Population and Community Ecology

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Icon objectives.jpg
  • Give some examples of abiotic factors
  • Look at the concepts of habitat and niche with an emphasis on tolerance
  • Briefly look at food chains and food webs

Abiotic Factors

Factor Description Examples
Sunlight different plants require different amounts of sunlight/shade oak, honeysuckle
Temperature most species tolerate only a narrow range of temperatures penguin, palm
Precipitation some prefer dry climates, while others prefer wet climates cactus, frogs
Latitude climate and vegetation vary with latitude, many species are restricted by to specific latitudes polar bears, mangroves
Altitude altitude has an effect similar to latitude mountain goat
Fire frequency some species require fire as part of their life cycle jack pine
Soil different types of soil have different types and quantities of nutrients wheat, rice

Aquatic organism have some additional factors:

Factor Description Examples
Water currents Speed of water can affect where an organism lives salmon, perch
Dissolved oxygen Fish do not breathe air, but instead take dissolved oxygen out of the water
Dissolved nutrient Nutrient requirement vary widely between organisms
Suspended soils Soil and other particles suspended in the water can reduce the amount of sunlight penetrating the water

Limiting factors

Limiting Factor 
One factor which is more important than other factors in determining the success of a population

Example 1 If there is enough water, shelter, and space for 20 rabbits, but only food for 10; then the rabbit population will not exceed 10.

Example 2 If there are food for 1000 birds, but nesting sites for 100, the population of birds will be limited.

In Examples 1 and 2, food and nesting sites are the limiting factors, respectively.

Habitat and Niche


the place where a population lives
(pronounced nitch) is the role which a species has within an ecosystem.

Note that the habitat can be considered the address and the niche as the occupation.


Broad versus Narrow

Generalist species - Broad niche

lives in many different places, eats a variety of food, or tolerates wide range of environmental conditions
examples: cockroach, humans, dogs

Specialist species - Narrow niche

lives in only one type of habitat, tolerates narrow range of environmental conditions, or uses only one or a few types of food
example: giant panda (eats only bamboo, lives in very specific habitat)


Species have a specific range of tolerance to physical, chemical, and biological conditions and resources.

This range forms the fundamental niche.

However, pressures from other species form a narrower niche, called a realized niche.

Niche Differentiation

The key to understanding the concept of niche is understanding why different niches exist.

Competitive Exclusion Principle 
Two species competing for the same resource cannot coexist if other ecological factors are constant

In other words, different species cannot exist in the same niche.

How different niches can live in the same area is due to resource partitioning

Resource Partitioning 
Dividing up resources so that species do not directly compete

Types of resource partitioning:

Temporal - species compete for same resources at different times
Example: hawks hunt during the day, owls hunt at night
Spatial - species occupy different areas
  • Different species of warblers feed in different parts of the same trees
  • Giraffes feed at the top of trees, while deer and antelope feed near the bottom of the tree

Food Chains

A food chain is a series of organisms each of which is a source of food for the next one

A Food Chain

Trophic level 
each level in a food chain

Major roles

Organisms which use energy to produce their own food
Organisms which get their food by consuming other organisms
Primary consumers or Herbivores 
Animal that eat producers
Secondary consumers or Carnivores  
Animal that eat other consumers
Tertiary consumers or Carnivores  
Animal that eat aecondary consumers
Animals which are both herbivores and carnivores
Organisms which get their energy decomposing organic matter
Organisms which get their energy by feeding on organic debris (for example, in soil)
Organisms which eat meat of animals killed by others

Food Webs

The description of a food chain is too simplistic. Organism eat many other organisms. This leads to a further concept.

Food web 
A food web is a set of interconnected food chains

Chesapeake Waterbird Food Web.jpg

Energy Flow

As energy is transferred through the food chain, the concept of energy flow applies.

Energy flow through a food chain

The efficiency of each trophic level is about 5 - 20%. (10% is typical).

This means that carnivores have about 1% of the energy that plants capture.

The energy can be described in terms of biomass

This energy flow has an effect on the number of organisms at each level.

Energy Pyramid show decreasing biomass and number of species per trophic level

Icon activity.jpg
Purpose:An important part of science is the abililty to observe and take notes.

Objectives: Observe wildlife in order to appreciate ecology and to hone your observation skills.

Preparation: Print this observation form

Activity: Go to either a park or to a wildlife area where there is little outside disturbances (such as cars or people). Then sit quietly and watch an animal (bird, mammal, reptile, or amphibian) for about one to two hours taking notes (using the observation form) of everything you see and hear.

Note:This could be done individually or in groups. However, groups of more than three can make the activity difficult. Follow your instructors guidelines.

Hints: You may not see any activity at first, but after a while things will start to appear. Do not forget to note the physical environment, plants, and insects. It is also important to look at the interactions between organisms.