Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 11

Jon P. Sadler1 and Eva Panagiotakopulu2

1 School of Geography, Earth and Environmental Sciences, The University of Birmingham, Birmingham, UK

2 School of GeoSciences, University of Edinburgh, Edinburgh, UK

There is no escaping the fact that the island biogeography of the North Atlantic Region is singularly peculiar. While it has aspects of the characteristics of many island groups in terms of disharmonic and impoverished species pools (Coope 1986), it lacks true endemics (Buckland 1988; Downes 1988, but cf. Böcher 1988), although it is home to a wide number of putative subspecies and races (e.g. Lindroth 1968; Löve and Löve 1963). Sitting in the north of the Atlantic Ocean these islands have been subjected to large‐scale shifts in climate over the last few million years, unlike the other island groups further south which were likely buffered from the vicissitudes of Quaternary climate changes (Sadler 2001). Unlike island groups elsewhere, there is only one documented extinction on these island groups (the Great Auk) and those in the insects are local events relating to species that are distributed throughout the Palaearctic region. Over half the insect species in Iceland and Greenland are non‐indigenous and many of these were first introduced to the islands by the Norse colonists (Buckland 1988; Panagiotakopulu 2014; Sadler 1990). The faunas, excluding Greenland (Böcher 1988, are predominantly Palaearctic (Buckland 1988; Downes 1988; Lindroth 1931), and have close affinities with the faunas of the British Isles and Scandinavia. These unique physical and biological characteristics have interested biologists and biogeographers for centuries (Hooker 1862).

In their seminal book North Atlantic Biota and Their History, Löve and Löve (1963) concluded that plants survived the heavily glaciated Atlantic areas, in ‘glacial refuges’ or ‘nunataks’, dismissing the earlier ‘now merely historical’ tabula rasa idea – the hypothesis that the current Atlantic floras were established following postglacial immigration from source areas situated outside the ice sheets. The virtually complete consensus among plant biogeographers (with notable exceptions among some palaeoecologists) at that time was based on two main arguments, the occurrence of (a) disjunct and (b) endemic taxa in the North Atlantic region. Firstly, the presence of disjuncts was believed to necessitate in situ glacial survival because long‐distance dispersal in general and trans‐oceanic dispersal in particular was considered to be extremely rare, if not impossible, because many amphi‐Atlantic disjuncts lacked adaptations for long‐distance dispersal. Secondly, the occurrence of endemics was believed to necessitate in situ glacial survival because the postglacial period was considered to be far too short to allow for evolution of endemic taxa, an argument that has been supported in later studies (e.g. Dahl 1987). The pervading view was to postulate in situ glacial survival not only during the last glaciation but throughout the entire Pleistocene, to explain the occurrence of disjuncts and endemics in the North Atlantic region. Other research on insects, principally by Lindroth (1957), also argued for early immigration via landbridges and glacial refugia to explain the biogeographical affinities of the biota. More recent work also supports the refugial hypothesis (Maggs et al. 2008).

Löve and Löve (1963) perceptively pointed to the fact that conclusive tests of these varying colonization hypotheses must await a more extensive fossil record. Indeed, recent investigations of a rich plant fossil record seem to support the view that a North Atlantic Land Bridge facilitated plant migration between North America and Europe until the late Miocene (e.g. Brochmann et al. 2003). The Quaternary fossil record is now very extensive and has been used by many to support tabula rasa and post‐glacial colonization via ice rafting at the terminal phase of the last glaciation (Buckland et al. 1988; Coope 1986). Moreover, the fossil beetle fauna demonstrates that even at the beginning of the previous interglacial period a similar situation might have occurred, capable of transporting European taxa over the North Atlantic to Greenland (Bennike and Böcher 1994; Böcher 2012).

