Competition
• The natural theologians were fascinated by the balance of nature
• What cosmic forces kept the delicate balance between organisms and their environment?
• Darwin also worried about this - why aren’t we up to our neck in house flies??
• Populations of most organisms have an incredible potential for growth
• Consider the lowly house fly
• House flies have seven generations per year, 120 flies per generation - what would happen if
they all lived?
• Forget nasty little flies, what about cute little robins?
• A female robin lays four eggs per clutch
• She can lay two clutches in one year
• What if all 8 baby robins survived?
• End of one year = 64 robins
• End of ten years = 24,414,060 robins
• At the end of 30 years, the entire planet Earth would be buried under a blanket of robins 4.5
miles thick !!!
• Needless to say, most organisms don’t live long enough to reproduce
• There are many limiting factors in nature that regulate the growth of populations
• Limiting factors can act from outside the population - extrinsic limiting factors - can be
physical (abiotic) factors:
> Sunlight
> Water
> Nutrients
> Food
> Resources
• Limiting factors can act from outside the population - extrinsic limiting factors - can be biotic
factors:
> Competition
> Predation
> Symbiosis
• Limiting factors can also act from inside the population - intrinsic limiting factors
> Changes in reproductive physiology
> Changes in behavior
• Limiting factors can act in proportion to how dense the population has become - density-
dependent limiting factors
• Limiting factors can have the same effect regardless of how dense the population has become
(forest fires, tidal waves) - density-independent limiting factors
• Most populations are regulated by a combination of limiting factors - they never reach their
full reproductive potential
• Charles Elton proposed that equilibrium in nature resulted from a balance of interactive
forces, especially competition and predation
• Modern ecologists are fascinated by non-equilibrium theory, which stresses the importance of
disturbance
• Disturbances are forces that disrupt a natural ecosystem
> Abiotic - forest fires, floods
> Biotic - diseases, parasites
• Elton’s focus on predation and competition was ahead of his time
• Both predation and competition are important forces in regulating the growth of natural
populations
• Competition occurs when two or more organisms use the same resource in a way that affects
the birth rate or death rate of the competitors
• Which is true competition, limited tickets for a Peter Paul and Mary reunion, or a Black
Sabbath reunion concert??
• The intensity of the competition will depend to a large degree on the density of the population
in the parking lot outside the arena
• Competition is an extrinsic, density-dependent limiting factor
• Competition can be intraspecific, between members of the same species
• Competition can be interspecific, between members of different species
• Which type of competition will be the most intense??
• Intraspecific competition!
• Why??
• Intraspecific competition is most intense, because your needs exactly match the needs of other
members of your species
• Niche is the ecological role that a species plays in a biological community, the sum total of its
needs and the parameters within which it can survive (niche = job, habitat = address)
• Individuals of different species will occupy a different niche
• The intensity of the competition between them depends on the extent to which their niches
overlap
• Competition limits the ability of either species to realize its full potential, its fundamental
niche
• Competition forces organisms into a much narrower niche - the realized niche
• Consider the flour beetle, Tribolium
• Larvae are found in grain storage areas - look like the closely related mealworms sold in pet
stores
• Thomas Park raised flour beetles under several different sets of conditions
• Set up six types of environments in six tubes full of wheat flour
> Hot / temperate /cold
> Dry / moist
• Added Tribolium castaneum to one set of vials, Tribolium confusum to another
• Put equal numbers of both in another set
• When grown alone, each species could thrive under any set of conditions
• When grown together, one species proved a superior competitor, depending on conditions
• When grown alone, each species showed the same fundamental niche
• When grown together, competition forced them into a realized niche
• Consider two species of Paramecium, grown in a jar
• Either P. aurelia or P. caudatum can do equally well under the same conditions
• But P. aurelia is a superior competitor, and when raised together it always eliminates P.
caudatum
• Two species cannot coexist if they share the same limiting resource
• Competitive exclusion occurs when one species is a better competitor than another, and forces
it into local extinction
• Scotland - intertidal zone has two competing species of barnacles - Semibalanus and
Cthamalus
• When the ecologist Connell removed Semibalanus from the rock, Cthamalus would fill the
entire space
• Both have overlapping fundamental niche
• Semibalanus is the better competitor - overgrows Cthamalus and smothers it, or grows under
it and pries it off the rock!
• But they coexist on the same rock
• Cthamalus can tolerate drier conditions that Semibalanus, so holds on in high tide zone
• We have always been fascinated by competition, the struggle for existence
• But most species manage to coexist peacefully in nature
• How do species avoid or minimize competition with one another?
• There are several ways in which species can coexist with one another
> Live in different geographic areas - don’t meet, don’t compete - North American Bison
and Australian Kangaroo are both grazing herbivores on grassy plains, but never encounter
one another in the wild
• There are several ways in which species can coexist with one another
> Live in same geographic area, but in a different habitat
– Like the deer mouse Peromyscus in the forest and Peromyscus in the meadows
– Like grazing mammals on a mountain peak
> Live in same geographic area, and same habitat, but use it at a different time of day
– Like the night herons and other wading birds
– Like birds (diurnal) and bats (nocturnal)
> Live in same geographic area, same habitat, use it at same time of day, but exploit the
resource in a different way - resource partitioning
– Like mixed species foraging flocks
• Mixed species foraging flocks have higher rates of food capture than solitary birds
> Specialize in different feeding zones
> Some specialize in tops or bottoms of leaves
> Some glean insects from cracks in the trunk
> Some work along the main branches
• There are several ways in which species can coexist with one another
> Modify your physical shape through natural selection - character displacement
– Geospiza fortis alone, beak size ~8-12 mm, same as G. fulginosa
– G. fortis beak size is 11-15 mm on Santa Cruz, where it competes with G. fulginosa
• In 1982 a breeding population of G. magnisrostris arrived on Daphne, began competing with
G. fortis for large size seeds
• Grant 2006 (Science) reports 2003/2004 droughts hit G. fortis with bigger beaks, population
now has smaller average beak size
• Coexistence and competition are both important factors in the regulation of natural
populations
• Competition is inevitable if two organisms need the same thing to survive or reproduce
• Competition can take many forms
> Scramble competition - exploit resources by using them up (exploitative)
> Contest competition - engage in a face to face contest over limited resources (interference)
• Scramble competition - exploit resources by using them up (exploitative)
• Everyone gets at least some of the available resources - examples??\
• Mardi Gras !! - Zulu parade
• Contest competition - engage in a face to face contest over limited resources (interference)
• Winner takes all - one organism gets all the resource, the other competitor gets none - Rex
parade!!
• Contest competition is typical of animals that defend a territory
• Territory = any area that an animal defends against other animals
• Usually defend territory against members of your own species - except mockingbirds!!
• Territories vary greatly in size - depends on the needs of the species
• Most often defend feeding territories, breeding territories, nesting territories
• Sometimes defend courtship territories (leks)
• Limited number of high quality territories
• Only territory holders will mate
• Bachelor males will become “floaters”
• High stakes competition, but injuries and deaths are relatively rare
• Advertise your ability to defend your territory with recognized signals - body postures,
vocalizations, plumage displays
• Territory defense can be very costly - Why?
• Stereotyped, ritualized behaviors are used to defend territories - pushing, shoving, hollering,
butting heads…
• Means of minimizing the physical effects of intraspecific competition
• Humans like to think they are above the fray, but we are one on the most territorial animals on
Earth!!
• Phallus stones used to mark property in jolly old England - analogous to primate displays of
erect penis!