The Canaries: an important biogeographical meeting place

Abstract

We were delighted on two counts to be able to act as the conveners of the third biennial meeting of the International Biogeography Society (IBS) in Puerto de la Cruz, Tenerife in January 2007. First, the conference facilities generously provided for our use by the Cabildo Insular de Tenerife made an excellent venue for the free-flowing discussion with colleagues old and new that is the hallmark of a good academic meeting, while the symposia and poster presentations alike were of a uniformly high standard. In short, it was acknowledged by those attending to be a thoroughly successful meeting, which showed biogeography to be a dynamic and exciting discipline and demonstrated that the still recent initiative of launching the IBS in 2000 was long overdue. Second, holding the meeting in the Canary Islands provided an opportunity to bring the archipelago to the attention of the biogeographical community, hopefully generating wider interest in the biogeography, ecology, evolution and conservation of the wonderful diversity of ecosystems and species found within the archipelago. This brief introduction is divided into two parts. First, we provide a brief comment focused on the content of this Special Issue, and second, we provide a short overview of the special biogeographical setting of the Canary Islands, emphasizing the pressures on the natural environment today and some of the measures in place for the conservation of nature within the archipelago. The conference was organized around five thematic sessions under the headings of (1) An integrative view of ecogeographic ‘rules’, (2) Quaternary impacts on Holarctic biogeography, (3) Island biogeography, (4) Maritime connectivity: reconciling models of dispersal and vicariance with evidence of biogeographical structure in a continuous environment, and (5) Separating historical from environmental effects on species distributions. An indication of the diversity and substance of the presentations at the meeting is represented in this collection of papers, which are drawn from each of these themes. Most of the papers herein derive from key-note presentations, but one derives from a poster presentation, one from a presentation during a panel discussion on the theme ‘Biogeography in the public eye’, and finally we have included a specially commissioned Commentary (Mackey, 2008) focusing on the contribution by Patten & Smith-Patten (2007). Geographical gradients in species richness have long fascinated biogeographers and ecologists, with recent work capitalizing on newly available spatial data sets and advances in spatial analytical techniques (e.g. Diniz-Filho & Bini, 2005; Rangel et al., 2006) to offer substantial advances in understanding. In this Special Issue, Svenning et al. (2008) provide an illustration of a further recent development (Hawkins et al., 2006), integrating evolutionary structure with the spatial analysis in their examination of palm species richness gradients across the New World. While finding support for the significance of climate, especially the mechanisms invoked in water–energy dynamics theory (O’Brien, 2006), their analysis also emphasizes the role of long-term environmental change, in the form of Late Tertiary orogeny. The importance of the changing elevation of landmasses is even more strongly emphasized in Patten & Smith-Patten’s (2007) analyses of biotic boundaries in Neotropical avifauna. Their paper demonstrates a contrasting approach to that of Svenning et al. (2008) in that they use species location data rather than trusting in the species-range map data commonly used in spatial analyses of diversity gradients, and it demonstrates a contrasting approach to spatial analysis in their use of Monmonier’s algorithm. In a further illustration of the methodological diversity of biogeographical analyses of relationships between taxa and area, both Waters (2007) and Sanmartín et al. (2008) offer approaches to using molecular phylogenies to infer the relative roles of dispersal and vicariance between and among continents and islands. Their work is part of a growing resurgence of interest in long-distance dispersal as a valid mechanism (alongside vicariance mechanisms) within biogeographical science, and of a growing awareness of the importance of incorporating general patterns of environmental history in island biogeographical and evolutionary models (e.g. Carine et al., 2004; Carine, 2005; Whittaker et al., 2007, 2008). Also on the theme of environmental change, but with a more recent focus, both Magri (2007) and Bhagwat & Willis (2008) provide substantive contributions to an understanding of the role of glacial/inter-glacial climate cycles in