Syllabus Edition
First teaching 2015
Last exams 2025
Abiotic & Biotic Factors
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Abiotic & Biotic Factors
Abiotic Factors
The non-living, physical factors that influence ecosystems and the communities of organisms within them are termed abiotic factors
These include factors such as:
Temperature
Sunlight
pH
Salinity
Precipitation
Changes in abiotic factors can affect the survival and reproduction of organisms, and the overall functioning of ecosystems
Examples of Abiotic Factors
Abiotic Factor | How it Affects Communities |
Light intensity | Light is needed by plants for photosynthesis. More light leads to an increase in the rate of photosynthesis and an increase in plant growth rates. |
Temperature | Affects the rate of photosynthesis in plants. Also affects the rate of metabolism, growth and reproduction of organisms. Certain species have adapted to specific temperature ranges and cannot survive outside of those ranges. |
Moisture and precipitation levels | Determines the amount of water available to organisms, which can impact their survival, growth and reproduction. Some species are adapted to areas with high precipitation, while others are adapted to arid environments. |
Soil pH and mineral content | Different species of plants are adapted to different soil pH levels and nutrient concentration levels. |
Wind intensity | Wind speed affects the transpiration rate in plants. Transpiration affects the rate of photosynthesis as it ensures water and mineral ions are transported to the leaves. |
Carbon dioxide levels for plants | CO2 is required for photosynthesis in plants. CO2 concentration affects the rate of photosynthesis. |
Water chemistry | Affects the health and survival of aquatic organisms, particularly those that are sensitive to changes in pH, salinity, and dissolved oxygen levels. Some aquatic animals, such as fish, can only survive in water with high oxygen concentrations. |
Biotic Factors
The living, biological factors that influence ecosystems and the communities of organisms within them are termed biotic factors
In other words, biotic factors are the interactions between the organisms within a community
These interactions include:
Competition
Predation
Herbivory
Parasitism
Mutualism
Disease
These biotic interactions mean that each species in an ecosystem has an influence on the population dynamics of the other species
Examples of Biotic Factors
Biotic Factor | How it Affects Communities | Example |
Availability of food | More food means organisms have a higher chance of surviving and reproducing. This means their populations can increase. | Rainforest ecosystems have a rich food supply and this allows many species to live there. Deserts have a poor food supply and this allows fewer species to live there. |
New predators | In balanced ecosystems, predators catch enough prey to survive but not so many that they wipe out the prey population. If a new predator is introduced to the ecosystem, it may become unbalanced. | Red foxes were introduced for recreational hunting in Australia in the 1800s but have since caused the decline of many native species that they feed on, such as small mammals and birds. This has also reduced the food supply for native predators. |
New pathogens | If a new pathogen enters an ecosystem, the populations living there will have no immunity or resistance to it and the population may decline or be wiped out. | Avian flu can cause population declines in wild bird species - an outbreak of the H5N1 virus in the bar-headed goose (Anser indicus) in Qinghai Lake, China in 2005 caused the deaths of over 6,000 birds in the area, representing a significant proportion of the bar-headed goose population. |
Competition | If two species compete for the same resource(s) and one is better adapted to take advantage of these resources, then that species will outcompete the other. This may continue until there are too few members of the less well-adapted species to breed successfully. | North American grey squirrels were introduced to the UK in the 1800s and have since caused the decline in our native red squirrel population. Grey squirrels have outcompeted red squirrels for resources such as food and nest-sites. They also carry a virus (a new pathogen) that red squirrels have no resistance to. |
Worked Example
A study recorded the number of red and grey squirrels in a particular woodland habitat for 20 years. Grey squirrels were introduced to the habitat in year 5 of the study. What conclusions can you draw from the graph about the effect of introducing grey squirrels to a habitat that is occupied by red squirrels? Explain why this might have occurred.
Answer
As the number of grey squirrels increases, the number of red squirrels decreases. This might have occurred because the two squirrel species are competing for one or more of the same resources. Grey squirrels are better adapted to use these resources and have outcompeted the red squirrels until eventually there are too few red squirrels left to breed successfully. At this point there are no red squirrels left in the habitat - they have become locally extinct.
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