Nutrients and Extinction

Nutrients and productivity may affect diversity on evolutionary timescales not only through their effects on the origin of species, but also through their effects on the extinction of species. A number of studies have noted a correlation between trophic pattern and susceptibility to extinction in benthic marine invertebrates, specifically the apparently higher extinction rate of suspension feeders compared with nonsuspension feeders, although interpretations for the cause of these correlations vary. Levinton (1974) noted that suspensionfeeding bivalves showed lower generic survivorship (and therefore higher extinction probability) over the entire Phanerozoic than deposit-feeding bivalves and attributed this difference to the greater "instability" of the suspension feeders' food source. (He reversed himself in 1996, arguing that differences in extinction rates among bivalve trophic patterns may relate to other, nontrophic factors.) Similarly, Paleozoic gastropod genera inferred to have been suspension feeders show higher extinction rates (during both mass and background extinction) than do nonsuspension feeders (Allmon et al. 1992). For Cretaceous mollusks, Kauffman (1972,1977) found that suspension feeding bivalves and gastropods show much shorter species durations than deposit-feeding bivalves. Sheehan and Hansen (1986) note that suspension feeders show higher extinction rates than deposit feeders across the Cretaceous-Tertiary boundary and suggest that this was due to decimation of plankton in the water column. This interpretation is disputed by Levinton

(1996). This line of reasoning has also been applied at higher trophic levels. Jeppson (1990) suggests that diversity of Silurian conodonts (presumed to be nektonic carnivores) was negatively impacted by decline in nutrient supplies around Baltica.

The strongest evidence for a linkage between nutrients and extinction comes not from studies of individual clades, but from more general studies attempting to understand paleoenvironmental conditions around mass extinction boundaries. Many such studies have concluded that at least some mass extinction events were associated with massive collapses of marine productivity (e.g., Vermeij 1995). A number of authors have concluded that the Cretaceous-Tertiary boundary is coincident with a dramatic reduction in primary productivity in the oceans (Bramlette 1965; Tappan 1968,1986; Sheehan and Hansen 1986; Arthur, Zachos, and Jones 1987; Corfield and Shackleton 1988; Zachos,Arthur, and Dean 1989; Paul and Mitchell 1994; Martin 1998a,b; Smith and Jeffery 1998), a condition that persisted for at least several hundred thousand years (Caldeira and Rampino 1993; Hollander, McKenzie, and Hsu 1993) and perhaps as long as three million years (D'Hondt et al. 1998). Similar nutrient decreases of various magnitude have also been claimed for the Permo-Triassic extinction event (Wang et al. 1994; Martin 1996,1998a,b), the Cenomanian-Turonian event (Paul and Mitchell 1994), the Paleocene-Eocene event (Rea et al. 1990), and for the Pliocene event in the Western Atlantic (Allmon 1992b; Allmon et al. 1996a,b). Increased instability in seasonal productivity has been implicated in the Late Eocene event (Purton and Brasier 1997). As mentioned previously, for taxa favoring oligotrophic conditions, a rapid increase in nutrient levels might lead to increased extinction; this has been suggested for photosymbiont-bearing foraminifera in the late Paleozoic (Martin 1995,1998a,b) and Late Eocene (Brasier 1995).

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