ASPECTOS METODOLÓGICOS

According to the Convention on Biological Diver- sity, biological diversity (or biodiversity) means the variability among living organisms from all sources, including inter alia, terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part; this includes diversity within species, between species, and of ecosystems. There are a variety of other definitions of biodiversity, but this one has considerable importance because many countries have signed and ratified the Convention on Biological Diversity, which has as its objectives ‘‘The conservation of biological diversity, the sustainable use of its components, and the fair and equitable sharing of the benefits arising from the utilization of genetic resources.’’

Although biodiversity includes all levels in the gene- alogical (genes to higher taxa) and ecological (indivi- duals to ecosystems) hierarchies, there are significant relationships between various components of biodiver- sity. For example, strong relationships exist between numbers of species (richness) and numbers of higher taxa such as genera, families, and orders, between numbers of species and numbers of characters (or mor- phological features), and between numbers of species and the diversity of functional roles in an ecosystem that any one assemblage might have. Much present knowledge of biodiversity concerns patterns in spe- cies-richness variation across space and through time. Therefore, the relationships between species richness and other components of biodiversity mean that at least the fundamental patterns of this variation, if not the mechanisms underlying it, are reasonably well understood.

Spatial patterns in biodiversity are complex and three-dimensional. Species richness in marine, terres- trial, and freshwater systems varies with both latitude and longitude. Arguably the best-known pattern in species-richness variation is the latitudinal gradient in diversity, which encompasses a decline in richness from the tropics to the poles. It was first documented by Alexander von Humboldt (in 1804), and has been the subject of intense debate ever since. In marine and freshwater lake systems richness also varies with depth, whilst in terrestrial and freshwater stream sys- tems, richness also changes with altitude. In virtually all cases, the gradients are neither linear nor symmet- ric, but show substantial variation depending on the region, the system (marine, freshwater, terrestrial), and the organisms of interest.

Ultimately, the species richness of an area depends on four main processes: speciation, extinction, immi- gration, and emigration. The relative importance of these processes depends on both the spatial and BIODIVERSITY, TERRESTRIAL

temporal scale of interest. More proximate determi- nants of richness variation include available energy (e.g., coming in as infrared, visible, and UV radiation, and often measured as temperature or net primary productivity), area (larger areas have more species for a variety of reasons), evolutionary rates (faster life cycles and high-energy environments lead to more speciation), and the effects of history of an area, which might substantially influence present di- versity. The proximate determinants also include a null model, which posits that because of the con- straints of a bounded domain, richness must be high- est in its centre, so accounting for tropical peaks in richness.

From a terrestrial and freshwater perspective, the low-energy environment and isolation of Antarctica mean that it is species poor. Indeed, terrestrial and freshwater systems of Antarctica represent the end- point of the global cline in diversity. Where life can exist, richness is typically low, and continental Antarc- tic systems represent some of the simplest in the world. For example, some endolithic communities are re- stricted to a few species of algae, fungi, and bacteria.

In Ellsworth Land (75
–77
S) the simplest faunal sys-

tem comprises five tardigrade and two rotifer species. Even the nematode worms, which are known from harsh, poor environments such as the Dry Valleys, are absent here. In lakes the systems are dominated by phytoplankton and the microbial loop, with few invertebrates and no fish. Nonetheless, gradients in diversity exist on the continent in terrestrial and freshwater systems and are especially pronounced along the Antarctic Peninsula. The sharp distinction between the biotas of the peninsula and the continental Antarctic is also an important terrestrial biogeo- graphic feature, with the peninsula biota typically rep- resenting more recent colonists and that of the continental sites including several Gondwana relics. Richness variation in continental Antarctica is spatial- ly complex, reflecting regional and local variation in conditions as much as it does the increasingly low energy and isolated environment towards the pole. This variation is poorly understood owing to limited sampling in many areas, especially of the East Antarc- tic. Nonetheless, it is apparent that in many groups the species are wholly endemic (i.e., restricted to the region) to Antarctica (nematode worms) or nearly so. Although the terrestrial and freshwater environ- ments are species poor, the species often occupy envir- onments that are otherwise not utilized elsewhere, show a range of remarkable strategies for overcoming Antarctic environmental challenges, and inhabit eco- systems which are often dominated by groups which are less noticeable elsewhere on the planet. For exam- ple, molecular techniques are now starting to reveal

