Chapter 4: Biological Communities
and Species Interactions
Understand the fundamental factors
driving community development
I. Who Lives Where and Why?

A. Critical Factors and Tolerance Limits
– Examples of limiting conditions are
temperature, moisture levels, nutrient supply,
and soil
– Liebig’s principle (law) states, the single factor
in shortest supply relative to demand is the
critical determinant to species distribution
• Called the principle of limiting factors
I. Who Lives Where and Why?

A. (cont)
– Shelford used Liebig’s principle to formulate
that there are maximums and minimums for
environmental factors (resources)
• Called tolerance limits
• Zones of intolerance are areas of species extinctions
in that habitat
• The factor closest to the zone of intolerance
determines where an organism can survive
• Sometimes called Shelford’s Law
I. Who Lives Where and Why?

A. (cont)
– Some variations of the 2 rules exist
• Could be multiple factors working together to limit
distribution
– Some organisms have a specific critical factor
• Passenger Pigeons (land), Saguaro Cacti (cold)
– Some limitations may occur during a specific
portion of the life cycle
• Desert Pupfish (temperature and salinity levels for
juveniles only)
I. Who Lives Where and Why?

A (cont)
– Indicator species are species with defined
tolerance limits and are used to indicate the
health of the habitat
• For some, if missing there is a problem
• For some, if present there is a problem
– Environmental indicators are species of
organisms which can give specific information
about a habitat
• May indicate specific nutrients present or missing
• May indicate pollution
I. Who Lives Where and Why?

B. Natural Selection, Adaptation, and
Evolution
– 1. General Information
• Organisms adapt to special conditions
– One form of adaptation is acclimation
• Organisms experiences physiological
modifications or changes
• Non-permanent, reversible
– Another form is genetic, part of evolution
• Will change the population
• Inheritance of specific traits
• Natural selection allows the organism “best
suited” for an environment to reproduce
Darwin’s Finches
I. Who Lives Where and Why?

B. (cont)
• Acts on pre-existing genetic diversity
– Mutations can add to the genetic diversity
• Genes that suit the environment will
become the dominant trait over time
• Darwin’s finches is a good example
– Common, general ancestor becoming specialized
multiple current species
• Also called selective pressure
I. Who Lives Where and Why?

B. (cont)
– Factors affecting selective pressure are
• Physiological stress due to inappropriate
levels of some critical factors
• Predation, parasitism and disease
• Competition
• Luck?
• Geologic isolation can aid in different gene
expression
– Possibly leading to speciation
I. Who Lives Where and Why?

B. (cont)
• Natural Selection and Adaptation can cause
similar species or 2 groups of the same
species to drift genetically apart
– Called Divergent Evolution
• Natural Selection and Adaptation can cause
2 different species to drift genetically
together (considered the same species)
– They look and act alike
– Called Convergent Evolution
I. Who Lives Where and Why?

C. The Ecological Niche
– 1. General Information
• Habitat is the place where an organism lives
• Ecological niche is a description of the role
of a species in a biological community
– Niches can change as physical characteristics
change
I. Who Lives Where and Why?

C. (cont)
– 2. Law of competitive exclusion
• States that no two species will occupy the
same niche and compete fro exactly the
same resources in the same habitat (for a
long period of time)
• Creates niche specialization, which creates
behavioral separation, when two niches
overlap
Paramecium Graph
Resource partitioning and niche
specialization
I. Who Lives Where and Why?

C. (cont)
– 2 (cont)
• The number of niches is determined by the
resources and the extent by which they can be
separated
– Some animals can share resources, but use them at
different times
• Ex. Owls and Hawks, Bats and Mockingbirds
– Some animals can use the same resources, but use
different portions of the same resource
• Ex. Finches, MacArthur's Warblers, Flickers and
Woodpeckers
MacArthur's Warblers : Splitting the
same resource
Left to right: Cape May, Yellow-rumped, Black-throated Green,
Blackburnian, and Bay-breasted Warblers. Black areas in stylized conifers
show where feeding is concentrated.
II. Species Interactions and
Population Dynamics

A. Predation
– An organism that feeds directly on
another organism (living)
• Yes, Herbivores are predators!
• Scavengers, detritovores and decomposers
(that feed on dead organisms) are NOT
predators
• Parasites? , pathogens
– Predation is an influence on population
balance in a community
II. Species Interactions and
Population Dynamics

A. (cont.)
• Involves 3 scenerios
– 1. Influences all stages of the life cycle for both predators and prey
– 2. influences food obtaining mechanisms
– Influences prey- predator adaptations to resist or encourage
predation
– As prey species mature, the predators change
– As predators mature, the prey species change
– Tend to be the most successful with the old and the
young (book says least fit)
– Some prey have created defenses
• Spines, thorns, thicker bark, poisonous chemical mimicry,
speed, etc
II. Species Interactions and
Population Dynamics

B. Keystone Species
– A species or group of species whose impact on
its community is much larger and more
influential than would be expected from mere
abundance
– At one time they were thought to be top
predators
– May be a species that has a significant impact
on other organisms
• Ex: tropical figs, sea otters, prairie dogs
Keystone Species: Prairie Dogs
Keystone Species: Sea Otters
Keystone Species: American Beaver
II. Species Interactions and
Population Dynamics

B. (cont.)
– In some conditions microscopic organisms
may be the keystone species
• Ex: mycorrhzae (root fungus)

C. Competition
– When organisms compete over resources
– 2 types:
• Interspecific: between organisms of different species
• Intraspecific- between organisms of the same species
Interspecific Competition
Intraspecific Competition: Territories
II. Species Interactions and
Population Dynamics

