Habitat & Niche

Introduction:

The International Union for Conservation of Nature (IUCN) lists nearly 470,000 species as vulnerable or endangered. These species typically have narrow niches and specific habitat requirement. Therefore, habitat loss is a major cause of extinction. To protect endangered species, it is necessary to have a good understanding of their niche characteristics and nature of the habitats they require.

An organism’s habitat is the place an organism lives. Often general types of environments are referred to as habitats because they are associated with certain assemblages of plants, animals and other organisms. For example, a sandy beach habitat would have specific organisms associated with it that might be absent or rare in a deciduous forest habitat, or a cold-water stream habitat. Each kind of habitat supports many kinds of organisms that must share common resources.

An organism’s niche is the role an organism plays in its ecosystem. A niche is a very abstract concept that includes everything that affects a species of organisms and all the impacts the organism has on its surroundings. To clarify the distinction between an organism’s habitat and niche, let’s look at a couple examples. The habitat of a bison is grassland. Its niche includes its role as a consumer of grass, begin a host to parasites, and migrating to find food. Other niche characteristics include feeding habits, breeding habits, competition with other grazing animals and physiological constraints. The habitat of cattails is a wetland. Its niche includes carrying on photosynthesis, providing hiding places for many kinds of animals, providing nesting sites for redwing blackbirds, and many other functions.

A community consists of many different species of organisms that interact with one another. Those with similar niches will compete with one another. A concept known as the competitive exclusion principle states that no two organisms can occupy the same niche at the same time. Species with the same niche requirements will be in intense competition with one another and this competition is harmful to both organisms. In some cases, one of the two competing organisms may simply not be able to survive and will be excluded from the area. Natural selection also plays a role. When two species are in intense competition, natural selection may lead to one species being driven to extinction. Alternatively, natural selection may cause the two species to evolve into slightly different niches, which reduces the intensity of competition. Thus, even organisms that appear to have similar niches have some important niche differences. This separation of organisms into distinct niches is known as niche differentiation. For example, there are many species of birds that feed on insects. However, they may not compete directly because they feed on different kinds of insects, look for insects in different places, or feed at different times of day. Thus, niche differentiation allows many species of insect-eating birds to coexist without intense competition.

The range of conditions in which an organism can be successful is known as niche breadth. Some species can live in a wide variety of habitats and have very broad niches, while other species require very special set of environmental conditions and have very narrow niches. For example, the American robin has a very broad niche. It eats many kinds of foods, can survive in a wide variety of environmental conditions, and is found throughout North America. However, the saguaro cactus has a very narrow niche. It requires summer rains for seeds to germinate, rocky soil to anchor its roots, and very mild winter temperatures. Thus, it is found in a very small areas of the desert southwest where the proper environmental conditions occur.

In this lab you will evaluate niche breadth by comparing the number of individuals of each species across the different habitats you survey. Species that deviate most from the average have narrow niches and may be found only in one habitat. This species may be termed a habitat specialist. However, if a species exists in all surveyed habitats in approximately the same numbers, the average for that species will be close to the number found in each habitat. Species that have a population in all surveyed habitats that are close the average have broad niches and are considered habitat generalists.

Niche overlap is the degree to which different species play similar roles in their communities. All plants have a certain degree of niche overlap because they all carry on photosynthesis. All plants that have insect-pollinated flowers have a certain amount of niche overlap. However, there is also niche differentiation among plants that use different kinds of insects as pollinators. Similarly, the niches of wind-pollinated plants do not overlap with those of insect pollinated plants. The difference in method of pollination (niche differentiation) reduces niche overlap and reduces competition.

In this lab you will also evaluate the degree to which the niches of the different species overlap in a habitat. The greatest degree of niche overlap occurs in habitats where the number for each species are similar.

Materials:

• 4 pieces of surveyor’s ribbon
• Colored pencils
• 50-meter tape measure
• Two meter sticks
• Data sheets
• Calculators

Procedure:

1. Your instructor will describe six plant species that are common in your area and that occur in at least one of the three habitats listed in Table 1.2.
2. Your instructor will divide the class into 6 groups of 4. Each group will be assigned one of the plant species and will be responsible for counting only that species in each habitat.
3. At each of the three habitats, use a 50-meter tape to measure a 30-meter by 30-meter square plot and mark the corners with surveyor’s ribbon.
4. At each habitat, students will characterize the site by gathering the requested data on Table 1.1.
5. Each group is responsible for identifying and counting every plant of their assigned species within the 30 x 30-meter plot in each of the habitats and recording the number counted on Table 1.2.
   1. The instructor will assist students in identifying the species. Images of species can be found on Data Sheet 1.4
6. After surveying all three habitats, each group will share its results, and all students will record the results on Table 1.2 and calculate the average for each species.
7. Create a bar graph of the number of each species using Graph 1.1 on Data Sheet 1.2. Use different colors or shading to plot the numbers of each species.
8. The bar graph will help you evaluate niche breadth and niche overlap.

Data Sheet 1.1

Table 1.1 Site Characteristics

Characterize each of the three habitats and record the data in the spaces provided.

Table 1.2 Number of Plants Found in Each Habitat

Record the total number of individuals of your plant species found in the study plot of each of three habitats. After returning to the classroom, enter the data collected by the other groups about the remaining species.

Data Sheet 1.2

Use the data from Table 1.2 to construct a graph (below) of the different species distribution and each species’ average. You may wish to enter the data from Table 1.2 into an Excel spread sheet to construct the graph.

Graph 1.1 Distribution of Species in Three Different Habitats

Data Sheet 1.3

Analysis

Although there are statistical methods for evaluating the niche breadth of a species and the degree of niche overlap between species, they are somewhat cumbersome so we will evaluate niche breadth and nice overlap by examining the bar Graph 1.1 of species abundance you produced. See the discussion of niche breadth and overlap at the beginning of this exercise.

1. Which of the 5 species examined were generalists? How do you know they are generalists?

2. Which of the 5 species examined were specialists? How do you know they are specialists?

3. For the specialist species, which of the environmental factors you assessed appeared to be the most important to the success of the specialist species?

4. What characteristics of the generalist species appear to have accounted for their success in the different habitats?

5. In which habitat was the greatest amount of niche overlap observed?

6. Those species that occur together show some degree of niche overlap and are in competition with one another. What evidence do you have that there has been niche differentiation?