Water and Ecological Adaptations
Water makes up a large proportion of the bodies of plants and animals, whether they live on land or in water. Active cytoplasm holds about 70–90 per cent of water. It has several important physiological properties. There exists a strong relationship between the water status of soil, plant and atmosphere. The rooting zone of the soil (zone of soil in which the water absorbing organs, roots, root hairs are present), the plant body and the lower layer of atmosphere behave as a continuum, called spac or soil plant atmosphere continuum in relation to water transfer (Phillip, 1966). Solar radiation is the primary energy source for the water transport process in the SPAC. On the other hand, animals obtain water (i) by drinking (ii) by absorbing it through their skin from contact with some damp ground, (iii) directly from their food or (iv) from water produced by metabolism. The method of obtaining water and the relation to the supply of liquid water as well as resistance to the drying effects of the surrounding atmosphere are important in determining the distribution of animals.
The scarcity or abundance of water brings about adaptations in living organisms. Plants which grow in areas where water is available in plenty, are classified as mesophytes and terrestrial animals living under such conditions are called mesocoles. Plants growing in water are called hydrophytes, while animals that live in the aquatic environment are called aquatic animals or hydrocoles. Some plants can grow in ecosystems where water is scarce and where the day temperature is very high. These plants are called xerophytes and the animals living in such xeric conditions are called desert animals or xerocoles. Xerophytes living in physiologically dry soils, i.e., saline soils with high concentrations of salts such as NaCl, Mg Cl2 and Mg SO4, are called halophytes. Based on their specific habitat, halophytes can be further classified into lithophilous, psammophilous, pelophilous and helophilous plants growing on rock and stones, sand, mud and swamp, respectively. Helophilous helophytes include mangroves of sea shores of Bombay such as Rhizophora mucronata and Sonneratia.
(1) Hydrophytes and hydrocoles and their adaptations. Hydrophytes include : (a) freefloating hydrophytes (e.g., Wolffia, Lemna, Spirodella, Azolla, Eichhornia crassipes, Salvinia and Pistia), (b) rooted hydrophytes with floating leaves (e.g., Trapa, Nelumbo, Nymphaea, Marsilea, etc.), (c) Submerged floating hydrophytes (e.g., Ceratophyllm, Utricularia, Najas, etc.), (d) rooted submerged hydrophytes (e.g., Hydrilla, Chara, Vallisneria, etc.), (e) rooted emergent hydrophytes or amphibious plants (e.g., Sagittaria, Ranunculus, etc.). Tenagophytes are amphibious plants—they grow in water bodies as well as in water logged soil. These hydrophytes grow in hydric conditions and show the following general adaptive features: They possess poor mechanical, absorbing, conductive and protective tissues.
They also contain an extensive development of air spaces (aerenchyma) in the tissues. Roots are either absent (e.g., Wolffia) or poorly developed (e.g., Hydrilla). Roots may not have root hairs, root cap (instead of root cap Eichhornia has root pockets) and vascular tissue. Roots of hydrophytes are generally fibrous and adventitious, when present. The stem of hydrophytes is weak, slender and spongy. In some it is like a horizontal rhizome covered with mucilage, while it may be hard, as in Nelumbo. The aerial leaves may be broad but the submerged leaves are thin, long or ribbon-shaped. Stomata are completely absent in submerged leaves (e.g., Anacharis), but in floating forms, stomata are confined only to the upper surfaces of leaves as in Nymphaea.
Aquatic animals or hydrocoles in general exhibit an elongated stream-lined body having a compressed head, body and tail. Hydrocoles include fishes, sea turtles, mammals such as whales, and many others. There are also amphibious forms such as frog, toad, crocodile, etc., and many birds which visit water bodies either for reproduction or for collection of food.
(2) Xerophytes and xerocoles and their adaptations. Xerophytes grow in conditions of water scarcity, high temperature, strong winds, high transpiration rate and evaporation higher than precipitation. The soil is very dry and porous. The essential adaptations of xerophytes involve increased water absorption by roots, storing of water and retardation of transpiration. Thus, in search of water, xerophytic trees may go very deep in the soil and have extensive root hairs to absorb it. The roots of plants such as Calotropis procera, Ficus, and Acacia nilotica may go as deep as 10 to 16 metres and may reach the water table.
As a consequence, these plants survive in deserts or arid conditions even if their rate of transpiration is higher. The storage of water is facilitated either by modifications of leaves, as in Mesembryanthemum and in the malacophyllous xerophytes (in which leaves contain turgescent parenchymatous cells) as Aloe, Begonia, Bryophyllum, Agave, Yucca, etc., or modification of stems, as in cacti such as Opuntia (phylloclade) and Euphorbia. In some xerophytes the water is stored in their roots as in Asparagus and Ceiba parvifolia. All these xerophytes are called succulents because they possess thick, fleshy, water storage organs such as stems, leaves and roots.
