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Tundra
Ecosystems | Tundra 3633
Tundra R Harmsen, Queen’s University, Kingston, ON, Canada ª 2008 Elsevier B.V. All rights reserved.
Introduction The Periglacial Environment Landscape and Species Diversity Vegetation and Succession
Ecosystem Structure and Function Special Adaptations to Tundra Conditions Global Warming and Other Anthropogenic Effects Further Reading
Introduction
The Periglacial Environment
Tundra ecosystems are widely distributed over all continents. Tundra is characterized by climatic stress consisting of low temperatures, strong winds, low precipitation, frost action, and long periods of shortage of liquid water caused by freezing and/or drought. These stresses combine to create what is called the periglacial environment, which is defined by repeated effects of freezing and thawing on soils and water bodies. Tundra comes in many different kinds. The two main categories are arctic and alpine tundra. Each of these can be divided into subcategories or can be seen as gradients from a richly vegetated tundra with tall shrubbery adjacent to the ‘tree line’ through categories with less vegetation to barren areas with only a minimum of vegetation adjacent to ice-fields and permanently frozen polar or alpine areas (see Boreal Forest and Alpine Forest). Included in the tundra biome are tundra ponds, lakes, streams, marshes, and other wetlands. Since tundra is found at the cold limit of life forms on Earth, climatic changes of the past have had major effects on tundra ecosystems and the plant and animal species of these systems. With each Pleistocene ice age, big areas of arctic tundra were eradicated, while others shifted southwards, as entirely new areas of forest or prairie became tundra, only to be reversed with the subsequent interglacials. Similar changes would have occurred in mountainous regions. These major changes resulted in the extinction of species and in the disruption of coevolved, interactive plant, and animal assemblages. These changes in tundra communities persist today, resulting in low species diversity and the scarcity of complex food-chains. During the current interglacial, many areas that were pushed down by the weight of the ice were first flooded by the rise in sea level, but have subsequently, in part at least, re-bounded and developed into tundra. Some parts of Beringia (eastern Siberia, northern Alaska, and into the Yukon and Banks Island) were not glaciated, and retained a far northern tundra during the last glacial period. Species of plants and animals that now form arctic tundra communities survived the ice ages either south of the glaciated land area or in unglaciated refuges such as Beringia.
Periglacial conditions are the result of current or geologically recent frost and ice formations on a landscape. Glaciers affect landscapes in major ways, which can have long-lasting effects on geomorphology, drainage systems, and soils. But even temperature regimes that cause frequent freeze–thaw cycles – for example, annually in the high arctic and daily on high tropical mountains – affects not only plants and animals directly, but also have indirect effects on soils and water, which results in specific types of erosion and the formation of characteristic landscapes. Furthermore, the usual presence of permafrost under tundra ecosystems is of critical importance, in that it forms a permanently impenetrable floor, preventing biological penetration and vertical movement of water and nutrients. The freeze–thaw cycle causes expansion and contraction of soils and water, while the gradual freezing of wet soils will also cause a nonrandom redistribution of water into ice lenses and ice wedges. These processes can result in frost heave and long-term vertical and horizontal movements of soils, debris, and even large rocks, creating typical landscape features (such as polygons), frost mounds (such as palsas and pingos), slope solifluction, and others. These land forms in turn affect the vegetation and all other life forms. The permafrost is ubiquitous in the arctic tundra, but less frequently found in alpine tundra sites, as alpine landscapes are more diverse and the summers are warmer. It is not found in any but the highest tropical mountains. During spring, the thawing of the soil starts from the surface down, gradually releasing the vegetation from the grip of the frozen soil. There is usually an overlap between snow melt and the thawing of the soil, especially in undulating landscapes. All melt water must run off, accumulate in low areas, or evaporate, as no vertical movement of water is possible due to the impervious permafrost. This can cause erosion, affecting plants and small animals. During the summer, thawing of the permafrost continues till autumn, when the surface may already start to refreeze. During later autumn and early winter,
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the frost will penetrate deeper into the soil from the surface, as it also comes up from the main body of the permafrost. This process can cause considerable expansion and result in frost heave and can cause much damage to root systems and animal burrows. In many areas, tundra soils are low in nutrients, because the permafrost prevents vertical movement of soil water. Some lakes (e.g., kettle lakes and moraine lakes) have their origins in major ice formations dating from the latest ice age, while others are recent formations. Tundra lakes and ponds are severely affected by annual freezing, especially those lakes that freeze each winter right to the bottom and beyond to the permafrost. Freezing of lake ice causes expansion and results in the shoreline with its vegetation being elevated above the surrounding lowlands. Lake sediments are often high in inorganic matter from spring runoff, but low in organic matter due to low productivity reflecting low nutrient levels. Frost action and wind effects on ice tend to disturb lake sediments in shallow lakes.
Landscape and Species Diversity Many parts of the arctic tundra are flat, especially in areas adjacent to the sea. These areas are often covered with ponds and shallow lakes, separated by marshes and connected by meandering streams and rivers. These areas can accumulate peat and develop into fens. Along the seashore these habitats tend to merge into salt marshes, brackish lagoons, and beach ridges. On higher ground, with hills and rock outcrops, the landscape diversity is much greater, especially since north and south facing slopes have very different microclimates, and hence, very different biological communities. Here one can also find deep lakes and fast-flowing rivers. Both erosion and the underlying rock type will also add to ecosystem diversity. In mountainous areas the arctic tundra merges into an alpine version. The diversity of alpine tundra worldwide is enormous, as it is found on all continents and in many climatic zones. Snow accumulation during winter, combined with slope, wind, and summer climate affect the length of the growing season of alpine tundra ecosystems. Tropical alpine tundra occurs only at very high altitudes, with unique climates varying from desert to some of the wettest conditions on Earth (Figure 1). It should also be noted that many of the alpine tundra zones are isolated from other such zones by hundreds or even thousands of kilometers, so that they have undergone independent evolution of their flora and fauna. Especially geologically old high mountains contain many endemic species derived from local forest or savannah species. For instance, the Southern Alps of New Zealand have over 600 species of
Figure 1 Mount Kenya, tropical Africa. High tropical alpine tundra. In the foreground a boulder moraine with lichens, mosses, scattered tussock grasses, and a few rosettes of the large Seneciodendron keniensis. In the middleground a sparse stand of the yet larger Seneciodendron keniodendron. The genus Seneciodendron is endemic to the east-central African mountains. In the background the Tyndall Glacier. Photo by W. C. Mahaney.
alpine plants, very few of which are found elsewhere on Earth. Roughly 5% of the Earth’s surface is covered with arctic vegetation and 3% with alpine vegetation. The alpine tundra worldwide, as well as per hectare for most alpine systems, has a much higher biodiversity than the arctic lowland tundra. Species richness declines with altitude on mountains and with latitude in the arctic, and also is dependent on local climatic conditions, nutrient availability, etc.
