C4.1 - Populations and communities

What is a population?

One species in a specific area at a given time. These species must be reproductively isolated

Why is it difficult to count all members of a population?

• not enough time

• Destructive - damages environment / habitat

• motile animals move in and out

• bias

How can population size be estimated?

• Random quadrat sampling to estimate sessile populations (ones that cannot move)

• Capture-Mark-Release-Recapture to estimate motile populations (ones that move) —> then estimated using lincoln index formula

What is carrying capacity?

The maximum population size that an environment can long-term support

What factors affect carrying capacity?

• Food and water

• Shelter

• Space

• Predators

• Climate

• Disease

What are density dependent factors that affect population growth?

These are usually biotic, so living:

• competition for resources

• predators

• disease

Usually, as population growth increases, so do density dependent factors, which in turn decreases the population again (negative feedback)

What are density independent factors that affect population growth?

These are not dependent or influenced by population density. These factors are usually abiotic:

• Climate events and natural disasters

• Habitat destruction

• Seasonal changes

How is competition for resources, predators and disease an example of negative feedback loops?

Competition for resources —> if population density increases, there is more fight for resources. Better adapted animals win the fight, while the others die, decreasing the population

Predators —> if population density increases, there is more prey for predators, causing predator population to increase, so more animals which feed on prey, decreases the population size

Disease —> if population density increases, there is easier spread of diseases, but this causes more animals to die off, decreasing population size

What are factors affecting population growth?(Remember: Growth, not density!)

• Natality → births

• Mortality → deaths

• Immigration → entering

• Emigration → leaving

What is the formula for changes in population size?

Population size = (Natality + Immigration) - (Mortality + Emigration)

What is a sigmoid population growth curve?

A curve showing population growth, with an exponential increase, a transitional phase and then a plateau phase

Analyse the sigmoid population growth curve

What is intraspecific competition?

Competition for resources between members of the same species

Give examples of resources which animals of the same species compete for

• Food and water

• Shelter

• Mates for reproducing

• Social dominance

What is intraspecific cooperation?

When members of a species work together, such as group hunting or foraging, defense against predators and parenting.

What is a community?

Many different populations of species living and interacting with each other in an ecosystem

What are interspecific interactions and give examples?

These are interactions between different species within a community.

• Herbivory - animals eating plants

• Predation - animals killing other animals

• Competition - when different species compete for the same resources

• Mutualism - a close relationship between organisms of different species e.g., when bees pollinate plants (Mutualism benefits both species)

• Parasites - when parasites harm animals

• Pathogens - when pathogens harm animals