Tag Archives: limiting factors

Limiting Factors and the Ecological Niche

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BACKGROUND:
normalcurveEnvironmental abiotic and biotic factors can also be termed “Limiting Factors.“ They are limiting in that they tend to select only for those organisms which have the best tolerance, or adaptation to the factor. At different times of the year, some abiotic factors take on more importance than others. Water is certainly not a limiting factor at Race Rocks in the months of December and January compared to the dry months of July and August. In this assignment, we will examine data records and determine what other factors are variable in their importance in various times of the year. This assignment is paired with the assignment on Abiotic Factors

Objectives:

1. Present an argument for what you consider to be the most important abiotic factor in determining the distributions of organisms at Race Rocks , and contrast this with what you consider to be the most important factor in determining the distribution of organisms where you live.

2. Use the following terms to explain with graphs, examples of the concept of Limiting Factors.

  • Euryhaline and Stenohaline
  • Eurythermal and Stenothermal

3. Describe how Natural Selection of species occurs as the result of Limiting Factors.

4. Demonstrate how Limiting Factors of the environment determine and define the ecological niche of an organism.

5. Produce a graph demonstrating the Ecological Niche of an organism.

6. Discuss how our built up environments with cats, lawns, and other introduced species limit the ecological niches available and thus impact negatively on Biodiversity.

PROCEDURE:

1.Introduction: Examine this graph representing the Normal Curve or a Bell Curve of the level of population of a species, in a certain area, or the density. If you think of the blue line here representing the number of individuals of animals or plants in a population, which exist in an environment with abiotic conditions favoring those at the central value of 0, then you can see that there is actually more positive selection for the central area being exerted than for either of the extremes. Species evolve in much this way. Those having the right amount of insulation, the right amount of ability to survive dessication or drying up, the right abilities to survive in a certain salinity or level of dissolved oxygen, or the right ability to tolerate a high level of wind in their environment, are the ones who are more successful in reproducing and thus their traits get passed along to more offspring. So the X-axis can represent the abiotic variables in an environment.

2. Correlate the month of occurrence of a species and the population levels for that month.

3. We can see from the assignment on abiotic factors, that there may be dozens of such graphs you could make for any organism.musshand At Race Rocks, we have two species of the molluscs called mussels. They are Mytilus californianus and Mytilus trossulus . Visualize the x-axis above being a scale of temperature, for a marine animal such as a mussel, lets say the ideal temperature for mussels is 10 degrees C. Draw a similar graph with 10 in place of the 0 . Show the scale going up and down from 10. Lets say that the Mytilus californianus mussels cannot reproduce at a temperature higher than 12 degrees nor lower than 8 degrees C, so the blue line tapers down and ends at 8 degrees C.
4. Now plot another graph on the same scale, This one is for the other local mussel, Mytilus trossulus. musselpool4It lives in the intermediate level tidepools and can survive in water up to 16 degrees and down to 4 degrees. In the sample above we say that the M.californianus is a Stenothermal ( narrow range of tolerance to heat) animal, whereas the M.trossulus is a Eurythermal (wide range of tolerance to heat) organism
5. Next we will do a similar plot for salinity. Here 30 parts per thousand for salinity becomes the central measurement and above and below that amount make the rest of the scale. M.Californianus prefers to live in water that is 27 parts per thousand. But it can live in up to 30 or down to 25. M.trossulus also prefers 27 parts per thousand, but can live in water that ranges from 32 down to 22 parts per thousand. Now if we combine the same prefixes: Eury and Steno,with the word haline, we will get a word that describes the organisms. So now you have two new words that help describe the variation the same abiotic factor can have on two closely related species.
6. Natural Selection which can lead to the evolution of separate species, is often determined by the way an organism is able to tolerate variations in the abiotic factors of it’s environment.variabilityJust when that seemed so straightforward, we have to recognize that selection pressure may affect the same species group in different ways. Two species of mussel may on the other hand prefer different ranges of temperature or salinty instead of the 10 degrees. The picture B could represent that situation. What would the skewed distribution in C therefore represent in terms of selective pressures on the population?

mus7. The Ecological Niche is defined by limiting abiotic and biotic factors: In this assignment, we deal with the problem of defining what are the ideal conditions for an organism’s existence. At Race Rocks, the position on a shoreline within a few centimeters can determine survival of an organism. Use these references to find out about how you can model an ecological niche of an organism. Choose an organism in your own ecosystem and produce a graph representing it’s ecological niche.

