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Ecology Primer: Biological
Interactions
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Nicklaus Kruger
NISL
[email protected]
Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/
Biological Interactions
 Ecology is the study of the interactions between living organisms
and their environments
 Other organisms are part of that environment
 The interactions between organisms are numerous, but they can all
be fitted into a framework of a handful of interactions
Interspecific Interactions
Relationship
Effect on species 1
Effect on species 2
Competition
- (competitor)
- (competitor)
Predation
+ (predator)
- (prey)
Mutualism
+ (partner)
+ (partner)
Commensualism
+ (epiphyte)
O (host)
Amensalism
+ (secretor)
- (other)
Parasitism
+ (parasite)
- (host)
Competition
 The concept of competition is fundamental to the science of
ecology
 Competition occurs when two (or more) organisms require the
same limiting resource
 Intraspecific competition refers to competition between
conspecifics
 Interspecific competition refers to competition between members of
different species
 The more similar the fundamental niche of two organisms, the more
potential for competition between them
Competition (cont)
 Competition leads to niche differentiationn or competitive exclusion
 Competitive release occurs when one of the competitive species is
removed the area, and its competitor species expands its niche and
occupation of the habitat
 Laboratory experiments demonstrate that two species cannot
coexist if they require similar resources
Competition (cont…)
 Closely related species rarely coexist for long in the laboratory
 If two species are forced to live off the same resource, inevitably
one persists and the other dies out
 The competitive exclusion principle holds that two species cannot
coexist on the same limiting resource
 Only resources that limit population growth can provide the basis
for competition
 Non-limiting resources are superabundant compared to the needs
of organisms
Predator/Prey Dynamics
 Organisms must consume food to survive
 Some organisms are able to derive chemical energy from inorganic
molecules
 Other organisms get their carbon from living organisms
 Predation has received more attention from ecologists than any
other form of biological interaction (except competition)
Predator/Prey Dynamics
 When prey is plentiful, predator numbers can increase
 But predators have an obvious adverse effect on prey
population numbers
 Ecologists debate whether ecosystems are regulated more by the
dynamics within predator populations, or the dynamics within the
populations of their prey
 Individual predators tend to be larger than their prey, and also less
abundant
Herbivores and Plants
 While we often view herbivore relationships differently from classic
predator/prey relationships, functionally they are rather similar
 Herbivores tend to be larger than carnivores, or to have larger
populations, as a result of the way energy flows through an
ecosystem
Herbivores and Plants (cont)
 The nature of the plant has profound effects on the evolution of its
herbivores
 Plants also adapt to the pressures of herbivory, and develop
strategies to escape predation
 Plants and herbivores are locked in a coevolutionary
arms race
Parasitism
 Parasitism is similar to predation, except that the parasite tends not
to kill its host as swiftly (if at all)
 Endoparasites live within the tissues or their hosts
 Ectoparasites live outside the tissues of their hosts, but attached to
them
 Epiparasites are parasites that parasitise parasites
 Social parasites exploit the interactions between members of their
host species
Parasitism: The malaria parasite
Mutualism
 In mutualism, both of the interacting species benefit
 When the species form a lifelong association in close physical and
biochemical content, this form of mutualism is known as symbiosis
 Obligate mutualism occurs when both species are dependent on
their partners, and cannot survive without them
 Facultative mutualism occurs when the partners are capable of
surviving separately under certain conditions
Symbiois: Lichens
• Lichens are symbiotic
associations between algae and
fungi
• They are more successful
together than apart
• Many lichens are obligate
mutualists
Endosymbiosis
 Obligate mutualism can sometimes lead to the evolution of a new
unity
 Endosymbiogenesis is the theory that this has happened several
times in the evolution of the eukaryotic cell
 The mitochondria that power our cells are thought to have once
been bacterial endosymbionts
Commensalism
 In this form of interaction, one species benefits, while the other is
not affected at all
 Real and well-attested examples of commensalism are hard to
come by
 Certain epiphytes are thought to be involved in commensal
relationships with their hosts (algae covering barnacles, for
example)
Coevolution
 Co-evolution is the mutual evolutionary influence between two or
more species
 In a co-evolutionary relationship, organisms exert selective
pressure on one another
 Effectively, co-evolution refers to biological interactions on an
evolutionary timescale
Coevolutionary arms races
 Co-evolutionary arms races are positive feedback loops
 The evolution of one participant leads to the evolution of the other,
which in turn feeds back to the original participant
 The classic example of this is in predator-prey relations
 Below is an illustration of positive feedback and its involvement in
the greenhouse effect
Bibliography
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