Vegetation and Succession Whether one climbs a mountain and crosses the timberline, or travels northwards in the arctic, and crossing the tree line, one enters the low tundra, which is characterized by shrubs. A combination of low temperatures,
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shallow soils, and strong winds prevents tree growth, but a tight shrub cover manages to thrive under such conditions. On each mountainous area on Earth shrub tundras can be found, which are superficially quite similar to other isolated alpine shrub tundra communities; even many of the individual species have a remarkably similar appearance. However, mostly unrelated species form such shrub communities in different parts of the world. For instance, most of the species of the shrub vegetation on East Africa’s Mount Kenya, New Guinea’s Mount Wilhelm, and Pico Mucun˜uque of the Venezuelan Andes belong to different families. This is a good example of convergent evolution acting on divergent taxa, causing adaptation to a specific environment. The shrub zone in the Canadian arctic has a more impoverished vegetation than the shrub zones on tropical mountains. It is dominated by several species of willow and birch, and a smattering of other species (Figure 2). Again, the arctic tundra in Greenland, Scandinavia, or Siberia also looks very similar, but in this case the species are all close relatives or even the same circumpolar species on the different continents. Another difference is that on tropical mountains there are a lot of shrub species that are not found below the tree line, whereas in the arctic many of the shrub species are also found south of the tree line. These differences are the result of the different effects of the ice ages, which on mountains merely caused the vertical movement of more or less entire plant communities up and down alpine valleys and slopes, while in the arctic, changes in the climate can cause north–south displacements of the conditions suitable for shrub tundra of over hundreds of kilometers. The more typical graminoid, forb, and moss tundra found higher up the mountains and further north in the arctic is adapted to extreme cold, long periods of temperatures permanently below freezing (and permanent darkness in the arctic) and strong winds. It is the strong
Figure 2 Hudson Bay Lowlands, Northern Manitoba, Canada 60 N. Low arctic willow (Salix spp.) and graminoid tundra. Note the radio-collar on the polar bear.
winds blowing ice crystals which abrade any vegetation above snow level, combined with desiccation that makes tree and tall shrub growth impossible in high arctic and alpine tundra. Especially in arctic deserts, where snow cover is low, vegetation remains very low to the ground (Figure 3). For instance on Banks Island at 70 N, arctic willow (Salix arctica) grows horizontally along the ground, forming matted areas of intertwining branches that form catkins and leaves in summer. One such willow can live and grow for decades. All grasses, sedges, and forbs die back in autumn and survive the winter as belowground root masses, or as ground hugging rosettes. One advantage of being a plant in a dense, low to the ground plant community is that on cool, sunny summer days radiant heat from the 24 h solar radiation is trapped within the air between the plants, keeping temperatures high enough for growth and maturation of seed. There are very few annual plants in the high arctic tundra, because the season is not long enough to germinate, grow, and reproduce. A few very small species, such as Koenigia islandica and Montia lamprosperma, maintain an annual life strategy. Uniquely a few species of semiparasitic members of the Scrophulariaceae, such as Euphrasia arctica, do so as well. These species have a distinct early season advantage being able to grow very rapidly by gaining nutrients and photosynthate from neighboring perennials. The frequent disturbances due to the freeze–thaw cycles often lead to local eradication of vegetation. This creates openings for reinvasion and subsequent succession. One of the most interesting examples of this is the result of solifluction of soil clumps on south facing slopes in the arctic. Soil clumps with vegetation surrounded by clefts get heated by the sun on the downslope side, causing them to thaw out and slump downwards, burying
Figure 3 Banks Island, North West Territories, Canada 70 N. Upland high arctic tundra, also described as arctic desert. The vegetation is dominated by mountain avens (Dryas integrifolia) and various species of arctic vetch (Oxytropis spp. and Astragalus spp.) and scattered clumps of small graminoids. In the background is the Thomsen River valley with sedge meadow tundra and tundra ponds.
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Figure 4 Banks Island, North West Territories, Canada 70 N. Two types of high alpine tundra. In the foreground a wet graminoid tundra fed by snowmelt water. On the opposite slope a sparsely vegetated dry tundra showing solifluction. The muskoxen feed primarily on the graminoid slope, but will venture onto drier tundra types to feed on high nitrogen species such as arctic vetch.
the lowest vegetation, while at the same time exposing a small strip of upslope bare soil (Figure 4). It takes up to 30 years for the clump to make one entire downhill rotation. On each clump, one can see a successional sequence of plant maturity, species composition, and diversity, as the oldest community gets buried and an opening appears at the top end for reinvasion. Succession on a larger scale occurs after slope collapses, frost mounting, stream erosion, mud deposits after flooding, etc.
Ecosystem Structure and Function Very few species remain active within arctic tundra ecosystems during the winter. Only most mammals such as the muskox (Ovibos moschatus), the reindeer (Rangifer tarandus), the arctic hare (Lepus arcticus), lemmings, and the wolf (Canis lupus arctos) remain fully active. A few birds, for example, raven (Corvus corax) and rock ptarmigan (Lagopus mutus), manage as well. During the autumn and early winter months, soil microbial metabolic activity continues down to at least 12 C. The vast majority of organisms that spend the winter on the tundra do so in some form of dormancy. Alpine tundra, being much more diverse, and much of it having periods of daylight throughout the year, varies greatly in the degree of winter activity of the fauna. The brief summer on the tundra is enormously productive, and provides food for a wide variety of organisms. The vegetation starts to bloom and grow as soon as the snow starts to melt. At that time of year the sun hardly sets if at all and temperatures rise quickly. Dormant overwintering insect larvae start to feed and eggs eclose to add innumerable larvae in snow melt ponds, in the soil, and on the new vegetation. The
Figure 5 Nest of snowy owl (Nyctea scandiana) with six eggs and one hatchling. Snowy owls start incubating as soon as their first egg is laid, so that the young are hatched sequentially. Note the seven dead lemmings surrounding the nest, intended as food for the hatchlings. Later that summer, the lemming population crashed. Only the two eldest hatchlings survived to fledge, the others were eaten by the older ones.
