Predation-Amensalism Summary
• Gause did early predator-prey experiments, and concluded
that cycles in nature result from constant migration,
because he couldn’t get coexistence in his experiments.
• Huffaker found habitat complexity allowed coexistence
• Holling studied Functional response – relationship
between prey density and the rate at which an individual
predator consumes prey and Numerical response- increase
in predator numbers with increases in prey abundance
•
Predation-Amensalism Summary
• 3 types of functional response curves, I, II, and III
• Search image- Only when the prey population increases
above some threshold level does the predator form a search
image and begin to recognize that prey item as a valuable
food source. The predator then focuses on and exploits
that food source heavily
• Prudent predation occurs without altruism
• Predation can cause: changes in size distribution; both
decreases and increases in diversity; morphological
modifications (spines, mimicry, crypsis)
Predation-Amensalism Summary
• Paine’s exp. led to keystone species concept
• Optimal foraging-comcerns types of feeding behaviors that
maximize food (energy( intake rate
• Inducible defenses are brought on by predation threat and
serve to deter predators
• Indirect effects of predation: sub-lethal predation; TMII;
trophic cascades
• Parasites are either ecto- or endo-parasites
Predation-Amensalism Summary
• Parasites exploit host behavior to maximize transmission
• Host defenses are behavioral, structural or immunological
• Herbivory is the first step in the transfer of energy in food
webs; provides for the cycling of nutrients; and can affect
the productivity and structure of plant communities. It
increases prevalence of species with:
– Low nutritive value (low nitrogen)
– Chemical Defenses (secondary compounds)
– Structural Defenses (calcareous skeletons)
– Shifts in functional groups (from fast to slow growers)
Predation-Amensalism Summary
• Secondary compounds can deter herbivores but entail
tradeoffs in energy allocation
• Mutualisms- usually have one species providing nutrition
while the other provides protection or cleaning services.
Can be obligatory or facultative
• Mutualisms common in tropics and likely evolved from
host-parasite relationships
• Commensalism and trophic amensalism less common
Review Questions
Community Ecology
• A community is a group of interacting populations, all
living in the same place at the same time
– the focus is on the interactions between species or
populations including competition, predation ,
succession, invasion, mutualism, predation, etc.
Community Structure and
Change
• Community Structure - a description of the
community members (species list) and their
relative abundances
• Community Dynamics - the changes that occur
over time and space in a community. (Even
though communities have an underlying structure,
the structure may change over time
Emergent Properties
• Properties not predictable from study of component
populations
• Only apparent at level of community
Why is this important?
Appropriate unit of study:
- If the community is more than the sum
of its parts, then we must study the entire
community
(holistic approach)
- If not then entire picture can be put
together from individual pieces
(reductionist approach)
The Study of Ecological
Communities
• Properties &
patterns
– Diversity (Number
of species)
– Species’ relative
abundances
– Morphology
– Succession
• Processes
– Disturbances
– Trophic
interactions
– Competition
– Mutualism
– Indirect effects
Two Views on Communities
• Community as a superorganism (equilibrium
community, Clements)
• Species not replaceable
• Species need one another to survive
• Community as a group of individual species
(non-equilibrium community, Gleason)
• Species are replaceable
• Random association of species
Community Dynamics:
Succession
• Succession - The change in numbers and kinds
of organisms in an area leading to a stable
(climax) community. Replacement of
communities.
• Pioneer community - the first community to
develop in a successional sequence
• Sere - any successional community between
pioneer and climax community
Types of Succession
1. Primary – situation where barren substrate
is available for habitation (inorganic
substrates= lava flows/ spreading centers
2. Secondary – occurs in areas where
communities have previously existed (after
fires or hurricane; much more rapid)
Succession in community traits
 increasing size and longevity of organisms
 shift from predominantly "r-selected" to
