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SPECIES DIVERSITY IN THE ARCTIC (Chapter 2)
Lead Authors: David C. Payer, Alf B. Josefson and Jon Fjeldså
Species richness is generally lower in the Arctic than at lower latitudes, and richness also tends to decline from the low to high Arctic. However, patterns of species richness vary spatially and include significant patchiness. Further, there are differences among taxonomic groups, with certain groups being most diverse in the Arctic.
Many hypotheses have been advanced to explain the overall decline of biodiversity with increasing latitude, although there is still no consensus about a mechanistic explanation. Observed patterns are likely the result of complex interactions between various biotic and abiotic factors. Abiotic factors include lower available energy and area at high latitudes, and the relatively young age of Arctic ecosystems. Among biotic factors, latitudinal differences in rates of diversification have been suggested, but empirical evidence for this as a general principle is lacking. Recent evidence suggests that ‘tropical niche conservatism’ plays a role in structuring latitudinal
When there is an earthquake, we say that the mammoth are running. We have even a word for this, holgot. Vyacheslav Shadrin, Yukaghir Council of Elders, Kolyma River Basin, Russia; Mustonen 2009.
Physical characteristics of the Arctic important for structuring biodiversity include extreme seasonality, short growing seasons with low temperatures, presence of permafrost causing ponding of surface water, and annual to multi-annual sea-ice cover. The Arctic comprises heterogeneous habitats created by gradients of geomorphology, latitude, proximity to coasts and oceanic currents, among others. Superimposed on this is spatial variation in geological history, resulting in differences in elapsed time for speciation.
Over 21,000 species of animals, plants and fungi have been recorded in the Arctic. A large portion of these are endemic to the Arctic or shared with the boreal zone, but climate-driven range dynamics have left little room for lasting specialization to local conditions and speciation on local spatial scales. Consequently, there are few species with very small distributions. In terrestrial regions, high-latitude forests were replaced by tundra about 3 million years ago. Early Quaternary Arctic flora included species that evolved from forest vegetation plus those that immigrated from temperate alpine habitats, but the most intensive speciation took place in situ in the Beringian region, associated with alternating opportunities for dispersal (over the Bering land bridge, when sea levels were low) and isolation (during high sea levels). In the marine realm, the evolutionary origin of many species can be traced to the Pacific Ocean at the time of the opening of the Bering Strait, about 3.5 million years ago.
More than 20 cycles of Pleistocene glaciation forced species to migrate, adapt or go extinct. Many terrestrial species occupied southern refugia during glaciations and recolonized northern areas during interglacials. Ice-free refugia persisted within the Arctic proper; species occupying these refugia diverged in isolation, promoting Arctic diversification. The most significant Arctic refugium was Beringia and adjacent parts of Siberia. Pleistocene glaciations also resulted in a series of extinction and immigration events in the Arctic Ocean. During interglacials, marine species immigrated mainly though the Arctic gateways from the Pacific and Atlantic Oceans, a process that continues today.
Throughout the Pleistocene, Arctic species responded to climatic cycles by shifting their distributions, becoming extirpated or extinct, persisting in glacial refugia, and evolving in situ. Although the last 10,000 years have been characterized by climatic stability, the Earth has now entered a period of rapid anthropogenic climate change that is amplified in the Arctic. Generalism and high vagility typical of many Arctic species impart resilience in the face of climate change. However, additional anthropogenic stressors including human habitation, overharvest, industrial and agricultural activities, contaminants, altered food webs and the introduction of invasive species pose new challenges. The consequences of current warming for Arctic biodiversity are therefore not readily predicted from past periods of climate change.
Arctic ecosystems are relatively young in a geological sense, having developed mainly in the last 3 million years (Murray 1995), although some Arctic species’ lineages diverged and adapted to cold, polar conditions much earlier (see Section 2.3). In general, species richness is lower in the Arctic than in more southerly regions (Fig. 2.1). This is consistent with the general observation that biodiversity declines from the Equator to the poles (Rosenzweig 1995, Gaston & Blackburn 2000, Willig et al. 2003). The strength and slope of latitudinal biodiversity gradients differ between regions and are more pronounced in terrestrial and marine systems than in freshwater environments, and, in general, most pronounced in organisms with greater body mass and those occupying higher trophic levels (Hillebrand 2004). With the recent development of global distributional and phylogenetic datasets, however, it has become apparent that the pattern is much more complex than previously assumed (Jetz et al. 2012).
