For the last 6 million years or so, the Earth has been subjected to drastic temperature fluctuations. During the Pliocene, the Earth underwent cycles of warm wet and cooler dry periods. Substantial continental ice began to form in the Northern Hemisphere around 3 million years ago (Mya), before the planet was plunged into a series of ice ages. Although you may think of an "ice age" as a snowy period after which woolly mammoths and neanderthals disappeared, you may not know that currently we are simply enjoying a "summer" break from this ice age. The last 2 millions years of Earth's history has consisted of various glacial periods interspersed by warmer times, known as interglacial periods.
NB - Collectively these fluctuations have been referred to as the Quaternary or Pleistocene Glaciation, although it is also common to simply refer to the glacial periods as Pleistocene glaciations or glacial maxima.
During glacial periods, average global temperatures cooled 5-10ºC. Images of a snowy iceworld spring to mind. However, glacial periods did not affect everywhere in the same way. Certainly, most of Europe and North America were engulfed by ice, but the tropics, subtropics, and even parts of Southern Europe remained relatively warmer. The ice-free lands of Southern Europe provided sanctuary for temperate species which would have otherwise gone extinct during the glacial periods. The fluctuating climate caused species distributions to expand and retreat in accordance with their habitat. Genetic signatures of population bottlenecks can be found in some species, such as small canids and felids of Europe, while other animals, such as wolves, may not have been greatly affected. The pockets of habitat temperate-adapted species were restricted to during glacial periods, and those cold-adapted species were restricted to during interglacial periods, are known as refugia. The tropics and subtropics also retained refugia, where rainforest species would survive the spread of dry savannah during global cooling, and montane species would descend to escape spreading ice caps.
All this happened on land – what about marine habitats? Water temperatures and sea levels dropped as the higher latitudes froze. Unsurprisingly, this also altered ocean currents and had a major affect on marine life. However, even in the ocean there were refugia for species accustomed to water climes. The Indo-Pacific Warm Pool is thought to have provided refuge for corals and reef fish during the Earth's cooler periods. The discovery of a species of dinoflagellate originally thought to have gone extinct at the end of the Pliocene supports this theory. Dinoflagellates are a type of unicellular protists which are important components of planktonic communities. Dapsilidinium pastielsii is a species which was once widespread throughout the world's ocean during warmer epochs of geologic history. It was thought to have gone extinct as the Earth began to slip into much cooler conditions. Mertens and colleagues revealed that this little dinoflagellate appears to have survived in a few pockets of the Indo-Pacific Warm Pool, the largest area of warm water on the planet (with current average surface temperatures of 28ºC).
The heat-loving protist's current confinement is supposedly a result of ocean temperature fluctuations during the last 3 My and raises interesting questions with regards to the future of our planet's climate. As temperatures increase as a result of human-induced global warming, it is predicted that a great many species will be lost as a consequence. However, some species like D. pastielsii may increase in range. The authors who rediscovered D. pastielsii emphasise the role of refugia in the survival of warm-adapted species during cooler times and their contribution to biodiversity hotspots. Perhaps a more interesting message that can be taken from this study is the importance of using certain planktonic species as indicators of global warming in the future. Plankton are often used as indicators of aquatic ecosystem health and some can cause huge ecological problems when out of control, such as algal blooms. Though it appears D. pastielsii likes particularly warm waters, the population dynamics of other planktonic species can provide useful information about the state and health of aquatic ecosystems, which will become increasingly important in a rapidly changing world.
Beaugrand, G. (2005). Monitoring pelagic ecosystems using plankton indicators. ICES J. Mar. Sci., 62 (3)
Didier Paillard, D. (1998). The timing of Pleistocene glaciations from a simple multiple-state climate model. Nature 391: 378-381
Gagan, M. K. et al (2004). Post-glacial evolution of the Indo-Paciﬁc Warm Pool and El Niño Southern oscillation. Quaternary International 118–119: 127–143
Haffer, J. (1987). Quaternary history of tropical America. In Biogeography and Quaternary History of Tropical America. Eds. Whitmore, T. C. and Prance, G. T. 1—16. Claredon Press, Oxford.
Lawrence, K. T. et al. (2006). Evolution of the Eastern Tropical Pacific Through Plio-Pleistocene Glaciation Science 312: 79-83
Mertens, K. N. et al. (2014). Living fossils in the Indo-Pacific warm pool: A refuge for thermophilic dinoflagellates during glaciations. Geology, online 10 April 2014, DOI: 10.1130/G35456.1
Sommer, R. and Benecke, N. (2005). Late-Pleistocene and early Holocene history of the canid fauna of Europe (Canidae). Mammalian Biology, 70(4): 227-241.
Sommer, R. and Benecke, N. (2006). Late Pleistocene and Holocene development of the felid fauna (Felidae) of Europe: a review. Journal of Zoology, 269: 7–19