tigure 8.2 The number of species (species richness) of breeding birds and mammals declines with increasing altitude in the Nepalese Himalayas (redrawn from Begon et al., 1996, who give the source of the original data).
amount of solar radiation available to fuel the growth of plants and other photosynthetic organisms.
Mountains provide an environmental gradient that occurs without a change in latitude. There are very few organisms that can survive the harsh conditions at the top of high mountains, but, at low altitude, depending on their location, there may be diverse communities of organisms. Several groups of organisms show a decline in species richness with increasing altitude (Figure 8.2). It is easy to accept that the environment becomes more harsh as you ascend a mountain, but the change is not a simple one and, in some ways, mirrors the changes seen with increasing latitude. Average temperatures decrease but temperature ranges increase with altitude. Organisms living at high altitude on tropical mountains experience sudden switches between very hot days o <u a
21- 41- 61- 81- 101- 121- 141- 161- 181- >200 40 60 80 100 120 140 160 180 200
Salinity (parts per 1000)
figure 8.3 The number of animal species found in saline lakes of southeastern Australia declines with increasing salinity. This graph is based on the ranges of salinity tolerances of animals as given in Bayly & Williams (1973).
and freezing nights. Water and oxygen availability decreases with altitude, while winds and other sources of physical disturbance may increase. The area available for organisms to colonise decreases with increasing altitude and this could also be responsible for the observed decline in species richness.
Saline lakes vary in the concentrations of salts found dissolved in their waters. There are relatively few organisms that can live in highly saline lakes (those that have a salt concentration greater than 50 parts per 1000 or 5 per cent salt). The number of animal species found in the saline lakes of southeastern Australia declines with increasing salinity (Figure 8.3). For these lakes, there are no fish at salinities above 70 parts per 1000, no insects above 95 parts per 1000 and no molluscs above 112 parts per 1000. The most salt-tolerant animal in these lakes is a crustacean Parartemia zietziana, which has been found at salinities of up to 353 parts per 1000. There are no animals or higher plants living in the
Dead Sea, which has an average salinity of 370 parts per 1000, and it only supports one species of alga, a few species of halobacteria and some viruses. Thus, there seems to be a decline in the species richness of saline lakes with increasing salinity. However, the survival of organisms in saline lakes may be affected by factors in addition to salinity. Saline lakes may have high concentrations of substances other than sodium chloride. The Dead Sea, for example, has high levels of magnesium and calcium, which appear to be particularly difficult for organisms to cope with. High salinity also affects various physical characteristics of the water, including its density, freezing point and its ability to hold oxygen.
Productivity is another measure that ecologists use to categorise communities of organisms. This is the rate of production of new biomass (the total mass of organisms) and is related to factors such as the availability of light and nutrients, temperature and the length of the growing season. In general, we expect diversity to decrease with increasing harshness of the environment, but the pattern of change in productivity may be more complex (Figure 8.4). Dr Dev Niyogi, currently a postdoctoral fellow at the University of Otago, kindly provided me with Figure 8.4 and with some of the ideas on which the following interpretation is based. As environmental stress increases, diversity decreases, but productivity, as measured by biomass, initially increases. The reason for this is that, in a complex community of organisms, there are a number of levels in the food chain or food web. Plants use the energy of sunlight to produce organic material (they are the primary producers), the plants are eaten by herbivores and the herbivores are eaten by predators, which, in turn, may be eaten by bigger predators. Herbivores use the energy and organic material contained in the plants they eat to build their own bodies. This process is not efficient. The biomass of herbivores is always less than the biomass of the plants they have consumed. A community that consisted solely of plants (primary producers) would thus produce a greater total biomass than one which included herbivores and predators (all other things being equal). As the harshness of the environment increases, not only are there fewer species of organisms that can live in the conditions but
figure 8.4 As the harshness of the environment increases, the biodiversity (e.g. number of species) decreases, but the biomass (total mass of organisms) initially increases before declining. Based on a drawing by Dr Dev Niyogi, University of Otago.
whole levels in the food chain disappear. Large predators are the first to go, followed by smaller predators and then herbivores. Ultimately, only the primary producers are left, plus perhaps some decomposers (such as bacteria) which rely on the dead bodies, or products of, the primary producers. The Dead Sea, for example, has just one primary producer (the alga Dunaliellaparva) and several species of halobacteria that rely on its products. Finally, the conditions may become so harsh that no organisms can survive and the biomass declines to zero.
