1977] 
Porter — Mesostenines 
91 
tition. On the other hand, drastically stressed habitats, where the 
main selective pressure is toward mere survival, may have few spe- 
cies, none of which is outstandingly abundant. The arid Subandino, 
with its relict mesostenine populations surviving in scattered humid 
refugia, doubtless approximates this last model. Finally, the Lower 
Rio Grande Valley, because of its moderately large fauna including 
many scarce and two superabundant species, seems consistent with 
the penultimate case. Here the main natural stress is rather low 
and very irregularly distributed rainfall but to this are added the 
potent anthropogenic factors of habitat destruction and pesticide 
contamination. Very little natural flora remains in the Valley and 
the existing parks and wildlife refuges are surrounded by heavily 
sprayed citrus groves, truck farms, and other crop systems. Under 
these conditions, it is not surprising that a few exceptionally toler- 
ant species, such as Agonocryptus discoidaloides, which can thrive 
in disturbed as well as natural areas, have attained unusually high 
abundance. 
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