in many
species as a result of predator-prey interactions.
• Plants have both morphological and chemical defence
mechanisms designed to keep them from getting eaten.
– Morphological defences include physical features such
as thorns, hooks and needles.
– Chemical defences include substances that are
distasteful or unpleasant. Some may even mimic
hormones of other organisms that scare off the thing
that wants to eat the plant!
• It is interesting to note that as the plants change to defend
themselves, the herbivores in turn, change to be able to
continue feeding on the plants. It is an interesting case of
coevolution in some ways.
Defence Mechanisms
Animals
• Animals may have passive defences such as hiding or
active defences such as fleeing from their predators.
There is a greater energy cost to the animal that
practices active defences.
• Camouflage is passive and helps some animals blend
into their surroundings so they go undetected by
predators.
• Other animals use bright colourations or audible
warnings that make them conspicuous, but, they often
have very strong venoms.
• Mimicry may also be employed – this is when one
species takes on the appearance of another species that
its predator finds very unpleasant. The viceroy butterfly
looks like the monarch butterfly which birds find very
distasteful.
Defence Mechanisms
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Plant – Thorns
Plant – Needles
Plant – Sap
Animal – Fleeing
Animal – Hiding
Animal –
Camouflage
• Animal – Warning
• Animal – Mimicry
Equilibrium – A Delicate Balance
• Communities live in balance – they do not exceed their
carrying capacities. The animals that are sustained by
the community are able to survive for many generations
because the environment rarely changes (and even if it
does it is usually a very gradual change).
• The term used to describe the balance in an ecosystem
is equilibrium.
• The introduction of an exotic species can disturb the
equilibrium in an ecosystem. The exotic species may:
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Have no predators – so it flourishes.
Drains a resource so it is not available for others.
Outcompetes a native species – it flourishes and the native dies.
Economic effects as well – destruction of crops or livestock.
FIN!