Ecology: Distribution and Abundance

The factors that determine distribution and abundance can be divided into physical factors, chemical factors, biotic factors and dynamic factors.

Distribution refers to the region where an organism is found. Abundance is the number of individuals in the area. Both distribution and abundance can change seasonally or yearly. Abundance is usually described as density - the number on individuals in a certain area - as this gives an idea of crowding. 

The factors that determine distribution and abundance can be divided into physical factors, chemical factors, biotic factors and dynamic factors.

Physical factors:

Include temperature, soil composition, rainfall patterns, altitude, pressure, soil composition, light penetration etc. 

Most organisms prefer to live in environments with temperatures between 0 degrees to 45 degrees. Some animals can live in certain temperatures others cannot, this is what it means by temperature as a physical factor affecting abundance and distribution. 

Rainfall patterns include not only the amount of rain, but how it is spread over the year. 

Soil composition depends on the rock type, rainfall, temperature, and length of formation of the soil -- many plant species prefer specific soil types i.e: desert plants which are prone to lesser rainfall conditions and higher temperatures (desert plants cannot live in winter conditions).

Light penetration influences distribution and abundance as plants require light for photosynthesis. 

Biotic factors:

Include food source, predators, competitors, parasites, disease-causing pathogens, members of the same species and the impact of human activities.

If an organism is a consumer, it depends on other species as a food source, thus many organisms are only found where their food source is found.

The number of individuals of the same species can also affect survival rates i.e: when overcrowding occurs, fewer offspring are usually born and rabbits will drive out some of the rabbits when the warren reaches a certain size etc.

Underpopulation:

Can also be a problem as some species need a certain population size to function properly. For example, a wolf pack or hyena pack needs to be a certain size to make a kill. Competitors refer to other species that compete for the same resource, whether it is growing space for a plant, a nesting site, nesting materials or food source.

Chemical factors:

Include the pH of the soil/water and salinity levels. Most species have a specific tolerance for chemical factors i.e some plants prefer acidic soil while some prefer more alkaline ones--some are also more salt tolerant (this relating to salinity levels).

Dynamic factors: 

Include wind speed and patterns, and wave action. Only a few species are able to withstand harsh sea winds and salty spray, an example of one that can is the she-oak. 

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MEASURING THE DISTRIBUTION OF A SPECIES IN AN ECOSYSTEM:

Types of distribution:

Where an organism can be found is the distribution of that organism. Distribution may differ from species to species. Some may randomly distribute themselves around an ecosystem or they may be found distributed in patterns, determined by the resources around them. Some organisms can be found in clumped patters, others in uniform patterns.

Type of distribution:

Regular distribution (very rare in nature):

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Possible explanation for this type of distribution is: territorial species.

Random distribution:

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-    - -  -    - -

- -      -

-  ---          - --- -

Possible explanation for this type of distribution is: random distribution of resources (food to eat etc).

Clumped distribution: (possibly the most common pattern reflecting the fact that species need resources that are not distributed uniformly). 

0 0     0

0   0

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Possible explanation for this type of distribution: patch distribution of resources. 

Transects:

There are different techniques that may be used to measure the distribution of a species in an ecosystem. In large areas transects are commonly used to give an idea of the variation that may occur. A transect is a narrow strip that crosses the entire area being studied, from one side to the other. Transects provide an accurate and easy method of representing an area simply.

Two examples of transects are a plan sketch and a profile sketch.

A plan sketch is an aerial or surface view of a representative area within an ecosystem. It shows to scale the distribution of organisms in a measured and plotted view. 

A profile sketch is a side in view of an area showing to scale the distribution of organisms along a line.

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