Author Topic: The Nature of the Community: Continuity vs. Discontinuity  (Read 840 times)

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The Nature of the Community: Continuity vs. Discontinuity
« on: August 08, 2008, 11:31:03 PM »
The Nature of the Community: Continuity vs. Discontinuity

Allegheny College Department of Biology

Albrando  Lucino Jr, B.S, B.A


Introduction
   Community compositions are said to vary from stand to strand within a particular environment depending on the nutrients of the soil and the elevation of a particular stand within a forest or ridge. Questions do arise in the field of biology whether communities are integrated units or collections of populations that require the same environmental conditions. There are processes in which to tell whether vegetation is part of a continuum or is a distinct type have been to use community ordination techniques. Ordination is the arrangement or plotting of either species or samples on an axis or axes.  The axes may represent environmental gradients or purely mathematical constructs derived from a matrix of similarities between the quadrats or species. If there are simple environmental gradients, then it would be easier to use direct ordination, however if environmental gradients are difficult to perceive, indirect ordination techniques may be used; extract "new" axes so that instead of interpreting the occurrence of a species in terms of one environmental factor, it is interpreted in terms of a collection of environmental factors. The two hypotheses that we tested were that vegetation exhibits a continuous change in response to environmental gradients. The second hypothesis was that the environmental gradient controlling species distribution on the south facing slope of Cemetery Run is elevation.  In other words, indirect ordination of stands along an artificial construct will not be greatly different from direct ordination of stands along an elevation gradient.

Methods
   Working in a group of two people, sample trees within one meter of each side 50 meter line transect or within a 20 x 20 meter quadrat, both of which run parallel to the slope contour. After marking the specified transect, identified each tree to species, and measured its circumference at breast height. After finishing the sampling of the particular transect, calculated the basal area of each species encountered by using the equation for species dominance C2/4(PIE). To find relative dominance, divide the dominance measure for each species by the sum of the dominances of all species in the assigned quadrat/transect.

Retort


To calculate similarity coefficient, add the smaller of the two relative dominance values for each species comparison. To calculate dissimilarity coefficient, use 100% - similarity%. The method of placing a stand on a "new" axis requires the use of the following equation:
X = L2 + D12 - D22
           2L
D1 and D2 are the distances to the stand from left and right endpoint stands, and L is the distance between the endpoint stands. Figure 1 and 2 shows the direct and indirect ordination graphs of the different species in varying elevation gradient and Table 1 shows the total dominance of each species within a particular stand.
Discussion
   From analyzing the trends of figures 1 and 2 and the data observed in table 1, we assessed the proposed hypotheses. The first hypothesis that stated that vegetation exhibits a continuous change in response to environmental gradients was supported in that there indeed were changes in vegetation frequency as elevation increased or decreased. For example, figures 1 and 2 shows that hemlock and beech trees were observable in low elevation, but were lacking in presence in elevation levels of 3, 4, 5 and 6 but were again seen in higher elevation levels. Table 1 showcases the species dominance rates of each vegetative species within each elevation stand, and one observes that there are definitely a lot of changes in each species’ dominance as elevation levels increase or decrease. The second hypothesis that stated indirect ordination of stands along an artificial construct will not be greatly different from direct ordination of stands along an elevation gradient was supported in that Figure 2 is almost similar to that of Figure 1 in showing the relationship between elevation and species dominance.


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