Community Structure
Factors that affect Community Structure:
• Predation
• Symbiosis
– Mutualism
– Commensalism
– Parasitism
• Competition
– Keystone predators
• Succession
– Primary and Secondary
Predation
• Predators feed
on other living
organisms -- their
prey
• Predators do not
take up residence in
or on prey (unlike
parasites)
• Prey may or may not
die from relationship
Symbiosis
• A symbiotic relationship is one in which there is an
intimate, long-term relationship between two (or
more) species
– Involves one species living on or in another species for at
least part of life cycle
• A symbiotic relationship could be:
– Mutualistic
– Commensalistic
– Parasitic
Mutualism
• Mutualism is a
symbiotic relationship
in which both species
_benefit .
Above: Impala being
“groomed” by Oxpecker bird,
which gets a meal of insects.
Left: Lichen is really a
mutualistic symbiosis
between photosynthetic
algae (which makes sugars
for both species) and a
fungus, which “houses” the
algae and protects it from
water loss.
Above & Below: A
Clown fish makes its
home and hides from its
predators in an
anemone and in return
keeps the anemone
clean from parasites
and safe from its
predators.
Commensalism
• Commensalism is a
symbiotic relationship in
which one species
benefits and the other
Above: Remoras get a free ride from a Lemon
shark, but do no harm to the shark.
species is not affected
Below: Man-O-War fish (Nomeidae)
find shelter and safety among the
tentacles of a Portuguese Man-OWar jellyfish.
Right: Epiphytes
are plants that live
on tropical tree
limbs and obtain
sunlight & rainwater
without harming
their “home” since
their roots remain in
the air (not inside
tree bark or tissue).
Parasitism
• Parasitism is a symbiotic
relationship in which one
species benefits (parasite)
and the other species is
harmed (host).
Left:
Tapeworms
inhabit animal
digestive tracts,
living off of the
nutrients that
are intended
for the host.
Over time, this
weakens and
can kill the
host.
Right: Ergot
is a fungus that
grows on
grains such as
barley,
breaking down
the grain and
getting food
from it.
Above: Cowbirds are “brood parasites” they lay their eggs in another bird’s nest
and leave them to be hatched & raised by
the host species. Cowbird nestlings grow
rapidly, frequently out-competing the host's
offspring for food and parental care. This
adult Yellow-throat is feeding a cowbird
fledgling that's more than twice its size.
Competition
• Competition occurs when two or more
individuals attempt to use an essential common
resource that is in limited supply (food, water,
shelter, sunlight, etc.)
• Can be intraspecific (among individuals in a population)
or interspecific (between different species)
Ex) Oak trees and pine trees may
compete for sunlight and space in
a New England forest.
Species - Species Interactions
Interaction
Direct
Direct
Effect on Effect on
Species 1 Species 2
Neutral relationship
0
0
Predation of Sp2 by Sp1
+
-
Mutualism of Sp1 & Sp2 *
+
+
Commensalism of Sp1 w/ Sp2 *
+
0
Parasitism by Sp1 on Sp2 *
+
-
Competition betw. Sp1 & Sp2
-
-
* Symbiotic relationship
“Ancient Farmers of the
Amazon” video clip
•
•
•
•
•
Leaf cutter ants
Leaves (from trees)
Cultivated Fungus (Ant “garden”)
Mold
Bacteria -- that produce “antibiotic”
chemicals (really anti-mold)
Competition & the Niche
• Every org. has its own ecological role or “job” in
its community; this is called its ecological niche.
• This role description includes all the biotic &
abiotic aspects of the organism’s existence; it’s
physical habitat, it’s prey, it’s predators, it’s
competitors, and how it is influenced by light,
wind, water, etc.
• When resources are abundant, different species
can share a niche or role. Unlimited resources
are not often found in mature, stable communities,
however…
Inter-specific Competition
• More typically, species compete for resources
and a particular niche.
• Inter-specific competition between species with
overlapping niches can lead to competitive
exclusion. (ie removal of the less-fit species
from the shared niche in question…)
Competitive Exclusion
Laboratory Experiment by
Gause (1934):
Two species of paramecium
reach high population
densities in given conditions
when grown separately.
