Species interactions: Competition

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Species interactions Competition

• How do species compete for the resources they
  require?
  
• How does competition influence the ability of
  species to coexist?
  

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Came across intraspecific competition last week
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Competition Use or defense of a resource by one
individual that reduces its availability to
another individual of the same (intra-specific)
or other species (interspecific)
- mutually negative interaction
Resource any substance or factor that is
consumed or used by an organism and that supports
increased population growth as its availability
in the environment increases
What are resources?
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Why does competition influence species
coexistence?
Species often share the same resources (many
more species than resources) Some species may
be more efficient or effective at acquiring
resources than others superior competitors
Over time, superior competitors (even if only
marginally so) would be expected to drive
inferior ones to extinction
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How do we study competition?
Tansley (1917) first test of competition in
plants
Observation Closely related plant species often
grow in different habitats or microsites
Hypothesis Each species is adapted to particular
soil conditions such that it is the best
competitor for resources (soil nutrients) in the
microsite where it is most abundant
Prediction Grown singly, each species will grow
best in its native soil type. The outcome of
competition, when both spp are grown together
will depend on soil type a soil x species
interaction
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G. saxatile peat soil specialist
G. sylvestre chalk soil specialist
More on habitat specialization of plant species
later
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Consumer organisms also compete for resources
(food), but also for space Billions of blue bl
istering barnacles
Barnacles settle from plankton on to rock
surfaces
(where space is available)
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Joe Connell (1971) Distribution of two barnacle
species on the inter-tidal zone in Scotland
Observation Chthalamus barnacles live higher on
the intertidal zone than do Balanus barnacles,
yet larvae of the two species settle equally
across the zone
Hypothesis Barnacle species compete for space on
rock surfaces. Balanus has heavier shells and
faster growth and is able to displace Chthalamus
Prediction If allowed to grow in the absence of
interspecific competition then both species
should be able to grow equally well across the
inter-tidal zone. Grown together each species
should outcompete the other in its native
microsite (species x site interaction).
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Competition is important, but more complicated
predation and desiccation tolerance also
constrain Balanus distribution
Many examples of species zonation in coastal
habitats
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Competitive exclusion principle
Gause (1934) Grew two protozoa (Paramecium) in
culture. Each grown separately Grown in mix
ture only one survived
Why did P. caudatum disappear?
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Competitive exclusion principle
Many other experiments with different taxa (fruit
flies, plants etc) in which two species are
grown together show the same result.
Species differ in competitive ability populatio
ns are more effective at acquiring resources will
grow at the expense of a competitor
Two species cannot coexist indefinitely on the
same limiting resource
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Differences in resource requirements among spp
Resource level where no net population growth sp
A
Which is the superior competitor?
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Adding competition to population growth models
dN/dt rN(K-N/K) describes intraspecific
competition
What about the N of other species?
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Vito Volterra - incorporated interspecific
competition into the logistic growth model
2 species competition model Non mechanistic
model (ie no information on how species
compete) Effect on one species on another exp
ressed by effects on K - ie the resource pool
available to the other species
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Competition coefficients
K species 1
Proportion of species 1 resources used by
individual of sp 1
Each individual consumes/uses a portion of the
resource pool (the box K) But a competing speci
es (species 2) uses some or all of the resources
of species 1. Its individuals will therefore also
consume some of species 1s K
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Competition coefficients
K species 1
Proportion of species 1s resources used by
individual of sp 1(purple) or sp 2 (blue)
Individuals of the blue species consume 4 times
as much of the resources available to the purple
species as does purple sp itself.
Competition coefficient ?purple, blue 4
Read ?1,2 as effect of species 2 on species 1
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Adding competition coefficients to the logistic
growth equation Species 1 dN1/dt r1N1((K1-N
1-?12N2)/K1) Species 2 dN2/dt r2N2(( K2-N2-?2
1N1)/K2) Where ?ij competition coefficient fo
r the effect of species j on species i
Overall Proximity of each species i to its carr
ying capacity, K is dependent upon its current
population size, Ni, and the population size of
its competitor, Nj, weighted by the competition
coefficient, ?ij If ?ij ?ji 1, then effect
of individuals of each species are the same, and
species are
competitively equivalent
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If ?ij competitors have a weaker effect than intraspeci
fic competitors (note competitive interactions
need not be symmetrical) What conditions lead t
o stable coexistence for two species?
Same as asking under what conditions will the gr
owth rates (dN/dt) of both species 0 for popul
ation sizes (N0)?
Take case of species 1 first Set differential eq
uation to zero 0(K1-N1- ?12N2) Gives us zero-gro
wth isoclines So Equilibrium N1 K1- ?12N2
If N2 0, then N1 K1 If N1 0, then N2
K1/?12
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Zero growth isoclines - combination of abundances
of N1 and N2 at which growth of one species is
zero

1000/0.6
-ve
ve
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Zero growth isoclines - combination of abundances
of N1 and N2 at which growth of one species is
zero

1000/0.6
-ve
ve
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What is the outcome of competition?
?12 0.6 ?21 0.5
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What is the outcome of competition?
?12 0.5 ?21 1.2
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Generalizing the Lotka-Volterra competition model
Stable coexistence is likely when coefficients
of inter-specific competition ?12 and ?21 are Ie interspecific competition is weaker than
intra-specific competition How is this achiev
ed? By partitioning resources (ie species dont
share all the same resources) Can extend this
to multi-species competition - just add more
pairwise competition coefficients
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Summary
Competition takes place for any resource that
affects population growth (e.g nutrients, water,
space). Closely related species may compete str
ongly because of similar resource requirements.
However, If resource requirements are too similar
one (inferior) competitor will be excluded.
Competition can be modelled as an extension of t
he logistic growth model, requiring estimation of
competition coefficients If competition coeffic
ients of competing species pairs are both then intraspecific competition is stronger than
interspecific competition and species are
predicted to coexist.