Unlike several island archipelagos (e.g. Pacific and Indian oceans) and continental shelf islands (e.g. Magascar), Pre‐European settlement in the North Atlantic was restricted to Greenland where Inuit settlers colonized the islands in the mid‐Holocene (Andreasen 1996; Jensen 2006). Although some scholars have consistently argued for pre‐Viking settlement of the Faroe and Iceland (e.g. Church et al. 2013; Hermanns‐Audardóttir 1991), the first major European settlement of the North Atlantic region was undertaken by Norse or Viking colonists who reached Faroe by CE 850, Iceland by CE 870, Greenland by CE 986 and ultimately America (Newfoundland) by CE 1000 (Fitzhugh and Ward 2000). In Greenland, however, European settlements were abandoned by the sixteenth century (Arneborg 2003), although the island was recolonized by Norwegians some 200 years later. The role that these human colonists played in shaping biogeographical patterns shows pulses of European introductions linked to Norse arrival (e.g. Panagiotakopulu 2014). The impact of the settlers on the island ecosystems also had an indirect influence in shaping habitats and therefore assemblage dynamics. Palynological work has tracked the Landnám ‘footprints’ of the Norse settlers in the region (Edwards et al. 2008, 2011) and Dugmore et al. (2012) have recently presented a comprehensive review of the environmental impact of Norse farming practices on ecosystem management, soils and pasture management. Wholescale environmental modification of this magnitude has left its mark on the biota in terms of assemblage changes and local extinctions (Buckland and Panagiotakopulu 2010; McGovern et al. 2007).

Some 40 years have elapsed since Löve and Löve's (1963) volume was published and the key debates concerning the biogeography of the North Atlantic islands still rumble on. Was it cryptic refugia (Stuart and Lister 2001) or otherwise (Willis and Whittaker 2000), or tabula rasa and recolonization (Buckland and Dugmore 2010)? How important were human communities in shaping the existing biota and biogeographical patterns? Throw into this mix current concerns over global warming, we can now add how resilient is the biota to change, either natural or anthropogenic? This volume draws together a range of researchers with longstanding research interests in the region, from diverse academic backgrounds, to evaluate these questions.

This book is organized into sections each examining a particular theme. Section I focuses on the remote origins of the islands, diving deep into the early history of the region. Upton (Chapter 1) examines the opening of the North Atlantic from a geological perspective, charting its origin from super‐continent in the lower Palaeozoic, approximately 420–430 million years (Ma) ago, to the development of the North Atlantic Ocean as a late product of the disintegration of Laurasia, a part of Pangaea, which split to form North America, Greenland, Europe and Asia. The Cenozoic vegetation and phytogeography of the sub‐arctic areas are discussed by Grímsson et al. (Chapter 2). They examine the ‘Arcto‐Tertiary element’ hypothesis and present data that demonstrate that several north temperate tree taxa thrived in the sub‐arctic during the Paleogene, while also noting evidence for the presence of several ‘Arcto‐Tertiary elements’ in Greenland. They then go on to evaluate the possible role for the ‘The North Atlantic Land Bridge’, reviewing recent investigations of the rich plant fossil record, and demonstrate that the NALB facilitated plant migration between North America and Europe until the late Miocene. Moving forward in time, Bennike and Böcher (Chapter 3) examine the biotal record from the last interglacial period. They review the refugia‐tabula rasa debate in light of this record and point to areas where knowledge is lacking, such as the role of microclimate and insolation in supporting the former. They illustrate that during the interglacials there was a rich biota in suitable biotopes on the North Atlantic.

Section II of the volume examines the contentious issue of biotal origins. Brochmann and Alsos (Chapter 4) present new genetic evidence based on a total of 9018 plants from 1140 populations to re‐evaluate their earlier conclusions (Brochmann et al. 2003) on the origins and dispersal of the North Atlantic vascular plant floras. The data point towards postglacial immigration of a highly dispersive flora, although they note convincing molecular evidence suggesting in situ glacial persistence of some elements of the ‘west‐arctic’ species group. Gíslason (Chapter 5) examines the aquatic fauna of the North Atlantic islands with a particular emphasis on Iceland. He notes that the islands have few aquatic species, almost no aquatic endemics, but their faunas are closely related. He goes on to discuss how the proportion of continental (Norway and Britain) species present on the islands is much higher among crustaceans than other groups. Despite low levels of endemism found amongst crustaceans in subterranean groundwater systems, the patterns indicate a Holocene (post glacial) origin for the biota.