ecosystem seems to burst into active life. High availability of edible vegetation, exploding insect, bird and rodent populations, and young birds lasts till just before freezeup in autumn (Figure 4). Many bird species migrate annually from more southerly wintering sites to the tundra to breed, taking advantage of, and adding to, the burst of summer productivity. Some of these species arrive in extremely large numbers. Most of these birds are insectivorous or feed on pond crustaceans, some such as loons and grebes are pisciverous, falcons and hawks are predators, and geese are herbivorous. Especially the colonially nesting geese can have major destructive effects on the vegetation, which in turn can affect many other species. In some tundra ecosystems some small mammals, especially two species of lemmings, show extreme oscillations in population density, making them keystone species in the tundra ecosystem. For instance, on Banks Island in northern Canada both the collared lemming (Dicrostonyx torquatus) and the brown lemming (Lemmus sibiricus) undergo sharp population oscillations with a 3–5-year period. At peak populations the lemmings are all over the place, whereas the year after it is hard to find a single lemming. During the outbreak phase, several predatory birds, including snowy owls (Nyctea scandinaca), rough-legged hawks (Buteo lagopus), and jaegers (Stercorarius spp), migrate long distances and concentrate in the regions with high lemming populations. They lay large clutches and raise many young, only to disperse to other areas when the lemming population collapses (Figure 5). Mammalian predators are not as able to respond by migration. Arctic foxes (Alopex lagopus) and ermines (Mustella erminea) are the main mammalian
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predators; they also take advantage of lemming outbreak with large litters. However, this leaves relatively dense populations of these predators after the collapse of the lemming population. This has a major feed-forward effect in that the half-starved predators exert a strong negative effect on other less-favored prey species, mostly birds, from small passerines to ducklings and even goslings. Only after the predator population has collapsed can the lemming population start to grow again. The ultimate cause of the collapse of the lemming population is not the predation pressure, but the exhaustion of quality vegetation and a delay in nutrient cycling. However, once the lemming population has collapsed, the subsequently declining predator population can drive the lemming population further down to its minimum. The vegetation, litter layer, and soils are strongly affected by the lemming cycles. This is shown by the enormous difference between the tundra in northern Canada and central Greenland, as in Greenland there are no lemmings, much more accumulated litter, differences in relative abundance of plant species, and far fewer predators. Exclosure experiments in Canadian tundra have similar results.
Special Adaptations to Tundra Conditions Many species have evolved special adaptations to the rigorous, but often predictable conditions of the tundra. This article presents four cases of such adaptations as examples of this phenomenon: the muskox, two species of arctic bumblebee, an alpine lobelia, and two congeneric alpine beetle species. The muskox of Banks Island in Canada’s Northwest Territories The muskox (Ovibos moschatus) is a surviving species of the Pleistocene megafauna; it survived the ice age both in Beringia and south of the ice sheet in what is now southern Canada and the northern United States. It has a very long adaptive history in arctic conditions, which shows in a number of very effective adaptations to extreme cold. Besides the obvious anatomical features such as the extremely effective insulating wool under the shaggy guard hair and the front hooves that are perfectly shaped to scratch the hard arctic snow to expose vegetation, this animal has a set of integrated physiological and behavioral traits making up a unique reproductive strategy. A muskox cow responds to her nutritional condition in autumn by not going into heat when in poor condition, and only going into heat early in the rutting season when in excellent condition. This means that cows in poor condition, which would not have been able to survive the winter and produce a calf the next spring, will live
and have another chance at reproduction the next year. The cows that do get pregnant, when faced with a bad winter, will either abort their fetus or abandon the calf after birth. Since most calves are born well before snow melt and the reappearance of new fodder, the cows have to be in good shape to not only carry the calf to birth, but also lactate for several weeks. However, only calves born early in the year have a good chance of gaining enough weight and reserve fat to survive their first winter. Integrated with this strategy are some significant traits. At birth, the calf weight over cow weight ratio is one of the lowest among ungulates, making abortion or abandonment a relatively minor cost for the cow, which can then cut lactation. Once the calf is born and the cow is lactating, she licks the calf when it urinates and swallows the urine. The urea of the urine is rebuilt into protein by the cow’s gut flora and will eventually be available for milk production. This is important because storage of protein over the winter is difficult, and late winter forage is scarce and low in protein. As soon as new forage is available during snow melt, the cows graze selectively on high protein vegetation, such as willow catkins and sprouting rosettes of arctic vetch (Oxytropis spp.). In far northern parts of their range, muskox cows live long lives, but only reproduce every second or third year and still lose some of their calves. Two Species of Bumble Bee from the Canadian Arctic The author has a personal recollection of working in early July on the tundra on northern Banks Island when in the middle of a snow squall a bumblebee flew by. This seemingly incongruous event is explained by the fact that the common large bumblebee (Bombus polaris) has an unusually well insulated thorax, which allows it to keep its flight muscles at approximately 30 C even when the ambient drops to the freezing point. What is even more special about this species is that the queen keeps her abdomen also near 30 C, which presumably allows its eggs to develop faster. However, early in the season the queen also warms her eggs and larvae in the nest by inserting her abdomen into the middle of the nest and producing heat by vibrating her flight muscles and circulating the heat to her abdomen. The queen, after overwintering, builds the nest, often in an abandoned lemming burrow, using bits of dead vegetation and muskox wool. There she raises one brood of workers before switching to start raising reproductives for the next year. The other species of bumblebee (B. hyperboreus) found on Banks Island is an obligatory brood parasite of B. polaris. The queens of this species lay only eggs for reproductives, and lay them in the nest of their host species. This strategy is obviously adapted to the very short summer season in the high arctic tundra, but it also depends on the presence of
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B. polaris. The ratio of the densities of the two species is stabilized by frequency-dependent selection. Flightless Beetles of the Genus Parasystatus on Mount Kenya In the tussock grass alpine tundra of Mount Kenya between 3200 and 4000 m, there are six described species and at least one undescribed species of the genus Parasystatus. These large beetles must be adapted to the diurnal extremes of the climate, which has been described as summer each day and winter each night. Two of these species, P. elongates and one undescribed species, have been studied in some detail as to their adaptation to the nightly frost of that zone. P. elongates spends its entire larval and pupal development inside a tussock of the grass Festuca abyssinica, where it is not affected by the nightly frost. As an adult beetle, it is active by day, shielded from the intense solar radiation by inflated elytra and a shiny, reflective outer cuticle. At night, the beetle hides under vegetation to avoid the worst of the frost; it has an ineffectively high supercooling point, but an effective freeze tolerance. The other species of the same genus is active well into the night, and protects itself with a much lower supercooling point, but is freezing sensitive. (Cooling a liquid to below its freezing point without phase transition; here pertaining to the avoidance of ice formation due to the presence of antifreeze substances and/or the absence of crystallization nuclei.) These two different physiological adaptations to nightly frost within one genus indicate that the two species have independently invaded the alpine tundra, rather than having arisen through speciation in the alpine zone. Being flightless – a typical adaptation to mountain top ecosystems – also rules out invasion from another mountain The Giant Lobelia and Its Insect Commensals on Kilimanjaro Between 3000 and 4000 m on the slopes of Mount Kilimanjaro, the giant lobelia (Lobelia deckenii) also has to face the stress of nightly frost, which can be severe due to parts of the Kilimanjaro alpine tundra being relatively dry. The plant has evolved into a ball-shaped rosette consisting of a fleshy center surrounded by concave spiky leaves, which are arranged in such a manner as to trap rainwater. A single plant can contain, trapped in its rosette, a compartmentalized mass of several liters of water. This volume is large enough to prevent it from freezing right to the middle in any one night. Indeed, the center of the plant where the growing tip is located maintains a very even temperature throughout the diurnal cycle. Not surprisingly, this water mass of the lobelia plants with its relatively even temperature has become the breeding environment for a few species of insects with
aquatic larvae, the most abundant of which is a chironomid midge. The water in the lobelias also contains microorganisms, which feed on decomposing debris and are in turn the food for the insect larvae.
