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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.

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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 parentalcare. Most species have free-swimming larvae, but some guard or brood eggs, brood young within body cavities, or have live birth like mammals.

5) Inalgae 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) directly developed and 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
  • Why are not all sexual species hermaphroditic?
  • What are the “costs” and “benefits” of sex?
  • Under what circumstances does it make sense for a hermaphrodite to be protandrous? Protogynous?
  • What might be the value, if any, of long-distance dispersal across an ocean?
  • What are the advantages of having planktotrophic larvae capable of settling and metamorphosing upon any substratum? What are the disadvantages?
review questions1
  • Why are species with planktotrophic larvae more common in the tropics than at polar latitudes?
  • What is the value of planktonic feeding larval development?
  • What are the potential sources of mortality for planktonic larvae?
  • What effect does planktotrophic larval dispersal tend to have on the geographic range of a coastal marine invertebrate species?
  • 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
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
liebig s law of the minimum
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
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
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
Intraspecific Competition
  • Intraspecific competition among limpets
intraspecific competition self thinning
Intraspecific Competition (Self Thinning)
  • Reduced individual fitness
  • Reduced body size
  • Increased mortality
interspecific competition gause s competitive exclusion principle
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)


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?



r1N1 K1-N1


species 1




r2N2 K2-N2


species 2







How does interspecific competition affect N?

Lotka-Volterra equations



r1N1 K1-N1 - α12N2


species 1




r2N2 K2-N2- α21N1


species 2


What does it mean ifα12= 1? What ifα12= 0.5?

forms of interspecific competition
Forms of Interspecific Competition
  • Interference Competition
    • access to a resource is limited or denied by the dominant species
    • examples include antibiotics secreted by microorganisms, or territorial behavior
  • Exploitative 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
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”
  • Where do you see evidence of competition in the oceans?
    • As overgrowth by sessile organisms
    • By 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
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
Competition in unusual forms
  • Overgrowth competition-one species overgrows a second species
    • Some corals and sponges do this
  • Chemicals are used to defend access to a shared resource
    • Has big impacts on settlement
    • Ex: allelopathy in some sponges
how to assess competitive effects
How to assess competitive effects
  • Measure:
    • Changes in resource supplies
    • Number of competing individuals
  • Disadvantageous impacts on:
    • survival rate
    • growth rate
    • adult weight
    • fecundity

white shrimp

brown shrimp

Estuarine salinity gradient

How do we measure competition in nature?

1. Observe the patterns of distribution and see if they conform to predictions of competition theory



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 barnacles


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


Chthamalus juveniles

Chthamalus adults





Ha: Competition for space with Balanus prevents adult Chthalamus from occurring in the lower area

Ho: When present together Balanus has no effect on Chthalamus



  • 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





  • Chthamalus survival was much greater where Balanus was
  • excluded.
  • Most Chthamalus killed by being overgrown or undercut by
  • Balanus
experimental conclusions
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

competitive release – niche of the competitively-inferior species

expands in the absence of the

competitively-superior species





Chthamalus with




realized niche

fundamental niche




Location in intertidal zone


Character displacement

  • When two species occur in sympatry natural selection should favor the evolution of mechanisms that reduce competition if resources are limiting
  • This often takes the form of character displacement, where the two competing species diverge in a trait that reduces the strength of interspecific competition





Character displace-ment: mud snails




Hydrobia ventrosa

Hydrobia ulvae


the importance of intra and interspecific competition
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
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
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
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





broad diet

consumes many prey types

niches and natural selection




with a broad niche

Number of individuals


with a narrow niche



Region of

niche overlap

Resource use

Niches and Natural Selection

Competition for resources causes


CS Fig. 4.7