Transcript Marine Ecology 2010 final lecture 4 Competition

Lecture #3 Review
Reproduction, Migration and Dispersal
1) Sex is nearly universal, despite its considerable costs to organisms.
Its value is the maintenance of genetic diversity, which increases
adaptability to environmental challenges.
2) Sexes may be separate (in gonochoristic species), simultaneous,
or sequential in the same body. The contributions of different
sexes to the next generation determine the value of
hermaphroditism, as well as the relative sizes of males and
females. For example, if the relative advantage of a sequential
hermaphrodite functioning as a large female and small male have
an advantage in maximizing offspring production, this will select
for male-female transitions..
Reproduction, Migration and Dispersal (2)
3) Fertilization success is affected by the mode of sperm
transfer, the volume of gamete production, the distance
between males and females, water turbulence, the timing of
spawning, and behavior. Free spawning has a number of
costs, and planktonic gametes have special problems in
ensuring fertilization, but specialized sperm attractants exist.
4) Marine species differ in parental care. Most species have
free-swimming larvae, but some guard or brood eggs, brood
young within body cavities, or have live birth like mammals.
5) In algae and corals, nonsexual reproduction involves colonial
individuals (modules) that are connected to each other and
exchange nutrients.
Reproduction, Migration and Dispersal (3)
6) Gamete production and larval life must be timed to allow
settlement and promote dispersal, to avoid being swept to
inappropriate habitats, and to counter predation. Egg size decreases
with increasing egg numbers.
7) Marine invertebrate offspring may be (1) brooded or released as
small adults, (2) dispersed to a small degree by means of short-lived,
yolk-dependent lecithotrophic larvae, or (3) dispersed great
distances by longer-lived plankton-feeding planktotrophic larvae.
8) Despite the potential for dispersal, planktonic larvae often settle
near their origin, owing to behavior and cyclonic currents. On the
microscale, larvae use a number of cues to find their settlement site.
9) The geographic range of species with planktonic dispersal is
greater than the range of species without planktonic larvae. Genetic
variation can be used to identify barriers to coastal zone dispersal.
Reproduction, Migration and Dispersal (4)
10)Migratory patterns include: Anadromy - breeding in
freshwater and living in seawater (e.g., salmon, shad, sea
lamprey); catadromy - Adults living in freshwater then
migrate to seawater to spawn (e.g., eels); and
oceanodromy- living totally in seawater but migrating
from adult habitat to spawning habitat )e.g., herring and
cod)
REVIEW QUESTIONS
1.
Why are not all sexual species hermaphroditic?
2.
What are the “costs” and “benefits” of sex?
3.
Under what circumstances does it make sense for a
hermaphrodite to be protandrous? Protogynous?
4.
What might be the value, if any, of long-distance
dispersal across an ocean?
5.
What are the advantages of having planktotrophic larvae
capable of settling and metamorphosing upon any
substratum? What are the disadvantages?
REVIEW QUESTIONS
10. Why are species with planktotrophic larvae more common in the
tropics than at polar latitudes?
11. What is the value of planktonic feeding larval development?
12. What are the potential sources of mortality for planktonic larvae?
13. What effect does planktotrophic larval dispersal tend to have on
the geographic range of a coastal marine invertebrate species?
14. Anemones often occur in clones of large numbers that have
arisen by fission from a founder individual. What experiment
might be performed to determine the benefit of large numbers of
adjacent anemones as opposed to smaller groups or solitary
individuals?
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 are deprived- resulting in reduced
fitness, reflected by lower growth, fecundity
and survivorship
Intraspecific competition: an
example with limpets
Patella cochlear
Intraspecific Competition
• Intrapspecific competition among 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
Interspecific Competition:
Gause’s Competitive Exclusion
Principle
When two species compete for identical,
limited resources, one will be more
successful and will eventually eliminate the
other
A classic interspecific competition experiment
two species of Paramecium
predict the outcome of
interspecific competition
P. aurelia
P. caudata
Gause (1934)
How does interspecific competition affect N?
species 1
species 2
dN1
r1N1 K1-N1
dt =
K1
dN2 = r2N2 K2-N2
dt
K2
α12 – effect of an
indv of species 2 on
an indv of species 1
α21 – effect of an
indv of species 1 on
an indv of species 2
α21 & α21 = competition coefficients
How does interspecific competition affect N?
Lotka-Volterra equations
species 1
species 2
dN1
r1N1 K1-N1 - α12N2
dt =
K1
intraspecific
competition
interspecific
competition
dN2 = r2N2 K2-N2- α21N1
dt
K2
What does it mean if α12 = 1? What if α12 = 0.5?
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 obvious “winner”
Competition
• Where do you see evidence of
competition in the oceans?
– Overgrowth
– Aggressive behavior
• What’s the end result of strong
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 (only partially
successful)
• 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 nature?
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 only 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 snail (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
of character displacement,
where the two competing
species diverge in a trait
that reduces the strength of
interspecific competition
0.8
0.6
Use
• This often takes the form
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 and Competition
in Evolutionary Time
• Niche - the role of a species in a community,
defined in practice by measuring all possible
resources used and tolerance limits
• 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
Niches and Types of Species
• Generalist species have large niches, tolerate
wide range of environmental variations, and 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
Niche Breadth (width or size)
• Some plants and animals are more
specialized than others, and measures of
niche breadth attempt to measure this
quantitatively
Diet breadth
consumes only one prey type
narrow
diet
broad diet
specialist
generalist
consumes many prey types
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
Competition for resources causes
evolution
CS Fig. 4.7
42
Lotka-Volterra competition equations = including
interspecific competition in the logistic model.
Competition coefficients (’s) show per capita
competitive effect of each species on the other.
When ’s are less than 1, stable coexistence is possible.
 implies that niches don’t completely overlap