Global Warming and Other Anthropogenic Effects Extensive research in arctic and alpine regions including ice core analysis, paleolimnology, palynology, and geomorphology has provided a detailed picture of the climatic history of these regions. This allows us to conclude that, as well as the major changes at the end of the last ice age, frequent climate oscillations have subsequently occurred that caused major changes in tundra ecosystems. Furthermore, there have been times when tundra types existed that are no longer extant. The species complexes that now exist consist of species that have been sufficiently flexible and/or dispersible to have survived the climatic and landscape oscillations of the past. However, this does not necessarily bode well for the future of tundra ecosystems and species, as anthropogenic changes are certain to be increasingly imposed on the Earth. Already, the most likely reason for the extinction of most of the Pleistocene arctic megafauna is a combination of climate change and human hunting. The disappearance of the large herbivores at that time caused a major switch in plant dominance on tundra ecosystems from graminoids to mosses, with concomitant changes in long-term soil and peat formation. We must expect similar major changes in the coming century, associated with at least some extinctions. Climate change will be severe and direct human effects will also increase. Already, several species are declining due to pollution and over-hunting. Some of the most at risk tundras (and associated endemic tundra species) will be isolated alpine tundra systems on relatively low mountains, where climatic warming will cause the entire system to be replaced with forest. See also: Alpine Ecosystems and the High-Elevation Treeline; Alpine Forest; Biodiversity; Boreal Forest; Cycling and Cycling Indices; Dispersal–Migration; Freshwater Lakes; Grassland Models; Grazing Models; Grazing; Polar Terrestrial Ecology; Predation; Prey–Predator Models; Soil Formation; Steppes and Prairies; Succession; Water Availability; Wind Effects.
Further Reading Chapin FS and Ko¨rner C (eds.) (1995) Arctic and Alpine Biodiversity. Patterns, Causes and Ecosystem Consequences, 332pp. Berlin: Springer. Coe MJ (1967) The Ecology of the Alpine Zone of Mount Kenya, 136pp. The Hague: Junk.
Ecological Indicators | Turnover Time Craeford RMM (ed.) (1997) Disturbance and Recovery in Arctic Lands, 621pp. Dordrecht: Kluwer Academic. French HM and Williams P (2007) The Periglacial Environment, 478pp. Toronto: Wiley. Goulson D (2003) Bumblebees: Their Behavior and Ecology, 235pp. Oxford: Oxford University Press. Jones HG, Pomeroy JW, Walker DA, and Hoham RW (eds.) (2001) Snow Ecology: An Interdisciplanary Examination of Snow-Covered Ecosystems, 378pp. Cambridge University Press. Laws RM (ed.) (1984) Antarctic Ecology, vol. 1, 344pp. London: Academic Press.
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Mahaney WC (ed.) (1989) Quaternary and Environmental Research on East African Mountains, 483pp. Rotterdam: Balkema. Pienitz R, Douglas MSV, and Smol JP (eds.) (2004) Long-Term Environmental Change in Arctic and Antarctic Lakes, 562pp. Dordrecht: Springer. Rosswall T and Heal OW (eds.) (1975) Ecological Bulletin, Vol. 20: Structure and Function of Tundra Ecosystems, 450pp. Stockholm: Swedish Natural Science Research Council. Wielgolaski FE (ed.) (1997) Ecosystems of the World 3: Polar and Alpine Tundra, 920pp. Amsterdam: Elsevier.
Turnover Time E H Dettmann, US Environmental Protection Agency, Narragansett, RI, USA Published by Elsevier B.V.
Introduction Calculation of Turnover Time of Material and Energy Applications of Turnover Times in Ecosystem Analysis
Summary Further Reading
Introduction
biogeochemical cycle of an element. Turnover times of subsystems will differ from that of the larger system of which they are a part.
Turnover time refers to the amount of time required for turnover or replacement by flow-through of the energy or a material contained in a system. Ecological units are generally open to flow-through of energy and materials. Carbon, nitrogen, phosphorus, and other materials, as well as energy, enter the system, may be incorporated into system components and later be released by respiration, excretion, decomposition, and other processes, and eventually leave the system. The material of interest may undergo a single cycle of incorporation and release, or may be recycled many times before exiting. Some substances may be passively transported through the system. The concept of turnover is also applied to appearance and extinction of biological species in a local community, and can describe passage of individuals through a population through recruitment and harvesting. Turnover time provides a timescale for this replacement and is a tool for analyzing the significance of flow rates and the sizes of material and energy pools within a system or its components. The turnover time of a material in a system is generally defined as the ratio of the quantity of that material in the system to its throughput or flow-through rate. This definition assumes that the rates of inflow and outflow are equal, that is, the system is in equilibrium (steady state). It is an average quantity, since the calculation provides no way to account for the transit time of an individual entity (e.g., an atom or molecule) through the system. Turnover time can be calculated at various scales, for small ecosystems or large ones, such as the component subsystems of the global
Calculation of Turnover Time of Material and Energy Turnover time ( ) for a flow of material is the ratio of the quantity of material in the system (M) to its flow rate (F) through the system, that is, ¼
M F
½1
Turnover time is the inverse of the fraction of material in the system that leaves per unit time. It may also be viewed as the amount of time required to put into the ecosystem an amount of the substance equal to that currently residing there, or for that amount to leave the system. The units used to express the quantities in the numerator and denominator of eqn [1] must be consistent. For materials, the mass of the material in the system is M, and F is the flux (mass/time), with flux expressed using the same units for mass. If energy turnover is calculated, the ratio is of biomass to production, both expressed in consistent biomass or energy units. Other timescales used in analyzing the dynamics of material flow through systems are age and transit time. The age of an entity is the amount of time that has elapsed since its entry into the system; its transit time is the total time between its entry into and exit from the system.
