Population Biology: A
summary
• The term "population growth" refers to how the
number of individuals in a population increases
(or decreases) with time. This growth is controlled
by the rate at which new individuals are added to
the population -- the birth rate, and the rate at
which individuals leave the population -- the death
rate.
• There are many types of plants and animals, and
different types show different kinds of population
growth.
Population Biology: Geometric
and Exponential Growth
•
If a population has a
constant birth rate and is not
limited by food or disease, it
grows geometrically or
exponentially
– the birth rate alone controls
how fast (or slow) the
population grows.
•
•
Population declines (r < 0)
Population increases (r > 0)
Population does not change (r
= 0)
Population Biology: Logistic
Growth
• In most populations both
food and disease become
important as conditions
become crowded. There is
an upper limit to the
number of individuals the
environment can support.
The carrying capacity (K)
• Populations in this kind of
environment show what is
known as logistic growth.
Announcements
• Lectures posted on DISL share site
• Sample test questions will be posted next
week (feel free to remind me!)
• Midterm Thursday July 24
– Runs from 9AM to 12
– Lab notebooks due by 4 PM that afternoon
Population Interactions
• Competition (--) when both species suffer from
an association
• Predation (+-) when one benefits and one suffers
• Commensalism (+0) when one species benefits
from another and it is unaffected
• Amensalism (-0) when one species negatively
affects another and it is unaffected
• Mutualism (++) when both species benefit from
another
Competition
Liebig’s Law of the minimum
• Guiding principle: Liebigs's Law of the
Minimum
• The distribution of a species will be controlled
by that environmental factor for which the
organism has the narrowest range of
adaptability or control.
• The growth of a population of organisms
increases until the supply of a critical
resource becomes limiting
Liebig’s Law of the Minimum:
an example
• “growth of phytoplankton is dependent on the
minimum amount of nutrients/light present”
• whatever is in shortest supply will limit (and
may stop) growth
– nitrate, silica, phosphate, or iron limitation
– light limitation
• if a nutrient (or light) is at low levels, it may be
limiting growth
Limiting Resources: an example
Space is a limiting resource to these
colonies of colonial ascidians
Intraspecific competition:
Background
• Individuals of same species
• Shared resource demands and use of a
limiting resource- food, shelter, mates
• As individuals compete for resources
some deprived- results in reduced fitness,
reflected by lower growth, fecundity and
survivorship
Intraspecific competition: an
example
Patella cochlear
Intraspecific Competition
• Limpets
140
120
Biomass 100
(g/sq. m) or
80
Length
60
(mm)
40
20
0
Biomass
Length
0
200
400
600
800
Density
1000 1200 1400
Intraspecific Competition (Self
Thinning)
• Reduced individual
fitness
• Reduced body size
• Increased mortality
Gause’s Competitive Exclusion
Principle
When two species compete for identical
resources, one will be more successful and
will eventually eliminate the other
Forms of Interspecific
Competition
• Interference (Contest) Competition
– access to a resource is limited or denied by
the dominant species
– examples include antibiotics secreted by
microorganisms, or territorial behavior
• Exploitative (Scramble) Competition
– the direct use of a resource before a
competitor can use it, thus reducing its
availability simply by elimination
Nature of the interspecific
competitive interaction
• Direct competition- Interference competition
– Dominate resource
– Need for space - e.g. Sessile & territorial
– Prevent use by others, “winner takes all”
• Indirect competition- Exploitation competition
– Competitors can not dominate the resource
– Reduces resource availability by using it up
– Resource is “shared” - no outright “winner”
Competition
• Where do you see evidence of
competition in the oceans?
– Overgrowth
– Aggressive behavior
• What’s the result of interspecific
competition?
– Dominance or monopoly by a single species
in a given habitat
– Competitive exclusion
Persistence among competing
species
• Behavioral acclimations- learn to feed when
competitors are not present
• Character displacement-through time two closely
related species tend to be more distinct morphologically
and therefore use different portions of limiting
resources
• Change in habitat utilization
Competition in unusual forms
• Overgrowth competition-one species overgrows
a second species
– Some corals and sponges
• Chemicals are used to defend access to a shared
resource
– Big impacts on settlement
– Allelopathy in some sponges
How to assess competitive effects
• Measure:
– Resource dynamics
– Number of competing individuals
• Disadvantageous impacts on:
– survival rate
– growth rate
– adult weight
– fecundity
How do we measure competition in the field?
1. Observe the patterns of distribution and see if they conform to
predictions of competition theory
white shrimp
brown shrimp
Low
Estuarine salinity gradient
High
One conclusion is that white and brown shrimp are in competition with
each other (competition theory would predict this distribution)
Weak argument: there are multiple explanations for this distribution
Classic study in Experimental Ecology
Connell (1961)
Experiments with intertidal barnacles
Observations
Two species – Chthamalus stellatus and
Balanus balanoides (now Semibalanus
balanoides)
Chthamalus adults in upper zone,
juveniles in both upper and lower zone.
