Biology - Genetics, Populations, evolution and ecosystems.

Community:

Multiple populations (of different species) living and interacting in the same area

Ecosystem:

The interaction between a community (the living, biotic part) and the non-living (abiotic) parts of the environment

Ecology:

This is the study of the inter relationships between organisms and their environment. 

Population:

all the members of a single species which live in the same area.

Habitat:

The area in which an organism lives.

Niche:

Within a habitat, different organisms have a particular niche. A niche is the role than an organism plays in the eco-system and includes it’s biotic and abiotic interactions. If two different species occupy the same niches one will be out competed for by the other.

Factor’s which affect population size:

The abundance is defined as the number of individuals of one species in a particular area. It depends on both abiotic and biotic factors.

Abiotic factors that affect population size:

* light intensity
* water availability
* temperature

when these conditions are ideal then organisms can grow and reproduce succesfully.

Biotic Factors that affect population size:

* Interspecific competition
* Intraspecific competition
* Predation

Interspecific competition:

This describes competition for the same resources between different species. This means that resources have to be shared and therefore become less available to both species. As a result less energy will be used for growth and reproduction so population numbers will decrease.

Intraspecific competition:

This describes competition for the same resources between the same species. When resources are abundant the population size increases. This increases intraspecific competition since resources will need to be shared between more organisms, which cause population size to decrease.

Predation:

This affects the abundance of the prey species and vice versa. Whenever the population size of prey increases, this causes and increase in the population size of the predator.

Carrying Capacity:

The carrying capacity is the maximum stable population size that an ecosystem can support. It varies in response to abiotic factors and biotic factors.

Investigating population size:

To save time ecologists take a sample of a habitat and use this to estimate the size of the population in the whole habitat.

Quadrats:

* are used for species which don’t move such as plants
* they are placed randomly to avoid bias - either by using a tape measure to plot out the habitat as a grid and using a random number generator to plot co-ordinates. A large number of quadrats will be randomly placed throughout the habitat to ensure the data collected is reliable.

Systematic sampling:

Sometimes it may be better to place the quadrats non-randomly - this is called **systematic sampling**. For example quadrats can be placed one next to each other along a **transect** (see below). Systematic sampling is needed wherever there is an **environmental gradient** i.e. the abiotic factors change gradually from one end of the sample to the other.

Transects:

To see how the **distribution** of different species changes along a habitat, we use something called a **transect**

The different ways of using transects:

1. **Line transect** - a tape measure is placed along the line and any species which touch the tape measure will be recorded.
2. **Belt transect** - quadrats are placed along the transect one after another, so that each quadrat is touching
3. **Interrupted transect** - quadrats are placed at regular intervals along the transect

Mark- release- capture:

* capture a sample of population using traps
* mark them in a way that won’t cause harm
* release the sample and wait for some time
* set up traps and capture a second sample
* count the number of individuals in your second sample and count how many are marked.
* use the formula to estimate population size

Assumptions made in the mark-release-recapture method:

* The marking technique does not reduce the organism’s chances of **survival** (i.e. due to ink toxicity or increased visibility to predators).
* The mark has not **washed away** or worn off.
* No organisms have **died** between capturing the first and second samples.
* There is **no migration** in or out of the population.
* The first sample have become **fully distributed** in the habitat before the second sample was taken.

Succession:

describes the **change in an ecological community over time,** from a relatively sparse landscape to a stable community of several different plants and animals.

Types of succession:

* primary succession
* secondary succession

Primary succession:

**Primary succession** is when an ecological community develops in the **absence of soil** (i.e. from bare rock). This may happen after a volcanic eruption which results in the formation of new rock or if the sea level lowers and exposes new land.

Secondary succession:

**Secondary succession** is when an ecological community develops from a barren landscape in which **soil is present**.

Stages of succession:

The area is first colonised by the pioneer species such as lichens which are adapted to survive in such harsh conditions. As organisms die, they are decomposed by microorganisms thus adding humus, this in turn leads to formation of soil which makes the environment more suitable for more complex organisms. Over time, the soil becomes richer in minerals thus enabling larger plants such as shrubs to survive. Eventually, a climax community is established which is the final stage of succession, a self-sustaining and stable community of

Conservation:

Conservation of habitats (and the organisms they support) often involves the **management of succession**, preventing the formation of the climax community and preserving certain habitats.

How can succession be controlled?

* Grazing livestock - sheep and cattle grazing can prevent a climax community forming because tree seedlings can’t grow properly
* Controlled fires - after a fire the pioneer species are the first to grow back. By the time other species begin to take over they will be removed by the next controlled fire.

Ways in which conservation takes place:

* **Fishing quotas** – these place limits on the number of fish that are legally allowed to be caught, preventing population sizes from falling excessively.
* **Protecting land** – areas can be made into national parks and nature reserves. This restricts farming and development on the land and protects habitats.
* **Captive breeding** – endangered species can be bred in captivity (e.g. zoos) then returned to the wild.
* **Seedbanks** – these hold a collection of seeds from a variety of different plant species. If plants become extinct, the seeds can be used to reintroduce the plants into their native habitats.