This presentation summarizes the major concepts about interactions of organisms while highlighting the ecosystem, competition, symbiosis and the ecological niche.

1. Ecosystem and Organism
Interactions
Amos Watentena
wateamos@gmail.com

2. Ecosystem and Organism Interactions
• An ecosystem is a defined space in
which interactions take place between a
community, with all its complex
interrelationships, and the physical
environment.
• A community is an assemblage of all
interacting species of organisms in an
area.

3. • Competition
• Predation
• Symbiosis

4. • It is a struggle between organisms as they attempt
to use the same limited resource
• Occurs when two species occupy
the same niche
• Why can’t two species occupy the same niche?
 If two species occupy the same niche, they will
compete directly against each other and one
species will eventually die off

5. Competition
• It occurs when two or
more individuals seek
to utilize the same
limited resource
• An example is the
African driver ants
(Hymenoptera) that
out compete and
consume everything
that crosses its path,
even cows!!!

6. Competition
• Competition is a kind of interaction in
which two organisms strive to obtain
the same limited resource.
 Intraspecific competition is competition
between members of same species.
 Interspecific competition is competition
between members of different species.

7. Competition
• The competitive
exclusion principle
holds that no two
species can occupy
the same ecological
niche in the same
place at the same
time.
 Less-fit species must
evolve into a slightly
different niche.

8. Complete competitors cannot coexist
Competitive exclusion is reached more slowly
with higher resource abundances
 Stable coexistence requires niche
differentiation, such that members of each
species compete more strongly among themselves
than with members of the other species.
 Therefore the level of intraspecific competition
is always greater than interspecific competition

9. Symbiosis
• Symbiosis is a close
ecological relationship
between the individuals
of two or more species.
• Sometimes a symbiotic
relationship benefits
both species
• Mutualism,
commensalism,
parasitism, cooperation
• Bees (Hymenoptera) and
Plants

10. Symbiotic Relationships
• Symbiosis is a close, long-lasting, physical
relationship between two different
species. At least one species derives
benefit from the interaction.
• There are three categories of symbiotic
relationships:
 Parasitism
 Commensalism
 Mutualism

11. Symbiotic Relationships
• Parasitism is a relationship in which one
organism (parasite) lives in or on
another organism (host), from which it
derives nourishment.
 Ecto-parasites live on the host’s surface.
Fleas, lice, molds, mildews
 Endo-parasites live inside the body of the
host.
Tapeworms, malaria parasites, bacteria, fungi

12. Symbiotic Relationships
• Commensalism is a relationship in which one
organism benefits while the other is not
affected.
 Remoras and sharks
• Mutualism is a relationship in which both
species benefit. The relationship is obligatory
in many cases, as neither can exist without
the other.
 Mycorrhizae

13. Mutualism and Symbiosis
• Mutualism is an association
between organisms of two
different species in which
each member benefits.
• Ants (Hymenoptera)
protect the aphids
(Aphididae) and the aphids
provide honeydew for the
ants

14. Commensalism and Symbiosis
• Commensalism is a
symbiotic relationship
between two organisms of
different species in
which one derives some
benefit while the other is
unaffected.
• E.g. Pseudoscorpions
hitching ride on a fly’s
(Diptera) leg

15. Parasitism and Symbiosis
Parasitism is a form of symbiosis in which
one species benefits at the expense of
another species; similar to predation,
but acts more slowly than predators and
may not always kill the host.
Parasitized caterpillar (Lepidoptera),
covered with wasp (Hymenoptera) pupae
which have consumed all internal tissue
except vital organs

16. Symbionts
These are organisms that live in or on
other organisms.
More than half of the millions of species
that live on Earth are symbionts.
Human body is a habitat to many micro
species.

17. Important Facts
Some symbionts are mutualists, but the
majority are parasites.
A parasite consumes the tissues or body fluids
of the organism on which it lives, its host.
Pathogens are parasites that cause diseases.

18. Natural History of Parasites
As a group, parasites typically harm, but do
not immediately kill, the organisms they
eat (unlike predators). The degree of
harm to the host varies widely.
Macro-parasites are large, such as
arthropods and worms. Micro-parasites
are microscopic, such as bacteria.
Many Species Are Host to More Than One
Parasite Species.

19. Coevolution
This is when the host and the parasite
evolve together each in response to the
selection pressure imposed by the
other.
In this case, they both develop
adaptations aimed at increasing their
chances of survival.

20. Ecological Effects of Parasites
At the population level, harm done by
parasites translates into reduction of
population growth rates.
Parasites can drive local host populations
extinct and reduce their geographic
ranges.

21. Ecological Effects of Parasites
The physical environment can be changed
when a parasite attacks a species that is
an ecosystem engineer—a species whose
actions change the physical character of
its environment, as when a beaver builds
a dam.

22. Ecological Effects of Parasites
When the trematode parasite drives the
amphipod populations to extinction,
erosion increases, silt content increases,
and the islands disappear.
The amphipod Corophium is an ecosystem
engineer in the tidal mudflats, the
burrows it builds hold the mud together,
preventing erosion and forming “mud
islands” at low tide.

23. Cooperation
• Cooperation is the act of working or acting together
• Ants and bees colonies work together
• Polyergus samurai raid and capture pupae of Formica
japonica and use them as workers when they emerge

24. Predator and Prey
• Predation describes an
interaction where a
predator species kills
and eats other
organisms, known as
prey.
• Sometimes, predators
themselves become
preys e.g. when a
praying mantis
captures grasshopper
and the anole captures
and eats praying
mantis.

25. Ecological niche
A niche is a set of conditions
within which an organism
can maintain a viable
population
It is multi-dimensional
with as many
dimensions as their
are limiting conditions
temperature
light intensity
salinity
ecological
niche

26. Two types of ecological niche
Fundamental niche depends on physical
(abiotic) conditions.
Realized niche depends on biotic as well as
abiotic conditions.
The niche of a species may contract in the
presence of a competitor species, this
phenomenon leads to resource (niche)
partitioning and coexistence among functionally
similar species. The narrower niche resulting
from competition is called the realized niche.

27. growth
rate
Location in intertidal zone
low highmiddle
The realized niche of a barnacle
Balanus
realized
niche Chthamalu
s realized
niche
Balanus
and
Chthamalus

28. growth
rate
Location in intertidal zone
low highmiddle
Chthamalus alone
Balanus alone
The fundamental niche of a barnacle
Removal experiments – remove each species and see
where the other grows
Balanus
fundamental
niche
Chthamalus fundamental
niche

29. growth
rate
Location in intertidal zone
low highmiddle
Chthamalus
alone
When one species is removed from the system, this
results into a competitive release
niche of the competitively-inferior species expands in the
absence of the competitively-superior species
fundamental
niche
realized
niche
Chthamalus with
Balanus
competitive
release

30. Thank You.