Thórhallsdóttir (Chapter 6) evaluates and analyses data on the vascular floras of high‐latitude islands, again with special reference to Iceland, corralling independent lines of evidence that all favour the view that the Icelandic flora is young, i.e. of Holocene descent. In Quaternary vertebrates from the North Atlantic islands, Bennike and Wagner (Chapter 7) review the meagre fossil record of mammals from the main islands of Greenland, Iceland and Faroe, pointing out the incomplete record and the need for other independent lines of evidence such as genetic analyses. Panagiotakopulu (Chapter 8) reviews the North Atlantic insect fauna data revealing the effect of climate change and their early immigration to the islands but emphasizing the importance of the arrival of Europeans in the North Atlantic region in terms of introduction of species to the region but also the biological impact that they have had on the fauna.

Section III picks up on this key theme of human impact on the islands. Edwards et al. (Chapter 9) use the archaeological and palaeoecological records to examine the impact of Landnám and the North Atlantic flora. The review highlights the fact that the impacts of landnám on vegetation were broadly similar across that region, but that there are subtle differences in the Norse ‘footprint’ when examined at finer spatial scales which varied according to the interplay of the climatic, pedogenic, topographic and anthropogenic factors at each location.

Petersen and Olsen (Chapter 10) review the status of the bird fauna of Iceland and Faroe and discuss its colonization and Mainland and Harland (Chapter 11) explore the profound impact of farming on North Atlantic vertebrate biota, reviewing evidence for the introduction of domesticated faunas and rapid and widespread changes to the island landscapes and environments as a result of pastoralism and the exploitation of marine resources. The former (Chapter 10) characterizes the patterns in bird dispersal and extinctions and extirpations and its conservation significance of the avian fauna, while the latter (Chapter 11) presents comprehensive evidence showing that farming and fishing were vital to subsistence and trade as well as being core to island and community identity in the past, roles they continue to play out to the current day.

The prospects for the future environmental systems of the region is addressed in Section IV. Ogilvie et al. (Chapter 12) use a rich historical dataset to provide an elegant perspective on the significance and importance of sea ice patterns and flows to both historical and contemporary communities. Fosaa (Chapter 13) returns to a biodiversity theme and reviews the influence of both climate change and direct human impact on the flora of Faroe, including the threats posed by introduced species pointing to elements in Faroese flora that are of some conservation concern. The policy and legislative frameworks for biodiversity and conservation in Iceland under a changing climate is evaluated in considerable detail by Hauksson (Chapter 13). Johnsen and Heide‐Jørgensen (Chapter 14) examine the natural environment and its biodiversity in Greenland during the present climate change, presenting observations of the biological response related to an increasing greenhouse effect and stratospheric ozone depletion with an emphasis on terrestrial plant ecology.

It is appropriate that we conclude this introduction with a tribute to one of the region's leading scientists for whom this volume was conceived. Professor Paul C. Buckland blended his early (doctoral) training from the related fields of archaeology and geology, work on tephrochronology of East Africa lakes, a detailed evaluation of the value of insect fossils in the interpretations of archaeological deposits across the world, into a unique, innovative and complementary skillset for examining the biological conundrum that was (and to some extent still is) the biogeography of the North Atlantic. Like many researchers and colleagues (several authors of chapters in this volume), both editors of this book have been small cogs in this body of research and benefitted greatly from Paul Buckland's supervision and tutorage as doctoral researchers. Having developed a love for the environments, plants, animals and people of the North Atlantic region in a research career spanning some four decades, this book and contributions within it are a fitting tribute to his unique contribution to our understanding of the biogeography of the region.