90 
Psyche 
[March 
It would be desirable to compare equitability figures for the semi- 
arid Valley and the arid Subandino with data from a comparable 
long-term survey of mesostenines in some optimum Neotropic wet 
forest habitat. Although complete information of this type is not 
yet available, results of my 10 month 1973 Malaise project at Gen- 
eral Saavedra, Bolivia may be taken as fairly typical for a humid 
forest community. At Saavedra, 27 (64%) of the 42 species trapped 
were represented by one specimen each, so that there are almost 
twice as many very rare species at Saavedra as in the Subandino or 
south Texas. Of the additional 15 Saavedra mesostenines, 1 ac- 
counted for 10.4% of the total number obtained, 1 for 9.4%, 2 for 
7.3% each, 1 for 4.1%, 3 for 3.1% each, and 6 for 2.1% each. This 
fauna thus shows even more equitability of commoner species than 
the Subandino. The two most abundant Saavedra mesostenines, 
Diapetimorpha sp. 3 and Agonocryptus sp. 1, together account for 
10 and 9 specimens respectively, or only 20% of all specimens ob- 
tained. On the other hand, 96 specimens were trapped at Saavedra 
during 10 months and this shows much greater overall abundance 
than the Subandino records (56 specimens per year by Malaise 
Traps and net), being roughly comparable to my 12 month Bentsen 
Park Malaise catch of 138 specimens. The Saavedra fauna thus 
is numerically large but composed entirely of scarce to very rare 
species. 
We can only speculate why evenness shows such marked differ- 
ences among the three mesostenine communities studied. Saavedra 
with its many rare species has a benign, thermically and pluvially 
rather stable climate and the trap employed there was situated in 
a large patch of undisturbed wet forest, Such environments, where 
severe physical stress is absent, traditionally are supposed to ac- 
commodate large numbers of species in a complex variety of niches 
determined mainly by selective pressure of interspecific competi- 
tion. Under these circumstances, reproductive success is less im- 
portant than niche differentiation for avoidance of competition 
and many groups are represented by many species, each one of 
which may be relatively uncommon. Toward the other extreme of 
the equitability scale, stressed environments have fewer species, 
some of which are rare since they exist at the limits of their eco- 
logical tolerance while others may be disproportionately abundant 
because some special adaptation allows them to survive the burden 
of the stress, so that they flourish in a context of minimal compe- 
