Ecological Change, Range Fluctuations and Population

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Current Biology 19, R584–R594, July 28, 2009 ª2009 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2009.06.030 Ecological Change, Range Fluctuations and Review Population Dynamics during the Pleistocene Michael Hofreiter1,* and John Stewart2 For example, during times of changes from stadials to interstadials the average temperature changed by 5–10C within as little as a few decades [4]. Similarly, during glacial maxima, Apart from the current human-induced climate change, the temperature was up to 21 C colder compared to today the Holocene is notable for its stable climate. In contrast, [5]. Only during these times did the northern hemisphere the preceding age, the Pleistocene, was a time of intensive conform to the movie image of the Pleistocene with extensive climatic fluctuations, with temperature changes of up to glaciers covering large parts of Eurasia and North America. 15C occurring within a few decades. These climatic The large amounts of water bound in glaciers also resulted changes have substantially influenced both animal and in sea-levels being up to 120 m lower than today and thus plant populations. Until recently, the prevailing opinion in a much larger land area with, for example, the British Isles about the effect of these climatic fluctuations on species being connected to the European mainland and the North in Europe was that populations survived glacial maxima Sea being a large area of dry land. However, as recent in southern refugia and that populations died out outside research has shown, the extent of the northern glaciers was these refugia. However, some of the latest studies of much smaller during most of the Pleistocene [6]. Moreover, modern population genetics, the fossil record and espe- it should be noted that during some interglacials, such as cially ancient DNA reveal a more complex picture. There the Eemian (127,000–117,000 years ago [7]), temperatures is now strong evidence for additional local northern refugia were warmer than today. It was during these times that for a large number of species, including both plants and elephants (the extinct straight-tusked elephants Elephas animals. Furthermore, population genetic analyses using antiquus) and hippopotamus were living as far north as the ancient DNA have shown that genetic diversity and its British Isles (Figure 1, lower image) [8]. geographical structure changed more often and in more Clearly, these massive climatic and environmental unpredictable ways during the Pleistocene than had been changes significantly influenced the distribution and genetic inferred. Taken together, the Pleistocene is now seen as diversity of plants and animals. The idea that, during times of an extremely dynamic era, with rapid and large climatic adverse climate, species track their habitat goes back to fluctuations and correspondingly variable ecology. These Darwin [9], and the Pleistocene should represent an excel- changes were accompanied by similarly fast and some- lent opportunity to test this assumption. Generally, one times dramatic changes in population size and extensive would assume that Arctic species would expand their distri- gene flow mediated by population movements. Thus, the bution southwards during colder times and that temperate Pleistocene is an excellent model case for the effects of species would expand northwards during warmer times. rapid climate change, as we experience at the moment, While this is straightforward in North America, with mountain on the ecology of plants and animals. chains, which represent partial barriers to range shifts, running from north to south, in Europe a level of complexity Introduction is added with mountain chains running from east to west The Pleistocene is often characterised as a time of massive and the available land mass becoming smaller to the south glaciers covering much of Eurasia and North America with and being divided into several peninsulas bordering the a fauna consisting of enigmatic creatures such as mammoths Mediterranean. This geography, together with numerous and sabre-toothed cats (Figure 1). As always, reality is more studies that found geographical patterns in the genetic complex. The Pleistocene started about 1.8 million years ago diversity of many species consistent with colonization of and ended 10,000 radiocarbon years ago (we use radio- mid-latitude and northern Europe from the Iberian Peninsula, carbon years throughout the text when referring to years Italy and the Balkans (for review, see [10,11]) has resulted ago or before present) [1]. The Pleistocene is notable for in the classical ‘refugium theory’, which proposes that massive climatic fluctuations. The causes of these climatic temperate species survived the glacial maxima in southern fluctuations are still an area of active research, but are refugia with little gene flow among them and colonized the already known in some