PATTERN AND PROCESS IN CORIPETITION
29
total number of species must remain constant. Hutchinson (1957) notes
that this is likely in a homogeneously diverse environment, in which the
mosaic of structural features (e.g. logs, stones, bushes, etc.) is small
compared with the free paths of the organisms concerned, but is less
likely in a heterogeneously diverse environment where, for instance,
stands of woodland may be separated by areas of grassland.
Two examples of a correspondence between observed species-abundance distributions and the predictions of this case of the model have
been cited as evidence of competitive exclusion. MacArthur (1957,1958)
obtained a reasonable fit t o the model when he plotted data for birds in
the Quaker Run Valley of New York from component habitats (pasture,
orchards, mature oak-hickory forest) which were homogeneously diverse; and Kohn (1959) found a remarkably good correspondence for
some of his samples of snails (Conus) in Hawaii. However, these studies
would appear to involve two quite different kinds of interaction. The
birds studied by MacArthur are mostly territorial, they are relatively
uninfluenced by minor variations in environmental factors, particularly
microenvironment, and interfere with each other sufficiently to suggest
that competitive exclusion is a t least a distinct possibility. Kohn (1959),
however, found that the adult ecological niches of each species of Conus
differ significantly with respect to a t least two of the following characteristics: nature of the food, nature of and relation to the substratum,
and zonation of the marine environment. Kohn (1959) stated, “These
differences are concluded to be the primary factors by which the
ecological niches of species of Conus are differentiated. This is the
mechanism which enables the maintenance of populations of large
numbers of closely related, sympatric species of Conus in tropical
regions.” I n other words, the fundamental niches of birds are large and,
for the most part, independent of close environmental control, and the
results obtained by MacArthur (1957) may reflect a series of nonoverlapping realized niches determined by competitive exclusion. But
there is no evidence of territoriality or interference among different
species of Conus (A. J. Kohn, personal communication). They are so
sensitive to variations in the microenvironment that their fit to the
MacArthur model probably reflects a condition of specialization with
respect to food and substrate and mutually exclusive distributions that
are controlled by environmental discontinuities and not competition.
Correspondence with the predictions of the MacArthur model of nonoverlapping niches can be produced by (1) separate realized niches due
to competitive exclusion of one species from the intersection subset
of two intersecting fundamental niches (Fig. 3 B or C), (2) separate
realized niches due to environmental discontinuities that are greater
than the amount of intersection between fundamental niches, or (3)
B*
29
total number of species must remain constant. Hutchinson (1957) notes
that this is likely in a homogeneously diverse environment, in which the
mosaic of structural features (e.g. logs, stones, bushes, etc.) is small
compared with the free paths of the organisms concerned, but is less
likely in a heterogeneously diverse environment where, for instance,
stands of woodland may be separated by areas of grassland.
Two examples of a correspondence between observed species-abundance distributions and the predictions of this case of the model have
been cited as evidence of competitive exclusion. MacArthur (1957,1958)
obtained a reasonable fit t o the model when he plotted data for birds in
the Quaker Run Valley of New York from component habitats (pasture,
orchards, mature oak-hickory forest) which were homogeneously diverse; and Kohn (1959) found a remarkably good correspondence for
some of his samples of snails (Conus) in Hawaii. However, these studies
would appear to involve two quite different kinds of interaction. The
birds studied by MacArthur are mostly territorial, they are relatively
uninfluenced by minor variations in environmental factors, particularly
microenvironment, and interfere with each other sufficiently to suggest
that competitive exclusion is a t least a distinct possibility. Kohn (1959),
however, found that the adult ecological niches of each species of Conus
differ significantly with respect to a t least two of the following characteristics: nature of the food, nature of and relation to the substratum,
and zonation of the marine environment. Kohn (1959) stated, “These
differences are concluded to be the primary factors by which the
ecological niches of species of Conus are differentiated. This is the
mechanism which enables the maintenance of populations of large
numbers of closely related, sympatric species of Conus in tropical
regions.” I n other words, the fundamental niches of birds are large and,
for the most part, independent of close environmental control, and the
results obtained by MacArthur (1957) may reflect a series of nonoverlapping realized niches determined by competitive exclusion. But
there is no evidence of territoriality or interference among different
species of Conus (A. J. Kohn, personal communication). They are so
sensitive to variations in the microenvironment that their fit to the
MacArthur model probably reflects a condition of specialization with
respect to food and substrate and mutually exclusive distributions that
are controlled by environmental discontinuities and not competition.
Correspondence with the predictions of the MacArthur model of nonoverlapping niches can be produced by (1) separate realized niches due
to competitive exclusion of one species from the intersection subset
of two intersecting fundamental niches (Fig. 3 B or C), (2) separate
realized niches due to environmental discontinuities that are greater
than the amount of intersection between fundamental niches, or (3)
B*