6.6.2 Interaction Between Populations Flashcards
(7 cards)
What factors limit stable population size?
Populations always tend towards a natural, stable size
The stable population size is limited by a number of factors
Interactions between one population and another (of the same species) will limit the size of those populations through intraspecific interactions
Interactions between one species and another can also limit the size of populations through interspecific interactions such as competition and predation
What is intraspecific competition?
Intraspecific competition occurs when individuals from the same species compete for the same resources (“intra” means within)
For example:
When resources are plentiful, the population of grey squirrels increases
As the population increases, however, there are more individuals competing for these resources (e.g. food and shelter)
At some point, the resources become limiting and the population can no longer grow in size - the carrying capacity has been reached
What is interspecific competition?
Competition between different species for the same resource is described as interspecific competition (“inter” means between)
A well-known example of this occurs between the grey squirrel (Sciurus carolinensis) and the red squirrel (Sciurus vulgaris)
The introduction of the grey squirrel into the southern UK caused the native red squirrel to be outcompeted for food and shelter
The grey squirrel also carries a disease, parapoxvirus, that is harmless to itself but can be fatal to the red squirrel
Conservation efforts in northern England and Scotland have slowed the spread of the grey squirrel by
Protecting the red squirrels’ habitats and food
Giving the red squirrel legal protection
Reintroducing the pine marten, a natural predator of the grey squirrel
What is competitive exclusion in the effects of competition?
If two organisms occupy very similar niches, then competition can exist between them for resources
One species may be slightly better adapted to compete than the other, so the other species could starve or not find a habitat
The second species would then move to find a different niche, and possibly evolve over generations to adapt to its new niche
This is called competitive exclusion
What is resource partitioning for the effects of competition?
A particular resource eg. a fruit tree is divided up (partitioned) to satisfy the needs of different feeders
The fruit may be grazed by one species, the leaves by another
The two species are not competing directly for the fruit tree but coexisting on different parts of it
What is character displacement in the effects of competition?
Closely related species that inhabit the same geographical region and occupy similar niches differentiate in order to minimise niche overlap and avoid competitive exclusion
An example is beak size and shape in Galapagos finches, as observed by Charles Darwin
Each species differs in beak dimensions
Food, in the form of seeds, comes in different shapes and sizes from a variety of sources
The finches with the stronger beaks eat large, tough seeds, while the finches with smaller beaks eat the smaller, softer seeds
Natural selection therefore favours those species that compete less with other species
What is predation?
Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers who are themselves eaten by tertiary consumers
Consumers that kill and eat other animals are known as predators, and those eaten are known as prey
In a stable community, the numbers of predators and prey rise and fall in cycles, limiting the population sizes of both predators and prey
The graph below demonstrates some of the key patterns of predator-prey cycles:
Initially, the number of predators increases as there is more prey available
The number of prey then decreases as there are now more predators
Then the number of predators decreases as there is now less prey available
As a result, the number of prey increases as there are now fewer predators
The cycle now repeats