shaping the composition of European fauna and flora. Both papers emphasize that elements of the post-glacial biota of north-western Europe persisted through unfavourable glacial periods not just in distant southern refugia, but also farther north, dispersed across landscapes at low densities or in pockets of favourable microclimates. These findings deserve to be carefully considered by those attempting to use so-called bioclimatic envelope models to forecast future species responses to global climate change (e.g. Araújo & Guisan, 2006; Araújo et al., 2006; Randin et al., 2006). The pioneering attempt by Bhagwat & Willis (2008) to identify the species traits that favoured persistence in western Europe through the Pleistocene also deserve close attention: what sort of a guide do their findings provide on how temperate communities may respond not to a cooler climate but to a substantially warmer one? This seems an area worthy of further consideration from biogeographers. There has recently been a considerable resurgence of interest among biogeographers in ecogeographical rules such as Bergmann’s rule, Jordan’s rule, and the Island rule (e.g. Meiri & Dayan, 2003; Lomolino et al., 2006; Meiri et al., 2006; Price & Phillimore, 2007). Gaston et al. (2007) neatly define these ‘rules’ collectively as being about spatial patterns in biological traits. Many of these rules have been around for a long time, and there is generally something in them, but, as both Gaston et al. (2007) and McDowall (2007) emphasize, the patterns of variation involved typically turn out to be more complex than originally envisaged, and also typically involve co-variation with other biological traits on the one hand, and co-variation amongst potential causal environmental variables on the other (e.g. see Guillaumet et al., 2007; Meiri et al., 2008). Equally important is that these rules, however they may originally have been defined, are frequently being addressed at a number of taxonomic levels, typically both intra-specific, and inter-specific, thus posing significant challenges to those attempting to synthesize findings in this area. Both McDowall (2007) and Gaston et al. (2007) rise to these challenges, and identify some key research priorities for the future. Although much of the conference focused on the underlying pre-human signal inherent within biogeographical data, the application of biogeography in the Anthropocene was a theme continued from the previous meeting of the society (Riddle, 2006), represented in this Special Issue notably in the form of Blondel’s (2007) synthesis of human impacts on the ecology and environment in the Mediterranean, and in the advocacy for biogeographical analyses in conservation decision-making evident in other papers in the Special Issue (e.g. Patten & Smith-Patten, 2007; Bhagwat & Willis, 2008). However, of all of the papers in the Special Issue, perhaps the most important message for the biogeographical community is to be found in Ladle’s (2008) critique of the public representation of biogeography. His finding that biogeography has a low public profile will come as no great surprise to most biogeographers, but nonetheless provides painful evidence that ours is a largely cryptic discipline. The failure of practising biogeographers to promote public awareness of their work more effectively under the label biogeography may also be linked in some way, as Ladle suggests, to our failure to penetrate school curricula (at least this seems to be the case in the UK). Although the evidence from this Special Issue is of a vibrant, intellectually challenging and exciting discipline, Ladle demonstrates that we need to do much more outreach and public dissemination work to establish the relevance of biogeography in the public sphere (see also Whittaker et al., 2005). Certainly, we would anticipate public outreach to be an important emerging role for a maturing IBS, and one that we would hope to address further at the fourth biennial IBS meeting, which will take place on the Yucatan Peninsula of Mexico in January 2009. We return to this theme again, briefly, at the end of the second section of this Introduction to the Special Issue. This section of the paper is based on the first presentation at the IBS meeting, in which the first author (JMFP) provided an overview of the Canary Islands, their geo-environmental history, biodiversity value, human colonization, environmental challenges, and conservation responses. There is also much to say about recent advances in the understanding of the biogeography of the Canary Islands, in which great strides have been made in the last 20 years, in large measure through the application of modern molecular phylogenetic analyses. For further detail on