Interactions in the

Community
 Community structures
• A community consists of various populations
living within an area
• Populations affect each other in various ways.
Including predation, competition and close
interactions with one another (mutualism and
parasitism)
 Feeding levels in a community
• Autotrophs (primary producers)
– Use light to produce food (photosynthesis)
• Heterotrophs (consumers)
– Rely on an external source for food.
• Primary consumers: herbivores
• Secondary consumers: Carnivores, omnivores
• Tertiary consumers: carnivores that feed on
secondary consumers – scavengers
– Some worms and crabs feed on dead material
and are called detritivores
– Decomposers such as bacteria and fungi
breakdown organic material and use the
inorganic nutrients for food
 Feeding levels in a community

• The sun is the main source of energy, radiant

energy is trapped in chlorophyll and through
photosynthesis is converted into organic food.
• Animals feed on plants directly and indirectly.
This food is needed for growth, production of
organic compounds and activities
• Animals are active while plants are sedentary
(stationary) which means that animals require
more energy. However there are many more
plants with a greater biomass
Trophic levels
NOTE: This pyramid applies
to the number of animals as
well as the biomass. Which
is the total mass at each
trophic level.
Five main types of interactions in
a community
• Competition: occurs when a large number of
organisms depend on a common resource that is in
short supply
• Competition can either by intraspecific, competition
between organisms of the same species or interspecific
competition between organisms of different species
• As resources decreases, the intensity of the
competition increases
• Those that are successful survive those that aren’t
either die or have to move to another area which
decreases the population size
• It can this be seen that competition will influence
population growth.
 Ecological niche
• When species are similar their niches overlap.
Ecologists say that no two species that have the
same niche can exist together indefinitely in an
ecosystem
• One species outcompetes the other, this is
competitive exclusion principle
• Through natural selection there would be a
reduction in the existence of species that
complete with each other: very little overlap of
niches. This is known as resource partitioning,
this may include timing of feeding, location of
feeding and nesting patterns
 Ecological Niche
• The ecological role of a species within the
structure and function of a community is known
as the ecological niche. Takes into account the
biotic and abiotic aspects of the species
existence.
• Includes
– What the species eats
– What eats it
– What organisms it competes with
– How it interacts with the abiotic components
• The ability of a species to adapt will determine its
tolerance for environmental extremes. If any
feature lies outside the bounds of tolerance, then
the species cannot live there.
 Example of interspecific
competition
• The graph below shows how interspecific
competition, between two species of
protozoa, decreases the population of the one
species.
• This is due to Competitive exclusion principle
where one species outcompetes the other
species.
• The outcompeted species may be eliminated
or may have to move to a new habitat.
 Resource Partitioning
• The image below shows an example of
resource partitioning.
• In order for similar species to live in a similar
ecological niche, each species may show
features that allow it to utilize a particular
resource more effectively than others.
• Thus allowing all of the species to co-exist in
an ecosystem.
 Predation
• The size of the prey population will fluctuate
due to availability of resources. This in turn
affects the size of the predator population.
 Predation
• When the prey population increases so does the
predator population. The increase in the
predators increase mortality of the prey
decreasing the population size which in turn
decreases the predator population.
• When the predator population decreases the
prey population increases.
• Predation is a density-dependent factor since the
density of the prey influences the density of the
predators and visa-versa. They regulate the
population size
• The populations undergo a series of population
explosions and crashes
 Symbiosis
• Close association between two organisms that
one or both benefit from.
• Mutualism: Close association of two organisms
that both benefit from the relationship.
• Parasitism: An organism which lives in close
association with another organism, called a host
and usually harms the host while benefiting from
the host.
• Commensalism: Two organisms of different
species that are associates such that one benefits
and the other is not harmed.
 Introduction
• Described as the interaction between two or