Tundra R Harmsen, Queen’s University, Kingston, ON, Canada ª 2008 Elsevier B.V. All rights reserved.
Introduction The Periglacial Environment Landscape and Species Diversity Vegetation and Succession
Ecosystem Structure and Function Special Adaptations to Tundra Conditions Global Warming and Other Anthropogenic Effects Further Reading
Introduction
The Periglacial Environment
Tundra ecosystems are widely distributed over all continents. Tundra is characterized by climatic stress consisting of low temperatures, strong winds, low precipitation, frost action, and long periods of shortage of liquid water caused by freezing and/or drought. These stresses combine to create what is called the periglacial environment, which is defined by repeated effects of freezing and thawing on soils and water bodies. Tundra comes in many different kinds. The two main categories are arctic and alpine tundra. Each of these can be divided into subcategories or can be seen as gradients from a richly vegetated tundra with tall shrubbery adjacent to the ‘tree line’ through categories with less vegetation to barren areas with only a minimum of vegetation adjacent to ice-fields and permanently frozen polar or alpine areas (see Boreal Forest and Alpine Forest). Included in the tundra biome are tundra ponds, lakes, streams, marshes, and other wetlands. Since tundra is found at the cold limit of life forms on Earth, climatic changes of the past have had major effects on tundra ecosystems and the plant and animal species of these systems. With each Pleistocene ice age, big areas of arctic tundra were eradicated, while others shifted southwards, as entirely new areas of forest or prairie became tundra, only to be reversed with the subsequent interglacials. Similar changes would have occurred in mountainous regions. These major changes resulted in the extinction of species and in the disruption of coevolved, interactive plant, and animal assemblages. These changes in tundra communities persist today, resulting in low species diversity and the scarcity of complex food-chains. During the current interglacial, many areas that were pushed down by the weight of the ice were first flooded by the rise in sea level, but have subsequently, in part at least, re-bounded and developed into tundra. Some parts of Beringia (eastern Siberia, northern Alaska, and into the Yukon and Banks Island) were not glaciated, and retained a far northern tundra during the last glacial period. Species of plants and animals that now form arctic tundra communities survived the ice ages either south of the glaciated land area or in unglaciated refuges such as Beringia.
Periglacial conditions are the result of current or geologically recent frost and ice formations on a landscape. Glaciers affect landscapes in major ways, which can have long-lasting effects on geomorphology, drainage systems, and soils. But even temperature regimes that cause frequent freeze–thaw cycles – for example, annually in the high arctic and daily on high tropical mountains – affects not only plants and animals directly, but also have indirect effects on soils and water, which results in specific types of erosion and the formation of characteristic landscapes. Furthermore, the usual presence of permafrost under tundra ecosystems is of critical importance, in that it forms a permanently impenetrable floor, preventing biological penetration and vertical movement of water and nutrients. The freeze–thaw cycle causes expansion and contraction of soils and water, while the gradual freezing of wet soils will also cause a nonrandom redistribution of water into ice lenses and ice wedges. These processes can result in frost heave and long-term vertical and horizontal movements of soils, debris, and even large rocks, creating typical landscape features (such as polygons), frost mounds (such as palsas and pingos), slope solifluction, and others. These land forms in turn affect the vegetation and all other life forms. The permafrost is ubiquitous in the arctic tundra, but less frequently found in alpine tundra sites, as alpine landscapes are more diverse and the summers are warmer. It is not found in any but the highest tropical mountains. During spring, the thawing of the soil starts from the surface down, gradually releasing the vegetation from the grip of the frozen soil. There is usually an overlap between snow melt and the thawing of the soil, especially in undulating landscapes. All melt water must run off, accumulate in low areas, or evaporate, as no vertical movement of water is possible due to the impervious permafrost. This can cause erosion, affecting plants and small animals. During the summer, thawing of the permafrost continues till autumn, when the surface may already start to refreeze. During later autumn and early winter,
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the frost will penetrate deeper into the soil from the surface, as it also comes up from the main body of the permafrost. This process can cause considerable expansion and result in frost heave and can cause much damage to root systems and animal burrows. In many areas, tundra soils are low in nutrients, because the permafrost prevents vertical movement of soil water. Some lakes (e.g., kettle lakes and moraine lakes) have their origins in major ice formations dating from the latest ice age, while others are recent formations. Tundra lakes and ponds are severely affected by annual freezing, especially those lakes that freeze each winter right to the bottom and beyond to the permafrost. Freezing of lake ice causes expansion and results in the shoreline with its vegetation being elevated above the surrounding lowlands. Lake sediments are often high in inorganic matter from spring runoff, but low in organic matter due to low productivity reflecting low nutrient levels. Frost action and wind effects on ice tend to disturb lake sediments in shallow lakes.
Landscape and Species Diversity Many parts of the arctic tundra are flat, especially in areas adjacent to the sea. These areas are often covered with ponds and shallow lakes, separated by marshes and connected by meandering streams and rivers. These areas can accumulate peat and develop into fens. Along the seashore these habitats tend to merge into salt marshes, brackish lagoons, and beach ridges. On higher ground, with hills and rock outcrops, the landscape diversity is much greater, especially since north and south facing slopes have very different microclimates, and hence, very different biological communities. Here one can also find deep lakes and fast-flowing rivers. Both erosion and the underlying rock type will also add to ecosystem diversity. In mountainous areas the arctic tundra merges into an alpine version. The diversity of alpine tundra worldwide is enormous, as it is found on all continents and in many climatic zones. Snow accumulation during winter, combined with slope, wind, and summer climate affect the length of the growing season of alpine tundra ecosystems. Tropical alpine tundra occurs only at very high altitudes, with unique climates varying from desert to some of the wettest conditions on Earth (Figure 1). It should also be noted that many of the alpine tundra zones are isolated from other such zones by hundreds or even thousands of kilometers, so that they have undergone independent evolution of their flora and fauna. Especially geologically old high mountains contain many endemic species derived from local forest or savannah species. For instance, the Southern Alps of New Zealand have over 600 species of
Figure 1 Mount Kenya, tropical Africa. High tropical alpine tundra. In the foreground a boulder moraine with lichens, mosses, scattered tussock grasses, and a few rosettes of the large Seneciodendron keniensis. In the middleground a sparse stand of the yet larger Seneciodendron keniodendron. The genus Seneciodendron is endemic to the east-central African mountains. In the background the Tyndall Glacier. Photo by W. C. Mahaney.
alpine plants, very few of which are found elsewhere on Earth. Roughly 5% of the Earth’s surface is covered with arctic vegetation and 3% with alpine vegetation. The alpine tundra worldwide, as well as per hectare for most alpine systems, has a much higher biodiversity than the arctic lowland tundra. Species richness declines with altitude on mountains and with latitude in the arctic, and also is dependent on local climatic conditions, nutrient availability, etc.