The decline in diversity and the increase in productivity in extreme environments suggests that the organisms which are able to colonise them gain a very great prize. The success of an organism, in terms of its numbers or biomass, depends not just on the physical conditions of its environment but also on its interactions with other organisms. It may m
Harshness of the environment figure 8.5 As the harshness of the environment increases, the amount of abiotic stress (physical factors such as temperature, pH and, salinity) increases (-), but biotic stress (predation, grazing, competition, parasites and diseases) decreases be eaten by other organisms, it may have parasites and diseases, and it may face competition for resources from other organisms. The increased productivity of extreme environments may be associated with a reduction in these biological pressures (Figure 8.5). The alga D. parva more or less has the Dead Sea to itself, with no other organisms present that eat it or compete with it. An extreme organism may thus be more successful (it can produce a greater biomass and/or occupy a greater proportion of the available habitat) than one which lives in a less extreme environment.
The first organism to colonise an extreme environment may be able to exclude other organisms that seek to do so (a sitting tenant effect). If an organism develops an adaptation which enables it to survive the conditions, although the solution may not be an efficient one at first, the process of natural selection may ensure that the adaptation becomes optimised. Any competitor which develops a similar adaptation will not have had the opportunity for it to become optimised and will be outcompeted by the sitting tenant. Perhaps the sitting tenant will only be evicted if the interloper comes up with a more efficient solution to the problem of coping with the conditions.
Although it seems to be generally true that species diversity is low in extreme environments, this might be driven by factors other than the harshness of the environment. Some extreme environments might be considered to be islands surrounded by less extreme conditions (as is the case with hot springs, hydrothermal vents and some saline lakes) or by even more extreme conditions (as in the ice-free areas of the terrestrial Antarctic that support moss and algal growth). These environments are small, rare and with a low complexity in terms of the sorts of habitats they provide within them. These sorts of island effects could in themselves be responsible for the low species diversity observed, rather than the extreme physical conditions per se.
The distribution and dispersal of extreme organisms Extreme organisms are usually limited in their distribution by a requirement for the extreme conditions to which they have become adapted. Some may, however, be capable of colonising less extreme conditions but are prevented from doing so by competition from, or predation by, other organisms. Populations of the brine shrimp Artemia, for example, are limited to saline lakes and ponds that are too salty to support the fish, and other predators, which would eat them. The extreme conditions thus represent islands of habitat that are surrounded by a 'sea' of unsuitable habitat. Suitable habitats may be separated by quite large distances. Organisms need to be able to disperse and colonise new areas. If they stayed in the same place, they would be vulnerable to catastrophes that could wipe out their population and cause them to become extinct. How do extreme organisms cope with the necessity for, and problems of, dispersal?
The fauna of deep-sea hydrothermal vents illustrate the problem.
Hydrothermal vents are temporary, opening and closing as the tectonic forces that create them shift. The animals living around hydrothermal vents must colonise new vent habitats, if they are to survive catastrophes and leave descendants in the long term. New vents sometimes appear hundreds of kilometres away from any existing ones and yet are rapidly colonised by animals. Vent animals are specialised to live there and have to cope with the extremes and gradients of temperatures and the high concentrations of metals and sulphides they experience. They rely on sulphide oxidation by bacteria for their sources of organic materials, rather than the products of photosynthesis by algae - as do animals living in the rest of the sea. The vents are thus widely separated by an environment that will not support the growth of adult vent animals. Like most marine animals, the animals of hydrothermal vents disperse by producing larvae. These are not capable of actively swimming the distances involved and must rely on ocean currents to carry them from one vent site to another.
Terrestrial Antarctic organisms (such as mosses, nematodes, spring-tails and mites) are limited to areas of ice-free ground which receive sufficient moisture from melting snow and ice to support their growth. Suitable sites are rare and may be separated by large distances. Many of these organisms are capable of anhydrobiosis, at least in some stages of their life cycle. In a dry state, they can survive being blown around in the air and could be transported large distances by this method. Springtails have water-repellent cuticles and rafts of springtails have been observed floating on the surface of water. They could perhaps disperse along coasts by this method. Some Antarctic mites can survive prolonged immersion in seawater. Transfer on the legs of seabirds (e.g. skuas, gulls and terns) is another possibility.
