SUCCESSIVE APPROXIMATION I N DESCRIPTIVE ECOLOGY
49
of description and classification have been only too ready t o accept
their own subjective evaluation of the situation, and have in general
not troubled to go beyond their general visual impression of homogeneity in the stands they are studying”. Nevertheless most practising
plant sociologists are well aware of the problems of pattern and their
causes; they are also well aware of the limitations of the concept of
“minimal area”, which are so closely associated with pattern.
It is well at this stage to examine exactly how the plant sociologist
works, and how the application of (‘successive approximation” reduces
the faults of what may seem to be arbitrary procedures.
Full details of the actual field methods are given for example in
Nordhagen (1928 and 1943), Poore (195513) and Ellenberg (1956). Only
a very short priccis will be given here. After extensive reconnaissance
the worker chooses stands which are as far as possible uniform in
general physiognomy, ecology and species composition, as far as these
may be determined by inspection. They should also be stands of communities which are typical of the region and occur frequently in it.
The actual area occupied by them is less important. This is the first
approximation. Many assumptions have already been made. Are they
justifiable? One cannot say until the data have been examined more
closely, when it will be possible to reject certain descriptions which are
clearly mixtures or are atypical.
It is possible to make objective tests for “homogeneity”. This was
done by Dahl, for example, in certain of his communities using the distribution of the dominant as the criterion of homogeneity by comparing
the distribution of numbers of shoots with the Poisson series. Nordhagen
assesses homogeneity by describing ten plots of a given size in each community and examining the resulting constancy diagrams.
The “minimal area” curve is frequently used t o obtain some assessment of homogeneity as well as t o ensure that a “representative” portion of the community is described. A standard and simple technique
for determining the “minimal area” of the community is described by
Braun-Blanquet (1951), Cain and de Oliviera Castro (1959), Poore
(1955a) and Ellenberg (1956). The concept has been criticized by Goodall
(1952, 1954a,b), Cain and de Oliviera Castro (1959), Dahl (1956) and
others. It is unlikely that any truly satisfactory rigid definition will be
found for the minimal area. It is probable that it is an ideal concept t o
which natural communities only partially approximate, because surfaces which are large enough and are absolutely uniform in their effect
on all the potential species of the community are never or very rarely
found in nature. Evans and Caiii (1952) suggest that some arbitrary
point on the species-area curve should he chosen such as the area at
which a 10% increase in area leads t o a 10% increase in number of
49
of description and classification have been only too ready t o accept
their own subjective evaluation of the situation, and have in general
not troubled to go beyond their general visual impression of homogeneity in the stands they are studying”. Nevertheless most practising
plant sociologists are well aware of the problems of pattern and their
causes; they are also well aware of the limitations of the concept of
“minimal area”, which are so closely associated with pattern.
It is well at this stage to examine exactly how the plant sociologist
works, and how the application of (‘successive approximation” reduces
the faults of what may seem to be arbitrary procedures.
Full details of the actual field methods are given for example in
Nordhagen (1928 and 1943), Poore (195513) and Ellenberg (1956). Only
a very short priccis will be given here. After extensive reconnaissance
the worker chooses stands which are as far as possible uniform in
general physiognomy, ecology and species composition, as far as these
may be determined by inspection. They should also be stands of communities which are typical of the region and occur frequently in it.
The actual area occupied by them is less important. This is the first
approximation. Many assumptions have already been made. Are they
justifiable? One cannot say until the data have been examined more
closely, when it will be possible to reject certain descriptions which are
clearly mixtures or are atypical.
It is possible to make objective tests for “homogeneity”. This was
done by Dahl, for example, in certain of his communities using the distribution of the dominant as the criterion of homogeneity by comparing
the distribution of numbers of shoots with the Poisson series. Nordhagen
assesses homogeneity by describing ten plots of a given size in each community and examining the resulting constancy diagrams.
The “minimal area” curve is frequently used t o obtain some assessment of homogeneity as well as t o ensure that a “representative” portion of the community is described. A standard and simple technique
for determining the “minimal area” of the community is described by
Braun-Blanquet (1951), Cain and de Oliviera Castro (1959), Poore
(1955a) and Ellenberg (1956). The concept has been criticized by Goodall
(1952, 1954a,b), Cain and de Oliviera Castro (1959), Dahl (1956) and
others. It is unlikely that any truly satisfactory rigid definition will be
found for the minimal area. It is probable that it is an ideal concept t o
which natural communities only partially approximate, because surfaces which are large enough and are absolutely uniform in their effect
on all the potential species of the community are never or very rarely
found in nature. Evans and Caiii (1952) suggest that some arbitrary
point on the species-area curve should he chosen such as the area at
which a 10% increase in area leads t o a 10% increase in number of