9.2: a/biotic factors affecting communities

What is an abiotic factor?

Non-living factors affecting abundance and distribution of organisms

Examples of abiotic factors

Light intensity, temperature, moisture levels, soil pH content, soil mineral content, wind intensity, wind direction, carbon dioxide levels (for plants), oxygen levels for aquatic animals, and levels of pollutants

How does light intensity affect the abundance and distribution of organisms?

Some plants have evolved for optimum growth in bright sunlight, such as a cactus which lives in the desert

Some plants have evolved for optimum growth on trees in darker conditions, such as orchids many of which live in rainforests

The rate of photosynthesis also determines how fast plants can grow, affecting other species as well, as plants can act as food or shelter for other organisms

How does temperature affect the abundance and distribution of organisms?

Plants and animals have evolved to grow at their optimum temperatures - for humans, it is around 37 degrees Celsius. This is because temperature controls the rate of photosynthesis in plants, and the rate of reactions such as digestion in animals (too high and the enzyme denatures, too low and the enzyme works slowly)

How does moisture levels affect the abundance and distribution of organisms?

Most plants grow best in drier soils (not waterlogged, since their roots are unable to respire so they rot and the plant dies), however some plants live in bogs, so have evolved to grow well in areas of high water content. However, even plants that live in deserts need some moisture content in soil, as all organisms need water to survive

How does soil pH content affect the abundance and distribution of organisms?

Some plants grow best in acidic soil, such as azaleas - these will die if planted in alkaline soils, but clematis grow best in alkaline soil conditions. There are a few species which have adapted to both, such as hydrangea (their flower colour changes in soils of different pHs - pink for alkaline and blue for acidic)

The pH of water also affects aquatic animals - these can have varying levels of tolerance, with frogs having a large tolerance for water pH between 7 and 4, whereas clams can only survive in pHs of 7 to 6

Soil pH affects the rate of decay, therefore how fast minerals are returned to the soil in decomposition

How does soil mineral content affect the abundance and distribution of organisms?

Many plants need soils with high mineral content, as magnesium is required to produce chlorophyll and other minerals are needed too. Carnivorous plants have adapted to catch insects to supplement low levels of minerals in nearby soils

How does wind intensity and direction affect the abundance and distribution of organisms?

Many organisms prefer more sheltered regions, where the seeds can settle and germinate, and the animals on which they depend grow close by. If there is a strong wind, it can negatively impact organisms also.

Wind affects the rate of transpiration and the rate of photosynthesis in plants

How does carbon dioxide levels for plants affect the abundance and distribution of organisms?

Planets need CO2 to survive as it is a reactant in photosynthesis. Areas with more CO2 are likely to have healthy plants growing in them, so farmers often release CO2 within their greenhouses to maximise crop yield. Woodlands often have higher CO2 levels so plants living in open areas have evolved mechanisms to get around having not enough CO2

How does oxygen levels for aquatic animals affect the abundance and distribution of organisms?

Oxygen dissolves into water, sustaining populations of aquatic animals - healthy lakes and rivers have a high oxygen content, while polluted water has lower levels. This means that fewer species can live in polluted water, such as sludge worms, which act as bioindicator species as their presence or absence informs us about the conditions in the habitat

How does levels of pollutants affect the abundance and distribution of organisms?

Lichens are bioindicators of levels of pollution in the air, since they cannot grow if the concentration of sulfur dioxide is high, but they will be abundant in clean, unpolluted air

What is a biotic factor?

Living factors which affect abundance and distribution of organisms

Examples of biotic factors

Availability of food, new predators, new pathogens, and competition

How does availability of food impact the abundance and distribution of organisms?

All animals need food to survive, and more food means that organisms can breed more successfully and the population can increase. Some habitats have rich food supplies naturally, such as rainforests, whereas other habitats have less food available, like in deserts, and therefore have less biodiversity and abundance of organisms

How does new predators impact the abundance and distribution of organisms?

This will reduce the population numbers of most species affected (either eaten or further down in the food chain). This leads to adaptations in species otherwise the species might go extinct. Introducing new predators can cause a rapid decline in numbers of prey, which also reduces the food supply for predators which ate that prey. This can upset the previously balanced ecosystem. There are also some cases where species will increase in number - when the numbers of prey decrease, the animals/plants that would have been eaten by the prey can increase.

How does new pathogens impact the abundance and distribution of organisms?

When a new pathogen arises, the population has no/very little initial resistance to it and faces being wiped out. An example of this is chalara ash dieback, which was introduced to the UK in 2012, and has killed many ash trees since.

However, some pathogens are introduced on purpose, to try to limit a population. An example of this is myxomatosis, which affects rabbits. When it was released, approx 99% of rabbits died, but then they developed immunity and the population has now returned to previous levels

How does competition impact the abundance and distribution of organisms?

If one species is better adapted to the environment than another, it will outcompete the original species until their numbers are insufficient to breed. Grey squirrels were brought over from America, where they have outcompeted red squirrels, as their grey counterparts can store more fat and survive harsher winters. This caused the numbers of red squirrels and the places they live to decrease