Ecology. Michael Begon

maydis) in a corn field in Connecticut, the plants closest to the trees that were shaded for the longest periods were the most heavily diseased (Harper, 1955).

      competition

      Competition between species can also be profoundly influenced by environmental conditions, especially temperature. Two stream salmonid fishes, Salvelinus malma and S. leucomaenis, coexist at intermediate altitudes (and therefore intermediate temperatures) on Hokkaido Island, Japan, whereas only the former lives at higher altitudes (lower temperatures) and only the latter at lower altitudes. A reversal, by a change in temperature, of the outcome of competition between the species plays a key role in this pattern (discussed more fully in Section 8.2.3).

      temperature and water availability

      Many of the interactions between temperature and other physical conditions are so strong that it is not sensible to consider them separately. The relative humidity of the atmosphere, for example, is an important condition in the life of terrestrial organisms because it plays a major part in determining the rate at which they lose water. In practice, it is rarely possible to make a clean distinction between the effects of relative humidity and of temperature. This is simply because a rise in temperature leads to an increased rate of evaporation. A relative humidity that is acceptable to an organism at a low temperature may therefore be unacceptable at a higher temperature. Microclimatic variations in relative humidity can be even more marked than those involving temperature. For instance, it is not unusual for the relative humidity to be almost 100% at ground level amongst dense vegetation and within the soil, whilst the air immediately above, perhaps 40 cm away, has a relative humidity of only 50%. The organisms most obviously affected by humidity in their distribution are those ‘terrestrial’ animals that are actually, in terms of the way they control their water balance, ‘aquatic’. Amphibians, terrestrial isopods, nematodes, earthworms and molluscs are all, at least in their active stages, confined to microenvironments where the relative humidity is at or very close to 100%. The major group of animals to escape such confinement are the terrestrial arthropods, especially insects. Even here though, the evaporative loss of water often confines their activities to habitats (e.g. woodlands) or times of day (e.g. dusk) when relative humidity is relatively high.

      APPLICATION 2.6 Farmers’ choice of cover crops in relation to temperature and soil water potential

      

Farmers have a wide choice of species that can be sown as cover crops during fallow periods to improve soil quality and reduce soil erosion and runoff. But which should they choose? Seed germination is a key stage in plant establishment, particularly when sowing occurs in summer, when temperatures are high and water availability low, and germination for 34 species of potential cover crops in four families was monitored in the laboratory at temperatures ranging from 4.5 to 43°C and at four water potentials (Figure 2.21). Optimal temperatures for germination of seeds varied from 21.3 to 37.2°C; maximum temperatures at which the species could germinate varied from 27.7 to 43.0°C; and base water potentials, the lowest water potential at which a seed can germinate, varied from –0.1 to –2.6 MPa. (Note that at a potential of 0 MPa soil is in a state of saturation, while at –1.5 MPa soil is at its permanent wilting point (see Section 3.3.2).)

Figure 2.21 Niches of cover crops in terms of temperature and base water potential. (a) Response curves to temperature for selected species of cover crops in terms of percentage of seeds that germinate. (b) Niches in one dimension for various species of cover crop. Base water potential is the lowest water potential at which a seed can germinate. At one extreme, Vicia faba is very sensitive to water potential for germination while, at the other, Secale cereale can germinate at very low water potentials.

      Source: From Tribouillois et al. (2016).

      Most of the cover crops were adapted to summer sowing with a high mean optimal temperature for germination, but some, such as Vicia sativa (Figure 2.21a), were more sensitive to high temperatures. Others, such as Secale cereale (Figure 2.21b), were more resistant to water deficit and germinated even when water potential was very low. Tribouillois et al. (2016) classified the cover crops into functional groups that are of value to farmers when choosing species appropriate for their particular conditions. Thus, functional group 1, which includes Guizotia abyssinica and Setaria italica, has a minimal temperature of 10ºC, a maximal temperature of 41.2ºC and a base water potential of –0.9 KPa. Functional group 4, on the other hand, which includes Brassica rapa and Secale cereale, has a minimal temperature of 0.4ºC, a maximal temperature of 38.6ºC and a base water potential of –2.4 KPa.

      

2.5 pH of soil and water

      The pH of soil in terrestrial environments or of water in aquatic ones is a condition that can exert a powerful influence on the distribution and abundance of organisms. The protoplasm of the root cells of most vascular plants is damaged as a direct result of toxic concentrations of H⁺ or OH⁻ ions in soils below pH 3 or above pH 9, respectively. Further, indirect effects occur because soil pH influences the availability of nutrients and/or the concentration of toxins.

Increased acidity (low pH) may act in three ways: (i) directly, by upsetting osmoregulation, enzyme activity or gaseous exchange across respiratory surfaces; (ii) indirectly, by increasing the concentration of toxic heavy metals at higher pHs, particularly aluminium (Al³⁺) but also manganese (Mn²⁺) and iron (Fe³⁺), which are essential plant nutrients; and (iii) indirectly, by reducing the quality and range of food sources available to animals. Tolerance limits for pH vary amongst plant species, but only a minority are able to grow and reproduce at a pH below about 4.5.

In alkaline soils, iron (Fe³⁺) and phosphate (), and certain trace elements such as manganese (Mn²⁺), are fixed in relatively insoluble compounds, and plants may then suffer because there is too little rather than too much of them. For example, calcifuge plants (those characteristic of acid soils) commonly show symptoms of iron deficiency when they are transplanted to more alkaline soils. In general, however, soils and waters with a pH above 7 tend to be hospitable to many more species than those that are more acid. Chalk and limestone grasslands carry a much richer flora (and associated fauna) than acid grasslands and the situation is similar for animals inhabiting streams, ponds and lakes.

      Some Archaea can tolerate and even grow best in environments with a pH far outside the range tolerated by eukaryotes. Such environments are rare, but occur in volcanic lakes and geothermal springs where they are dominated by sulphur‐oxidising bacteria whose pH optima lie between 2 and 4 and which cannot grow at neutrality (Stolp, 1988). Thiobacillus ferroxidans occurs in the waste from industrial metal‐leaching processes and tolerates pH 1; T. thiooxidans cannot only tolerate but can grow at pH 0. Towards the other end of the pH range are the alkaline environments of soda lakes with pH values of 9–11, which are inhabited by cyanobacteria such as Anabaenopsis arnoldii and Spirulina platensis.

2.6 Salinity

      For terrestrial