When grown under the
same conditions together,
however, one species is
out-competed by the other,
and eventually goes extinct.
http://www.sci.sdsu.edu/classes/bio100/Lectures/Lect21/Image294.gif
more
Competitive Exclusion
There once were two species of barnacle that inhabited the same rock…
For the story, visit: http://nortonbooks.com/college/biology/animations/ch35a02.htm
• Potential niche v. Realized niche
Species-Species Interactions: Practice Questions
Communities are shaped by all the
interactions that occur within them
• Ecologists used to think that competition was the
most important relationship that determined
community structure (both the number of species in a
community and the size of each population).
• Further study has proven that this is not the case, all
types of species-species interactions, as well as
biotic-abiotic interactions, are highly influential in
determining community structure.
Community stability is an outcome
of forces that have come into
“uneasy balance”
• Resources are sustained, as long as populations do
not grow beyond capacity.
• Predators & prey coexist, as long as neither wins.
• Competitors have no sense of fair play.
• Mutualists are stingy.
– Plants produce as little nectar as needed to attract
pollinators
– Pollinators take as much nectar as they can for the least
effort
Disturbance can destabilize a
community
• Short-term, disturbance can slow growth of some
populations
• Long-term changes (like climate) can also destabilize
• If instability becomes great enough, community may
change in ways that persist even when disturbance
ends or is reversed
• If some community member species are rare or weak
competitors, they may become extinct.
Keystone Species are “key” to
maintaining community structure
• Keystone Species: a species that is critical
in determining the nature of an entire
community, usually by influencing amount of
available food, water, or other resource.
• Keystone species may be present in relatively small
numbers, but still affect whole community structure.
• Ex: Fig trees in tropical rainforests of C. & S. Am.
– Fig trees produce a continuous crop, year-round
– Figs sustain fruit-eating vertebrates at times of year when other fruits
less available
– No figs  no fruit-eating vertebrates
– No fruit-eaters  less seed dispersal in fruited plants
– Less seed dispersal  reduced distribution of fruit-bearing plants
Keystone Predators
• What is the role of a keystone predator?
– reduces the density of the strongest
competitors in a community
– helps maintain species diversity in a
community by preventing competitive exclusion
of weaker competitors, which in turn contributes
to community stability (think about why this is
so).
Ex1: Gray Wolf
Ex2: Sea Star (Pisaster)
Ex3: Common periwinkle (Littorina littorea)
Examples of Keystone Predators
• Ex1: Gray Wolf
– Wolves hunted to extinction in some areas
 elk, deer pops explode due to loss of key predator
 overgrazing of vegetation by deer
 decline of smaller animals (rodents, rabbits,
insects) which rely on vegetation
 decline of foxes, hawks, owls, badgers which rely
on small animal prey
 also see decline in scavengers (ravens, eagles)
that eat wolf kill
Examples of Keystone Predators
• Ex2: Sea star (Pisaster) preys on blue mussels,
limpets, chitons, & various barnacles
– When Pisaster was removed from community, saw
decline from 15 species to 8 species overall, because
Mytilus was left to dominate and crowd out other
invertebrates!
Another Example
• Ex3: Common periwinkle
(a type of snail)
– Periwinkes live in the rocky intertidal zone and eat algae.
– In tidepools, they eat the dominant algal species, like the
yummy tender filamentous green algae.
– This predation of filamentous green algae by periwinkles
keeps this dominant species in check and allows other
less competitive species, like the tough, unpalatable red
algae Irish moss to survive.
Common periwinkle
Filamentous green algae
“Irish moss” – a red algae
Species-Species Interactions:
More Questions for Thought & Practice
What does the word
succession mean??
There’s Royal succession…
Ecological Succession
A series of predictable changes in community
structure over time, typically culminating in a
mature, relatively stable “climax community.”
Primary Ecological Succession
Primary succession describes the process by which life can
colonize virgin territory and turn barren land (no true soil
present) into a thriving ecosystem, over a period of time.
Pioneer species:
the first species to
inhabit an area
Primary Ecological Succession
Secondary Succession
Secondary Succession occurs where vegetation has been
removed but soil is intact (instead of starting with bare rock).
Succession vocab: Disturbance, Pioneer Species, Climax Community
Photo Credits
http://www.worldbuilders.org/lessons/less/biomes/introbiomes.html
http://www.micro.utexas.edu/courses/levin/bio304/
ecosystems/ecology.html
http://www.aloha.com/~lifeguards/portugue.html
Pioneer species:
first species to
inhabit an area
Climax
Community:
mature, stable
community
Secondary Succession
Secondary Succession occurs where vegetation has been removed but
soil is intact.