Ecology. Michael Begon
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in the soil water offers osmotic resistance to water uptake. The most extreme saline conditions occur in arid zones where the predominant movement of soil water is towards the surface and crystalline salt accumulates. This occurs especially when crops have been grown in arid regions under irrigation; salt pans then develop and the land is lost to agriculture. The main effect of salinity is to create the same kind of osmoregulatory problems as drought and freezing and the problems are countered in much the same ways. For example, many of the higher plants that live in saline environments (halophytes) accumulate electrolytes in their vacuoles, but maintain a low concentration in the cytoplasm and organelles. Such plants maintain high osmotic pressures and so remain turgid, and are protected from the damaging action of the accumulated electrolytes by polyols and membrane protectants.
Freshwater environments present a set of specialised environmental conditions because water tends to move into organisms from the environment and this needs to be resisted. In marine habitats, the majority of organisms are isotonic to their environment so that there is no net flow of water, but there are many that are hypotonic so that water flows out from the organism to the environment, putting them in a similar position to terrestrial organisms. Thus, for many aquatic organisms the regulation of body fluid concentration is a vital and sometimes an energetically expensive process. The salinity of an aquatic environment can have an important influence on distribution and abundance, especially in places like estuaries where there is a particularly sharp gradient between truly marine and freshwater habitats.
The freshwater shrimps Palaemonetes pugio and P. vulgaris, for example, co‐occur in estuaries on the eastern coast of the USA at a wide range of salinities, but the former seems to be more tolerant of lower salinities than the latter, occupying some habitats from which the latter is absent. Figure 2.22 shows the mechanism likely to be underlying this (Rowe, 2002). Over the low salinity range (though not at the effectively lethal lowest salinity) metabolic expenditure was significantly lower in P. pugio. P. vulgaris requires far more energy simply to maintain itself, putting it at a severe disadvantage in competition with P. pugio even when it is able to sustain such expenditure.
Figure 2.22 Metabolic expenditure in relation to salinity for two shrimp species. Standard metabolic expenditure (estimated through minimum oxygen consumption) in Palaemonetes pugio and P. vulgaris at a range of salinities. Note that there was significant mortality of both species over the experimental period at 0.5 ppt (parts per thousand), especially in P. vulgaris (75% compared with 25%).
Source: After Rowe (2002).
2.6.1 Conditions at the boundary between the sea and land
Salinity has important effects on the distribution of organisms in intertidal areas but it does so through interactions with other conditions – notably exposure to the air and the nature of the substrate.
algae and higher plants
Algae of all types have found suitable habitats permanently immersed in the sea, but permanently submerged higher plants are almost completely absent. This is a striking contrast with submerged freshwater habitats where a variety of flowering plants have a conspicuous role. The main reason seems to be that higher plants require a substrate in which their roots can find anchorage. Large marine algae, which are continuously submerged except at extremely low tides, largely take their place in marine communities. These do not have roots but attach themselves to rocks by specialised ‘holdfasts’. They are excluded from regions where the substrates are soft and holdfasts cannot ‘hold fast’. It is in such regions that the few truly marine flowering plants, for example sea grasses such as Zostera and Posidonia, form submerged communities that support complex animal communities.
Most species of higher plants that root in seawater have leaves and shoots that are exposed to the atmosphere for a large part of the tidal cycle, such as mangroves, species of the grass genus Spartina and extreme halophytes such as species of Salicornia that have aerial shoots but whose roots are exposed to the full salinity of seawater. Where there is a stable substrate in which plants can root, communities of flowering plants may extend right through the intertidal zone in a continuum extending from those continuously immersed in full‐strength seawater (like the sea grasses) through to totally non‐saline conditions. Salt marshes, in particular, encompass a range of salt concentrations running from full‐strength seawater down to totally non‐saline conditions.
Higher plants are absent from intertidal rocky seashores except where pockets of soft substrate may have formed in crevices. Instead, such habitats are dominated by the algae, which give way to lichens at and above the high tide level where the exposure to desiccation is highest. The plants and animals that live on rocky seashores are influenced by environmental conditions in a very profound and often particularly obvious way by the extent to which they tolerate exposure to the aerial environment and the forces of waves and storms. This expresses itself in the zonation of the organisms, with different species at different heights up the shore (Figure 2.23).
Figure 2.23 General zonation scheme for the seashore determined by relative lengths of exposure to the air and to the action of waves. The littoral zone extends between the extreme high water and extreme low water of spring tides (upper and lower dashed lines).
Source: After Raffaelli & Hawkins (1999).
zonation
The extent of the intertidal zone depends on the height of tides and the slope of the shore. Away from the shore, the tidal rise and fall are rarely greater than 1 m, but closer to shore, the shape of the landmass can funnel the ebb and flow of the water to produce extraordinary spring tidal ranges of, for example, nearly 20 m in the Bay of Fundy (between Nova Scotia and New Brunswick, Canada). In contrast, the shores of the Mediterranean Sea experience scarcely any tidal range. On steep shores and rocky cliffs the intertidal zone is very short and zonation is compressed.
To talk of ‘zonation as a result of exposure’, however, is to oversimplify the matter greatly (Raffaelli & Hawkins, 1999). In the first place, ‘exposure’ can mean a variety, or a combination of, many different things: desiccation, extremes of temperature, changes in salinity, excessive illumination and the sheer physical forces of pounding waves and storms (to which we turn in Section 2.7). Furthermore, ‘exposure’ only really explains the upper limits of these essentially marine species, and yet zonation depends on them having lower limits too. For some species there can be too little exposure in the lower zones. For instance, green algae would be starved of blue and especially red light if they were submerged for long periods too low down the shore. For many other species though, a lower limit to distribution is set by competition and predation. The seaweed Fucus spiralis will readily extend lower down the shore than usual in Great Britain whenever other competing midshore fucoid seaweeds are scarce.
2.7 Hazards, disasters