General Ecology: Lecture 13

General Ecology: Lecture 13 October 31, 2005

I. Concept of a niche A. Definitions 1. Theodore Giesel 2. Hutchinson: For each environmental variable it faces in its world, an organism has a range of values for which it can grow and multiply (what is this similar to?) B. Problems 1. Infinite number of variables that define a space of n-dimensions a) The most we can depict visually are 3 dimensions as a volume [Fig. 14.15] 2. Assume variables can be ordered and measured for limits a) Think about how this relates to Shelford’s Law of tolerance 3. Each diagram depicts an instant in time C. Usefulness of concept 1. For a pair of important variables, can more fully define as a contour diagram to determine “prime” vs. “marginal” conditions, thus helping to explain the distribution and abundance of a particular organism. a) Example: Feeding niche of the blue-gray gnatcatcher [Fig. 14.17] 2. Can determine regions of species overlap for variables determined to be important [Fig. 14.19] a) Note that when more than one variable is considered, overlap of niches is apparently reduced [Fig. 14.21] D. Fundamental vs. realized niche [Fig. 14.16] 1. Fundamental niche a) Defined theoretically b) Converted to a concrete reality 2. Realized niche II. From the perspective of niches, how do species deal with competition? A. Gause’s hypothesis/competitive exclusion principle (i.e. Species 1 or Species 2 wins) 1. Gause’s hypothesis: “As a result of competition, two similar species scarcely ever occupy similar niches, but displace each other in such a manner that each takes possession of certainly particular kinds of food and modes of life in which it has an advantage over its competitor.” a) Refer back to laboratory experiments on Paramecium and the differences in results depending upon the similarities of the two species competing. 2. Hardin: Competitive exclusion principle: “Complete competitors cannot coexist.” a) Which of the Lotka-Volterra phase-plane diagrams predicts competitive exclusion? B. Resource partitioning 1. Defined/described [Fig. 14.9] a) When alone, a species may use a wide range of a particular type of resource. Whereas when resources are shared, organisms might only utilize a subset of those resources. 2. Examples a) Example 1: Partitioning of soil resources by plants [Fig. 14.10] b) Example 2: Resource partitioning of terns on Christmas Island (figure from a different book) C. Niche compression 1. Defined: Competitors force a once wider-ranging species to retreat back to optimal patches of habitat. a) Example: dove species in New Guinea (see text for complete example) 2. Similarities to resource partitioning. 3. Different from resource partitioning D. Niche shift 1. Defined: A shift from one realized niche to another within the fundamental niche. a) Example: sunfish in artificial ponds with different vegetation zones (text example) E. Ecological release 1. Defined: This is essentially the opposite of niche compression. In this scenario, a species will expand to fill unoccupied habitats a) Example: Species arrives on an uncolonized island. F. Character displacement (evolutionary time scale) a) Defined: If competition persists between two species, their characteristics will diverge due to natural selection b) Classic example: Darwin’s finches in the Galapagos [Fig. 14.23] c) How does this differ from resource partitioning?

Study questions

1. Niches a. What is a niche? Compare the Hutchinson vs. Seussian version. b. What is some of the confusion surrounding the definition of a niche? c. What are some of the practical problems in defining a niche in Hutchinson’s way? d. How does the concept of a niche relate to Shelford’s law of tolerance?

2. What point about niches is being illustrated in figure 14.21?

3. What is the difference between the fundamental niche and the realized niche?

4. Define each of the following terms in terms of the species’ use of the niche in general, changes in geographical distribution, and whether there is an actual change in behavioral or morphological characteristics of the species involved. Also, provide an example of each from the lecture or text. Bottom line: You should know the differences between these and be able to recognize examples (either ones you have already seen, or completely novel examples…) a. Resource partitioning b. Niche compression c. Niche shift d. Character displacement

5. How is niche compression similar to resource partitioning? How does it differ? Under what conditions would the two appear to be the same thing?

6. What is ecological release, and how does it compare to niche compression?

7. Character displacement is often confused with resource partitioning. What is the difference? Could one be the result of the other? Explain.