NEOGENE INCREASE OF WITHIN HABITAT BENTHIC FORAMINIFERAL SPECIES DIVERSITY: HIGHER RATE IN TROPICAL COMPARED TO TEMPERATE AMERICA

Most modern organisms, including foraminifera, exhibit a gradient of increasing species richness from high to low latitudes. On a global basis species richness, including the foraminifera, has increased with time, but the question as to whether or not the rate of change is the same at high and low latitudes has not been previously addressed. Here, we examine this question through the within habitat diversity of benthic foraminifera from temperate and tropical latitudes. To measure within habitat diversity we used Fisher's alpha instead of the number of species because the latter is highly correlated with the number of individuals in a sample. For temperate latitudes, we examined within habitat diversity of Eocene to Recent sediments from the central Atlantic Coastal Plain. A regression (N=298) of alpha versus time was significant indicating an increase in diversity with time. An ANOVA on five time intervals was significant and a contrast indicated a significant difference between mean values of alpha for the Neogene and Paleogene. Contrasts were also significant between the Recent and the Miocene, but not for Recent versus Pliocene or Pliocene versus Miocene. The analyses demonstrate a gradual increase of within habitat diversity during the Cenozoic. For tropical latitudes, we examined the within habitat diversity of Miocene to Recent sediments from southern Central America. A regression (N=75) of alpha versus time was significant indicating an increase in diversity with time. An ANOVA on three time intervals was significant and all contrasts were significant. The analyses indicate a large increase of within habitat diversity during the Neogene. During the Neogene the central Atlantic Coastal Plain average alpha increased from 10 (Miocene), to 11 (Pliocene) to 14 (Recent), while the southern Central America alpha increased from 17 (Miocene), to 28 (Pliocene) to 37 (Recent). The temperate alpha increased 1.4 times or 40%, while the tropical alpha increased 2.2 times or 118%. Although both areas show an increase of within habitat diversity with time, the tropics have clearly increased at a much faster rate. Thus, while a difference in within habitat diversity between temperate and tropical areas has existed throughout the Neogene, the difference has become larger with time and is more accentuated today than in the geologic past. Whether or not this is a general ecological phenomenon exhibited elsewhere in the world and by other groups of organisms is unknown.