Competition and coexistence
Muzvondiwa J.V.
Intoduction
• Competition in nature takes a number of
forms. Intraspecific competition is
competition within the same species, and
interspecific competition is competition
between different species.
• Intraspecific competition is common in nature
and in plants is often described by a 3/2
thinning law.
Introduction
• Interspecific competition is also common, and
laboratory experiments have shown how
outcomes can be changed by variations in the
physical environment or by the presence of
other species, such as parasites or predators,
that affect one species, but not the other.
• Competition exists among about 55% to 75%
of species and the most common mechanism,
is over use of the same resource.
Introduction
• Mathematical models called Lotka–Volterra
models predict four outcomes of competition:
One species is eliminated, the other is
eliminated, both species coexist, or either
species is eliminated, depending on starting
conditions.
• Competing species can sometimes coexist if
they partition resources between them.
Species Interactions
Name of Interaction
Species 1
Species 2
Mutualism
+
+
Commensalism
+
0
Herbivory
+
-
Predation
+
-
Parasitism
-
-
Allelopathy
-
0
Competition
-
-
+ = positive effect; 0 = no effect; - = deleterious effect
Summary of biotic interactions
• Herbivory, predation, parasitism
– Positive for one population
– Negative for the other population
• Amensalism
– One-sided competition
– One species has a negative effect on another, but the reverse
is not true.
• Neutralism
– Coexistence of noninteracting species
– Probably rare
Summary of biotic interactions
• Mutualism and commensalism
–
–
–
–
–
Less common
Symbiotic relationships
Species are intimately associated with one another
Both species may NOT benefit from relationship
Not harmful, as is the case with parasitism
• Competition
– Negative effect for both species
Types of competition
• Interspecific
– Competition between two or more different
species
• Intraspecific
– Competition between members of the same
species
Characterizing competition
• Resource competition (scramble competition)
Occurs when a number of organisms utlize common
resources that are in short supply
Organisms compete for a limiting resource
Plants (light, nutrients, water and pollinators)
Animals (water, food, mates, space e.g space for
nesting in birds and sites that are safe from predators)
Characterizing competition
• Interference competition (contest)
Occurs when the organisms seeking a resource harm
one another in the process even if resource is not in
short supply
Individuals harm one another directly by physical force
Intraspecific Competition
• Quantifying competition in plants vs. animals
•
•
•
•
For plants, expressed as change in biomass
For animals, expressed as change in numbers
Plants cannot escape competition
Animals can move away from competition
• Yoda (1963)
– Quantify competition between plants
– Yoda’s Law or self-thinning rule; 3/2 power rule
– Describes the increase in biomass of individual plants as the
number of plant competitors decrease.
– Log w = -3/2 (log N) + log c
» w = mean plant weight
» N = plant density
» c = constant
– w = cN3/2
Interspecific Competition
• Park (1948) explored interspecies competition
using two floor beetle
• Variable studied in this early work were,
– Constant
• Climate
• Initial density
• Food
– Varied
• Volume of flour
• Presence or absence of Adelina, a sporozoan parasite
Park’s experiment, 1948
Park’s findings
• Space did not affect greatly the pattern of
growth or the outcome of competition
between these two species
• Cultivated together, T. confusum won over T.
castaneum (66 of 74 times) regardless of the
amount of space in the culture.
• Cultures were infested with a parasite Adelina
• T. confusum won 89% of the time
• Without the parasite, no clear winner
Findings
Findings
• Adelina was found to have an important effect
on T. castaneum but not on T. confusum as
indicated in Fig 13.11 above
• Mean densities of larvae, pupae and adults
(number per gram of flour)
Density (no. /g flour)
T. confusum
T. castaneum
With Adelina
19.2
13.3
Withoput Adelina
18.9
33.5
Experiment
• Park varied the climate
• Constant (Space, initial density, food, absence
of Adelina)
• Six combinations of temperature and humidity
were investigated and the following
generalised results were obtained
Experiment varying climate
Temp
Relative Climate
Humidity
(%)
Single species
numbers
Mixed species (% wins)
34
70
Hot-moist
Confusum=castaneum 0
100
34
30
Hot-dry
Confusum>castaneum 90
10
29
70
Temperate –
moist
Confusum<castaneum 14
86
29
30
Temperatedry
Confusum>castaneum 87
13
24
70
Cold-moist
Confusum<castaneum 71
29
24
30
Cold-dry
Confusum>castaneum 100
0
T.
confusum
T.
castaneum
Findings-Microclimate effects
• T. confusum did better in dry environments
• T. castaneum did better in moist environments
What is the mechanisms of this competition?
Adult and larval Tribolium cannibalize their own eggs and pupae.
This cannibalistic predation is a complex process and is
responsible for most of the mortality of these flour beetles
(park et al., 1965) T. Castaneum is more calibalistic than T.
Confusum in general. The competition between these beetles
is not over food but a special form of mutual predation
Experiment varying genetics of
populations
• Parks also varied the genetics of population
• Constant (space, initial density,food, climate,
absence of adelina)
• Used 8 genetic strains as indicated
Experiment
Findings
Findings
• Strains varied greatly in their competitive
ability
• Strain T. castaneum determined the outcome
• E.g strin cl of T. castaneum always wins and
strain of clII always loses
Conclusion
• Outcome of competition is affected both by
extrinsic agencies like weather and parasites
and by extrinsic properties like the genetic
composition of the competing populations