(5) Examples of biotic factors (living factors) - changes in community populations

Any changes in a biotic factor can also change population size up or down and also affect geographical distribution of species.

 

Competition between species for resources

In a given environment, all organisms are competing with each other for the same resources they need to survive and reproduce.

Plants need mineral ion nutrients from the soil, light and carbon dioxide for photosynthesis, space to grow and water and nutrients from the soil.

Weeds compete with crop plants for light, nutrients and water.

Unfortunately, this leads to the use of weedkillers to reduce the crop's competitors and allow the crop to have a greater access to the resources.

Animals need food (plant or animal), mates for offspring, water and territory - the space for their habit.

One species may out compete for food or territory leading to the rise of one species population and the decline of another to the point where breeding is low that the population is unsustainable.

e.g. In most areas grey squirrels have outcompeted native red squirrels in the UK so their population falls and the population of grey squirrels rises.

In some cases, a species outcompeted might move to another area where there is less competition.

The availability of food is one of the most important biotic factors that supports a stable population of a species.

The more abundant the food supply, the more organisms can survive and reproduce to maintain or increase their population.

The food ranges from plants for animals, animal prey for animal predators higher up the food chain.

 

The abiotic factor of the weather can considerably affect food availability.

Milder winter weather allows populations of birds to keep at higher levels - less dying form low temperatures.

Poor weather might mean less food is available for the birds to survive and reproduce, increasing competition and decreasing population numbers.

 

Predators and prey - predation biotic factors

Predation: Wild animals like lions, cheetahs and hyenas on the African savanna are competing for the same prey (food resources e.g. gazelle, zebra), same grass land territory and drinking water.

The relative populations of predators and their prey will affect each other.

If a predator eats all their chosen prey, it will die out for lack of food or move to another habitat.

In stable communities the numbers of predators and preys rises and falls but in a balanced way.

If a prey is abundant at one point in time, then the population of a predator can increase because there is more food available to eat.

However, in doing so, the population of the prey will decline soon after.

This leads to complex cycles in the rise and fall of the populations of the species involved.

A classic example is the relationship between rabbit (the prey) and fox (predator) populations in a particular community.

The population of a species (e.g. foxes) is often limited by the amount of food available (e.g. rabbits).

If the population of prey increases e.g. lots of rich green grass for the rabbits, their population can increase - the crests in the upper graph line.

This means more food for the foxes!, so their population increases too - upper crests on the lower graph line. There is a time lag to allow for reproduction!

BUT, the extra foxes eat more rabbits and so the number of rabbits decreases - troughs in the upper graph line.

The decline in rabbits once again limits the food for the foxes, so their population begins to decrease - troughs on the lower graph line.

This rise and fall in prey-predator populations is very typical and produces these 'wave-like' graphs and is a classic case of interdependence.

The two interdependent populations forming a predator-prey cycle will always be 'out of phase' with each other because there will be a time lag (dotted line) as one change in population gradually affects the other population - one population cannot respond immediately to the change in the other.

 

New predators - new biotic factors

If a new predator arrives in an ecosystem, it can displace another species that cannot compete with the arrival.

There might be similar species competing for the same food resource and if the 'invasion' is rapid the native population of the original organism might not be able to adapt in time before its population declines.

Competition for food is a biotic factor - see prey and predators in the previous section.

 

New pathogens - new biotic factors

If a new pathogen enters an ecosystem, organisms may be very susceptible to attack.

An animals immune system might not be able to cope leading to disease and death.

This happened to humans when explorers went to places like South America, taking with them 'western diseases' that led to the deaths of many native people.

The gene pool of an organism might not produce enough mutations fast enough to evolve species that can survive the new pathogen.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key ideas about biotic factors

Biotic Factors Affecting Ecosystem Population Changes

Biotic factors are living components of an ecosystem that influence population dynamics.

These factors interact with organisms and impact their survival, reproduction, and distribution.

Understanding biotic factors is essential in analyzing how populations change within a community.

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1. Examples of Biotic Factors Affecting Ecosystem Populations

A. Competition (Intraspecific & Interspecific Competition)

Competition occurs when organisms fight for limited resources such as food, water, territory, light, or mates.

It can happen between individuals of the same species (intraspecific) or different species (interspecific).

• Intraspecific Competition:

  • Individuals of the same species compete for identical resources.

  • Can lead to reduced reproduction rates and survival if resources are scarce.

  • Example: Trees in a forest competing for sunlight; weaker trees may die.

• Interspecific Competition:

  • Different species compete for overlapping resources.

  • Often results in one species dominating while others decline in population size.

  • Example: Red squirrels vs. grey squirrels in the UK - grey squirrels outcompete red squirrels due to better adaptability.

B. Predation (Predator-Prey Relationships)

Predators control prey populations by hunting and consuming them.

This relationship maintains balance in ecosystems and prevents overpopulation of prey species.

• When predator numbers increase, prey populations decrease due to higher predation pressure.

• If prey numbers decline too much, predators may struggle to find food, leading to their population decreasing as well.

• Example: Wolves preying on deer - fewer deer lead to a decline in wolf numbers.

C. Pathogens (Disease & Infection)

Pathogens - bacteria, viruses, fungi, and parasites - cause diseases that affect population sizes.

• Infectious diseases reduce populations by lowering survival rates and reproductive success.

• Overcrowding increases disease transmission rates, making populations more vulnerable.

• Example: Myxomatosis in rabbit populations - viral disease drastically reduces numbers.

D. Availability of Food Sources

The availability of food directly impacts population sizes.

• More food → population increases due to higher survival and reproduction.

• Less food → population decreases due to starvation or migration.

• Example: Seasonal changes affecting bird migration - birds move when food becomes scarce in colder months.

E. Symbiotic Relationships (Mutualism, Parasitism, Commensalism)

Species interact in various ways beyond competition and predation:

1. Mutualism – Both species benefit (e.g., bees pollinating flowers).

2. Parasitism – One species benefits, while the other suffers (e.g., tapeworms in human intestines).

3. Commensalism – One benefits, but the other is unaffected (e.g., barnacles on whales).

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2. Importance of Understanding Biotic Factors in Population Changes

• Ecosystem Stability: Prevents uncontrolled population growth and maintains biodiversity.

• Conservation Efforts: Helps manage endangered species by controlling factors affecting their survival.

• Agricultural Impact: Knowing how diseases and competition affect crops and livestock.

• Food Web Dynamics: Understanding predator-prey cycles aids in ecological studies.

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Understanding biotic factors gives insight into how populations shift within an ecosystem and helps predict environmental changes.

These interactions form the foundation of ecological balance.

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Summary of learning objectives and key words or phrases

Be able to describe and understand changes in biotic factors will cause changes in community populations, affect competition between species for resources, predators prey predation competition and effects of new pathogens or predators on the population of a community.

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