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ZOL 3702 LEARNING UNIT 6INTERSPECIFIC COMPETITION
Textbook reference: pg 227 - 265
Learning outcomes:
Explain the nature and characteristics of interspecific competition.
Deduce and explain the logistic equation - the lotke-volterra model - for interspecific competition.
Explain the competitive exclusion principle.
Discuss heterogeneity , colonization, pre-emptive competition, and apparent competition.
Explain the ecological and evolutionary effect of interspecific competition using examples.
Describe niche differences.
Interspecific competition occurs when individuals of 1 species are influenced regarding fecundity,
survivorship, or growth due to resource exploitation or interference by the individuals of another species.
Interspecific competition influences population dynamics of the species, their distribution and evolution.
THE NATURE AND CHARACTERISTICS OF INTERSPECIFIC COMPETITION pg 227-234
Competition between salmonid fish:
- An example where two species coexist but influence each other negatively.
- Salvelinus malma (Dolly Varden charr) and S. leucomaenis (white-spotted charr) are morphologically
similar and closely related fish of the same family.
- The 2 species are found together in streams on islands in Japan, but Dolly Varden are distributed at
higher altitudes (further upstream) than white spotted charr, with a zone of overlap at intermediate
altitudes.
- In streams where one species happens to be absent, the other expands its range, indicating that the
distributions may be maintained by competition. Each species suffers and is therefore excluded from
certain sites , in the presence of the other species.
- Water temperature, which has consequences for fish ecology , increases downstream.
- Experiments in artificial streams showed when either species was tested alone, higher temps led to
increased aggression. But this effect was reversed for Dolly Varden when in the presence of white
spotted charr.
- Reflecting this, at the higher temp, Dolly Varden were suppressed from obtaining favorable foraging
positions when white spotted charr were present, and suffered lower growth rates and a lower
probability of survival.
- The experiments support the idea that Dolly Varden and white spotted charr compete - one species
suffers directly from the presence of the other.
- They coexist in the same River, but on a finer scale their distributions overlap very little.
Competition between barnacles:
- A study of two species of Barnacle in Scotland: Chthamalus stellatus and Balanus balanoides.
- They are often found together on the same Atlantic rocky shores of northwest Europe.
- Adult Chthamalus generally occur in an intertidal zone that is higher up the shore than the adult
Balanus, even though young ones settle in considerable numbers in the Balanus zone.
- The usual cause of mortality in young Chthamalus was not the increased submergence times of the
lower zones, but competition from Balanus in these zones.
- Direct observation confirmed that Balanus smothered, undercut, or crushed Chthamalus, and the
greatest Chthamalus mortality occurred during seasons of most rapid Balanus growth.
- The few Chthamalus individuals that survived one year of Balanus crowding were much smaller than
uncrowded ones, showing , since smaller barnacles produce fewer offspring, but interspecific
competition was also reducing fecundity.
- Therefore, barnacle species compete. They coexist on the same shore but on a finer scale their
distributions overlap very little.
- Balanus outcompetes and excludes Chthamalus from the lower zones, but Chthamalus can survive in
the upper zones where Balanus cannot.
, Competition between bed straws - grass species:
- Galium hercynicum is a species which grows naturally in Great Britain at acidic sites.
- G. pumilum Is confined to more calcareous soils.
- In experiments, it was found that as long as they were both grown alone, both species would thrive on
both the acidic soil and a calcareous soil.
- However, if the species were grown together, only Galium hercynicum grew successfully in the acidic
soil and only G. pumilum grew successfully in the calcareous soil.
- Therefore, it seems that when they grow together the species compete and one species wins while the
other loses to the extent that it is competitively excluded from the site.
- The outcome depends on the habitat in which the competition occurs.
Competition between paramecium species:
- Three species of protozoan paramecium were tested.
- All three species grew all alone, reaching stable carrying capacities in tubes of liquid medium. They
consumed bacteria or yeast cells which lived on regularly replenished oatmeal.
- When P. aurelia and P. caudatum were grown together, P. caudatum always decline to the point of
extinction, leaving P. aurelia as the Victor.
- P. caudatum would not normally have starved to death as quickly as it did , but the experimental
procedure involved the daily removal of 10% of the culture and animals. Therefore, P. aurelia was
successful in competition because near the point where its population size leveled off, it was still
increasing by 10% per day , while P. caudatum was only increasing by 1.5% payday.
- When P. caudatum and P. bursaria were grown together, neither species suffered a decline to the
point of extinction - they coexisted.
- However, their stable densities were much lower than when grown alone, indicating that they were in
competition with one another.
Coexistence among birds: blue tits
- Closely related species of birds often coexist in the same habitat.
- For example, 5 Parus species occur together in English broadleaved Woodlands: the blue tit, great tit,
marsh tit, willow tit and coal tit.
- All have short beaks and hunt for food chiefly on leaves and twigs, but at times on the ground ; All eat
insects throughout the year, and all eat seeds in winter; and all nest in holes, normally in trees.
- The closer we look at ecology of these coexisting species, the more likely we will find ecological
differences.
- Eg. in precisely where within trees they feed, in size of their insect prey and hardness of seeds they eat
- We may be tempted to conclude that these species compete but coexist by eating slightly different
resources. However, a scientific approach to determine the current role of competition requires the
removal of one or more of the competing species and monitoring the responses of those that remain.
- In a study of the orange crowned warbler and Virginia's warbler whose breeding territories overlap in
central Arizona, on a plot where one of the two species had been removed, the remaining warblers
fledged between 78 and 129% more young per nest.
- The improved performance was due to improved access to preferred nest sites and consequent
decreased losses of nestlings to predators.
Competition between diatoms:
- An investigation of two species of freshwater diatom: Asterionella formosa and Synedra ulna.
- Both these algal species require silicate in the construction of their cell walls.
- The investigation was unusual because at the same time as population densities were being
monitored, the impact of the species on their limiting resource (silicate) was being recorded.
- When either species was cultured alone in liquid medium to which resources were continually being
added, it reached a stable carrying capacity while maintaining the silicate at constant low
concentration.
- However, in exploiting this resource, Synedra reduced silicate concentration to a lower level than
Asterionella did.
- Synedra therefore competitively excluded Asterionella from mixed cultures.
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