Essential Idea:
Community structure is an
emergent property of an ecosystem
Understandings:
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The distribution of species is affected by limiting factors
Community structure can be strongly affected by keystone species
Each species plays a unique role within a community because of the unique
combination of its spatial habitat and interactions with other species
Interactions between species in a community can be classified according to their
effect
Two species cannot survive indefinitely in the same habitat if their niches are
identical
Applications:
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Distribution of one animal and one plant species to illustrate limits of tolerance
and zones of stress
Local examples to illustrate the range of ways in which species can interact
within a community
The symbiotic relationship between Zooxanthellae and reef-building coral reef
species
Skills:
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Analysis of a data set that illustrates the distinction between fundamental and
realized niche
 Use of a transect to correlate the distribution of plant or animal species with an
abiotic variable
Understanding:
• Interactions between species in a community can be classified according to their
effect
In nature, no species exist in total isolation – all organisms interact with both the abiotic
environment and other organisms
 The interactions between species in a
community can be classified according to their
effect on the organisms involved
Herbivory
Herbivory is the act of eating only plant matter (e.g. primary consumers are considered
herbivores)
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Herbivores may employ different feeding
strategies (e.g. mucivores feed on plant sap,
granivores feed on seeds, etc.)
Herbivory can be either harmful or beneficial to the plant species as a whole:
 Certain types of beetle may feed voraciously
on the leaves / foliage of crop plants
(folivores), causing crop failure
 Fruit-eating animals (frugivores) spread the
seeds from a fruit in their faeces, promoting
overall seed dispersal
Examples of Herbivores
Predation
Predation is a biological interaction whereby one organism (predator) hunts and feeds
on another organism (prey)
Because the predator relies on the prey as a food source, their population levels are
inextricably intertwined
 If the prey population drops (e.g. due to overfeeding), predator numbers will dwindle as
intra-specific competition increases
 If the prey population rises, predator numbers
will increase as a result of the over-abundance
of a food source
Predator-Prey Relationship (Arctic Fox vs Snowshoe Hare)
Symbiosis
Symbiosis describes the close and persistent (long-term) interaction between two
species
 Symbiotic relationships can be obligate
(required for survival) or facultative
(advantageous without being strictly
necessary)
Symbiotic relationships can be beneficial to either one or both organisms in the
partnership:
 Mutualism – Both species benefit from the
interaction (anemone protects clownfish,
clownfish provides fecal matter for food)
 Commensalism – One species benefits, the
other is unaffected (barnacles transported to
plankton-rich waters by whales)
 Parasitism – One species benefits to the
detriment of the other species (ticks or fleas
feed on the blood of their canine host)
Types of Symbiotic Relationships
Application:
• Local examples to illustrate the range of ways in which species can interact within a
community
Mutualism
Mutualism describes an ongoing interaction between two species whereby both species
benefit from the interaction
 Honey bees gather food (nectar) from flowers
and distributes pollen between plants
(mediating plant life cycle)
 Plover birds pick food morsels from between
the jaws of crocodiles, cleaning the crocodiles
teeth in the process
 Zooxanthellae (algae) photosynthesise within
the protective environment of the polyp’s
endodermis (feeding the coral)
Commensalism
Commensalism describes an ongoing interaction between two species whereby one
benefits and the other is unaffected
 Remora attach to the underside of larger
predatory fish (e.g. sharks) and feed off the
uneaten food scraps
 Monarch butterflies can safely store poisonous
chemicals produced by milkweeds,
discouraging predators from eating it
 Decorator crabs remove small fragments of
tissue from sea sponges and uses them as a
source of camouflage
Parasitism
Parasitism describes an ongoing interaction between two species whereby one species
benefits at the other's expense
 Ticks infest the skin and fur of host animals
(such as humans), feeding off the host and
potentially causing disease
 Leeches attach to the skin and drinks the
blood of the host animal until fully engorged
 Tongue-eating louses eat the tongue of a fish
(it may then function as a replacement tongue
– stealing ingested food)
Application:
• The symbiotic relationship between Zooxanthellae and reef-building coral reef species
Reef-building coral will form a symbiotic relationship with the photosynthetic unicellular
algae – Zooxanthellae
 Coral are colonial organisms made up of
individual polyps that are connected by a layer
of living tissue
 The algae lives within the cells of the coral’s
endodermis (innermost lining of the animal)
The coral provides the algae with a protective environment and source of inorganic
compounds:
 Coral polyps secrete calcium carbonate to
build a stony skeleton which encases the
polyps (and zooxanthellae)
 Coral polyps also recycle the waste products
of the algae and supplies the zooxanthellae
with carbon dioxide
The zooxanthellae, in turn, provides the coral polyps with a necessary source of
nutrition:
 The algae supplies the coral with oxygen,
glucose and other organic molecules
(produced via photosynthesis)
 The algae also helps the coral to remove
necessary waste products
Mutualistic Relationship Between Algae and Coral
Coral Bleaching
It is the zooxanthellae within the polyp endodermis which gives the coral its
vibrant pigmentation
 When there is a large scale loss of
zooxanthellae from the coral (due to
environmental stress), bleaching occurs
 When bleaching occurs, coral begins to starve
and will die unless the zooxanthellae are
restored
Conditions which can cause coral bleaching include:
 Changes in light availability (e.g.
sedimentation may increase the opacity of the
oceanic waters)
 Temperature increases (water temperatures in
excess of 30ºC can irrevocably stress the
zooxanthellae)
 Ocean acidification (the build up of carbon
dioxide concentrations in the ocean can lower
pH and stress the algae)
Healthy versus Bleached Coral
https://ib.bioninja.com.au/options/option-c-ecology-and-conser/c1-species-and-communities/speciesinteractions.html