Biodiversity and Ecosystems for the ESAT

Investigating Distribution and Abundance

Ecological studies require biologists to quantify the number and location of organisms within a specific habitat. Because it is rarely possible to count every individual in a large ecosystem, sampling techniques are used to gather representative data. Two primary methods for this are the use of quadrats and belt transects.

A quadrat is a square frame, typically measuring $0.25 m^2$ or $1 m^2$, used to investigate the abundance of stationary or slow moving organisms like plants or limpets. To ensure the data is representative and to eliminate investigator bias, quadrats must be placed randomly. This is achieved by creating a grid over the study area and using a random number generator to select coordinates for placement.

A belt transect is used to investigate the distribution of organisms across a habitat where environmental conditions change, such as the transition from a forest into a meadow or across a rocky shore. A line or tape measure (the transect) is laid out across the gradient, and quadrats are placed at regular intervals along it. This allows researchers to record how species abundance changes in relation to abiotic factors like light intensity, soil pH, or water depth.

Determining the Number of Organisms in a Given Area

To estimate the total population of a species in a given area, you must follow a systematic calculation based on sample data. The process involves finding the mean density of the species in your samples and scaling it up to the total area of the habitat.

1. Count the number of individuals of the target species in several randomly placed quadrats.
2. Calculate the mean number of organisms per quadrat.
3. Divide the total area of the habitat by the area of one quadrat to determine the total number of possible quadrat positions.
4. Multiply the mean number of organisms per quadrat by the total number of possible positions.

For example, if a student uses a $0.5 m$ by $0.5 m$ quadrat (area = $0.25 m^2$) and finds a mean of $8$ dandelions per sample in a field of $400 m^2$, the calculation is as follows:

ext{Total number of possible quadrats} = rac{400}{0.25} = 1600

$ext{Estimated population} = 8 imes 1600 = 12800$ dandelions.

Human Interactions and Biodiversity

Human activities can have both positive and negative impacts on biodiversity, which is the variety of different species in a habitat. High biodiversity is essential for maintaining stable and resilient ecosystems. Positive interactions include conservation efforts such as reforestation, the creation of national parks to protect habitats, and recycling programmes that reduce the need for raw material extraction.

However, industrial and agricultural expansion often leads to negative interactions. Key examples include fish farming, acid rain, and eutrophication, all of which can significantly reduce biodiversity.

Impacts of Fish Farming, Acid Rain, and Eutrophication

Fish farming (aquaculture) helps meet food demand but poses several ecological risks. High densities of fish in cages produce large amounts of organic waste (faeces) and uneaten food, which can lead to eutrophication in the surrounding water. Furthermore, diseases and parasites like sea lice spread rapidly in crowded conditions and can infect wild fish populations. If farmed fish escape, they may outcompete wild species for resources or interbreed with them, reducing the genetic diversity of natural stocks.

Acid rain results from the combustion of fossil fuels, which releases sulphur dioxide and nitrogen oxides. These gases react with water vapour in the atmosphere to form dilute sulphuric and nitric acids. When this falls as rain, it lowers the pH of aquatic ecosystems, often killing fish and plants. In terrestrial habitats, acid rain damages tree leaves directly and leaches essential mineral ions, such as magnesium, from the soil, leading to nutrient deficiencies that stunt plant growth.

Eutrophication is the enrichment of water with nutrients, often from fertiliser runoff or sewage. This process leads to a loss of biodiversity through the following steps:

1. Nutrients cause an algal bloom at the water surface.
2. The algae block sunlight, causing submerged plants to die as they cannot photosynthesise.
3. Decomposer bacteria feed on the dead plant matter and multiply rapidly.
4. These bacteria use up dissolved oxygen through aerobic respiration.
5. Fish and other aquatic organisms die due to the lack of oxygen, creating a dead zone.