Nonsucculent xerophytes such as Calotropis, Prosopis, Acacia, Zizyphus, Casuarina, Nerium, Saccharum and Pinus possess other sort of xerophytic adaptations, viz., extensive root system, high osmotic pressure and other modifications in the leaves. Reduced transpiration is achieved by decreasing the leaf surface, as in Casurina, Acacia and Asparagus or by modifying the leaves into spines and barbed bristles, as in cacti, or by having thick, leathery, thick cuticle or wax-coating bearing leaves with welldeveloped hypoderma and sunken stomata to reduce transpiration, as in Calotropis and Nerium. Halophytes (e.g mangroves) resemble xerophytes and have high osmotic pressure; succulent organs; thin, evergreen, small leathery leaves with water storing tissues and thick cuticles and special air-breathing roots called pneumatophores.
Different animals have evolved the following adaptive features to live in arid environment :
- Nocturnal life style. Most desert animals are nocturnal and seek shade or burrow deep in the soil in the day time to avoid excessive heat and dryness. Some xerocole rodents passively lose heat through conduction by pressing their bodies against the burrow walls.
- Deceptive colouration. Desert animals are usually grey, brown or red matching with the colour of the sand or rock.
- Suspended animation. Certain animals, usually with simpler organization, such as rotifers, nematodes, tardigrades, desert snails, etc., retain their vitality in long dry environment. Other forms (frogs, toads, etc.) aestivate during droughts and are active during moist season of the year.
- Fast movement. Desert animals move much faster than other land animals, since they have to travel long distances in search of food and water.
- Migration. Many birds and mammals of arid zones migrate when water becomes scarce or as a result of drought or for other reasons, the food supply is less.
- Heat loss by radiation. Animals such as jack-rabbits (Lepus) and fox (Vulpes velox) have large ears that reduce the need of water evaporation to regulate the body temperature. Their ears function as efficient radiators to the cooler desert sky, which on clear days may have a radiation temperature 25°C below than that of the animal body. By seeking shade and sitting in depressions, Lepus could radiate 5 kcal/day through its two large ears (400 cm2).
- Impervious skin. The drier habitats (deserts, etc., are invaded by only those animals which contain a thick impermeable body covering. Such integuments occur in many insects, birds and mammals. Some mammals such as men, apes and horses lose much water (and salt) through sweat glands in heat regulation. Most rodents and some ruminants such as antelope nearly or completely lack sweat glands. Moist skinned forms (most amphibians and earthworms), certain mites and soft-bodied insects are restricted to swamps, stream margins, moist soils and other similarly damp places.
- Upturned nostrils. Desert animals have nostrils directed upwards; this may provide a protection from clogging by wind blown sand.
- Water from food and from metabolism. Most herbivores and carnivores live on the moisture obtained with food. Many insects utilize the high water contents of plants to meet their water requirements. In fact, most animals make use of water released during metabolism when fats and carbohydrates are broken. Kangaroo rat, which seals its burrow by day to keep its chamber moist, can live throughout year without drinking water. It obtains its water from its own metabolic processes and from hygroscopic water in its food.
- Internal lungs or tracheal system. Mode of respiration has some correlation with water. Crustaceans, with their gills covered by a water-retaining carapace, carry with them a liquid environment for their gills. The scaly body covering of a fish may be practically impermeable to water and exchange of gases may be limited to gills and gut. Internal lungs, whether in pulmonate snails, land isopods, spiders or higher vertebrates (amphibians, reptiles, birds and mammals) together with the internal tracheal system of insects are water saving. Much water is lost in breathing even in animals having internal lungs.
- Dry excretion. A further water-saving device is the excretion of concentrated, relatively dry nitrogenous and faecal waste materials. Water-saving insects, reptiles and birds excrete nitrogenous waste as solid uric acid.
Camel and African antelope, Oryx provide good examples of xerocoles or drought-resistant animals. Camel can go without food and water as long as 10 days at a stretch. For water conservation, camel has following adaptations :
(1) When camel drinks water, it can take in up to 50 litres in one gulp and this water is evenly distributed all over its body in tissues and not in specific pockets or organs (e.g., in stomach) as the common misconception holds.
(2) Camel excretes highly concentrated urine. Its dung also contains very little water, compared to the dung of the donkey or the cow (both of which are xerocoles).
(3) It perspires very little and its breathing rhythm is very slow.
(4) Camel can withstand dehydration up to 25 per cent of body weight and it loses water from body tissues rather than from blood.
(5) Body temperature of camel is labile, dropping to 33.8°C overnight and raising to 40.6°C by day, at which point it begins to sweat. Due to such thermoregulation, the amount of water loss by perspiration and in other ways is greatly reduced.
(6) Camel accumulates its fat in the hump rather than all over body. This speeds heat flow away from the body and its thick coat prevents the flow of heat inward towards the body.