22
RICHARD 5. MILLER
two niches would be precisely contiguous and non-intersecting (D),
this is a theoretical possibility which must at least be considered as a
possible cause of observed cases of contiguous allopatry in spatial
distributions.
The terms allopatry, sympatry and contiguous allopatry are conventionally used to describe geographic distributions, but analogous
patterns occur at all levels of distribution and it is not always possible
to distinguish between ecological events which affect geographic, habitat
or microhabitat distributions (Miller, 1964a). The related questions of
spatial distribution and coexistence are relative terms, depending on
the level of spatial distribution chosen as a criterion and the sensitivity
of sampling procedures in relation to this criterion. For this reason, it
seems advisable to consider the general phenomenon of spatial distribution as a single event which can be qualified according to specific
circumstances. When it is stated, according to the Gause axiom, that
no two species can coexist indefinitely in the same locality, we beg the
question of what is meant by “locality”. Populations which appear to
be sympatric in their geographic distributions may diverge ecologically
so that they are effectively allopatric in their local distributions, even
in the absence of a competitive interaction. We know that closely
related species frequently coexist in the same habitats, but there is
usually some level of spatial distribution at which they are separate.
What is required is a critical comparison between the spatial distributions of two supposed competitors and the amount of intersection in
their fundamental niches. Unless it can be shown that their spatial
distributions, at any level, reflect realized niches which are less than
what would be expected from an unrestricted intersection of their
fundamental niches, there is no reason to infer that competition is a
contributing factor.
Although there is no opportunity for competition to occur between
allopatric populations, this situation illustrates the difficulty of drawing
conclusions about niche relationships from observed distribution patterns. Allopatry may be due to an intervening barrier to dispersal or
habitat occupation, in spite of intersection between two fundamental
niches (Fig. 3 B); or the allopatric distribution may reflect a difference
in fundamental niches (Fig. 3 A) such that N, and N, are separate sets,
or at least do not intersect in the niche elements that affect spatial
distribution. The Grst of these relationships is shown in the distribution
of two races of song sparrow (Passerella melalia) in the San Francisco
Bay area. One race occupies tidal marshes and the other is confined to
nearby fresh-water habitats (Marshall, 1948). The intervention of unsuitable environments forces them to behave like allopatric species, but
there is hybridization between them wherever there is contact between
RICHARD 5. MILLER
two niches would be precisely contiguous and non-intersecting (D),
this is a theoretical possibility which must at least be considered as a
possible cause of observed cases of contiguous allopatry in spatial
distributions.
The terms allopatry, sympatry and contiguous allopatry are conventionally used to describe geographic distributions, but analogous
patterns occur at all levels of distribution and it is not always possible
to distinguish between ecological events which affect geographic, habitat
or microhabitat distributions (Miller, 1964a). The related questions of
spatial distribution and coexistence are relative terms, depending on
the level of spatial distribution chosen as a criterion and the sensitivity
of sampling procedures in relation to this criterion. For this reason, it
seems advisable to consider the general phenomenon of spatial distribution as a single event which can be qualified according to specific
circumstances. When it is stated, according to the Gause axiom, that
no two species can coexist indefinitely in the same locality, we beg the
question of what is meant by “locality”. Populations which appear to
be sympatric in their geographic distributions may diverge ecologically
so that they are effectively allopatric in their local distributions, even
in the absence of a competitive interaction. We know that closely
related species frequently coexist in the same habitats, but there is
usually some level of spatial distribution at which they are separate.
What is required is a critical comparison between the spatial distributions of two supposed competitors and the amount of intersection in
their fundamental niches. Unless it can be shown that their spatial
distributions, at any level, reflect realized niches which are less than
what would be expected from an unrestricted intersection of their
fundamental niches, there is no reason to infer that competition is a
contributing factor.
Although there is no opportunity for competition to occur between
allopatric populations, this situation illustrates the difficulty of drawing
conclusions about niche relationships from observed distribution patterns. Allopatry may be due to an intervening barrier to dispersal or
habitat occupation, in spite of intersection between two fundamental
niches (Fig. 3 B); or the allopatric distribution may reflect a difference
in fundamental niches (Fig. 3 A) such that N, and N, are separate sets,
or at least do not intersect in the niche elements that affect spatial
distribution. The Grst of these relationships is shown in the distribution
of two races of song sparrow (Passerella melalia) in the San Francisco
Bay area. One race occupies tidal marshes and the other is confined to
nearby fresh-water habitats (Marshall, 1948). The intervention of unsuitable environments forces them to behave like allopatric species, but
there is hybridization between them wherever there is contact between