Permalink : http://hdl.handle.net/2115/38437

KEYWORDS : Fossil birds;Climate change;Evolution;Extinction

The fossil record in North America indicates that considerable turnover in avian species occurred during a series of Ice Ages that began at approximately 2.5 million years ago (Ma) and demonstrates the impact of climate change on speciation and extinction in birds. Prior to the onset of these Ice Ages, recent research on early Pliocene fossil birds from Kansas has shown that speciation also can be associated with long periods of climatic stasis, indicating that mechanisms other than climate change may lead to diversification in birds. Fossils of seven living songbirds from the early Pliocene Rexroad Formation evince that diversification in longspurs (Emberizidae: Calcarius spp.), pipits (Anthus spp.), and meadowlarks (Sturnella spp.) had occurred by this time, perhaps in relation to the concurrent formation of rich grassland habitats with high seasonal productivity in the mid-continental region. Continuous episodes of climate change during the Ice Ages caused further speciation, as well as many extinctions, in birds. This pattern is particularly evident in Florida, where the fossil record is relatively complete. Dozens of sites are known from this region that range in age from late Pliocene to late Pleistocene and Holocene. This record is comprised of 239 fossil and living taxa and previous analysis has shown that sea-level changes associated with climate change are correlated with originations and extinctions of birds in conjunction with loss or gain of land area. During interglacial periods, sea level rise affected coastal wetland habitats and caused extinctions of wetland birds, especially in south Florida, due to loss of habitat area. During glacial periods, falling sea levels increased exposure of the shallow continental shelf in the Gulf of Mexico and facilitated range expansions of Neotropical and western continental species into Florida. These range expansions probably caused competition and predation between native and immigrant species, resulting in additional extinctions and extirpations. Thus, climate change has caused a step-wise loss in avian diversity in Florida since the beginning of the Ice Ages, despite originations that also occurred during this time. Finally, the warming period at the late Pleistocene-Holocene transition caused further losses of avian diversity in North America, as modern communities developed by 6000 B. P. All these records indicate a steady loss of avian diversity in North America over the past 2.5 Ma and counter arguments that the Ice Ages had minimal impact on species extinctions. Current threats to avian diversity from rapid climate change are due to human-induced impacts, either direct or indirect, and portend a relatively greater loss to avian biodiversity than during any of the natural events that have occurred in the past.