Factors Affecting Populations (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Biotic & Abiotic Factors
Factors that determine the distribution of a population can be abiotic or biotic
Biotic refers to the living components of an ecosystem
Abiotic refers to non-living, physical factors that may influence organisms
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 population or community
Biotic factors include:
Predation
Herbivory
Parasitism
Mutualism
Disease
Competition
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, which 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, the system 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 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 a 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.
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 (soil and water)
Salinity
Dissolved oxygen
Soil texture
Moisture and precipitation levels
Minerals and nutrients
Wind intensity
Carbon dioxide levels (for plants)
Changes in abiotic factors can affect the survival and reproduction of organisms, and the overall functioning of ecosystems
Abiotic factors can be quantified (measured) to help determine the distribution of species in aquatic or terrestrial ecosystems
Examples of Abiotic Factors
Abiotic Factor | How it Affects Communities |
|---|---|
Temperature | Affects the rate of photosynthesis in plants. It 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. |
Sunlight | Plants require light for photosynthesis. More light leads to an increase in the rate of photosynthesis and an increase in plant growth rates. |
pH (soil and water) | pH levels affect the availability of nutrients in soil and water, influencing plant growth and the survival of aquatic organisms. Certain species are adapted to specific pH ranges. |
Salinity | It affects the health and survival of aquatic organisms, particularly those that are adapted to specific salinity levels. |
Dissolved oxygen | Essential for the survival of aquatic organisms, particularly fish. Low oxygen levels can lead to hypoxia and negatively impact aquatic ecosystems. |
Soil texture | Influences water retention, nutrient availability, and root penetration, affecting plant growth and the distribution of soil-dwelling organisms. |
Moisture and precipitation | Determines the amount of water available to organisms, which can impact their survival, growth, and reproduction. |
Minerals and nutrients | Different species of plants are adapted to different soil mineral contents and nutrient concentrations, influencing plant growth and community composition. |
Wind intensity | Wind speed affects the transpiration rate in plants and can disperse seeds and pollen, influencing plant distribution and reproduction. |
Carbon dioxide levels | CO2 is required for photosynthesis in plants. CO2 concentration affects the rate of photosynthesis and overall plant growth. |
Ecological Niches
A niche describes the particular set of abiotic and biotic conditions and resources to which an organism or population responds and upon which an organism or population depends
Each individual species has its own distinct niche because only one species can occupy a given niche.
If two species try to occupy the same niche, they will compete with each other for the same resources
One of the species will be more successful and out-compete the other species until only one species is left and the other is either forced to occupy a new, slightly different niche or to go extinct from the habitat or ecosystem altogether
For example, the three North American warbler species shown below all occupy the same habitat (spruces and other conifer trees) but occupy slightly different niches as each species feeds at a different height within the trees
This avoids competition between the three species, allowing them to co-exist closely with each other in the same habitat
Examiner Tips and Tricks
It can be difficult to understand the concept of a niche. It can help to think of it as the specific role a species plays within its habitat, including its behaviours, the resources it consumes, and the specific places where it feeds and shelters.
Population Interactions
A population is a group of organisms of the same species living in the same area at the same time
Populations are characterised through:
Size
Density
Distribution
Age structure
Growth rate
Interaction with each other
Ecosystems consist of many populations of numerous different species interacting with each other
Populations interact in ecosystems through:
Herbivory
Predation
Parasitism
Mutualism
Disease
Competition
Resulting in ecological, behavioural and evolutionary consequences
Herbivory
When an organism (known as an herbivore) feeds on a plant
The carrying capacity of herbivore species is affected by the number of plants they feed on
An area with more plant resources will have a higher carrying capacity for herbivore species
This can have negative feedback effects (i.e. the carrying capacity of the herbivore species may decrease if herbivory rates are too high and the plant population decreases too much)
Predation
When one animal eats (preys upon) another
This lowers the carrying capacity of the prey species
This can also have negative feedback effects, lowering the carrying capacity of the predator species due to a decrease in prey numbers
In a stable community, the numbers of predators and prey rise and fall in cycles, limiting the carrying capacity of both predator and prey populations
The graph below demonstrates some of the key patterns of predator-prey cycles:
The number of predators increases as more prey is available
The number of prey then decreases as there are now more predators
The number of predators decreases as there is now less prey available
The number of prey increases as there are now fewer predators
The cycle now repeats
Parasitism
Parasites are organisms that are adapted to live very closely with another species, known as the host species
A parasitic relationship is one in which the parasitic organism benefits from the host organism
The parasite lives either in or on the body of the host species and gains resources (i.e. what it needs in order to survive) from the host, including food, shelter and a suitable location to reproduce (where offspring can feed and grow)
However, the host does not benefit from this relationship and parasites often harm the host in some way
This can lower the host's carrying capacity
An example of a parasitic relationship is fleas being parasites on mammals (e.g. dogs)—the fleas feed on the host's blood but don't provide anything to the host in return and may transmit diseases to the host
Another example is the parasite that causes malaria
This parasite infects red blood cells in humans, causes recurrent episodes of fever and can be fatal in certain instances
The malarial parasite has a life cycle that includes the mosquito as a vector
Mutualism
A mutualistic relationship between species is one in which both species benefit
This increases the carrying capacity of both species in the relationship
An example of a mutualistic relationship is the one that exists between bees and many species of flowering plants
Bees gain nectar (i.e. food to provide them with energy) from flowers
When bees visit flowers, pollen is transferred to their bodies
As bees visit multiple different flowers, they spread the pollen to these flowers, pollinating them
In this way, the flowers gain help in reproducing
Disease
Pathogens (bacteria, viruses, fungi, and protozoa) are organisms that cause diseases
These diseases lower the carrying capacity of the species that the pathogens infect
Changes in the incidence of diseases can cause populations to fluctuate around their carrying capacity
Competition
Competition can be divided into intraspecific competition (competition between members of the same species) and interspecific competition (competition between members of different species)
Intraspecific competition can lower the carrying capacity of a population due to a decrease in food availability caused by high population density
Interspecific competition occurs between species with similar niches, causing a decrease in the carrying capacity of one or both species
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