that the diversity of microbes in the Antarctic is com- plex and rich. Just how diverse it is by comparison with other systems is difficult to ascertain given that the techniques used for studying such diversity are novel. However, it seems likely that the microbes of Antarctica will prove to be extraordinary from a va- riety of perspectives. In the case of another group, the algae, the diversity of lifestyles is remarkable, includ- ing species which live within rocks with no obvious connection to the outside, as well as those that occupy small, seasonally liquid depressions on the surface of glaciers. In lakes, many protozoan species are mixo- trophic such that they can alternate between particu- late feeding and photosynthesis. In the most southerly lakes, these species are dominant and feed on bacteria during the winter and produce energy by photosyn- thesis during the summer.

Terrestrial and freshwater diversity increases rap- idly from the maritime Antarctic to the sub-Antarctic islands. On the latter, variation in richness is compar- atively well understood, especially in the case of vas- cular plants and insects, and to some extent in the bryophytes. Although the ways in which species have come to be present on the islands has been a contro- versial topic, there is increasing evidence for the importance of dispersal by wind, especially for the bryophytes and for some of the insects. Vicariance (or the presence of the species on a landmass as a consequence of its connection once to another land- mass) is also though to be important for some groups. Richness variation of the indigenous vascular plants and insects is a consequence of energy availability, island area, and to a lesser extent isolation. The dif- ferential effects of glaciation, once though to be im- portant for this variation, are less significant than variation in available energy. Thus, the largest and most northerly islands generally have the highest plant and insect species richness.

In marine systems the picture is much more com- plex. The biogeography of the region is influenced by the oceanic fronts, such that pelagic plankton can be divided into Antarctic, sub-Antarctic, and sub-Tropi- cal assemblages. However, species-richness gradients are not straightforward. In some groups, such as the decapod and stomatopod crustaceans and bivalves, richness is low, and brachyuran crabs (i.e., typical crabs) are entirely absent from the region. However, in others there seems to be almost no decline in Ant- arctic waters (or a converse pattern). Thus, the pyc- nogonids (sea spiders), echinoderms (sea urchins and their relatives), and polychaetes (segmented worms) are especially rich in the Antarctic region. Indeed, in the case of the sea spiders the Antarctic region may be the area of highest richness globally. Other groups, such as the notothenioid fish, have also diversified BIODIVERSITY, TERRESTRIAL

dramatically in the Antarctic. Although the Antarctic fish fauna is notably species poor, this is not true of the notothenioids. Of the 174 benthic or demersal fish species known from the Southern Ocean, 55% are notothenioids, and these fish commonly represent more than 90% of the individuals collected. The group has undergone a remarkable adaptive radiation in shallow water around Antarctica and dispersal through the Polar Front has resulted in divergence of other sub-Antarctic notothenioid species. These fish are endemic to the Antarctic region, although recently a Patagonian toothfish was discovered off Greenland, lending support to the idea that transe- quatorial dispersal events can take place in deep, cold water.

Although the distribution of benthic and pelagic diversity across the Southern Ocean is poorly under- stood for most groups owing to the paucity of infor- mation, especially for the East Antarctic, one group is especially well known. The seabirds, which are essen- tially pelagic species that return to land to breed, have been well documented both in terms of their breeding populations and their at-sea distributions. Species richness peaks in the vicinity of the Polar Front, and globally the proximate determinants of this richness are primary productivity, sea-surface temperature, wind speed, and to a lesser extent the accessibility of breeding sites. The majority of these variables reflect the overwhelming importance of energy availability in determining richness of this pelagic group of animals.