Vegetation and Succession Whether one climbs a mountain and crosses the timberline, or travels northwards in the arctic, and crossing the tree line, one enters the low tundra, which is characterized by shrubs. A combination of low temperatures,
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shallow soils, and strong winds prevents tree growth, but a tight shrub cover manages to thrive under such conditions. On each mountainous area on Earth shrub tundras can be found, which are superficially quite similar to other isolated alpine shrub tundra communities; even many of the individual species have a remarkably similar appearance. However, mostly unrelated species form such shrub communities in different parts of the world. For instance, most of the species of the shrub vegetation on East Africa’s Mount Kenya, New Guinea’s Mount Wilhelm, and Pico Mucun˜uque of the Venezuelan Andes belong to different families. This is a good example of convergent evolution acting on divergent taxa, causing adaptation to a specific environment. The shrub zone in the Canadian arctic has a more impoverished vegetation than the shrub zones on tropical mountains. It is dominated by several species of willow and birch, and a smattering of other species (Figure 2). Again, the arctic tundra in Greenland, Scandinavia, or Siberia also looks very similar, but in this case the species are all close relatives or even the same circumpolar species on the different continents. Another difference is that on tropical mountains there are a lot of shrub species that are not found below the tree line, whereas in the arctic many of the shrub species are also found south of the tree line. These differences are the result of the different effects of the ice ages, which on mountains merely caused the vertical movement of more or less entire plant communities up and down alpine valleys and slopes, while in the arctic, changes in the climate can cause north–south displacements of the conditions suitable for shrub tundra of over hundreds of kilometers. The more typical graminoid, forb, and moss tundra found higher up the mountains and further north in the arctic is adapted to extreme cold, long periods of temperatures permanently below freezing (and permanent darkness in the arctic) and strong winds. It is the strong
Figure 2 Hudson Bay Lowlands, Northern Manitoba, Canada 60 N. Low arctic willow (Salix spp.) and graminoid tundra. Note the radio-collar on the polar bear.
winds blowing ice crystals which abrade any vegetation above snow level, combined with desiccation that makes tree and tall shrub growth impossible in high arctic and alpine tundra. Especially in arctic deserts, where snow cover is low, vegetation remains very low to the ground (Figure 3). For instance on Banks Island at 70 N, arctic willow (Salix arctica) grows horizontally along the ground, forming matted areas of intertwining branches that form catkins and leaves in summer. One such willow can live and grow for decades. All grasses, sedges, and forbs die back in autumn and survive the winter as belowground root masses, or as ground hugging rosettes. One advantage of being a plant in a dense, low to the ground plant community is that on cool, sunny summer days radiant heat from the 24 h solar radiation is trapped within the air between the plants, keeping temperatures high enough for growth and maturation of seed. There are very few annual plants in the high arctic tundra, because the season is not long enough to germinate, grow, and reproduce. A few very small species, such as Koenigia islandica and Montia lamprosperma, maintain an annual life strategy. Uniquely a few species of semiparasitic members of the Scrophulariaceae, such as Euphrasia arctica, do so as well. These species have a distinct early season advantage being able to grow very rapidly by gaining nutrients and photosynthate from neighboring perennials. The frequent disturbances due to the freeze–thaw cycles often lead to local eradication of vegetation. This creates openings for reinvasion and subsequent succession. One of the most interesting examples of this is the result of solifluction of soil clumps on south facing slopes in the arctic. Soil clumps with vegetation surrounded by clefts get heated by the sun on the downslope side, causing them to thaw out and slump downwards, burying
Figure 3 Banks Island, North West Territories, Canada 70 N. Upland high arctic tundra, also described as arctic desert. The vegetation is dominated by mountain avens (Dryas integrifolia) and various species of arctic vetch (Oxytropis spp. and Astragalus spp.) and scattered clumps of small graminoids. In the background is the Thomsen River valley with sedge meadow tundra and tundra ponds.
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Figure 4 Banks Island, North West Territories, Canada 70 N. Two types of high alpine tundra. In the foreground a wet graminoid tundra fed by snowmelt water. On the opposite slope a sparsely vegetated dry tundra showing solifluction. The muskoxen feed primarily on the graminoid slope, but will venture onto drier tundra types to feed on high nitrogen species such as arctic vetch.
the lowest vegetation, while at the same time exposing a small strip of upslope bare soil (Figure 4). It takes up to 30 years for the clump to make one entire downhill rotation. On each clump, one can see a successional sequence of plant maturity, species composition, and diversity, as the oldest community gets buried and an opening appears at the top end for reinvasion. Succession on a larger scale occurs after slope collapses, frost mounting, stream erosion, mud deposits after flooding, etc.
Ecosystem Structure and Function Very few species remain active within arctic tundra ecosystems during the winter. Only most mammals such as the muskox (Ovibos moschatus), the reindeer (Rangifer tarandus), the arctic hare (Lepus arcticus), lemmings, and the wolf (Canis lupus arctos) remain fully active. A few birds, for example, raven (Corvus corax) and rock ptarmigan (Lagopus mutus), manage as well. During the autumn and early winter months, soil microbial metabolic activity continues down to at least 12 C. The vast majority of organisms that spend the winter on the tundra do so in some form of dormancy. Alpine tundra, being much more diverse, and much of it having periods of daylight throughout the year, varies greatly in the degree of winter activity of the fauna. The brief summer on the tundra is enormously productive, and provides food for a wide variety of organisms. The vegetation starts to bloom and grow as soon as the snow starts to melt. At that time of year the sun hardly sets if at all and temperatures rise quickly. Dormant overwintering insect larvae start to feed and eggs eclose to add innumerable larvae in snow melt ponds, in the soil, and on the new vegetation. The
Figure 5 Nest of snowy owl (Nyctea scandiana) with six eggs and one hatchling. Snowy owls start incubating as soon as their first egg is laid, so that the young are hatched sequentially. Note the seven dead lemmings surrounding the nest, intended as food for the hatchlings. Later that summer, the lemming population crashed. Only the two eldest hatchlings survived to fledge, the others were eaten by the older ones.