predominantly "K-selected" species
 increasing biomass
 increasing independence of physical/chemical
environment
Succession in community traits (2)
 decreasing rate of change
 increasing species diversity and complexity
of physical and trophic structures
 increasing habitat modification and
buffering of environmental extremes
 increasing complexity of energy and
nutrient flows
 increasingly closed system re-cycling of
organic and inorganic materials
Opposing Views of Communities
Superorganism View
Individualistic View
(Clements, 1916)
(Gleason, 1925)
- tightly evolved, interacting
- randomly assembled
- functions as a single
organism
- Similar resource
requirements
- developmental process
(succession)
- homeostasis
(self maintaining – stable)
- underlying “balance of
nature”
Types of Species
Early successional
Late Successional
- good colonizers
- rapid growth
- short lived
(r-selected)
-poor colonizers
- slow growth
- long lived
(k-selected)
Early
Late
Under Equilibrium Models
• Community returns to same position after
disturbance
• At equilibrium, processes that structure the
community produce no net change
Equilibrium Theory
Single stable state
Multiple
stable states
Outcomes of integrated view
• Equilibrium assumed (not tested)
• Explained succession
• Super-organism concept widely accepted
• Dominated community ecology until the
1950’s and beyond
Non-equilibrium models
• Disturbance is the norm rather than the
exception
• Disturbed patches provide opportunities for
colonization by dispersive species
• Patchiness promotes diversity on a larger scale
Evidence for each view:
• Superorganism:
• remove plants or autotrophs, the community will
disappear
• mutualisms and symbiotic relationships are
common (example: herbivore gut bacteria)
• Non-equilibrium
• high-level consumers can sometimes be removed
without major effects on community
• disturbances often play a role in determining
community structure; these are random
Alternative succession models
Connell and Slatyer (1977) – outlined 3 models:
1. Facilitation – Clementsian succession
2. Tolerance
3. Inhibition
Based on effect of initial spp. on subsequent spp.
Facilitation Model
Early Stand
Recruitment
E L
E
Growth
E
E
E
Facilitation
Disturbance
Recruitment
E L
L
L
L
L
L
Late Successionals only
Mortality
L
E
E
L
Mixed Stand
L
Tolerance Model
Mixed Stand
Recruitment
E L
L
Growth
E
L
L
Tolerance
Recruitment
Disturbance
E
E L
L
L
L
L
L
L
Late Successionals only
Mortality
L E
L E
Inhibition Model
Mixed Stand
Recruitment
E L
L
Growth
E
L
E L
L
L
L
L
L
Late Successionals only
Mortality
L
Inhibition
Recruitment
Disturbance
E
L
E
L E
Succession
•Can occur without invoking the existence
of a “Super-organism”
•Sequential replacement a consequence of
individual species properties
Physical disturbance
What are the components of
disturbance?
• The frequency of a
disturbance
• The intensity of the
disturbance
• The timing of the
disturbance
– Influences the
availability of larvae
to recolonize the
disturbed area
Intermediate Disturbance Hypothesis
(Connell 1972)
• Disturbance (eg, tree falls, storms) creates patchiness
and new space to be colonized
•Patchwork is created across the landscape with
- early and late successional species
- inferior and superior competitors
This theory is a non-equilibrium view of how
natural communities are structured because landscape is a
patchwork of different stages of succession.
Intermediate Disturbance Hypothesis (2)
Disturbance is critically important in structuring communities
because it can prevent competitively dominant species from
excluding others.
Weak/infrequent disturbances are insufficient to prevent
competitive exclusion
Intense/frequent disturbances exclude species sensitive to
disturbance
Highest diversity might therefore be expected at intermediate
frequency or intensities of disturbance
Intermediate Disturbance
Hypothesis (Connell)
Top-down vs. bottom-up control
Community structure could be controlled from
the bottom-up by nutrients:
predators
herbivores
community
structure can be
changed by
manipulating the
lower levels
autotrophs
nutrients
numbers of
autotrophs
are limited
by mineral
nutrients
Community structure could be controlled
top-down by predators (trophic cascade model)
predicts a series
of +/- effects if
upper levels are
manipulated
predators
herbivores
autotrophs
nutrients
numbers of
herbivores
are controlled
by predators
Trophic cascades
Reintroduction
and protection
of otters has
reduced urchin
barrens
Predation by
orcas has
increased
urchin
barrens
Species-area Relationships