In terms of functioning, terrestrial ecosystems are influenced predominantly by temperature and water availability, though nutrient availability is also im- portant. Both human (cloche-type enclosures) and natural (heated-ground) experiments have demon- strated the influence of temperature and water avail- ability on the identity and abundance of species in Antarctic habitats. Terrestrial systems are dominated by particulate feeding and detritivorous heterotrophs, and one or two species are most abundant at a given site. Predation is not uncommon, but it is rarer than elsewhere, and parasitism is infrequent. Functional roles typical of warmer climates, such as pollination, are poorly represented in the Antarctic. In freshwater ecosystems, food webs tend to be simple, and connec- tions may span several levels. Marine systems in the Southern Ocean are complex and incorporate a wide range of ecological processes, ranging from distur- bance such as ice scour in shallow-water environ- ments to considerable trophic complexity associated with pelagic top predators such as whales, seals, and penguins. The sea-ice environment plays an especially important role in ecosystem dynamics in the Southern Ocean, and recent changes to sea ice are having

far-reaching biodiversity consequences. Genomic stud- ies are proving instrumental in unravelling survival strategies and ecological interactions in marine and terrestrial systems.

Antarctica was not always glaciated. Indeed, the first continental ice sheets appeared in Antarctica about 34 Ma. In consequence, the fact that Antarctica supported a diverse terrestrial biota, including plants, insects, and even dinosaurs is not surprising. Howev- er, the extent of this fossil biota is only beginning to be explored, and remains poorly known for many groups. For example, the first Cretaceous fossil that can definitely be placed within the modern bird radia- tion hails from Antarctica. From about 10 Ma, Ant- arctic ice sheets were substantial and well established, and would have extirpated the bulk of the terrestrial and freshwater biotas, though some terrestrial relict species remain. Orbital forcing is thought to have driven expansion and contraction of the Antarctic ice sheet, so periodically covering and exposing conti- nental shelf habitat in the sea surrounding Antarctica. These changes are likely to have subdivided popula- tions and caused substantial changes to their ranges, promoting speciation. This mechanism is thought to be one of the causes of high diversity in several Ant- arctic marine groups.

Subsequent to the development of the Antarctic Circumpolar Current, the continent has remained rel- atively isolated. However, this isolation is not com- plete and organisms have migrated both into and out of the region. Aeolian (wind) transport of propagules (cysts, eggs, larvae, seeds, spores, plant parts) from South America to the Peninsula region are not un- common, especially under stormy weather conditions, and kelp rafts (and more recently floating plastic debris) may form another source of transport of colo- nists across the Southern Ocean. In addition, a wide range of pelagic mammal and seabird species regular- ly migrate into and out of the region, so overcoming the isolation barrier. However, even for those propa- gules that can cross the barriers which separate Antarctica from the other continents, the low temper- ature and low productivity environment (in the winter in marine systems) represent a significant challenge to establishment and survival.

More recently, the routes for colonization have increased substantially thanks to human traffic to Antarctica, first by ship and now by aircraft too. Thus, humans have introduced a wide range of alien, and in many cases invasive, species to Antarc- tica and the sub-Antarctic islands. These include microbes (and the diseases they cause in some cases), algae, fungi, bryophytes, vascular plants, invertebrates, fish, birds, and mammals. These species have come to survive, and in some cases dominate, terrestrial, BIODIVERSITY, TERRESTRIAL

freshwater, and marine habitats, in some regions caus- ing considerable damage by way of species extirpations and wholesale alteration of ecosystems. On the sub- Antarctic islands the effects of invasive species are well known. For example, rats and cats have caused the disappearance of seabird species from several islands, whilst eradication of the alien species has seen the return of the birds. Alien plants reduce local diversity by as much as 40%, and invasive rodents have caused wholesale alterations to ecosystem functioning. The indirect effects of species such as insects, mice, rein- deer, and salmonid fish are only beginning to be appre- ciated. Alien species arrive in a multitude of ways: in clothing and personal baggage, attached to fresh vege- tables, in vehicles, affixed to the hulls of ships and inflatable rubber boats, and as unwanted passengers on anchor chains, in sea chests, and in ballast water.

Across the Southern Ocean islands there is a strong relationship between the numbers of humans that visit an island and the numbers of species that have successfully been introduced to an island. This rela- tionship between visitor frequency and numbers of alien and invasive species has been established else- where and seems to be a general rule. Moreover, there is also a strong relationship between energy availabil- ity and numbers of alien species on the Southern Ocean Islands, suggesting not only that warmer islands are more at risk, but also that climatic amelio- ration is likely to enhance the risks of invasion.

