Observed species diversity trends have been explained by the time-stability model of Slobodkin and Sanders. Their model predicts low rates of speciation 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 Mandinga, Veracruz, Mexico (19°04^primeN lat., 96°04^primeW long.), lies south of the Tropic of Cancer, and is a small (20.39 sq mi o 53 sq km) tidal lagoon indenting the coastal plain of Mexico about 5 mi (8 km) southeast of the port of Veracruz; and Bay St. Louis, Mississippi (30°20^primeN lat., 89°20^primeW long.), is a shallow bay of slightly smaller area (16.52 sq mi or 43 sq km) about 13 mi (21 km) southwest of Gulfport. These two brackish-water bodies are separated by 11°16^prime lat. (~716 mi or 1,146 km) and 6°44^prime long. (~422 mi or 675 km). Both water bodies have been quantitatively sampled for benthic ostracodes. Total (live + dead) populations from 35 sample stations in Laguna Mandinga and 23 stations in Bay St. Louis (^Sgr = 58) furnish data which provide an estimate of latitudinal diversity gradients between these two depositional environments. Diversity measures calculated for ea h water body include S (number of species), H(S) (Shannon-Wiener 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 (1.214) are also higher than in the Bay (0.633). 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-s ability hypothesis, and suggest the tropical Mexican ostracode fauna is slightly older, hence more diverse than the temperate northern Gulf fauna.

End_of_Article - Last_Page 1562------------