Known for a long time that there is a
relationship between the size of an island and
the number of species present on the island.

This relationship, which exists for all taxa
studied to date, whether on land or in the sea,
is known as the Species-area Relationship.
Species-area relationships
• Species-area curve - the larger the
geographic area, the greater the number of
species
• Larger areas have
more diverse
habitat
• This can be used
to predict how
habitat loss may
affect key species
fig 53.25
Species Area Relationships
• As a rule of thumb for every 10x increase in
habitat area you can expect a doubling in species
number
• this relationship is best described by the regression
formula S=cAz
– where: S = the number of species, c= a constant
measuring the number of species/unit area, A=
habitat area, and z is another constant measuring
the shape of the line relating S & A
Often linearized
• ln (S ) and ln (A )
• ln (S ) = ln (c ) + z ln (A )
– z is now the slope
– ln (c ) is now the intercept
ln (S )
ln (A )
Top: Species-area curve for corals in coral reefs on Rasdu Atoll, Maldives, and
on Heron Island, Great Barrier Reef. Adapted from Scheer (1978).
Bottom: Relation of number of species and number of individuals in a sample,
based on twenty samples of benthic invertebrates collected from Buzzards Bay,
Why do Species-Area Relationships Exist?

Habitat heterogeneity - as area increases so will
habitat number, and species number
 Area per se - extinction rates will go down with
increasing area as populations increase
 Passive sampling - as area increases there is a
larger “target for immigrants to “hit” –
 Disturbance - smaller areas will be subject to more
disturbance (DI mortality) and species number
will be frequently “set back”
Importance of Islands in Ecology

Islands can provide opportunities for natural
experiments because different islands in an
archipelago can have different species of potential
competitors, or lack certain predators. Thus, the
effects of processes such as competition and
predation can be easily studied on islands.

Islands are also widespread, even on land, because
any isolated patch of habitat is effectively an
island (e.g., lakes, coral reefs, kelp beds) for the
species living there.
Island Biogeography

Because of the generality of the species-area
relationship, Preston (1962) and MacArthur &
Wilson (1963, 1967) proposed that islands were
supporting as many species as possible.
 Since islands continuously receive immigrants, yet
species number stays constant, there must be a
balance between immigration and extinction.
 Preston and MacArthur & Wilson proposed that the
number of species on an island is in a dynamic
equilibrium between immigration and extinction.
Island Biogeography (MacArthur and
Wilson, 1960’s)
The number of species on an island is in a dynamic
equilibrium determined by imm. and ext. rates
• immigration rate decreases with Sp. N since
it becomes more likely that immigrants will
not be new species
• extinction rate increases with Sp. N because
of greater incidence of competitive exclusion
• equilibrium reached when immigration and
extinction rates are equal
• equilibrium number is correlated with area
and distance from mainland
fig 53.26a
Island Biogeography (2)
 This “dynamic equilibrium” between
immigration and extinction was developed
into a quantitative theory that was termed
The Theory of Island Biogeography.
 The theory of Island Biogeography has two
major points: the area and distance effects.
Area effect
B
A
Mainland
Area Effect
Island size influences immigration and extinction rates
because……
• larger islands are more likely to be
found by immigrants which
increases immigration rate
• organisms are less likely to go
extinct on larger islands because
there is more available habitat
• equilibrium number is higher on
larger islands because of both
higher immigration and lower
extinction
fig 53.26b
B
Distance effect
A
Mainland
Distance Effect
Distance from the mainland influences immigration and
extinction rates
• given islands of the same size,
immigration will be higher on
near islands since they are more
likely to be found by
immigrants
• extinction rates the same (same
size islands)
• equilibrium number is higher
on near islands because of
higher immigration
fig 53.26c
Island biogeography is a simple
model and we must also take into
account abiotic disturbance,
adaptive changes, and speciation
events
Latitudinal species richness gradients
• Species richness of many taxa declines from
equator to poles
Land birds
• Why? NOT CLEAR
Could be evolutionary or
ecological factors, or both?
fig 53.23
Diversity along geographical gradients. Corals from the Great Barrier Reef;
copepods from the Pacific; remaining data from all oceans. After Thorson (1957)
Factors Proposed to Explain
Latitudinal Diversity Gradients
• History (more time permits more speciation
• Spatial Heterogeneity (more complex habitats provide
more niches and permit more species to exist)
• Competition (competition favors reduced niche
breadth, but competition can also eliminate species!)
• Predation (predation retards competitive exclusion)
Factors Proposed to Explain
Latitudinal Diversity Gradients(2)
• Climate (climatically favorable conditions allow more
species to co-exist)
• Climate Stability (stable climates allow specialization to
occur)
• Productivity (Diversity is limited by the amount of
energy that can be partitioned)
• Disturbance (moderate disturbance retards competitive
exclusion= intermediate disturbance hypothesis)
Recent Explanations for
Latitudinal Diversity Gradients


increased area of the tropics
increased effective evolutionary time due
to shorter generation times in the tropics
The world’s tropical lands cover about four times the area s the world’s
second largest biome, the tundra. Tropical oceans also cover more
surface than oceans in other climate zones. From Rosenzweig (1992).
Island Biogeography and Conservation


In many areas, (1) the total area of natural
habitats is shrinking, and (2) formerly
contiguous habitats are being fragmented.
In island biogeographic terms, this means
that island areas are shrinking and large
islands are being broken into archipelagos.
Island Biogeography and
Conservation (2)

