C4.1 Populations and Communities

Population

A group of organisms of the same species who live in the same area at the same time able to interbreed with each other.

Quadrat sampling

Only suitable for plants or small slow moving animals. It eliminates bias as the quadrat is placed randomly.

Capture, mark, release, recapture (Lincoln index)

Used to estimate the size of an animal population. Can be used to see how population size has changed within a period of time.

Carrying capacity

The maximum size of a population that an environment can support. Beyond this limit the population cannot increase anymore.

Density dependent factors

Factors that affect a population depending on its population size (eg. food, water)

Density independent factors

Factors that affect the size of a population of living things regardless of the density of its population size (eg. wildfire, deforestation)

Negative feedback control/loop

A type of self regulating system where any deviation from a steady state is counteracted to promote stability.

Immigration

Increases to population size from external populations (joining from other areas)

Emigration

Decreases to population size as a result of loss to external populations (leaving the population)

Natality

Increases to population size through reproduction

Mortality

Decreases to population size as a result of death

Population size

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

Community

A group of populations living together in an area and interacting with each other

Intraspecific

Relationships between members of the same species

Interspecific

Relationships between members of different species

Herbivory

The consumption of plant material by animals

Predation

Where one organism (the predator) kills and eats another organism (its prey)

Interspecific competition

Two or more species using the same resource with the amount taken by one species reducing the amount available for the others

Mutualism

When two organisms of different species "work together", each benefiting from the relationship

Parasitism

When a parasite lives on or in another organism, causing harm to the host while the parasite benefits

Pathogenicity

When a pathogen lives inside the host, causing a disease

Top-down control

Species occupying the highest trophic level exert a controlling influence on the species at the next lower level and so forth down the trophic ladder

Bottom-up control

Control of trophic interactions by resources (resources control community)

Allelopathy

Toxins released by plants are given off into the soil, preventing other plants from growing in the same area

Production of antibiotics

Prevents the growth of other species (like bacteria) by interfering with their cell wall, causing them to die

Examples of allelopathy

Sunflowers and walnut trees

Examples of plants which secrete antibiotics

Rosemary and penicillin

Endemic species

Species that only exist in one geographical location. Their population size is naturally regulated by density dependent factors

Invasive species

Species that are introduced accidentally or deliberately into an environment that they are not native to

Chi-squared test

Used to measure the difference between observed and expected data

Null hypothesis (H0)

Two species are distributed independently, there is no relationship between them

Alternate hypothesis (H1)

There is an association between the two species

Expected values

= (row total x column total)/grand total

Chi2 value

= Sum of all ((observed - expected)2)/expected

Degrees of freedom

= (Number of rows - 1) x (Number of columns -1)

Chi squared test: Positive association

Suggests species are found in the same habitat and have an association

Chi squared test: Negative association

Suggests there is no significant association between the species

Root nodules in legumes

Many legumes have a symbiotic relationship with bacteria, which live in nodules attached to plant roots. The bacteria converts nitrogen gas into ammonia which can then be converted into nitrates. Nitrates are used by plants to build essential biological molecules. The bacteria benefits as it gains carbohydrates that are produced by the plant in photosynthesis

Mycorrhizae in orchids

The fungi form long, thin filaments known as hyphae, which interact with the roots of the plants .This increases the surface area of the root system, increasing the amount of water and mineral ions that can be absorbed by plant roots. The fungi receives organic compounds from the plant

Zooxanthellae in hard corals

Polyps have tentacles which contain the zooxanthellae cells. The polyps secrete calcium carbonate which forms the hard structure of the coral skeleton. The polyp's body provides shelter and protection for the algae. The algae carries out photosynthesis and produces carbon compounds which can be used by the polyp