6) 3
PATTEI~N A N D PR(W;SS IN c o a i r i m ~ ~ w ~
Y
the two habitats. The fundamental niches of the two populations
intersect but their realized niches are expressed in allopatric distributions because of ecological barriers.
In the absence of barriers to dispersal and habitat occupancy, intersection in the spatial elements of two fundamental niches will be
expressed in various dimensions of sympatry, depending on the nature
of the competitive interaction that results from intersection and the
spatial requirements of the species concerned. There is considerable
evidence showing that competitive exclusion at the level of broad
habitat or geographic distributions is invariably accompanied by competitive interference. Species which compete primarily through exploitation are more often controlled by physical factlors which affect their
distribution and abundance and are less likely to be spatially exclusive,
except at the level of mxrohabitat distribution. They are also more
likely to show complete differentiation in some critical element of their
fundamental niches.
Townes and Townes (1960) remarked on the apparent coexistence of
three species of ichneumon wasp (Negarhyssa atrata, M . macrurus and
M . greenei), all of which parasitize the same host, the wood-boring larva
of the pigeon tremex (Tremex columba). A female Megarhyssa detects
a host larva or pupa in a dead log or stump and inserts her ovipositor
full length into the wood to deposit an egg on the host. The ovipositor
is directed a t a right angle to the surface of the wood and all ovipositions
require the complete insertion of the ovipositor (Heatwole and Davis,
1965). Tremex larvae occur a t various depths in the wood, but once
established an individual larva tends to remain a t a constant depth.
Thus the larva to be parasitized must be at a specified depth corresponding to the length of the extended ovipositor of the female wasp.
Figure 4 shows a comparison of the ovipositor lengths of these three
species of Megarhyssa. There is no overlap in the range of ovipositor
lengths for M . atruta and M . macrurus and only a slight overlap between
M . macrurus and M . greenei. There is a significant statistical difference
in their means and standard deviations (Heatwole and Davis, 1965)
with respect to this structural feature, and it is evident that they do
not compete for the same larvae but parasitize different segments of
the total host population. I n spite of their apparent sympatry, even
to the extent of the adults using the same foods, resting places and
oviposition sites and the larvae using the same resource in the same
habitat, there is no intersection with respect to the critical factor of
ovipositor length. The relationship between the fundamental niches of
pairs of these three species is described by Fig. 3 A or D.
It is possible that competition w-as a historical factor in the ecological
differentiation that now exists anioiig these three species, but its action
PATTEI~N A N D PR(W;SS IN c o a i r i m ~ ~ w ~
Y
the two habitats. The fundamental niches of the two populations
intersect but their realized niches are expressed in allopatric distributions because of ecological barriers.
In the absence of barriers to dispersal and habitat occupancy, intersection in the spatial elements of two fundamental niches will be
expressed in various dimensions of sympatry, depending on the nature
of the competitive interaction that results from intersection and the
spatial requirements of the species concerned. There is considerable
evidence showing that competitive exclusion at the level of broad
habitat or geographic distributions is invariably accompanied by competitive interference. Species which compete primarily through exploitation are more often controlled by physical factlors which affect their
distribution and abundance and are less likely to be spatially exclusive,
except at the level of mxrohabitat distribution. They are also more
likely to show complete differentiation in some critical element of their
fundamental niches.
Townes and Townes (1960) remarked on the apparent coexistence of
three species of ichneumon wasp (Negarhyssa atrata, M . macrurus and
M . greenei), all of which parasitize the same host, the wood-boring larva
of the pigeon tremex (Tremex columba). A female Megarhyssa detects
a host larva or pupa in a dead log or stump and inserts her ovipositor
full length into the wood to deposit an egg on the host. The ovipositor
is directed a t a right angle to the surface of the wood and all ovipositions
require the complete insertion of the ovipositor (Heatwole and Davis,
1965). Tremex larvae occur a t various depths in the wood, but once
established an individual larva tends to remain a t a constant depth.
Thus the larva to be parasitized must be at a specified depth corresponding to the length of the extended ovipositor of the female wasp.
Figure 4 shows a comparison of the ovipositor lengths of these three
species of Megarhyssa. There is no overlap in the range of ovipositor
lengths for M . atruta and M . macrurus and only a slight overlap between
M . macrurus and M . greenei. There is a significant statistical difference
in their means and standard deviations (Heatwole and Davis, 1965)
with respect to this structural feature, and it is evident that they do
not compete for the same larvae but parasitize different segments of
the total host population. I n spite of their apparent sympatry, even
to the extent of the adults using the same foods, resting places and
oviposition sites and the larvae using the same resource in the same
habitat, there is no intersection with respect to the critical factor of
ovipositor length. The relationship between the fundamental niches of
pairs of these three species is described by Fig. 3 A or D.
It is possible that competition w-as a historical factor in the ecological
differentiation that now exists anioiig these three species, but its action