Intraspecific and Interspecific competition Flashcards
Competition
Competition – an interaction between individuals, brought about by a shared requirement for a resource, leading to a reduction in survivorship, growth, and/or reproduction of at least some of the competing individuals concerned
Intraspecific competition
No organism lives in isolation, each is a member of a population composed of individuals of its own species
Each therefore has similar requirements
Their combined demand for a resource may exceed the immediate supply = intraspecific competition
Leads to depletion of a shared resource
Increased energy expenditure + reduced food intake = reduced contribution to the next grasshopper generation
Large seed output = high fecundity
Lower growth rate and lower seed output = reduced fecundity
Intraspecific competition leads to decreased resource intake per individual, consequently decreased growth rate, fecundity or survivorship = survival of the fittest
Individuals respond to the level of a resource
Do not always directly interact
Resource depletion zones
Exploitation – each individual affected by the amount of resource that remains
Interference – direct interaction
One individuals prevents another from exploiting a resource
One-sided competition
Weak competitors make a smaller contribution to the next generation = survival of the fittest
Density dependent mortality and fecundity
Low mortality at low densities (independent)
High densities = intraspecific competition leads to mortality (dependent)
Intensity of intraspecific competition experienced by an individual is not determined by population density as a whole – rather, by the extent to which it is crowded or inhibited by immediate neighbours
Density dependent growth = the law of the constant final yield
When density is sufficiently high and resources become limited, the effects of competition will result in a constant biomass due to a proportional decrease in the size of the individuals
Territoriality – active interference between individuals to defend an exclusive area (territory) against intruders using a recognisable pattern of behaviour = a contest with winners and losers, territory availability varies from year to year, depending on environmental conditions, population regulated by number of territory holders, failure to occupy a territory = failure to reproduce, a cost-benefit strategy, not just winners and losers, not all territories are equally valuable, habitat quality and patch availability
Territory defence must be offset against the benefit of occupation
Sometimes fierce combat
Or more subtle mutual recognition; song or scent
Still require energy to maintain / patrol
Benefits must exceed these costs if territoriality to be favoured by natural selection
Immediate patch size
Interspecific competition
Interspecific competition – one individual or a species deprives another (of a different species) of a vital resource, leading to slower growth, fewer progeny or greater risk of death of the inferior competitor
Resources that can be consumed to depletion = food, water in terrestrial habitats, light and nutrients for plants, space for sessile and mobile organisms
Species are also influenced by factors such as temperature, pH and salinity = conditions
Physical factors affect population growth rates but are not consumed or depleted
Competitions should increase in intensity when resources are scarce
Competitions in plants expected to increase in importance when growing in nutrient-poor soils
Competition has significant effects on a wide range of organisms
Competition, whether direct or indirect, can limit distributions and abundances of competing species
Exploitation competition – species compete indirectly through their mutual effects on the availability of a shared resource e.g diatoms
Interference competition – species compete directly for access to a resource, can also occur in sessile species
Individuals may perform antagonistic actions (e.g when two predators fight over a prey item)
Allelopathy – a form of interference competition in which individuals of one species release toxins that harm another species
In many cases the effects of competition are asymmetrical, and one species is harmed more than the other e.g when one species drives another to extinction
Competition can also occur between distantly related species
Competition can also limit distribution and abundance of species
Competing species are more likely to coexist when they use resources in different ways
If the overall ecological requirements of a species – its ecological niche – are very similar to those of a superior competitor, that competitor may drive it to extinction
The competitive exclusion principle = two species that use a limiting resource in the same way cannot coexist
The outcome of competition can be altered by environmental conditions, species interactions, disturbance and evolution
Environmental conditions can result in a competitive reversal – the species that was the inferior competitor in one habitat becomes the superior competitor in another e.g presence of herbivores can lead to competitive reversals
Disturbances such as fires or storms can kill or damage individuals but create opportunities for others e.g some forest plant species require abundant sunlight and are found only where disturbance has opened the tree canopy, as trees recolonise and create shade, these plants cannot persist in the patch = such species are called fugitive species because they must disperse as conditions change
Competition has the potential to cause evolutionary change, and evolution has the potential to alter the outcome of competition
Natural selection can influence the morphology of competing species and result in character displacement
Natural selection results in the forms of competition species becoming more different over time
In spite of competition, natural communities contain many species sharing scarce resources
Resource partitioning is one explanation for this
Other mechanisms include environmental variation and disturbance
Species may coexist if different species are superior competitors under different environmental conditions
Character displacement
Resource partitioning
Species use a limited resource in different ways