Organisms vary in the way they reproduce and conduct their lives. This includes aspects of their life history such as their fecundity (how many offspring they produce), how long a period they reproduce for, their age before they start reproducing, how often they reproduce and the size of their eggs (or other reproductive stages). The most successful pattern of reproduction might be to produce lots of large, well-provisioned eggs, frequently and early in the lifespan of the parent and to produce them for a long period of time. In practice, no organism can achieve such a strategy since food, and other resources, are limited. Organisms have thus had to evolve a compromise between the different characteristics of their life history. One trade-off is that parents who put a lot of resources into producing offspring might have to pay for this by having a shorter lifespan (or less growth) themselves. The resources they devote to their offspring cannot be assigned to supporting their own growth or survival. For example, a tree might produce more seeds or it might grow more quickly, but it cannot do both if it has limited resources. Another commonly recognised trade-off is between the number of eggs an organism produces and the size and amount of nutrients supplied by the parent to each egg.
Ecologists have proposed several schemes for classifying the life-history patterns observed in organisms. One commonly used scheme is to divide populations of organisms into 'r' and 'K' strategists (these refer to the characteristics of the growth curves of the organism's population). An r-selected population of organisms is thought to be adapted to short-lived or unpredictable environments by being able to reproduce quickly. These organisms are small in size, become reproduc-tively mature early on, they may have a single large breeding event and produce lots of small offspring. A K-selected population of organisms is thought to be adapted to competing with other organisms in a stable environment. These organisms have a larger size, become reproduc-tively mature later, may reproduce several times and produce fewer, but larger, offspring. For organisms in extreme environments, the patterns found in their life history depend on the predictability of the extreme conditions. An environment that is constantly and predictably extreme, would be expected to favour K-selected features, whereas an environment that is unpredictably extreme (or rather unpredictably favourable for growth) should favour r-selected features.
Organisms in extreme environments may have further characteristics that enable them to survive in harsh conditions. These are referred to as being A/S selected (adversity/stress selected). A/S-selected organisms take a long time to complete their life cycle, have low growth rates, low fecundity and rates of reproduction, a poor ability to compete with other species and an investment in survival strategies. Peter Convey of the British Antarctic Survey considers that terrestrial Antarctic plants and animals show the A/S selection pattern well. They have long life cycles, which are associated with low average temperatures, low water availability and short growing seasons. They also have physiological and biochemical mechanisms that enable them to survive low temperatures and desiccation.
In extreme environments, there may be three-way trade-offs between survival, growth and reproduction (Figure 8.6). Survival of low temperatures in terrestrial Antarctic organisms, for example, requires the production of sugars or polyols (trehalose, glycerol) and proteins (antifreeze proteins). Resources spent on producing these survival compounds cannot be spent on growth or reproduction. Where the severity of the environment varies with the season, there will be a seasonal shift in the production of survival compounds. Resources will be transferred entirely into survival during entry into winter and then shifted back into growth and then reproduction during the spring and summer (Figure 8.6A). Polar fish produce antifreeze proteins which enable them to survive the risk of freezing in polar waters. Since they are constantly exposed to this risk, the production of antifreeze proteins needs to be maintained. In a constantly extreme environment, the organism will achieve some sort of balance between the demands of survival, growth and reproduction (Figure 8.6B). This latter pattern may also be needed in environments where the stress is rapid and unpredictable, requiring the continual production of survival compounds to meet the risk. This might be the case in the more extreme terrestrial Antarctic habitats, or in rapidly drying habitats, where extreme conditions can occur at any time of the year and the change is too rapid to allow the production of protective compounds. Survival mechanisms thus have to operate continuously for the organism to persist.
Summer growth reproduction
growth reproduction ► survival figure 8.6 The allocation of resources to reproduction, growth and survival by organisms in extreme environments.
In (A), there is a seasonal shift in the allocation of resources as conditions become less extreme during summer and more favourable for growth and reproduction.
In (B), the conditions are constant and a balance between the resources allocated to survival, growth and reproduction is achieved.
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