A number of hypotheses have been advanced to explain the latitudinal trend of biodiversity, although no consensus exists for a mechanistic explanation (Currie et al. 2004). Hypotheses may be grouped into those based on ecological mechanisms of species co-occurrence, evolutionary mechanisms governing rates of diversification, and earth history (Mittlebach et al. 2007). Until recently, ecological hypotheses have dominated the discussion, but with the development of large DNA-based phylogenies there is now more focus on understanding the underlying historical processes. The hypotheses proposed to date are not necessarily mutually exclusive, and observed patterns are likely the result of complex interactions between various biotic and abiotic factors.
The decline of available energy (Allen et al. 2002) and decreasing biome area (Rosenzweig 1995) with increasing latitude should both contribute to declining species richness in the North. Rohde (1992) posited that the ultimate cause could be a positive relationship between temperature and evolutionary speed. Relative to the tropics, the Arctic has limited insolation (lower solar energy input and thus colder temperatures) and a shorter elapsed time for diversification. In Rohde’s (1992) view, all latitudes could support more species than currently exist, and, given adequate evolutionary time, the Arctic could support biodiversity rivaling that of lower latitudes. Because of great variation in speciation rates, however, the number of species in taxonomic groups is uncoupled from the age of groups (Rabosky et al. 2012). Further, several Arctic groups (notably waterfowl and gulls) underwent significant recent increases in speciation rates (Jetz et al. 2012). Thus, there is no general latitudinal change in speciation rates (as assumed, e.g. by Wiens et al. ), and Jetz et al. (2012) instead point out hemispheric or even more local differences.
The recently proposed ‘tropical niche conservatism’ hypothesis may reconcile some of these diverging tendencies. This hypothesis assumes that most organismal groups originated during times when the global climate was warm paratropical), and these groups tend to retain their adaptations to such conditions (Webb et al. 2002). Thus, as the global climate became cooler during the Oligocene, and again in the late Miocene, the ancient groups contracted their geographical distributions towards the Equator to maintain their original niches. The long time or speciation in tropical environments, compared with cold environments, would explain the large accumulation of species, and phylogenetically overdispersed communities, in the humid tropics (Wiens 2004). The most significant increases in speciation rates are associated with ecological shifts to new habitats that arose outside the humid tropics, notably in montane regions and archipelagos (Fjeldså et al. 2012, Jetz et al. 2012; see also Budd & Pandolfi 2010). However, only some groups have (yet) responded by adapting to these new environments, resulting in small and phylogenetically clustered communities in the Arctic.
The diversification process within the Arctic may have been strongly affected by the climatic shifts caused by variations in Earth’s orbit known as Milankovitch oscillations. This includes a tilt in the Earth’s axis that varies on a 41,000-year cycle (precession), an eccentricity in Earth’s orbit that varies on a 100,000-year cycle, and 23,000 and 19,000-year cycles in the seasonal occurrence of the minimum Earth-Sun distance (perihelion; Berger 1988). Milankovitch Oscillations cause variations in the amount of solar energy reaching Earth, and these variations interact with characteristics of the Earth’s atmosphere such as greenhouse gas concentration and surface albedo, resulting in rapid, nonlinear climatic change (Imbrie et al. 1993). The present interglacial period, which has extended over the last 10,000 years, is a period of exceptional climatic stability; stable conditions have typically lasted only a few thousand years, and > 90% of the Quaternary Period (2.6 Million years ago to present) has been characterized by more climatically dynamic glacial periods (Kukla 2000).
