60
RICHARD S. MILLER
where both species do coexist, 0. virilis occupies rocks and 0. irnmunis
is found on mud. More concrete evidence for interspecies competition
was provided by laboratory experiments in which substrate preferences
were tested with both species in the same tank. A series of contests
took place in which the less aggressive individuals were evicted from
the more desirable crevices between stones. The stream species, 0.
virilis, eventually occupied the rocks, leaving the mud substrate to the
pond species, 0. immunis. Thus, 0. virilis has a more restricted niche,
by virtue of its inferior ability to burrow in mud and to tolerate low
oxygen tensions, but it is the more aggressive species and survives by
excluding 0. immunis.
E. I N S E C T S
The ant's Messor barbarus and M . aegyptiacus are harvesters which
make long processions from their nests to harvest grass seeds. Pickles
(1944) studied the relationships between these two species in Algeria
and found that the foraging territory of a nest of M . aegyptiacus was
2 348 yd2 (1 963 m2) with a foraging distance of 82 ft (25 m). This nest
was within a larger space occupied by M . barbarus, which had a foraging
territory of 7 857 yd2 (6 569 m2) and a foraging distance of 45 ft (14 m).
Processions of M . aegy@iacus frequently led over the nest of the M .
barbarus and beyond it, showing that the presence of M . barbarus did
not deter M . aegyptiacus from foraging in this direction. On only a few
occasions, however, did M . barbarus forage in the direction of the M .
aegyptiacus nest, and then only a few single individuals were involved.
Eventually a series of battles took place during which there was considerable damage to members of the M . barbarus population and they
abandoned the old nest and constructed a new one farther away. These
two species apparently eat the same foods and have very similar habit'at
requirements. M . aegyptiacus can occupy areas within the foraging
territories of M . barbarus because of its greater competitive ability,
which in this case involves direct aggression. M . barbarus can, at the
same time, survive outside the immediate foraging territory of M .
aegyptiacus because M . barbarus has a greater foraging distance. This
system of relationships therefore allows coexistence of both species
within the same broad habitat.
While it is unlikely that the niche of one species will be entirely
contained within the niche of another, it is evident that the critical
factors which affect the outcome of competition between two species
can often be reduced to a few simple variables which do have this
relationship. This would seem to be especially true for homiotherms
which, because of their adaptations to climatic factors, are subject to
fewer controls and are perhaps more likely to have evolved a strong
RICHARD S. MILLER
where both species do coexist, 0. virilis occupies rocks and 0. irnmunis
is found on mud. More concrete evidence for interspecies competition
was provided by laboratory experiments in which substrate preferences
were tested with both species in the same tank. A series of contests
took place in which the less aggressive individuals were evicted from
the more desirable crevices between stones. The stream species, 0.
virilis, eventually occupied the rocks, leaving the mud substrate to the
pond species, 0. immunis. Thus, 0. virilis has a more restricted niche,
by virtue of its inferior ability to burrow in mud and to tolerate low
oxygen tensions, but it is the more aggressive species and survives by
excluding 0. immunis.
E. I N S E C T S
The ant's Messor barbarus and M . aegyptiacus are harvesters which
make long processions from their nests to harvest grass seeds. Pickles
(1944) studied the relationships between these two species in Algeria
and found that the foraging territory of a nest of M . aegyptiacus was
2 348 yd2 (1 963 m2) with a foraging distance of 82 ft (25 m). This nest
was within a larger space occupied by M . barbarus, which had a foraging
territory of 7 857 yd2 (6 569 m2) and a foraging distance of 45 ft (14 m).
Processions of M . aegy@iacus frequently led over the nest of the M .
barbarus and beyond it, showing that the presence of M . barbarus did
not deter M . aegyptiacus from foraging in this direction. On only a few
occasions, however, did M . barbarus forage in the direction of the M .
aegyptiacus nest, and then only a few single individuals were involved.
Eventually a series of battles took place during which there was considerable damage to members of the M . barbarus population and they
abandoned the old nest and constructed a new one farther away. These
two species apparently eat the same foods and have very similar habit'at
requirements. M . aegyptiacus can occupy areas within the foraging
territories of M . barbarus because of its greater competitive ability,
which in this case involves direct aggression. M . barbarus can, at the
same time, survive outside the immediate foraging territory of M .
aegyptiacus because M . barbarus has a greater foraging distance. This
system of relationships therefore allows coexistence of both species
within the same broad habitat.
While it is unlikely that the niche of one species will be entirely
contained within the niche of another, it is evident that the critical
factors which affect the outcome of competition between two species
can often be reduced to a few simple variables which do have this
relationship. This would seem to be especially true for homiotherms
which, because of their adaptations to climatic factors, are subject to
fewer controls and are perhaps more likely to have evolved a strong