Recent increases in human activity in the Antarctic have been considerable. For example, in the 2001– 2002 season, Treaty nations deployed 4390 personnel at 67 stations or field camps in Antarctica. They also made use of some sixty ships, departing from about thirty cities around the world, to offload per- sonnel and cargo. Tourist numbers doubled from approximately 6000 in 1992 to about 13,600 10 years later, most of whom visited the Peninsula region. From 1996 to 2006, the numbers of ships and passen- gers visiting South Georgia have increased threefold to fourfold. In 2001–2002, nearly 7000 people visited a single site: Whalers Bay, Deception Island.

Moreover, in the Peninsula Region, and at many sub-Antarctic sites, climates have warmed drama- tically over the past 50 years, making these envir- onments more benign than they once were. Thus, both the isolation and the climatic barriers that long meant low colonization rates of Antarctica are being influenced by human activities. In consequence, the chances of successful colonization of the Antarctic by alien species are likely to increase substantially unless appropriate mitigation measures are put in place. In this instance, prevention is more economically viable than eradication programmes, which are usually ex- pensive and often long-term.

Although the Convention on Biological Diversity calls for the conservation of biodiversity via a variety of measures, including the prevention of invasion and/or eradication of alien species, it does not apply to the Antarctic Treaty area. However, it does apply to the sub-Antarctic islands that have been annexed by those nations which have ratified the Convention. Many sub-Antarctic islands are managed as IUCN Category 1 Nature Reserves. Nonetheless, the conser- vation regulations at the islands vary widely, from extremely strict (e.g., no tourism, no fresh produce ashore at the Prince Edward Islands) to much less demanding (e.g., Iˆles Kerguelen and South Georgia). A formal, spatially explicit conservation assessment of the Southern Ocean islands has shown that by strictly conserving 15 of the Southern Ocean Islands, 90% of species richness can be captured, whilst only 50% of the alien species are included. Unfortunately, the generally strong relationship between alien and indigenous species richness means that some level of the latter will always be included. Moreover, planning for the biodiversity effects of climate change is made difficult both by the high levels of endemism in the region and by the likelihood that climate change will favour alien over indigenous species. Nonetheless, the formal analysis demonstrated that using spatially ex- plicit data and modern analytical methods provides a more robust approach than expert consultation, which in this particular case rarely proved better than simply picking islands at random.

In the Treaty Area, the conservation of biodiversity is undertaken by the Antarctic Treaty Consultative Parties under the auspices of the Committee for Envi- ronmental Protection, established under the Protocol on Environmental Protection to the Antarctic Treaty. Conservation is also facilitated by other organs of the Treaty system, such as the Convention for the Conser- vation of Antarctic Seals and the Convention for the Conservation of Antarctic Marine Living Resources. Whilst there are a wide range of measures in place for the conservation and sustainable use of biodiversity in the region, rational, spatially explicit conservation planning, which has been undertaken for many other parts of the globe, lags far behind in the Antarctic. In some cases, such as for marine benthic and pelagic diversity, this is a consequence of data deficiency, and only a sustained survey effort is likely to alter the situation. However, there are spatially explicit data for some groups, such as the seabirds and increas- ingly the seals, and much local-scale data on terres- trial biodiversity is available as a consequence of the requirements for the establishment of Antarctic Spe- cially Protected Areas (ASPAs) and Antarctic Specially Managed Areas (ASMAs). However, these data have yet to be used to develop a formal, spatially explicit BIODIVERSITY, TERRESTRIAL

conservation planning framework for Antarctica. If the biodiversity of the region is to be conserved and used in a sustainable manner, then such a framework, which incorporates the likely substantial changes that will be wrought to marine, freshwater, and terrestrial systems by climate change and the patterns of human use of the region, is urgently required.

STEVENL. CHOWN

See also Aerobiology; Algae; Antarctic Peninsula; Ant- arctic Treaty System; Benthic Communities in the Southern Ocean; Biodiversity, Marine; Biogeography;