ABSTRACT

Observed species diversity trends have been explained by the time-stability model of Slobodkin and Sanders (1969) which predicts low rates of specification from equable into hostile environments. Species radiation from tropical, unstressed (equable) environments into higher latitude stressed (unequable) zones should be relatively slow. Speciation from hostile (stressed) to equable (unstressed) biotopes should occur faster. These differences in rate will theoretically result in lower species diversity in hostile regions. This model is tested with ostracode population data collected from two modern lagoons in the Gulf of Mexico -- Laguna Madinga, Veracruz, Mexico, and Bay St. Louis, Mississippi. Both water bodies have been quantitatively sampled for benthic ostracodes (Krutak, 1971, 1978). Total (live + dead) populations from 35 sample stations in Laguna Mandinga and 23 stations in Bay St. Louis (, = 58) furnish data which provide an estimate of latitudinal diversity gradients between these two depositional environments. Diversity measures calculated for each water body include S (number of species), H(S) (Shannon function), and E (Equitability). Calculations show that average S values in Mandinga (5.171) are higher than those in Bay St. Louis (2.870). Average H(S) values in Mandinga are also higher than in the Bay (0.633), and average E values in Mandinga (0.673) are lower than those in the Bay (0.842). These diversity values indicate ostracode species are more prolific in Gulf of Mexico tropical lagoons; they demonstrate lesser species dominance in tropical Mexican lagoons; and they show less species equitability in the tropical environments. Temperate bays in the northern Gulf apparently support fewer ostracode species, show stronger species dominance, and are more equable in species distribution. These data support the time-stability hypothesis and suggest the tropical Mexican ostracode fauna is slightly older, hence more diverse than the temperate northern Gulf fauna.