these advances and innovative analyses based on these data, see Sanmartín et al. (2008). The Canary Islands constitute a volcanic archipelago located off the West Saharan coast of north-west Africa (Fig. 1). The islands are characterized by outstanding biodiversity, featuring high levels of endemism, including both palaeo- and neo-endemic forms, and spectacular radiations in many animal and plant lineages, distributed across an impressive array of major ecosystem types from semi-desert through sub-tropical broadleaved evergreen woodlands and xeric endemic pine woodlands to high-altitude sub-alpine and alpine environments. This diversity is underpinned and explained by a combination of geological and geographical characteristics (Table 1), including: (1) the subtropical location (27–29° N) of the archipelago; (2) the unusual longevity (16–20 Myr) of the older islands (for volcanic oceanic islands); (3) the high altitudes achieved by the central and western islands (> 1500 m a.s.l., with the highest point on Tenerife, the Teide peak of 3718 m a.s.l., also being the highest point in the Atlantic Ocean); (4) the influence of the North-East trade winds and of the Canarian cool marine current; and (5) the intermediate degree of isolation, varying between some 60 km at low sea-level stands and 95 km today (García Talavera, 1999), generating a less disharmonic biota (sensuWhittaker & Fernández-Palacios, 2007) than found in Hawaii, but enough restrictions on gene flow to enable the evolution of high in situ diversity (Table 2). The Canary Islands (a) today and (b) at the time of the sea-level minimum of the Last Glacial (modified from García-Talavera, 1999), and (c) in relief, showing the bathymetry of the archipelago (with the kind permission of Juan Acosta, Multibean Mapping Group, Instituto Español de Oceanografía, Madrid, Spain). Although the oldest parts of the archipelago date back over 10 Myr, the geological context of the islands is one of ever-changing circumstances. For instance, within just the last 2 Myr (the Pleistocene and Holocene) major events have included: (1) the emergence of new islands (La Palma and El Hierro) and islets (Alegranza, Montaña Clara, La Graciosa, Lobos) in the western and eastern extremes of the archipelago, respectively; (2) the partial destruction of some islands as a result of mega-landslides that have formed huge scar-valleys (La Orotava, Güímar, Icod in Tenerife; Taburiente, Cumbre Nueva in La Palma; and Las Playas, El Julan, El Golfo in El Hierro) (Whelan & Kelletat, 2003; Whittaker & Fernández-Palacios, 2007); and (3) the influence of the glacial/inter-glacial cycles of the Pleistocene. These cycles have driven eustatic sea-level fluctuations, which have periodically doubled and then halved the area of the archipelago, implying variations in the maximum island altitude of more than 100 m, and shortening the distances among islands and with the continent significantly. The low sea-level stands of the Pleistocene also resulted in: (1) the emergence of Amanay Island, currently a sand bank off the north coast of Fuerteventura; (2) the joining together of Lanzarote, Fuerteventura and their islets, to form the large East Canarian Ridge Island, also known today as Mahan (Fig. 1b, 1c); and (3) the emergence of a chain of stepping-stone islands that provided enhanced dispersal opportunities between the Canaries, Madeira, the Azores and the Iberian Peninsula (García Talavera, 1999). Carine’s (2005)‘colonization window hypothesis’ posits that the events listed above provide episodes in which the opportunities to colonize the Canary Islands are significantly enhanced, resulting in discrete waves of colonization, followed by subsequent evolutionary change: he exemplifies this model with data for Canarian Convolvulus. Along similar lines, but focusing just on the emergent geological pattern, Whittaker et al. (2007, 2008) argue that the sequential ontogeny of each island within such archipelagos drives changes in the dominant processes and patterns of island evolution, providing a long-term evolutionary model for oceanic islands, again illustrated (in part) by data from the Canaries. The catalogue of Canarian species includes 18,477 species, comprising 13,328 terrestrial (Izquierdo et al., 2004) and 5149 marine species (Moro et al., 2003). In addition to numerous Macaronesian endemic species, 121 genera, 3836 (3672 terrestrial + 164 marine) species and c. 600 subspecies are exclusive to the archipelago (Tables 2 & 3). In fact, despite some three centuries of attention from natural scientists, the species catalogue for the Canaries is far from being complete. New species or subspecies are being described from the archipelago at a rate of about one species every