. Bulletin of the British Museum (Natural History), Geology. BENTHIC LUDLOW COMMUNITIES 209 mapped. One can thus define a community by specifying either faunal composition or areal boundaries interchangeably. Jurgen Dorjes has applied a transect method to the areal definition of modern marine benthic communities (Dorjes et al. 1969, 1970; Dorjes 1971, 1972). An example of this work is reproduced in Fig. 13. Samples are taken along a line across a known environmental gradient, usually per- pendicular to a coastline and boundaries of sedimentary facies. The transect line is drawn on 10- CL T3 20
. Bulletin of the British Museum (Natural History), Geology. BENTHIC LUDLOW COMMUNITIES 209 mapped. One can thus define a community by specifying either faunal composition or areal boundaries interchangeably. Jurgen Dorjes has applied a transect method to the areal definition of modern marine benthic communities (Dorjes et al. 1969, 1970; Dorjes 1971, 1972). An example of this work is reproduced in Fig. 13. Samples are taken along a line across a known environmental gradient, usually per- pendicular to a coastline and boundaries of sedimentary facies. The transect line is drawn on 10- CL T3 20 sand BOTTOM PROFILE and SAMPLE LOCATIONS silty sand silt 100 200 300 405 510 610 765 945 distance from shore (m). Fig. 13 Distribution of mollusc species along a sedimentary-bathymetric gradient in the Gulf of Gaeta, Italy, illustrating the transect method of defining communities. Modified from Dorjes (1971 : fig. 5). paper, and occurrences of species are plotted along it according to sample to produce a graphic profile of faunal distribution. The profiles are then divided into contiguous segments which define communities, according to distinctive overlaps of species ranges along the line of transect (Fig. 13). The transect method provides a direct picture of community boundaries and gradations, and eliminates the need for statistical comparison of scattered samples. Some community boundaries represent very sharp changes in faunal composition, as where sand shoals border mud-filled troughs (Dorjes 1972 : pi. 2). In other transects, the faunas of communities grade into one another along continuous transitions of sediments (Fig. 13). This phenomenon, where community gradations reflect environmental gradients, has also been discussed by Johnson (1972).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British M
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