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G. E. Hutchinson on the Niche Concept
In 1957, in a paper entitled " " G. E. Hutchinson defined the niche concept formally. One could describe the activity range along every dimension of the environment. Physical and chemical factors such as temperature, humidity, salinity, and oxygen concentration, and biological factors such as prey species and resting background against which an individual may escape dection by predators, could be determined. Each of these dimensions could be thought of as one of n-dimensions in space. Visualizing a space with more than three dimensions is difficult, since the concept of the n-dimensional niche is an abstraction. We may, however, deal with multi-dimensional concepts mathmatically and statiscally, depicking their essense by physical or graphical representations in three or fewer dimensions. Ricklefs (1996) notes that "... for example, a graph relating biological activity to a single environmental gradient represents the distribution of a species' activity along one niche dimension. The level of activity, whether oxygen metabolism as a function of temperature or consumption rate as function of prey size, conveys the ability of an individual to exploit resources in a particular part of the niche space and, conversely, the degree to which the environment can support the population of that species. In two dimensions the individuals niche may be depicted as a hill, with contours representing the various levels of biological activity. In three dimensions, we must think of a cloud in space whose density conveys niche utilization. Beyond three the mind boggles."
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Hutchinson was the first to formally quantify the niche concept in terms of geometric space. For example, suppose the distribution of a given species of tree squirrel is determined primarily by 3 variables: branch diameter, acorn size and temperature. The "level of activity" describes the ability of the individual to exploit the resources in a given part of the niche space; in this case, number of squirrels foraging for a given level of each environmental factor. Then the niche space occupied by the species is the 3-dimensional space actually occupied by all individuals (Ricklefs 1996). This can be represented graphically as a contour plot. An empirical model ( Box and Draper 1989) can be obtained by the empirical determination of "niche occupancy" (e.g. density, number of individuals, etc.) in terms of n environmental variables (these may be both biotic and abiotic). This model may be formalized as a second-order polynomial equation; the eigenvalues calculated from the matrix of coefficients of cross-product terms formally quantify the response surface in the area of the optimum response. The simultaneous evaluation of multiple variables is important in biological systems where optimum responses usually consist of a range of values rather than a single point. |
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THE EXERCISE .1. For instance from this image from the first transect, A00501, (A0 being the Race Rocks location, 05 being the sample station location and 01 being the first transect at that location, 07 being the seventh quadrat from the top of the intertidal zone. ). Quadrat 07 looks like this. By clicking on this icon you will see this one sample of the actual photo from the transect file. 2. If we want to define the space that the mussels occupy in this quadrat, we have to measure the percent of the quadrat that they are covering. This could be done simply by cutting out a piece of acetate that has been made by xeroxing a piece of graph paper and overlaying it on the screen. 3. A more precise way of doing this is by using an Imaging program to help analyse different aspects of the photograph. Download this image by clicking on the full size image then pressing and holding the right mouse button on a PC or pressing and holding the mouse of a Macintosh . Note that the mussels occupy a portion of this quadrat, a meter stick on the left side gives you the size of the quadrat. |
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4. CONVERTING THE IMAGES: 5. Using GifConverter: Open the .jpg file obove that you have downloaded. Save it as a .PICT file. This is the format that is necessary to use in the next stage of the process, using NIH Image to measure features of the picture. 6. If you do not have a copy of the freeware NIH IMAGE. Download it from this site. (Both Mac and PC versions are available) 7. INSTRUCTIONS FOR NIH IMAGE: a) To calibrate the image in terms of real units: Use the straight line tool on the left panel, and draw it the length of the meter stick. 8. DOWNLOADING THE TRANSECT IMAGES: .. Go to the transect file.(http://www.pearson-college.uwc.ca/pearson/transect/transrrk.htm) 9. Select one of the transects, (it will take a few minutes to download one of the belt transect strips ). 10. USING A SPREADSHEET Here is a sample of the mussel distribution data as it appears on the spread sheet PLOTTING IN 3D This file on Ecological Niche Models was developed by Penny Reynolds, Richard Rosecrance and Garry Fletcher at the Bioquest Consortium Workshop, on WHAT CAN WE LEARN FROM CONTEMPORARY MATHEMATICS REFORM? June 21-29, 1997 It was supported by a grant from the Howard Hughes Medical Institute to :
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