six days over the last two decades (Martín et al., 2005b), among them vertebrates such as the Canarian shrew (Crocidura canariensis), the Teno (Tenerife) and La Gomera giant lizards (Gallotia intermedia and G. gomerana, respectively), as well as two trees, the Grancanarian dragon-tree (Dracaena tamaranae) and the round-leaf fire tree (Myrica rivas-martinezii). This trend is likely to continue in the near future, with new discoveries most likely in habitats such as steep cliffs, forest canopies and in the floor of the sea-channels separating the islands. Arguably, the Canarian biota can be considered the most biodiverse of any political unit within Spain or within the European Union, including not only large numbers of species, and of endemic species, but also outstanding examples of archipelagic radiations (Table 4) – in both the animal and plant kingdoms. For example, the Hemycicla snails and Laparocerus weevils respectively comprise 76 and 68 species within endemic monophyletic clades, and succulent rosette-forming members of the plant family Crassulaceae in the genera Aeonium, Monanthes, Aichryson and Greenovia include at least 50 species within endemic monophyletic clades (Izquierdo et al., 2004). The Canarian terrestrial zonal ecosystems include up to six recognized formations from coast to summit (Table 5): (1) the sub-desert coastal scrub, with strong affinities to the nearest African mainland ecosystems, dominated by succulent endemic Euphorbia shrubs and today highly threatened by the pressure of continuing urban expansion driven by the tourist industry; (2) the thermophilous woodlands, the most Mediterranean-like ecosystems of the Canaries, which have almost disappeared as a result of anthropogenic clearance; (3) the laurel-forest, the sub-tropical ecosystem of the archipelago, shared with Madeira and the Azores and a relict of a forest type formerly (in the Miocene and Pliocene) widely distributed in Southern Europe and North Africa, which is today recovering as a result of the abandonment of agriculture in the mid-altitudinal belt; (4) the Canarian pine forest, once greatly reduced but extensively reforested in the last sixty years; (5) the summit scrub, dominated by endemic cushion-like legumes; and, finally, (6) exclusively represented in the highest slopes of the Teide volcano, the alpine Peak ecosystem (Fernández-Palacios et al., 2004). Although the timing of human colonization remains uncertain, it is considered that, some time during the first millennium BC, people of Berberic culture arrived on the Canaries from North Africa (Cabrera, 2001). The Guanche, as they became known, introduced goats, sheep, pigs and dogs, and developed a society based largely on shepherding, which persisted until the Castilians conquered the archipelago during the 15th century. The introduction of large vertebrate herbivores to island ecosystems that had evolved for millions of years in the absence of predators, and the use of fire to convert woodland to other land use, including pasturages, undoubtedly had huge impacts on the ecology of the islands, including the disappearance of entire forest types and dominant tree species within the thermophilous woodland belt (L. de Nascimento et al., submitted). The Castilian conquerors shifted this animal-based subsistence model to a new agriculture-based development model that led to the logging of almost all of the mid-altitude forests (thermophilous woodlands and laurel forests), where their settlements were established (Cabrera, 2001), and the eventual loss of most of the pine woodlands. The agricultural model was highly dependent on the economic success of a limited array of export crops, (wine, sugar cane, bananas, tomatoes), which have shown a pattern of boom-and-bust over time that might be described as cycles of near-monoculture. Finally, the third great shift in the Canarian economy occurred only 50 years ago, when the mass-tourism model, today implanted in the eastern and central islands almost exclusively, replaced the agriculture model, with the latter persisting as a dominant force only in the three small western islands. This last shift in economic development has abruptly transformed both Canarian society and Canarian landscapes (Table 6), resulting in the emergence of new environmental problems that threaten both ecosystem and species diversity. For instance, although the birth rate of the Canarian population (1.26 children/women) is clearly under the replacement level, an annual population growth of c. 50,000 people has yielded a population of 2 million inhabitants, which is double the population of the archipelago as recently as the 1960s. Furthermore, the islands are visited by about 12 million visitors a year (0.3 million