ecosystem seems to burst into active life. High availability of edible vegetation, exploding insect, bird and rodent populations, and young birds lasts till just before freezeup in autumn (Figure 4). Many bird species migrate annually from more southerly wintering sites to the tundra to breed, taking advantage of, and adding to, the burst of summer productivity. Some of these species arrive in extremely large numbers. Most of these birds are insectivorous or feed on pond crustaceans, some such as loons and grebes are pisciverous, falcons and hawks are predators, and geese are herbivorous. Especially the colonially nesting geese can have major destructive effects on the vegetation, which in turn can affect many other species. In some tundra ecosystems some small mammals, especially two species of lemmings, show extreme oscillations in population density, making them keystone species in the tundra ecosystem. For instance, on Banks Island in northern Canada both the collared lemming (Dicrostonyx torquatus) and the brown lemming (Lemmus sibiricus) undergo sharp population oscillations with a 3–5-year period. At peak populations the lemmings are all over the place, whereas the year after it is hard to find a single lemming. During the outbreak phase, several predatory birds, including snowy owls (Nyctea scandinaca), rough-legged hawks (Buteo lagopus), and jaegers (Stercorarius spp), migrate long distances and concentrate in the regions with high lemming populations. They lay large clutches and raise many young, only to disperse to other areas when the lemming population collapses (Figure 5). Mammalian predators are not as able to respond by migration. Arctic foxes (Alopex lagopus) and ermines (Mustella erminea) are the main mammalian
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predators; they also take advantage of lemming outbreak with large litters. However, this leaves relatively dense populations of these predators after the collapse of the lemming population. This has a major feed-forward effect in that the half-starved predators exert a strong negative effect on other less-favored prey species, mostly birds, from small passerines to ducklings and even goslings. Only after the predator population has collapsed can the lemming population start to grow again. The ultimate cause of the collapse of the lemming population is not the predation pressure, but the exhaustion of quality vegetation and a delay in nutrient cycling. However, once the lemming population has collapsed, the subsequently declining predator population can drive the lemming population further down to its minimum. The vegetation, litter layer, and soils are strongly affected by the lemming cycles. This is shown by the enormous difference between the tundra in northern Canada and central Greenland, as in Greenland there are no lemmings, much more accumulated litter, differences in relative abundance of plant species, and far fewer predators. Exclosure experiments in Canadian tundra have similar results.
Special Adaptations to Tundra Conditions Many species have evolved special adaptations to the rigorous, but often predictable conditions of the tundra. This article presents four cases of such adaptations as examples of this phenomenon: the muskox, two species of arctic bumblebee, an alpine lobelia, and two congeneric alpine beetle species. The muskox of Banks Island in Canada’s Northwest Territories The muskox (Ovibos moschatus) is a surviving species of the Pleistocene megafauna; it survived the ice age both in Beringia and south of the ice sheet in what is now southern Canada and the northern United States. It has a very long adaptive history in arctic conditions, which shows in a number of very effective adaptations to extreme cold. Besides the obvious anatomical features such as the extremely effective insulating wool under the shaggy guard hair and the front hooves that are perfectly shaped to scratch the hard arctic snow to expose vegetation, this animal has a set of integrated physiological and behavioral traits making up a unique reproductive strategy. A muskox cow responds to her nutritional condition in autumn by not going into heat when in poor condition, and only going into heat early in the rutting season when in excellent condition. This means that cows in poor condition, which would not have been able to survive the winter and produce a calf the next spring, will live
and have another chance at reproduction the next year. The cows that do get pregnant, when faced with a bad winter, will either abort their fetus or abandon the calf after birth. Since most calves are born well before snow melt and the reappearance of new fodder, the cows have to be in good shape to not only carry the calf to birth, but also lactate for several weeks. However, only calves born early in the year have a good chance of gaining enough weight and reserve fat to survive their first winter. Integrated with this strategy are some significant traits. At birth, the calf weight over cow weight ratio is one of the lowest among ungulates, making abortion or abandonment a relatively minor cost for the cow, which can then cut lactation. Once the calf is born and the cow is lactating, she licks the calf when it urinates and swallows the urine. The urea of the urine is rebuilt into protein by the cow’s gut flora and will eventually be available for milk production. This is important because storage of protein over the winter is difficult, and late winter forage is scarce and low in protein. As soon as new forage is available during snow melt, the cows graze selectively on high protein vegetation, such as willow catkins and sprouting rosettes of arctic vetch (Oxytropis spp.). In far northern parts of their range, muskox cows live long lives, but only reproduce every second or third year and still lose some of their calves. Two Species of Bumble Bee from the Canadian Arctic The author has a personal recollection of working in early July on the tundra on northern Banks Island when in the middle of a snow squall a bumblebee flew by. This seemingly incongruous event is explained by the fact that the common large bumblebee (Bombus polaris) has an unusually well insulated thorax, which allows it to keep its flight muscles at approximately 30 C even when the ambient drops to the freezing point. What is even more special about this species is that the queen keeps her abdomen also near 30 C, which presumably allows its eggs to develop faster. However, early in the season the queen also warms her eggs and larvae in the nest by inserting her abdomen into the middle of the nest and producing heat by vibrating her flight muscles and circulating the heat to her abdomen. The queen, after overwintering, builds the nest, often in an abandoned lemming burrow, using bits of dead vegetation and muskox wool. There she raises one brood of workers before switching to start raising reproductives for the next year. The other species of bumblebee (B. hyperboreus) found on Banks Island is an obligatory brood parasite of B. polaris. The queens of this species lay only eggs for reproductives, and lay them in the nest of their host species. This strategy is obviously adapted to the very short summer season in the high arctic tundra, but it also depends on the presence of
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B. polaris. The ratio of the densities of the two species is stabilized by frequency-dependent selection. Flightless Beetles of the Genus Parasystatus on Mount Kenya In the tussock grass alpine tundra of Mount Kenya between 3200 and 4000 m, there are six described species and at least one undescribed species of the genus Parasystatus. These large beetles must be adapted to the diurnal extremes of the climate, which has been described as summer each day and winter each night. Two of these species, P. elongates and one undescribed species, have been studied in some detail as to their adaptation to the nightly frost of that zone. P. elongates spends its entire larval and pupal development inside a tussock of the grass Festuca abyssinica, where it is not affected by the nightly frost. As an adult beetle, it is active by day, shielded from the intense solar radiation by inflated elytra and a shiny, reflective outer cuticle. At night, the beetle hides under vegetation to avoid the worst of the frost; it has an ineffectively high supercooling point, but an effective freeze tolerance. The other species of the same genus is active well into the night, and protects itself with a much lower supercooling point, but is freezing sensitive. (Cooling a liquid to below its freezing point without phase transition; here pertaining to the avoidance of ice formation due to the presence of antifreeze substances and/or the absence of crystallization nuclei.) These two different physiological adaptations to nightly frost within one genus indicate that the two species have independently invaded the alpine tundra, rather than having arisen through speciation in the alpine zone. Being flightless – a typical adaptation to mountain top ecosystems – also rules out invasion from another mountain The Giant Lobelia and Its Insect Commensals on Kilimanjaro Between 3000 and 4000 m on the slopes of Mount Kilimanjaro, the giant lobelia (Lobelia deckenii) also has to face the stress of nightly frost, which can be severe due to parts of the Kilimanjaro alpine tundra being relatively dry. The plant has evolved into a ball-shaped rosette consisting of a fleshy center surrounded by concave spiky leaves, which are arranged in such a manner as to trap rainwater. A single plant can contain, trapped in its rosette, a compartmentalized mass of several liters of water. This volume is large enough to prevent it from freezing right to the middle in any one night. Indeed, the center of the plant where the growing tip is located maintains a very even temperature throughout the diurnal cycle. Not surprisingly, this water mass of the lobelia plants with its relatively even temperature has become the breeding environment for a few species of insects with
aquatic larvae, the most abundant of which is a chironomid midge. The water in the lobelias also contains microorganisms, which feed on decomposing debris and are in turn the food for the insect larvae.
