Interspecific Competition in theory and reality

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Interspecific Competition in theory and reality

• TBH Chapter 6
• Attempt Q3,5,7,9

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Graveyard Lab
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A struggle for existence inevitable follows from
the high rate at which all organic beings tend to
increase Chapter III
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Competition is the engine of Natural Selection
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Competition in action Example 1
When grown alone both diatoms diminish the
resource silica ?
?
?
When grown together one diatom drives its
competitor extinct ?
?
?
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Competition in action Example 2
Larvae settle in Balanus zone but are outcompeted
Zonation patterns of barnacles determined by
conditions or competition?
Desiccation
?
?
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General Observations on Competition

• Species that overlap in geographical range may
  divide space on a finer scale
• Species are excluded from sites where conditions
  are suitable for them.
• There is a difference between fundamental niche
  and realized niche
• Competition can be very one sided
• Competition is very difficult to demonstrate

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Competition in action Example 3
Alone both bumblebees can exploit both flowers
Together they are forced to specialize
What is the fundamental niche? What is the
realized niche?
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Coexistence can be attained when 2 species
specialize on separate resources
At low levels of silica Cylotella excludes
Asterionella?
?
?
At low levels of phosphate Asterionella
exludesCylotella ?
Notice all the examples so far have been
competition between close relatives
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Competition between 2 unrelated taxa, ants and
rodents, resulted in a division of resources
?
?
In the presence of the competitor the populations
diet was restricted and the abundances were lower
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Competitive Exclusion(Gauses Principle)

• If 2 species coexist in a stable environment,
  they do so as a result of niche differentiation
• If there is no differentiation then one species
  will eliminate the other
• N.B If species are coexisting with different
  niches it does not mean that this is the
  principal in operation

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Environmental Heterogeneity

• Exclusion of one species by another will occur if
  competition has run its course (referred to as
  Equilibrium)
• We have already seen from the Holling lazy eight
  succession model that we might anticipate
  environments to be dynamic and heterogeneous.
  That is, habitats can be patchy

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Because of its higher fecundity and earlier
maturity Physa the poorer competitor to Lymnaea,
persists by reproducing early in temporary ponds
Physa surviving in ephemeral ponds
Time
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Evolutionary Impacts of Interspecific
Competition Evolution in Action
We might expect species to avoid competition,
however each time we see niche differentiation we
must confirm that it is competition driving the
differences
Benthic
Limnetic
Evidence from sticklebacks in freshwater lakes of
character displacement
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Darwins Finches in Galapagos
Where G. fortis and G. fulginosa co-occur there
is a shift in beak size
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Interspecific Competition How may it structure
biological communities?

• Community an association of interacting species
  living in a particular area also often defined
  as all the organisms living in a particular area
• Community structure attributes of a community
  such as the number of species or the distribution
  of individuals among species within a community

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Interspecific Competition How may it structure
biological communities?
Index of Seed Size
Body Size
Species of ant specialized of on different seed
sizes
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Community structure e.g. 1
Coexistence at a particular body size is
facilitated by alternative foraging techniques
G - Group foraging
I - Individual foraging
The guild as a whole demonstrates niche
complementarity
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Community structure e.g. 2
Niche Complementarity among nectar feeding
bumblebees
4 groups of bumblebees
Long Proboscis
?
Medium Proboscis
?
Short Proboscis
?
Robber
?
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Interspecific competition plays a role in
structuring communities by acting within guilds
(groups of organisms exploiting the same class
of resource in a similar fashion)
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Community structure e.g. 3
Patterns of co-occurrence on very local scales
examined for plants living in very similar soil
Where the plants are unlikely to be found
together more often than not they have similar
niches
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Plants have very similar niche requirements and
there is less scope for resource partitioning
experiments can elucidate the effects of
competition for light and nutrients
Alone
Roots
Shoots
Both
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Surveys of competition show that it is prevalent,
but its extent may be exaggerated by a failure to
publish results documenting its unimportance