Webb & Bartlein (1992) noted that Milankovitch oscillations are associated with changes in size and location of species’ geographical distributions. Dynesius & Jansson (2000) called these recurrent changes “orbitally forced species’ range dynamics” (ORD), and noted that they constrain evolutionary processes acting on shorter time scales. The effects of Earth’s precession and orbital eccentricity on surface temperatures are greatest at high latitudes (Wright et al. 1993), resulting in increasing ORD along the latitudinal gradient from tropics to poles. Predicted evolutionary consequences of enhanced ORD are apparent in general characteristics of Arctic biota, including enhanced vagility and larger species’ geographic range sizes (Rapoport’s rule), and therefore increased mixing of locally-adapted populations, increased proportion of polyploids within plant taxa, and reduced rates of speciation (Dynesius & Jansson 2000, Jansson & Dynesius 2002). There is spatial variation in these processes within latitude, however, which must be considered when evaluating current diversity patterns. For example, the Pleistocene temperature amplitude was lower in East Siberia and the Bering Strait region than in areas around the North Atlantic, leading to less glaciation (Allen et al. 2010) and enhanced opportunities for speciation in the Siberian-Beringian region (see below).
Although ORD increases risk of extinction associated with habitat change, this is mitigated by enhanced generalism, vagility and genetic mixing at high latitudes (Dynesius & Jansson 2000). This has important implications for risk of extinction associated with climate change and other stressors, as will be discussed in subsequent chapters of this Assessment.
FUTURE PROSPECTS FOR ARCTIC BIODIVERSITY
Over the last 2.6 million years, throughout the cycles of Pleistocene glaciations, Arctic species have shifted their distributions, become extirpated or extinct, persisted in glacial refugia, undergone hybridization, and evolved in situ. Although the last 10,000 years have been characterized by a relatively high degree of climatic stability, the Earth has now entered a period of rapid anthropogenic climate change. Global temperatures have been warmer than today’s for less than 5% of the last three million years (Webb & Bartlein 1992) and are within 1 °C of the maximum over the last one million years (Hansen et al. 2006). Further, the rate and magnitude of warming is amplified in the Arctic (McBean 2005, IPCC 2007, AMAP 2009, AMAP 2011). This trend of accelerating climate change and Arctic amplification is expected to continue (Overland et al.2011). Global warming has caused species distributions to shift northward and to higher elevations for a wide range of taxa worldwide (Walther et al. 2002), including species occupying the Arctic (e.g. Sturm et al. 2001, Hinzman et al. 2005).
The Arctic, being a region with high ORD and therefore populated by species that have experienced selection pressure for generalism and high vagility (Jansson & Dynesius 2002), should have inherent resilience in the face of climate change. Some extant Arctic species have survived population bottlenecks driven by climatic change, including cetaceans (e.g. narwhal [Laidre & Heide-Jørgenson 2005]) and waders (Kraaijeveld & Nieboer 2000), further suggesting some degree of climate-change resiliency. However, the rapid rate of change occurring now and the amplification of this change at high latitudes pose unique challenges for Arctic species. The Arctic has experienced less anthropogenic habitat change and fragmentation than lower latitudes, which favors the ability of species to track shifting habitats. However, because of the limited area available in the polar regions, terrestrial Arctic biota have limited ability to respond to warming by northward displacement (MacDonald 2010). Kaplan & New (2006) predicted that Arctic tundra will experience a 42% reduction in area if global mean temperature is stabilized at 2 °C above pre-industrial levels. Although the rate of change is debated (e.g. Hofgaard et al. 2012), there is general agreement that area of tundra will be significantly reduced in this century.
In addition to rapid and accelerating climate change, Arctic species are experiencing anthropogenic stressors that did not exist during past periods of warming, including human habitation, overharvest, industrial and agricultural activities, anthropogenic contaminants, altered food webs, and the introduction of invasive species (Meltofte et al., Chapter 1). The many migratory species that occur only seasonally in the Arctic face additional and potentially cumulative anthropogenic stressors on migration routes and in overwintering areas that could further impact their ability to adapt. The suite of stressors experienced by Arctic species today is therefore novel, making past periods of climatic change an imperfect analogue for the challenges now facing Arctic biodiversity. Future efforts to preserve Arctic biodiversity must be similarly novel and broadreaching.