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Population
and community
ecology
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Population
ecology
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The study of ecology
Ecology is the study of interactions among organisms
and their relationship with the environment.
It is possible to study ecology at different levels. From the
micro to the macro levels, there exists:
• organism;
• population;
• community;
• ecosystem;
• biosphere.
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Ecology concepts: the habitat
Every organism presents two defining characteristics that
define it: habitat and niche.
The habitat is the particular environment in which an
organism lives.
The tropical forest is a habitat for thousands of plants,
insects and birds.
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Ecology concepts: the niche
A species’ niche is the set of physical and biological
conditions it requires to survive, grow, and reproduce.
A niche is therefore partly defined by the resources
available in the environment.
A plant’s niche includes how much direct sunlight it can
tolerate and the type of soil on which it thrives; for an
animal, niche involves elements such as the food it eats and
whether it is active at night or during the day.
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Describing a population
A population is a group of individuals of the same species
living in a habitat for a relatively long period.
Populations can be described using the following aspects:
• the size – the number of individuals in the population;
• the density – the number of individuals per unit area;
• the distribution in the environment – the pattern of dispersal
of individuals across the area;
• the age distribution – the number of individuals in different
age groups.
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Population dynamics
Demography is the study of changes in a population’s
density and structure – also known as population dynamics.
Birth and death rates are the main factors influencing
changes in the size and structure of a population, along with
migration.
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Patterns of population growth:
exponential growth
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Exponential growth is a type
of population growth that
accelerates over time.
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It is typical of conditions with
unlimited resources and no
limiting factors.
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Patterns of population growth:
logistic growth
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Logistic growth occurs when
the density of a population
reaches the carrying capacity
of the environment; growth slows
down, resulting in an S-shaped
curve.
It is influenced by limiting factors.
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Density-dependent factors
affect population size
Population size can be
influenced by factors that
depend on how many
individuals from that
population live in a given
habitat; these are densitydependent factors, such as
competition, predation and
parasitism.
In these cases, mortality increases as the density of the
population increases.
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Density-independent factors
affect population size
Also abiotic factors, like
hurricanes, floods and fires, can
influence population size;
these are density-independent
factors, which are not influenced
by population dynamics.
In these cases, mortality does not depend on density.
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Life history patterns:
r-strategy species
In order to survive in different environmental conditions,
species adopt different strategies.
Opportunistic species (r-strategist)
• Exponential population growth followed by periodic falls
in population size;
• Small individuals with short life spam;
• Many offspring, but with relatively low probability of
surviving to adulthood.
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Life history patterns:
K-strategy species
Equilibrium species (K-strategist)
• Logistic population growth;
• Larger individuals with long life spam;
• Fewer offspring, but with relatively high probability of
surviving to adulthood.
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Life history patterns
and the environment
Opportunistic species, like rabbits and
dandelions, are adapted to a fluctuating
environment.
Equilibrium species, like gorillas and
oaks, are adapted to a relatively stable
environment.
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Community
ecology
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Interactions of populations
in a community
A community is an assemblage of populations of different
species, living in the same area and interacting with each
other and with the environment.
There are different types of interactions:
• competition
• predation
• parasitism
• mutualism
• commensalism
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Competition
Competition occurs when two or more species use the
same resource. The result of competitive interactions
depends on the availability of the resource.
According to the competitive exclusion principle, no two
species can occupy the same ecological niche at the same
time.
Resource partitioning decreases competition between two
species.
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Predator-prey interactions /1
Predation occurs when individuals of a species (predators)
feed on individuals of a different species (prey).
More predators
More prey
Fewer prey
Fewer predators
Predator-prey interactions affect
the dynamics of both populations:
more predators result in fewer
prey; when the number of prey
declines, predators decrease too
and prey increase again.
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Predator-prey interactions /2
Prey have evolved defences to
escape predators: senses, speed,
protective body parts, and chemicals.
Some species are able to deceive
predators by camouflage, cryptic
coloration, and mimicry.
Camouflage of a Shield Mantis
(also called Leaf Mantis) in the
Peruvian Amazon.
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Parasitism
In parasitism, a population
receives benefits at another’s
expense. It occurs when individuals
of a species consume only part of
the tissues of individuals of another
species, usually without killing
them.
Monotropa uniflora, also known as
the ghost plant, does not contain
chlorophyll. Instead of generating
energy from sunlight, it is parasitic
to some mycorrhizas.
Some parasites are pathogens and
they cause a disease in the host.
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Commensalism and mutualism
Commensalism benefits one species without causing
damage to the other.
For example, scavengers have a commensalistic
interaction with some predators.
Mutualism is an interaction between species that results
in benefits for both.
Pollination is a typical example of mutualism between the
plant and the pollinator.
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Ecosystem
ecology
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Ecosystems
Ecosystems are composed of:
• Biotic components – living organisms;
• Abiotic components – - resources, like sunlight and
nutrients, as well as conditions such as soil, water,
temperature and wind.
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Trophic structure of the ecosystems
The trophic structure of an ecosystem describes the food
relationships inside a community.
It presents different
trophic levels, that
when taken together,
form a food web.
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Flux of energy through ecosystems
Primary producers:
autotrophs
Primary consumers:
herbivores heterotrophs
Secondary consumers:
carnivores heterotrophs
Tertiary consumers:
carnivores that feed on
other carnivores
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The main source of
energy for the Earth is
the Sun.
In the flux through
different trophic levels,
energy is consumed and
dissipated in the form of
heat.
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Chemical cycling through ecosystems
Chemical nutrients, like energy, pass from one trophic level
to the next until decomposers return them to the
environment where producers can take them up again.
Unlike energy, matter does not dissipate.
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Ecological pyramids
An ecological pyramid is the graphic representation of the
number of organisms, biomass or energy, content of trophic
levels.
Tertiary consumers
Secondary consumers
Primary consumers
Producers
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