daily), resulting in a de facto population of 2.3 million inhabitants. This equates to a population density of about 300 people per square kilometre, which is unevenly distributed within and between islands: some 87% of the population are concentrated in Gran Canaria and Tenerife, with densities of approximately 500 people per square kilometre. This population needs increasing space for residence and infrastructure, energy (so-called ‘clean energy’ accounting for only 1% of the production), food and water resources, and is simultaneously producing increasing volumes of domestic waste. The image of the construction crane stalking the coastal zone of Tenerife consuming territory is one familiar to the attendees of the IBS meeting, as it has been to any visitor in the past quarter century. As a consequence of this evidently unsustainable rush for growth, half of the Canarian agricultural area (50,000 ha) has been abandoned in the last five decades, whereas the coastal ecosystems have been and still are being systematically replaced by tourist resorts and infrastructure (highways, airports, harbours, golf courses, etc.). The energy consumption has multiplied by 10 in the last 50 years (from 0.7 to 7GW), with 99% of the electricity production based on imported fossil fuels. Furthermore, about 130 hm3 year−1 of waste water is produced, of which 60% is delivered to the sea without treatment, and the islands are home to some 1.2 million cars, more than 12,000 km of paved roads, and some 500,000 tourist beds. Today, it can be calculated that each Canarian citizen contributes to global climatic change by means of the emission of 25 kg CO2 day−1, and produces c. 5 kg waste day−1, of which 1.5 kg is classified as urban waste (Fernández-Palacios et al., 2004). The upshot of these changes has been a wholesale shift in the nature and geography of the human impact on Canarian landscapes, alongside a shift in which the archipelago has moved from being a net exporter to a net importer of food. On Tenerife, the upper regions, which were once devastated by overgrazing and cutting, have now largely been handed over to replanting (especially in the endemic Canary Island pine belt) and to conservation. The laurel forest belt, although reduced to perhaps 20% of its original area, is stable in area and has protected status. Many areas once in cultivation in the more humid parts of the lowlands are gradually being recolonized by a mix of native and exotic plants, or else are being built on. As current mass tourism favours the sunniest environments, it is in the dry lowlands that the pressure is now greatest, with some of the biggest tourist developments severely impacting on the most arid areas, previously only sparsely populated. Although many Canarian endemic species are today on the brink of extinction (Martín et al., 2005b) (Table 7), the list of known species extinctions is fortunately not as great as in other similar volcanic archipelagos, such as Hawaii, the Mascarenes and the Caribbean islands (Groombridge & Jenkins, 2002; Whittaker & Fernández-Palacios, 2007). Nevertheless, the high biodiversity value (especially high endemism) and high level of threat to Canarian biodiversity has led to the designation of Canarian sites in several prominent conservation prioritization schemes, and to the incorporation of the whole archipelago in the recently expanded 2005 version of Conservation International’s hotspots scheme (http://www.conservationinternational.org), as a part of an intrusion into the Atlantic Ocean of the so-called Mediterranean Basin hotspot, which now embraces the Macaronesian archipelagos of the Azores, Madeira, the Canaries and Cape Verde Islands. Two main approaches to the conservation of the Canarian natural heritage have been developed by the various administrations (European, Spanish, Canarian and Insular) active in the archipelago (Tables 8 & 9): (1) protection of the land and marine territories through the establishment of networks of protected areas; and (2) protection of species through the establishment of several catalogues of threatened species. There are three overlapping protected-area networks within the Canaries: (1) the Canarian network (Red Canaria de Espacios Naturales Protegidos); (2) the European Union Natura 2000 network; and (3) UNESCO sites (biosphere reserves, World Heritage sites and Ramsar wetlands). Together they comprise 13 distinct designations of protected area (Santana et al., 2006; Whittaker & Fernández-Palacios, 2007) (Table 9), which impose varying degrees of land-use planning and protection, from strict nature reserves on the one hand, to zones in which agriculture and livestock grazing are an intrinsic part of an integrated development model on