Island biogeographic theory allows predictions
to be made about the effects of reducing and
fragmenting habitats, and to make
recommendations for conservation

Areas of application: (1) How large should
preserves be? (2)How does isolation affect
species number in reserves? (3) What kinds of
species will survive if area is reduced?
Application of biogeographic
principles to the design of nature
preserves. In each pair of figures
the design on the left is preferred
over that on the right, even though
both incorporate the same area.
The concepts are: A, a continuous
reserve is better than a
fragmented one; B, the ratio of
area to perimeter should be
maximized; C, distance between
refuges should be minimized; and
D, dispersal corridors should be
provided between fragments.
(from Ecology and Evolution of Communities, ed. M. L.
Cody and J. M. Diamond, 1975
.
Ecosystems Ecology
Food Chains
• The energy flow from one trophic level to the
other is the food chain
• A food chain involves one type of organism at
each trophic level
–
–
–
–
–
Producers (Autotrophs)
Primary Consumers – eat producers
Secondary Consumers – eat the primary consumers
Tertiary Consumers – eat the secondary consumers
Decomposers – bacteria and fungi that break down
dead organisms and recycle materials
What is a Food Web?
• Describes which organisms in communities
eat other kinds of organisms
• Community food web is a description of
feeding habits of a set of organisms based
on taxonomy, location or other criteria
• Webs were derived from natural History
approaches to describing community
structure
What is a Food Web (2)?
• Food webs portray flows of matter and
energy within the community
• Web omits some information about
community properties
– e.g., minor energy flows, constraints on
predation, population dynamics
Food Webs: Methods
1. Identify component species
2. Sample to determine who is eating whom
3. Sampling and gut analysis to quantify
frequency of encounters
4. Exclosures and removals of species to
determine net effects
5. Stable isotopes
6. Mathematical models
Descriptive Food Webs
Interaction or functional food webs depict the
most influential link or dynamic in the
community
What is a Food Web (cont.):
Complexity meets reality
• Fallacy of linear food chains as a adequate
description of natural food webs
– Food webs are reticulate
– Discrete homogeneous trophic levels an abstraction or
an idealism
– omnivory is rampant
– ontogenetic diet shifts (sometimes called life history
omnivory)
– environmental diet shifts
– spatial & temporal heterogeneity in diet
Are trophic levels useful?
• Even if organisms are not strict herbivores,
primary carnivores, etc., as long as they are
mostly feeding at one trophic level, the
concept can have value (e.g., trophic
cascade concept).
What is a food web (cont.)?
• Modern Approaches to Food Web Analysis
– Connectivity relationships
– Importance of predators and interaction strength in
altering community composition and dynamics
Energy flow through
ecosystems
• Energy transfer between trophic levels is not 100%
efficient, and energy is lost as it passes up a food
chain.
• Herbivores eat a small proportion of total plant
biomass; they also use only a small proportion of
plant material consumed for their growth. The rest is
lost in feces or respiration
• Thus, less energy is available for the next trophic
levels
Trophic Basis of Production
• Assimilation efficiency varies with resource
–
–
–
–
–
–
10% for vascular plant detritus
30% for diatoms and filamentous algae
50% for fungi
70% for animals
50% for microbes (bacteria and protozoans)
27% for amorphous detritus
• Net Production Efficiency
production/assimilation ~ 40%
Marine Ecology: Food Webs
• Ecological efficiency is defined as the energy
supply available to trophic level N + 1, divided by
the energy consumed by trophic level N. You
might think of it as the efficiency of copepods at
converting plants into fish food.
• In general, only about 10% of the energy
consumed by one level is available to the next.=,
but this can vary substantially.
• Difficult to measure so scientists focus on
measures of assimilation efficiency for selected
groups of animals.
Food Webs
• A pyramid of biomass
represents the amount of
energy, fixed in biomass, at
different trophic levels for a
given point in time
• The amount of energy available
to any trophic level is limited by
the amount stored by the level
below.
• Because energy is lost in the
transfer between levels, there is
successively less total energy at
higher trophic levels.
Food Webs in the Ocean
• The oceans can be an exception,
because the total amount of biomass
in algae is usually small. A pyramid
of biomass for the oceans can appear
inverted
• However, a pyramid of energy,