Global Warming and Other Anthropogenic Effects Extensive research in arctic and alpine regions including ice core analysis, paleolimnology, palynology, and geomorphology has provided a detailed picture of the climatic history of these regions. This allows us to conclude that, as well as the major changes at the end of the last ice age, frequent climate oscillations have subsequently occurred that caused major changes in tundra ecosystems. Furthermore, there have been times when tundra types existed that are no longer extant. The species complexes that now exist consist of species that have been sufficiently flexible and/or dispersible to have survived the climatic and landscape oscillations of the past. However, this does not necessarily bode well for the future of tundra ecosystems and species, as anthropogenic changes are certain to be increasingly imposed on the Earth. Already, the most likely reason for the extinction of most of the Pleistocene arctic megafauna is a combination of climate change and human hunting. The disappearance of the large herbivores at that time caused a major switch in plant dominance on tundra ecosystems from graminoids to mosses, with concomitant changes in long-term soil and peat formation. We must expect similar major changes in the coming century, associated with at least some extinctions. Climate change will be severe and direct human effects will also increase. Already, several species are declining due to pollution and over-hunting. Some of the most at risk tundras (and associated endemic tundra species) will be isolated alpine tundra systems on relatively low mountains, where climatic warming will cause the entire system to be replaced with forest. See also: Alpine Ecosystems and the High-Elevation Treeline; Alpine Forest; Biodiversity; Boreal Forest; Cycling and Cycling Indices; Dispersal–Migration; Freshwater Lakes; Grassland Models; Grazing Models; Grazing; Polar Terrestrial Ecology; Predation; Prey–Predator Models; Soil Formation; Steppes and Prairies; Succession; Water Availability; Wind Effects.
Further Reading Chapin FS and Ko¨rner C (eds.) (1995) Arctic and Alpine Biodiversity. Patterns, Causes and Ecosystem Consequences, 332pp. Berlin: Springer. Coe MJ (1967) The Ecology of the Alpine Zone of Mount Kenya, 136pp. The Hague: Junk.
Ecological Indicators | Turnover Time Craeford RMM (ed.) (1997) Disturbance and Recovery in Arctic Lands, 621pp. Dordrecht: Kluwer Academic. French HM and Williams P (2007) The Periglacial Environment, 478pp. Toronto: Wiley. Goulson D (2003) Bumblebees: Their Behavior and Ecology, 235pp. Oxford: Oxford University Press. Jones HG, Pomeroy JW, Walker DA, and Hoham RW (eds.) (2001) Snow Ecology: An Interdisciplanary Examination of Snow-Covered Ecosystems, 378pp. Cambridge University Press. Laws RM (ed.) (1984) Antarctic Ecology, vol. 1, 344pp. London: Academic Press.
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Mahaney WC (ed.) (1989) Quaternary and Environmental Research on East African Mountains, 483pp. Rotterdam: Balkema. Pienitz R, Douglas MSV, and Smol JP (eds.) (2004) Long-Term Environmental Change in Arctic and Antarctic Lakes, 562pp. Dordrecht: Springer. Rosswall T and Heal OW (eds.) (1975) Ecological Bulletin, Vol. 20: Structure and Function of Tundra Ecosystems, 450pp. Stockholm: Swedish Natural Science Research Council. Wielgolaski FE (ed.) (1997) Ecosystems of the World 3: Polar and Alpine Tundra, 920pp. Amsterdam: Elsevier.
Turnover Time E H Dettmann, US Environmental Protection Agency, Narragansett, RI, USA Published by Elsevier B.V.
Introduction Calculation of Turnover Time of Material and Energy Applications of Turnover Times in Ecosystem Analysis
Summary Further Reading
Introduction
biogeochemical cycle of an element. Turnover times of subsystems will differ from that of the larger system of which they are a part.
Turnover time refers to the amount of time required for turnover or replacement by flow-through of the energy or a material contained in a system. Ecological units are generally open to flow-through of energy and materials. Carbon, nitrogen, phosphorus, and other materials, as well as energy, enter the system, may be incorporated into system components and later be released by respiration, excretion, decomposition, and other processes, and eventually leave the system. The material of interest may undergo a single cycle of incorporation and release, or may be recycled many times before exiting. Some substances may be passively transported through the system. The concept of turnover is also applied to appearance and extinction of biological species in a local community, and can describe passage of individuals through a population through recruitment and harvesting. Turnover time provides a timescale for this replacement and is a tool for analyzing the significance of flow rates and the sizes of material and energy pools within a system or its components. The turnover time of a material in a system is generally defined as the ratio of the quantity of that material in the system to its throughput or flow-through rate. This definition assumes that the rates of inflow and outflow are equal, that is, the system is in equilibrium (steady state). It is an average quantity, since the calculation provides no way to account for the transit time of an individual entity (e.g., an atom or molecule) through the system. Turnover time can be calculated at various scales, for small ecosystems or large ones, such as the component subsystems of the global
Calculation of Turnover Time of Material and Energy Turnover time ( ) for a flow of material is the ratio of the quantity of material in the system (M) to its flow rate (F) through the system, that is, ¼
M F
½1
Turnover time is the inverse of the fraction of material in the system that leaves per unit time. It may also be viewed as the amount of time required to put into the ecosystem an amount of the substance equal to that currently residing there, or for that amount to leave the system. The units used to express the quantities in the numerator and denominator of eqn [1] must be consistent. For materials, the mass of the material in the system is M, and F is the flux (mass/time), with flux expressed using the same units for mass. If energy turnover is calculated, the ratio is of biomass to production, both expressed in consistent biomass or energy units. Other timescales used in analyzing the dynamics of material flow through systems are age and transit time. The age of an entity is the amount of time that has elapsed since its entry into the system; its transit time is the total time between its entry into and exit from the system.