This document discusses key concepts about communities and population interactions. It identifies and provides examples of 3 major population interactions: 1) feeding relationships shown through food chains and webs, 2) competition between species over limited resources which can lead to competitive exclusion, and 3) symbiosis including mutualism, commensalism, and parasitism. Additional topics covered include keystone species, ecological succession, and factors influencing biodiversity like geographic location and community size.

1. Communities

2. Objective
• Yesterday: You needed
to define and give
examples of density
dependent and density
independent limiting
factors
• Today: you need to:
identify, describe, and
give specific examples
of the 3 major
population interactions
– Feeding relationships
– Competition
– Symbiosis

3. What is a Community?
• The interactions between all species living in a
given area
– The boundaries of the area are defined by the
researcher who is studying the community

4. • Trophic structure-the feeding relationships between organisms
– Food chain-the transfer
of energy between
trophic levels
• Decomposers are
a vital part, but aren’t
shown
• Arrows point in
the direction of energy
flow
1. feeding relationships
Primary
producers
Primary
consumers
2o
consumers
3o
consumers
4o
consumers
Autotrophs
Herbivores/omnivores
Carnivores
Carnivores

5. • Food Web- a bunch of interconnected food chains

6. Food chains are limited; there are rarely more than 5-
7 links from the 1o
producer to the top carnivore….
*The 10% rule-
only 10% of organic
matter at each level
is converted into
usable energy that is
passed to the next
level; the other 90%
is used by the
organism or lost as
heat

7. 2. Competition
Competitive
Exclusion
Ecological
Niches
Character
Displacement
Resource
Partitioning
competition
1 of 2 competing sps
eliminates the other
An organism’s job
in an ecosystem
Adaptation leading to
modified niche Sympatric populations
become more
divergent in traits

8. • Intraspecific – same species, often fighting
involved to “win” the resource
– Can be very intense

9. • Interspecific – different species, usually no
fighting, just better at getting it
– Less intense since species have slightly different
niches

10. Competitive Exclusion Principle
• No two species can occupy
exactly the same niche
because competition for
resources would be too
intense
• One species will be better
at getting the resources
• The other species must
leave, adapt, or die

11. Solving the Problem

12. Resource Partitioning
• Two species evolve
adaptations that allow
them to use the same
resource in different
ways, at different
times, or in different
places in order to
minimize competition

13. 3. Predation: +/- interaction meaning that one individual
(predator) benefits, and another individual (prey) is
negatively affected
– Evolutionary connection: Predator-prey interactions
may select for unique adaptations
• Cryptic coloration- AKA camouflage
• Aposematic coloration-warning colors
• Batesian mimicry- a harmless animal mimics a harmful or
unpalatable one
• Mullerian mimicy-two or more unpalatable species resemble
each other

14. Aposematic coloration-
poison arrow frog
Batesian mimicry Mullerian mimicry

15. 2 rules about coloration
• Small + beautiful = poisonous
• Beautiful + easy to catch = deadly

16. Coevolution
• When two species have lived together and
adapted to each other for a long time such that
changes in the gene pool of one species leads to
changes in the gene pool of the other species

17. 4. Herbivory: A +/- interaction; an herbivore eats
a plant or algae
– Evolutionary connection: many plants possess
adaptations that help them avoid herbivory
Strychnos toxifera is a vine that produces strychnine

18. 5. Disease: pathogens (things that
cause disease) are another example
of a +/- relationship
– Differ from parasites in that
they’re usually microscopic
– EX: bacteria, viruses, protists,
fungi, & prions
– Like parasites,they can have a
large, negative effect on
population size

19. 6. Parasitism: a +/- symbiotic relationship between a
parasite & its host
– Many parasites have
complex lifecycles with
multiple hosts
– Parasite
outbreaks
can also greatly
affect host
populations

20. 7.Mutualism: A +/+ symbiotic relationship where both
organisms benefit
The Acacia tree provides nectar
to these ants and the ants provide
protection to the tree

21. 8. Commensalism: A symbiotic +/0 interaction where
one organism benefits and the other is unaffected
– Example: these egrets follow grazing herbivores to eat
the insects that the herbivores kick up as they travel

22. Some species have a disproportionate impact on their community
• These species often affect the biodiversity-variety of
different kinds of organisms-in the community
• Dominant species-Species in a community which are the most
numerous or have the highest biomass

23. Nonnative species
• Aka alien, invasive, exotic
• Accidentally or deliberately introduced

24. • Not always bad - most crops and livestock
are nonnative species

25. • Sometimes - nonnative species takes over
– Often there are no predators for this species,
since it hasn’t evolved alongside them
– This gives the nonnative a competitive
advantage

26. Keystone species
• Have a large effect on the number and type of
other species in the ecosystem

27. Examples:
• Top predators - feed on and help regulate the
size of other populations
• Pollinators - control the size of plant
populations

28. Effects of losing a keystone species
• When a keystone species is lost - it leads to
population crashes and extinctions of other
species

29. Foundation Species
• Aka ecosystem engineers
• Play a major role in creating the
ecosystems in which they live

30. Examples:
• Elephants pull out or break small trees in
the African savannah
• This allows grass to grow and benefits
other grazers and open plains predators

31. Examples:
• Beavers - build dams to create ponds that
other species benefit from

32. What other factors affect the biodiversity of an area?
Two key factors: location & size of the community
– Equatorial-Polar Gradients- The further you move away from the
poles and closer to the equator, the greater the biodiversity of an
area.
• Why?? 1. Many polar and temperate regions have “started
over” due to glacier movement
2. Climate-the tropics receive more direct sunlight for longer
periods
» The differences in sun
exposure can be measured
by measuring evaporation
from soil & plants=
evapotranspiration

33. What other factors affect the biodiversity of an area?
• Two key factors: location & size of the community
– All other factors being equal, the larger the geographic area of a
community, the greater the number of species
• Why?? This is because the greater the area, the greater the
variety of available habitats

34. A special case: Islands
• In ecology, an island can be in the ocean, or any community that’s
otherwise isolated
• Island biodiversity is affected by the rate of species immigration vs.
extinction
– These two processes are affected by island size and distance
from the mainland
• If the colonization of an island is a chance event (for example
the probability that a seed will land on an island), then small
islands are less likely to be inhabited in this way.
• The closer an island is to the mainland, the more likely it is
that migration will occur between the mainland and the
island

35. When immigration is high, then extinction is low
since organisms are populating the island at a
higher rate

36. What is an ecological disturbance?
• Disturbance-storm, fire, drought/flood, overgrazing, or any
human activity which can remove organisms or resources from
a community
– A disturbance isn’t necessarily a negative event…some
communities rely on disturbances for the natural
progression of their “life cycle”
• Ex-Lodgepole pine trees require periodic fires in order to
release their seeds
Closed cone
Open cone after fire

37. What happens to a community after an
ecological disturbance?
• Ecological succession-the order in which an area is
recolonized after a major disturbance has stripped
away the existing vegetation

38. Two main types of succession:
• Primary succession- think of this as a blank canvas…
– The first organisms to colonize the area are autotrophic
prokaryotes, followed by lichens & mosses. Soil gradually
develop, more plants grow, and animals move in
• Community maturation could take 100s or 1000s of years
– When might primary succession occur?
When a new volcanic
island forms or in an area
where a glacier has
receded

39. Two main types of succession:
• Secondary succession- Occurs after a
disturbance, for example a fire, has wiped out a
community, but has left the existing soil intact
– Often, herbaceous plants colonize the area first,
followed by insects, larger animals, and trees