which shows rates of production
rather than biomass, must have the
pyramid shape. Algae can double in
days, while zooplankton might
double in months, and fish might only
reproduce once a year. Thus, a
pyramid of energy takes into
account turnover rate, and can never
be inverted.
Decomposition and Mineralization
• Most material is derived from plants
• Involves:
• Release of chemical energy
• Mineralization (= organic --> inorganic)
• Note immobilization = reverse of mineralization
• Net mineralization rate = mineralization immobilization
Terrestrial communities:
Nutrient sources
•
•
•
•
Weathering of rock (K, P, Ca and many others)
Fixation of CO2 (photosynthesis) and N2
Dryfall (particles in the atmosphere)
Wetfall (snow & rain); contains
– Oxides of S, N
– Aerosols
• particles high in Na, Mg, Cl, S
• produced by evaporation of droplets
– Dust particles from fires, volcanoes
• Ca, K, S
Terrestrial communities:
Nutrient losses
• Release to atmosphere
–
–
–
–
CO2 from respiration
Volatile hydrocarbons from leaves
Aerosols
NH3 (decomposition), N2 (denitrification)
• Loss in streamflow
– Dissolved nutrients
– Particles
Oceans
•
•
No outflow
Detritus sinks --> mineralization --> nutrients end
up
1. Being carried back to surface in upwelling currents, o
2. Trapped in bottom sediments (e.g., phosphorus: 1%
lost to sediment with each cycling)
CARBON CYCLE
4 PROCESSES MOVE
CARBON THROUGH
ITS CYCLE:
CO2
1) Biological
2) Geochemical
3) Mixed biochemical
4) Human Activity
CO2
NITROGEN CYCLE
N2
in Atmosphere
Nitrogen-containing
nutrients include:
1) Ammonia (NH3)
2) Nitrate (NO3-)
3) Nitrite (NO2-)
4) ORGANISMS NEED
NITROGEN TO MAKE
AMINO ACIDS FOR
BUILDING PROTEINS!!!
N03NH3
&
N02-
The nitrogen cycle
PHOSPHORUS CYCLE
PHOSPHORUS FORMS PART OF IMPORTANT LIFE-SUSTAINING
MOLECULES (ex. DNA & RNA)
The phosphorus cycle
We’re in the Driver’s Seat - Human Activities
Dominate Many Biogeochemical Cycles
Disturbance simplified
• The greater the
disturbance the more
habitat that will be
opened up
Factors Hypothesized to Influence Biodiversity (Factor/ Rationale)
Factor
Rationale
1. History
More time permits more complete colonization and the evolution of new species
2. Spatial heterogeneity
Physically or biologically complex habitats furnish more niches
3. Competition
a. Competition favors reduced niche breadth
b. Competitive exclusion eliminates species
4. Predation
Predation retards competitive exclusion
5. Climate
Climatically favorable conditions permit more species
6. Climatic variability
Stability permits specialization
7. Productivity
Richness is limited by the partitioning of production among species
8. Disturbance
Moderate disturbance retards competitive exclusion
Source: Modified after Pianka (1988) and Currie (1991).
Time 0
Disturbance opens space; slate wiped clean
Time 1
Only certain species can
establish themselves in
open space; Opportunists,
Fugitives, Weeds
Time 2
First colonists modify
environment so it
becomes less suitable for
their further recruitment
but more suitable for
other species
Time 3
Process continues until
residents no longer
facilitate recruitment of
other species
Model
FACILITATION
No special requirements for first colonizers
First colonists make
environment less
suitable for their own
further recruitment, but
this has little or no
effect on other species
First colonists make
environment less
suitable for all
subsequent species
Process continues until
no species can invade
and grow in presence
of residents
First colonists continue
to hold space and
exclude all others (First
Come, First Served)
TOLERANCE
INHIBITION
Tests of the Island
Biogeographic Theory

Lots of small scale colonization studies were
consistent with the Theory

Best know test is the “million dollar experiment”
of Simberloff and Wilson

Although the results of this study continue to be
cited in support of the Theory, Simberloff says
they only provide weak support.
Tests of Island Biogeographic
Theory (2)

This was because many of his extinctions were
found to be transients that could not survive on
his mangrove islands, or species that visited the
islands as part of a larger range (e.g., wasps).
Thus, much of the measured turnover was
“pseudoturnover”.

He concluded that the Theory still needed
verification, as have others since then.
Insect recolonization of four defaunated mangrove islands. The y axis indicates the
predefaunation species richness of each island. Most of the islands reached an
equilibrium species number after 250 days that was approximately the same as the initial
richness. (From Simberloff and Wilson 1969.)
F.E. Clements (1916, 1936) idea of succession
•
•
•
•
Succession
Sere
Climax
Ecosystem = superorganism
Succession
b
A
b
Early
Colonizing
c
B
c
Mid
Mixed
Time
c
C
Late
Climax
c