ENVIRONMENTAL AND ECOLOGICAL CORRELATES OF LATE PLEISTOCENE-RECENT GEOGRAPHIC RANGE SHIFTS IN THE CALIFORNIAN PROVINCE

Marine terraces of the California coast preserve an excellent record of the geographic distributions of rocky intertidal mollusks during Pleistocene interglacial highstands. Many species found only in the Oregonian or Panamic provinces today range into the Californian province during the Pleistocene. Such “extraprovincial” species are of great interest for considering future global change scenarios. Previous work has shown that extraprovincials in the Californian province are not randomly drawn from the Panamic and Oregonian provincial species pools. However, interpreting the relationship between Pleistocene climate change and range shifts has been hampered by coarse temporal resolution: it has not been possible to examine geographic ranges during interglacial highstands. To overcome this limitation we limited our analysis to molluscs from marine isotope (MIS) substages 5e (~120 kya) and 5a (~80 kya), and only those from terraces with ages that are well constrained by either uranium-thorium or amino acid dates.

To determine which traits are most closely associated with MIS 5 range shifts we examined a variety of potential predictors including adult body size, larval ecology and modern salinity, net primary productivity, depth, and temperature ranges. Preliminary results confirm that extraprovincial species are non-randomly drawn from the pool of potential extraprovincials, but the traits most closely associated with extraprovincial species vary between substages. Notably, the traits predicting extra-provinciality do not appear to be simple extrapolations of the traits predicting smaller, within-province geographic range shifts. For example, preliminary results indicate that stage 5a northern extralimitals (species that occur farther south in the Pleistocene than at present day) have broader depth ranges than other Californian province species, perhaps suggesting that cool temperatures during this stage permitted these species to emerge from offshore depths in their southern ranges to invade the rocky intertidal. The effect of depth range on northern extraprovincials in the 5a, however, is negligible and instead appears to be determined by environmental tolerances linked to provincial boundaries.