 

malesparrow8.This now brings us to a classical study in Adaptation which was done over a century ago! This study has provided the data for many studies on limiting factors of the environment. However, be careful about jumping to conclusions! This study points out the difficulties in attributing what may at first seem to be a simple cause and effect relationship. See Bumpus’ Sparrows

9. Extension Materials: Central Tendency and Variability

Also, you may wish to take this opportunity to get into an exercise on Standard Deviation. To draw such a curve as the Normal Curve at the top of the page, one needs to specify two parameters, the mean and the standard deviation.  The graph has a mean of zero and a standard deviation of 1, i.e., (m=0, s=1).  A Normal distribution with a mean of zero and a standard deviation of 1 is also known as the Standard Normal Distribution.

Limiting Factors: The Bumpus Paper

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malesparrowA classic study done over a century ago has provided the data for many studies on limiting factors of the environment, See this PDF, partially quoted here from The Condor Journal : For the complete article go to http://elibrary.unm.edu/sora/Condor/files/
issues/v094n04/p0944-p0954.pdf

 

 INTRODUCTION
Nearly a century ago, Hermon Bumpus received 136 House Sparrows (Passer domesticus) that had been collected after a severe winter storm in Providence, Rhode Island (Bumpus 1899). Over half of these birds revived in his laboratory and Bumpus proceeded to evaluate physical characteristics that might distinguish these survivors from their dead counterparts. He concluded that the storm had taken a greater toll on individuals whose morphometrics deviated most from the “ideal type” (Bumpus 1899). Bumpus was unabashed in claiming this pattern of differential survival to be due to the agency of natural selection. The provocative nature of his interpretation, coupled with publication of the complete data set on which it is based, has prompted repeated analysis of Bumpus’ study (e.g., Harris 1911, Calhoun 1947, Grant 1972, Johnston et al. 1972, Lande and Arnold 1983, Crespi and Bookstein 1989). Studies reappraising the Bumpus data generally agree that females suffered proportionately greater mortality than males and that female survivorship reflects stabilizing or normalizing selection (Grant 1972, Johnston et al. 1972, Lande and Arnold 1983). Disagreement persists, however, in deciding whether male survivorship reflects directional selection and, if so, whether this selection favors larger (Johnston et al. 1972) versus smaller individuals (Lande and Arnold 1983, Clutton-Brock 1988). Such contradictory conclusions from the same data set reflect differences in confidence that var- ious authors place in Bumpus’ morphometric measures. Those accepting at face value all nine of Bumpus’ morphometric measures conclude that the winter storm selected against larger adult males, because male survivors had significantly less mass and shorter total length than did their dead counterparts. On the other hand, some contend that his measures involving plumage (alar extent and total length) and mass may be biased, albeit for different reasons, and should not be considered when comparing characteristics of survivors to non-survivors (Calhoun 1947, Grant 1972, Johnston et al. 1972, Crespi and Bookstein 1989). When analysis is restricted to each of the six skeletal measures, adult male survivors and non-survivors cannot be distinguished from one another.

FOR FURTHER INVESTIGATION:
The following original papers present the arguments for natural selection by abiotic factors in the environment:BUMPUS, H. C. 1899. The elimination of the unfit as illustrated by the introduced House Sparrow, Passer domesticus. Biol. Lectures, Marine Biol. Lab., Woods Hole:209-226.

JOHNSTON, R. F., D. M. NILES, AND S. A. ROHWER. 1972. Hermon Bumpus and natural selection in Passer domesticus.

PRICE, T. D., P. R. GRANT, H. L. GIBBS, AND P. T. BOAG. 1984. Recurrent patterns of natural selection in a population of Darwin’s Finches. Nature 309:787-789.
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