more animals, usually of the same species.
• Some species that engage in social behaviour
form societies.
• A society is an actively co-operating group of
individuals belonging to the same species
– Flock of birds Brings about Co-operation between
– Pack of wolves different sexes, age groups or castes
and allows the species to achieve
– School of fish more. Communication is key in a
– Hive of bees society.
 Introduction
• This type of social behaviour can lead to an
increase in the population size which
increases the chance of the individuals
survival
• This is achieved by
– Efficient predation
– Protection from predators
– Dominance in breeding
– Efficient reproduction
– Food gathering
– Greater care for young
– Specialisation of individual roles
 Herds and Flocks
• Fish are found in schools, which aids in
confusing predators as an individual cannot be
seen. The schools move in various forms to
confuse predators
• Zebras. The stripes confuse predators as the
zebras move in herds. Individual cannot be
distinguished
 Division of labour
• Eg Bees:
• Through division of labour insects are able to construct
elaborate nests and raise young by mass production
methods
• Social structure of honeybees
– Single adult queen (fertile female)
– 80 000 worker bees (all female)
– A few drones (males)
• The society is controlled by a pheromone secreted by the
queen. Stops the worker bees from raising a new queen.
Prevents the development of ovaries in the female bees
• If the queen dies or the colony becomes too large then
the inhibiting effect of the pheromone decreases. In
these cases some larva are feed on special food that
allows them to develop into new queens
 Honeybees
• The queen bee uses stored sperm from previous matings
to fertilise eggs to form female workers
• Drones form from unfertilised eggs and are haploid
• Workers bees
– Nourish larvae (nurses)
– Produce wax ad build and maintain the wax cells (builders)
– Bring home nectar and pollen (food gatherers)
• Bees have specific movements forming a dance which
allows for communication. A round dance show
availability of food within 50m. Waggle dance shows
food available some distance away. The movement
indicates the distance and direction of the food.
• This has lead to great success
 Hunting in packs
• Use of strategy to capture prey
– Working co-operatively increase the chance of
success
– Choosing vulnerable animals
– Migrating to areas where prey are found
– Using camouflage
• A pack of wolves can overcome animals much
larger than themselves. The pack guards and
protects it territory decreasing competition
 Dominant breeding pairs
• In some cases only one male and one female will
breed
• This is achieved through territoriality. Those that
claim the territory have the advantage for
resources which decreases competition
• A male can secure and dominate a territory is
likely to attract a female for mating
• This means that the fittest offspring are born.
Favourable genes are passed onto the next
generation
• Pg 337 gives an example
 Ecological Succession
• Development of a community over time which
involves species in one stage being replaced
by different species.
• Primary Succession
– Refers to the change in a species composition over
time in a habitat that was not previously inhabited
by organisms. Sand dunes, new ponds or lakes,
bare rocks surfaces such as volcanic lava or rock
scraped clean by glaciers
– No soil exists when primary succession begins
 Ecological Succession
• Secondary succession
– Change in species composition that takes place after
some disturbance removes the existing vegetation. Eg
abandoned agricultural fields or open areas produced
by forest fires
• The changes that take place during any form of
succession depends on a variety of environmental
factors. Moisture, temperature and wind.
• Write down how succession takes place. Pg 357
 Succession usually takes
place as follows
• The organism that first inhabit an open or
disturbed area are called pioneers. The pioneer
species usually alters the environment in such a
way that conditions become more favourable for
a new species to move in.
• The pioneers are usually replaced by annual
plants and then herbaceous perennials followed
by shrubs and trees.
• The last species to appear is referred to as the
climax species.
 Succession usually takes
place as follows
• Together with the changing plant population are changes to
the animal population, since the introduction of new plant
species creates habitats and food from new animal species.
• Each animal species that appears also becomes potential
food for new animal species.
• At a certain time, no further changes take place in the
composition of the community. The succession is then said
to have reached a climax and the community is regarded as
stable.
• The only thing that can change the community now is a
disaster such as a forest fire or humans chopping down
trees.