The fundamental niche comprises the range of environmental variables under which a species can live. Specialist species have narrower fundamental niches than do generalists. Thus, specialists benefit from homogeneous environments while generalists can exploit heterogeneous ones. The challenge is distinguishing relative specialists from generalists in a sedimentary rock succession. This is overcome by taking replicate samples from several horizons in a heterogeneous formation and comparing species’ mean percentage abundances using analysis of variance (ANOVA). The mean proportional abundances of specialists will differ between horizons in a heterogeneous succession, while those of generalists will not. The Globorotalia fohsi robusta Zone Middle Miocene Cipero Formation of Trinidad, western central Atlantic Ocean, consists of alternating, diffuse layers of light to dark grey clays. Five replicates of 40 g each were taken from four horizons across a light- to dark-grey cycle. They were picked entirely clean of benthic foraminifera, of which they yielded 5186 specimens in 153 species. Total recovery was dominated by Brizalina cf. tectiformis, with lesser Globocassidulina subglobosa, Pleurostomella alazanensis and Planulina wuellerstorfi. ANOVA applied to each species in turn indicated three species groups: 64 accessory species (each represented by < 3 specimens, < 0.06% of total recovery); 21 specialist species, for which mean percentage abundance varied significantly between at least two horizons; and 68 generalist species, for which mean percentage abundances did not differ across the horizons. It might be thought that ANOVA would preferentially assign a specialist status to less abundant species. However, this did not happen; of the four most abundant species, P. alazanensis was a specialist, while B.cf. tectiformis, G. subglobosa and P. wuellerstorfi were generalists. The mean percentage abundance of the specialist species did not differ from that of the generalists. Both specialist and generalist species have palaeoenvironmental applications. The generalist nature of P. wuellerstorfi shows that bottom current strength probably did not differ across the four horizons. The occurrence of the specialist and opportunistic Epistominella exigua in the lowest horizon indicates an interlude of seasonal phytodetrital flux, while sequential increases in the mean percentage abundance of the specialists Oridorsalis umbonatus and Uvigerina auberiana show that the organic carbon flux increased across the four horizons.