detail. There are several layers of peri- more northern parts from there during interglacial times. odicity overlaying each other, at intervals of approximately While this model is theoretically sound and correct in many 100,000, 41,000 and 23,000 years [2], resulting in the aspects, recent studies on both modern and, especially, glacial–interglacial (cold–warm) cycles. Numerous additional ancient DNA diversity have shown that reality is much more cold and warm events overlie the main curve [3]. Therefore, complex and only very broadly follows a contraction–expan- during the Pleistocene, periods of cold and warm climate sion model for population dynamics, with many additional occurred at irregular intervals and with varying durations processes complicating the picture [12–16]. (Figure 2). Both the extent and the speed with which these Finally, the end of the Pleistocene is marked by a massive temperature fluctuations occurred are quite remarkable. extinction of large land vertebrates across most of the world (Box 1), with the exception of Africa [17]. Although these extinctions have long been known, their causes remain contro- 1Department of Biology, University of York, York YO10 5YW, UK. versial. While some authors blame humans [18], others deny 2Department of Palaeontology, Natural History Museum, Cromwell any human influence, at least on the continents, although Road, London SW7 5BD, UK. human-induced extinctions are widely accepted for islands *E-mail: [email protected] [19]. Again, recent research has revealed a great deal about Special Issue R585 Figure 1. Artistic renderings of Pleistocene landscapes in middle Europe. Upper: Mammoth steppe during a glacial phase with its namesake, woolly rhino and Neanderthals. Lower: Open woodland with straight tusk elephants, hippopotamus and cave hyenas during the Eemian. Images reproduced with permission from Natural History Museum, London. the timing and processes of these extinctions, showing that popular [10,11]. It certainly conforms to the notion that we not only mammoths [20,21], but also giant deer (deceivingly generally do not expect to find animals in large glaciated known as Irish elk) [22] and some Caribbean ground sloths areas. Moreover, this model leads to some predictions about [23], survived into the Holocene. However, when it comes the pattern of genetic diversity we expect. Thus, with an to the cause(s) of these extinctions, the verdict is still out. expansion into previously unoccupied territory, we expect In this review, we will discuss the dynamics of animal and genetic diversity to decrease from the south to the north plant populations during the Pleistocene, trying to outline [26], which is often what is observed. Similarly, the genetic how populations reacted to the rapid variations in climate. diversity of many species in Europe is divided geographi- We will restrict our analyses to the northern hemisphere, as cally, with the groups often broadly conforming to a western the majority of studies on Pleistocene DNA have been done group (derived from refugia in the Iberian Peninsula), on species from this region. a southern group (from Italy or the Balkans) and an Eastern group (whose refugial area was for a long time less clear; The Glacial-Refugium Theory e.g. [27]). Similar patterns have been shown for North Amer- It was first recognized 30 years ago that seemingly continu- ica, where, for example, modern brown bears can be divided ously distributed populations may show geographical struc- into several genetic groups with virtually no overlap in their ture in their genetic diversity [24]. Since then, this field, later geographical distribution. Finally, populations of temperate named phylogeography, has grown into an independent species in North America, in contrast to Amazonian species, discipline [11,25]. With increasing amounts of DNA sequence have been shown to display a signal of recent population data from a variety of species, a model of habitat contraction growth [28], indicating that conditions became more favour- to southern peninsulas during glaciations and expansion into able for temperate species relatively recently – an observa- northern parts during warmer times became increasingly tion compatible with habitat expansion in these species. Current Biology Vol 19 No 14 R586 –30 Holocene –32 1 24 –34 19 23 21 20 17 15 12 10 8 3 –36 22 16 14 7 4 18 13 11 9 6 5 2 –38 –40 Eem –42 –44 140 126 112 98 84 70 56 42 28 14 0 [ka cal BP] –32 –34 1 –36 3 4 2 –38 Holocene –40 –42 –44 5 4 3 2 1 30 28 26 24 22 20 18 16 14 12 10 8 6 24 0 [ka cal BP] Figure 2. Climate variation during the Pleistocene. The x- axes show time in calibrated thousands of years before present and the y-axes show variation in oxygen isotope ratio, whereby more nega- tive values indicate colder tempertaures. Top panel: Climate change during the last 140,000 years. Numbers show the Greenland interstadials, short warm periods during the last glacial.

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