Abstract

When surveying a modern community with a good fossil record, two components are recognizable: species with a fossil record (immigrants to the Recent, I) and species without a fossil record (originating in the Recent, O). How many species will emigrate into the future is, of course, unknown. If we treat a slice of time in the past as representing the “present”, older geologic time-slices as the past and younger geologic time-slices as the “future”, four components are recognizable. The I’s can be divided into immigrants which become emigrants, IE, and immigrants which become extinct, IX; while the O’s are those originating that emigrate, OE, and those originating which become extinct, OX. Thus, the total number of species observed is S = IE + IX + OE + OX. The “future” consists of emigrants in time, E = IE + OE. The question arises as to whether or not information gleaned from such an approach is transferable to modern communities. With this in mind, we selected five Cenozoic time-slices and examined these four components of benthic foraminifera in the Salisbury-Albemarle Embayment (SAE) of the North American Atlantic coastal plain.

In all the fossil communities the species are distributed about equally (± 10%) between O and I components. The OX component averages about half of the O species and are often rare. These species can be regarded as failed “evolutionary trials” and are a minimum estimate for extinction. Of the five fossil communities, three are balanced where OE = IX so that I = E and the total number of extinctions is TX = IX + OX = O. In balanced communities all of the components can easily be predicted, if one is known. In the two unbalanced communities, OE > IX so that E > I. In these two cases, IX is small and, consequently, TX ≈ OX.

Based on our observations of the fossil foraminiferal communities of the SAE, we would predict that about half of the observed species in a modern community will be extinct within two to three million years. A minimum estimate, the OX component, constitutes about a quarter of the observed species. While many of the OX species occur rarely, survivorship of species with a fossil record (I) cannot be predicted on the basis of abundance.

Consideration of the time slices also indicates that the local foraminiferal community depends on continual transfer of species into and out of a regional species pool. Consequently, if other organisms behave like foraminifera, any conservation efforts must be directed toward a large geographic area.

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