Ecological Communities
We wish to learn:
• What is an ecological community and what kinds of
interactions take place within it?
• How important are the various categories of species
interactions, including mutualisms, commensalisms,
competition, and predation?
• What kinds of interactions among species become
important when many species affect one another ?
• What consequences do these interactions have for the
maintenance of biodiversity?
Types of Species Interactions
• An ecological community is a group of
actually or potentially interacting species,
living in the same place
• A community is bound together by the
network of influences that species have on
one another.
• There are four main classes of two-way
interactions, and many possible pathways of
indirect interaction.
Types of Species Interactions
Type of
interaction
Sign
mutualism
+/+
commensalism +/0
competition
predation
(includes herbivory,
parasitism)
-/+/-
Effects
both species benefit
one species benefits,
one is unaffected
each is neg. affected
each is pos. affected
Food Webs
This river food
web illustrates
the complex
interconnections
between
producer
organisms at
the bottom,
and consumers
progressively
higher in the
web.
Note there are different organisms at the same feeding or “trophic” level
Food Webs and Species
Interactions
• A food web reminds us of the many ways that
species interact with one another.
• In addition to the direct, two-way interactions
already defined, there are ample opportunities
for more subtle, indirect effects, as when
species A affects species C via an
intermediary, species B.
• Such effects may cascade from higher to
lower trophic levels (a “top-down” effect”),
or from lower to higher (“bottom-up effect”).
Keystone Species Concept
In ecological communities there
are little players and big players.
The biggest players of all are
referred to as keystone species.
A keystone species may be
defined as one whose presence/
absence, or increase/decrease in
abundance, strongly affects other
species in the community.
Removal of the keystone in
the arch will cause the
structure to collapse.
Evidence usually comes from
addition or removal experiments.
Starfish as Keystone Predators
Photo by Raymond Seed
In the rocky intertidal zone, mussels are
superior are occupying space, crowding
out other species. Starfish are effective
predators of mussels, and thereby make
room for other species.
http://life.bio.sunysb.edu/marine
bio/rockyshore.html
Starfish as Keystone Predators
A hoard of hungry starfish converge on
a mussel bed. A starfish curls its body
around the mussel, using its tube feet to
pry apart the valves enough to insert its
extensible stomach. Digestive enzymes
break down the mussels’ muscles, and
the starfish consumes its prey.
A starfish removal experiment
in Washington State
demonstrated that the mussels
are competitive dominants.
Starfish predation opens up
enough space for various
barnacles, echinoderms and
other marine invertebrate
species to maintain a presence.
When starfish are
experimentally removed,
mussels take over and other
species are excluded.
Other Indirect Species
Interactions
• The keystone species effect is the bestknown example of indirect interactions. One
cannot have a keystone species effect
without the presence of indirect interactions.
– Keystone species produce strong indirect effects, out of
proportion to their abundance.
• If a predator strongly suppresses its prey
(e.g., herbivores), one expects the trophic
level below (e.g., plants) to benefit. Such
top-down trophic cascades are well-known in
lakes.
Domino Effects - A Conservation
Issue
• Removal of one species
causes other species to be
lost from the system
•Aka “ripple effect”
• When the dodo (a 25 kg
pigeon) was exterminated
on Mauritius, the tree
Calvaria major ceased to
recruit. Its seeds needed to
be abraded in the dodo’s
gizzard to germinate.
Mutualistic Interactions
• A mutualism is an
interaction where both
species benefit
• facultative mutualisms
are beneficial but not
essential to either
species’ survival
• obligate mutualisms are
essential to the survival
of one or both species
Pollination is a classic
mutualism. The plant gains
through gamete transfer.
The pollinator receives a
reward of nectar and pollen.
The Boran - Honeyguide Mutualism
• Example of a facultative
mutualism between an
African people, and a bird,
Indicator indicator.
• Borans find bee colonies
more quickly when guided
by the bird
• Honeyguides get larvae,
wax, and are in less danger
from bees
The bird informs on direction,
distance to, and arrival at bee
colony. Search time is
reduced from 8.9 to 3.2 hrs
The Ant - Acacia Mutualism
• Example of an obligate
mutualism between
ants and a thorny,
small tree of dry
tropical forests.
• Acacia provides
shelter (hollow thorns)
and food (nectar,
protein bodies) to ant.
Ants remove herbivorous
insects, vines, others plants,
and leaf litter (which
reduces risk of fire).
The Ant - Acacia Mutualism
An acacia ant (Pseudomyrmex ferruginea) sipping nectar from
the petiolar nectary of a swollen thorn acacia (Acacia collinsii)
in Costa Rica.
http://waynesword.palomar.edu/acacia.htm#antleafb.gif
The Ant - Acacia Mutualism
Plants also supply ants with protein and fat-rich food in the
form of “Beltian bodies”, shown here being harvested by ants
(arrows) from the tips of newly expanding leaflets of Acacia
cornigera (Photo by T.W. Sherry)
The Ant - Acacia Mutualism
•Newly developing bull’s
horns (evolutionarily
enlarged thorns)
•Filled with a pith that ants
easily remove, creating
hollow interiors
•Ants chew small hole into
each thorn for use as home
The Ant - Acacia Mutualism
Pseudomyrmex ants
provide two services to
Acacia trees:
•24-hour patrolling of
leaves for protection
against insects and
mammals
•Clearing of plants from
ground and from Acacia
trees themselves as
protection from
competitors and fire
Commensalism
• When one
species
benefits, and
the other
neither
benefits nor is
harmed.
• A “+/0”
interaction
Cattle egrets follow cattle, consuming the
insects disturbed by the cows’ movements.
It is unlikely that the cows benefit.
Summary
• Species interactions are complex.
– Four main categories of two-way interactions
include mutualism, commensalism, competition
and predation
– Indirect, multi-way interactions include
keystone species effects and trophic cacades
• Forthcoming lectures on competition and
predation will add detail to the rich
complexity of interactions
• Human-driven extinctions of individual;
species can ramify to affect many others.