SHOULD THE LONG-TERM BIOTIC RESPONSES TO ENVIRONMENTAL CHANGES BE LINEAR/EXPONENTIAL?: PERSPECTIVES FROM PLANKTIC FORAMINIFERA AND MARINE BIVALVES

Attempts to determine the role of environmental change in driving biotic responses has often been viewed through the lens that the expectation should be one of a linear or exponential process. This perspective has largely been driven by a desire to: (i) simplify incredible complexity and (ii) to have a level of prediction within biologic systems that has been fueled by a few well-established relationships as exemplified by the species-area effect. This expectation for a predictable response has, at least to a degree, resulted in an expectation that long-term environmental and biotic data, especially richness/diversity trends at various taxonomic levels, should correlate. Mismatches have been either ignored or attributed to various complications. Here, we examine this expectation using data compiled from two groups: the planktic Foraminifera as well as the benthic marine Bivalvia (Phylum Mollusca) and comparing them to various environmental proxies, including temperature and sea level. Our results suggest that initial phases in a clade’s diversification are driven largely by evolutionary controls and, therefore, the impact of various environmental factors is difficult to assess. This is followed by the adaptive radiation of the group (which occurred ~30 Ma and ~40 Ma after their initial appearance for foraminfers and bivalves, respectively). Once these groups reached relatively higher standing diversity levels, the overall climatic regime – the alternation between green- and icehouse climates – appears to play a substantial role in regulating the biotic response. These intervals are associated with different types of controls on biotic diversification (i.e., changes in both oceanic stratification and in oceanic circulation patterns) which substantially disrupt a linear/exponential response. Instead, the various ‘tipping points’ that result in relatively rapid shifts from one climatic mode to another result in a range of variables which impact the biota in a non-linear fashion. We suggest that the impact of dramatic shifts in richness/diversity across a broad spectrum of environmental proxies should be potentially re-examined from a non-linear-response perspective.