C. (cont.)
– Interspecific competition is responsible for
niche specificity
– Physically designed to tolerate conditions,
acquire foods, and reproduce at a time different
from competitors
• Animal kingdom’s “arms race”
• Bigger, stronger, faster, and smarter
– Avoids fighting as much as possible
– Where 2 different species that occupy the same
niche compete in a habitat, one species will out
compete the other
II. Species Interactions and
Population Dynamics

C. (cont)
• Described as the Lotka-Volterra Competition
Model
• Mathematical equations to predict which
species will out compete the other
• Depends on 2 factors
– 1. The number of individuals of species 2
present; and (2) the intensity of the interference
with species 1’s growth; or the intensity of the
competition of species 2 on species 1
– It will be a negative factor
Lotka-Volterra Competition Model
II. Species Interactions and
Population Dynamics

C. (cont.)
– Intraspecific competition is more
intense
• Battling with organisms with the exact same
set of needs
• Plants have to battle with mature adults
• Adaptive tendencies lead to greater
dispersal of seeds
• Territories are a direct result of intraspecific
competition
II. Species Interactions and
Population Dynamics

C. (cont.)
• Battle for an area with all of the needs of the
organism at all stages of the life cycle
– Those animals without all either don’t reproduce or don’t
reproduce successfully

D. Symbiosis
– Interactions between species
• Not always antagonistic
– Intimate living together of members of two or
more species
II. Species Interactions and
Population Dynamics

D. (cont.)
– 4 types of symbiosis
• Commensalism- one benefits while another
has no apparent effect
• Mutualism- both organisms benefit
• Predator/Prey- one benefits while the other
dies
• Parasitism- one benefits the other has no
effect or bad effect
II. Species Interactions and
Population Dynamics

E. Defensive Mechanisms
– Toxic chemicals, body armor, similar
coloration, and others to defend against
predation
• Poison ivy, thorns
• Batesian mimicry
• Mullerian mimicry, 2 different species
unpalatable and dangerous looking very
similar
III. Community Properties

General Information
– Try to understand the factors which make up
the properties involving communities
• Productivity, diversity, complexity, resilience,
stability, and structure

A. Productivity
– Photosynthetic rates are regulated by light
levels, temperature, moisture, and nutrient
availability.
III. Community Properties

A. (cont.)
– Corn and sugar cane; under ideal
conditions, in the tropics can approach
productivity as high as the rain forest
– A very small amount of available
sunlight is captured by photosynthetic
communities

B. Abundance and Diversity
– Abundance is the total number of
organisms of a species in an area
III. Community Properties

B. (cont.)
– Diversity is the number of different
species in an area
– Abundance and diversity are inversely
related
• High abundance means low diversity
• High diversity means low abundance
– Diversity decreases away from the
equator and toward the poles
III. Community Properties

B. (cont.)
– Abundance increases away from the
equator and toward the poles
– Diversities decreases moving upward in
attitude

C. Complexity and Connectedness
– Complexity is the number of species at
each trophic level and the number of
trophic levels in a community
III. Community Properties
 C.
(cont.)
• A diverse community may not be a
complex community
• Tropical rainforests have many trophic
levels that are compartmentalized
– Called guilds
• Species that occupy the same trophic
level
• Fruit enters, leaf nibbles, seed gnawers,
etc
III. Community Properties

D. Resilience and Stability
– 3 Types
• 1. constancy, lacks fluctuations in
composition and function
• 2. inertia, which indicates resistance to
perturbations
• 3. renewal, which is the ability to repair
damage after disturbances
III. Community Properties

D. (cont)
– MacArthur believed the more complex a
community the more stable and resilient
the community will be when disturbed
(studies show no real consistency with
this conclusion)
– Disturbances are based on the
organism
• Ex. Earthquakes, flooding, traveling,
spitting, etc.
III. Community Properties

E. Edges and Boundaries
– Areas between 2 adjacent and different
communities
– Edge effect is the relationship of communities
and the organisms that inhabit the edge of the
2 communities or habitats
• Considered secondary habitats
– Some boundaries are sharp and distinct, called
edges
III. Community Properties

E. (cont)
– When the 2 habitats or communities blend
together, it is called a boundary
– Edges and boundaries are also called
Ecotones
• Sharp divisions are called closed communities
• Boundary divisions where many species cross are
called open communities
– Adjacent communities may be important for
species that need both types during different
stages of development in the life cycle
IV. Communities in Transition

A. Ecological Succession
– Transition of communities in an area
over time
– 2 types
• Primary Succession – development starts
with a site that is newly broken rock or an
area unoccupied previously by organisms
– Starts with pioneer species, such as lichens and
bacteria
IV. Communities in Transition

A. (cont)
• Secondary Succession – an existing
community is disrupted and a new,
previous, community redevelops in the
habitat or community
– Caused by wildfires, or farmland restoration
• Starts with weeds or grasses, when dealing
with a fire (depends on the severity of the
fire)
• Starts with grasses and shrubs when dealing
with overgrown farmland
IV. Communities in Transition

A. (cont)
• Typical primary succession
– Rocks, lichens, weeds, grasses, shrubs, conifers,
deciduous trees (TDF)
• Typical secondary succession
– Climax community, fire, weeds, grasses, shrubs,
coniferous trees, deciduous trees
– May be much faster and may skip some steps,
depending on what community surrounds the fire
area and what plants are ready to spread seeds
into the open area
IV. Communities in Transition

B. Introduced Species and Community
Change
– Introduced species are non-native species
– They can replace existing organisms or (by out
competing them for the same resources) or
– They can wipe out an unintended species
• Ex. Mongooses in Hawaii