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[Vol. 88 
Pogonomyrmex rugosus and P. barbatus, essentially act like exact 
ecological equivalents (Holldobler 1976a). Davidson (1977a) has 
suggested that the distribution of several individually foraging 
Pogonomyrmex (maricopa, californicus, desertorum and magni- 
canthus) is consistent with the hypothesis that they replace each 
other between habitats. The pattern could also occur in some other, 
less completely documented cases, perhaps in Atta (Rockwood 
1973). 
Population Structure and Its Effects on the Spatial 
Distribution of Colonies 
Monogynous, queenright colonies are almost innevitably aggres- 
sive to conspecific nests or foragers, regardless of how territorial 
they are (Holldobler and Wilson 1977c). Polygynous colonies may 
or may not display internest aggression. Holldobler and Wilson 
(1977c) point out the importance of queen number in the mainte- 
nance of clear territorial borders. Species which commonly have 
polygynous colonies and/or those which adopt newly fertilized 
females to augment or replace females already in the nest do not 
always have strong intraspecific interactions; some do not form 
distinct colonies ( Formica yessensis, F. lugubris, Table 1). In these 
cases the location of nests should be predominantly determined by 
ecological factors, in particular the kind of resource defense the 
colony shows. Thus some species should retain overdispersed 
patterns of nest distribution while other show clumped or random 
patterns (see model and predictions). 
Examining this issue is complicated by the lack of population 
structure data for many species. Several Formica species which 
form unicolonial populations, but depend upon randomly and 
unpredictably distributed resources, are found in overdispersed 
arrays [those species found in fields: F. pratensis (provisionally), F. 
uralensis (provisionally), F. opaciventris, F. exsectoides, Table 1]; 
those which nest along the margins of a habitat and/or which 
defend persistent resources tend to have more random or clumped 
distributions ( F . ulkei, F. rufa, F. lugubris, Table 1). Territory size 
in some tropical tree ants is partially a result of population 
structure. Many dominant species are polygynous and are able to 
expand their territories almost indefinitely under good ecological 
conditions (Greenslade 1971, Majer 1976a,b). In some cases, single 