the other. The protected-area estate includes four National Parks (Cañadas del Teide in Tenerife, Caldera de Taburiente in La Palma, Garajonay in La Gomera and Timanfaya in Lanzarote), and, excluding the Biosphere Reserves designation, which includes the entire islands of Lanzarote, La Palma and El Hierro, and the south-west part of Gran Canaria, the protected territory embraces c. 45% of the terrestrial surface area of the archipelago and about 1800 km2 of marine protected areas. Considered by island, the percentage of protected areas varies from 28% for Fuerteventura to 60% for El Hierro. In terms of species protection efforts, Canarian species are listed within both the Spanish Catalogue of Threatened Species (Bañares et al., 2004), and the Canarian Catalogue of Threatened species, as well as within the annexes of both the Birds and Habitat European Union Directives (Martín et al., 2005a). Altogether, some 465 Canarian endemic species or Canarian populations of charismatic species (such as the Cetacean species inhabiting the sea channels between the islands) are protected (Table 8). Of these 465 species, approximately 175 are classed as threatened by extinction (some of these are listed in Table 7). A governmental proposal for the withdrawal of about 56% of the species included in the Canarian Catalogue has recently been published (Martín et al., 2005a). This action was based on a four-year monitoring and assessment project. The authors concluded that, in about 200 cases, although the species concerned were restricted to only a few populations, the populations were healthy and stable. In addition, some 40 species previously listed as endangered were removed from that list based on, for example, changed assessments of whether they were endemic, taxonomic clarification of status, etc. Although some species populations may be healthier than once thought, and although intensive conservation efforts may be pulling some endangered species back from the brink of extinction, many others species remain acutely at risk (Bañares et al., 2004). In addition, the islands support large numbers of non-native introduced species, and their continued introduction, alongside the continued wholesale transformation of natural environments, especially in the coastal zone, is to be anticipated. The political imperatives within the archipelago remain focused on short-term economic interests and a model of increased tourism, development and urbanization. This economic model casts a dense shadow of uncertainty over the future of the natural resource base and biodiversity of the archipelago, particularly of the warm, dry climate belt so popular with European tourists. Despite attempts to provide legal protection for biodiversity and to invest in environmental conservation at various political levels, the pressures on the natural resources of the Canary Islands continue to increase (García Falcón & Medina Muñoz, 1999). These pressures include efforts to reduce the protection afforded to particular protected areas that have potential commercial value for development. Nonetheless, the various networks of protected areas and other conservation measures show how it is possible to tailor protected-area models to an insular context; without these legal instruments, the future of many Canarian endemic species and ecosystems would indeed be bleak. Despite the general recognition of the Canaries as Europe’s most outstanding biodiversity centre, and despite the efforts of various administrations in the protection of this unique heritage, the levels of environmental concern shown by Canarian society as a whole currently appear insufficient to generate fundamental shifts in the pattern of resource exploitation. Given the continuation of an economic development model based on increasing concentrations of population, and the reception of huge numbers of tourists, without clear signals of a serious shift to a sustainable development model it seems that pressure can only grow on the natural resources, in terms of space, buildings, infrastructure, energy, water, and food. A substantial change in the direction of this pattern of development and consumption is surely needed if the unique natural heritage of these islands is not to be squandered, with the consequent detrimental costs for the quality of life on offer to the people of the Canaries. At the same time, Canarian society itself is in flux, with large movements of people in to and (to a degree) out of the archipelago, leading to cultural as well as economic and environmental change. The challenge for those interested in the conservation of nature and of biodiversity is how to promote an increased valuation of these natural resources amongst the public and polity.