Balanus in lower zone
Observations
Chthamalus juveniles
Chthamalus adults
Balanus
Model
Ha: Competition for space with Balanus prevents adult
Chthalamus from occurring in the lower area
H0:
When present together Balanus has no effect on
Chthalamus
Experiment
• Transplanted stones with Chthamalus to lower level
• Followed settlement of Balanus, removed them from
one half of each stone
• Recorded the fate of individual barnacles
Balanus
Chthamalus
Results
• Chthamalus survival was much greater where Balanus was
excluded.
• Most Chthamalus killed by being overgrown or undercut by
Balanus
Experimental conclusions
• Balanus
– upper limit set by physical environment
– lower limit set by Thais predation
• Chthamalus
– upper limit probably set by physical environment
– lower limit set by interspecific competition
• Asymmetry
competitive release – niche of the competitively-inferior species
expands in the absence of the
competitively-superior species
competitive
release
growth
rate
Chthamalus
alone
Chthamalus with
Balanus
realized
niche
fundamental niche
low
middle
high
Location in intertidal zone
Character displacement
• When
1
0.8
0.6
Use
two species occur in
sympatry natural selection
should favor the evolution of
mechanisms that reduce
competition if resources are
limiting
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
0
0.5
1
Resource
1.5
2
2.5
3
1
0.8
0.6
Use
• This often takes the form of
character displacement, where
the two competing species
diverge in a trait that reduces
the strength of interspecific
competition
0.4
0.2
0
Character displacement:
mud snails
A
S
S
Hydrobia ventrosa
Hydrobia ulvae
A
The importance of intra- and
interspecific competition
• Can have strong negative impact on the
population growth of inferior competitor
• Reduces the geographic distribution of
competing species
• Alter evolutionary trajectories.
The Niche Concept
• Niche - the role of a species in a community,
defined in practice by measuring all possible
resources used and tolerance limits; Elton
(1927) and Hutchinson (1958).
• Niche Breadth - The amount of a resource used
by an organism; this amount may change when
new species are introduced or removed from a
community
The Niche Concept (cont)
• Fundamental: all the environmental factors
relevant to a species survival and reproduction.
– Also called the n-dimensional hypervolume
• Realized: The actual set of factors observed to
determine the density of a species.
Niches and Types of Species
• Generalist species have large niches; tolerate
wide range of environmental variations; these
do better during changing environmental
conditions
• Specialist species have narrow niches; they are
more likely to become endangered; these do
better under consistent environmental
conditions
Diet breadth
consumes only one prey type
narrow
diet
broad diet
specialist
generalist
consumes many prey types
Niche Breadth (width or size)
• Some plants and animals are more
specialized than others, and measures of
niche breadth attempt to measure this
quantitatively
• It is typically measured by observing the
distribution of individual organisms within
a set of resource states
• Information is collected and presented in a
resource matrix
Number of individuals
Niches and Natural Selection
Niche
separation
Specialist species
with a narrow niche
Niche
breadth
Region of
niche overlap
Resource use
Generalist species
with a broad niche
Measuring an Ecological Niche
• To measure the niche, we must determine all
the biotic and abiotic factors that influence a
species’ abundance.
• Such factors can be plotted against species
abundance on graphs. The axes are called niche
axes.
• Niche breadth can be visualized on such a niche
axis graph.
Competition
• Intraspecific competition
– Competition between members of the same species;
all species exhibit this
• Interspecific competition
– Competition between members of different species.
Limiting Factors: Some
additional examples
•
•
•
•
•
•
•
•
•
Classes of abiotic variables
Direct effect
Temperature
Wind
Salinity
Resources (indirect effect)
Nutrients
Shelter
Light
Interspecific Competition
• Between different species
• Both have equal access but one exploits better
• Some individuals control access
Fundamental niche depends on physical (abiotic) conditions.
Realized niche depends on biotic as well as abiotic conditions.
What is the realized niche of each barnacle?
What is the fundamental niche of each?
How can we determine the realized niche of each barnacle?
Where do they grow when allowed to compete?
growth
rate
Balanus
realized
niche
low
Balanus
and
Chthamalus
Chthamalus
realized
niche
middle
high
Location in intertidal zone
Liebig’s Law: Continued
• A limiting factor is a factor that restricts the
fundamental niche of an organism
– temperature is one example- animals and plants are all limited
to some temperature zone of tolerance (tropical, temperate
species)
• Snook (Centropomus undecimalis) northern limit is at 15 oC
isotherm
• Pinfish (Lagodon rhomboides) cannot tolerate low temperature
(12 oC C)
– currents - marine invertebrate larvae are carried by currents
to suitable settlement sites; if carried to an inappropriate area
by the current, the larvae die
– salinity - copepod species differ in salinity tolerances
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