Although organisms respond to each condition in their environment, the effects of conditions may be determined largely by the responses of other community members. Temperature does not act on just one species: it also acts on its competitors, prey, parasites and so on. This, as we saw in Section 2.2, was the difference between a fundamental niche (where an organism could live) and a realized niche (where it actually lived). For example, an organism will suffer if its food is another species that cannot tolerate an environmental condition. This is illustrated by the distribution of the rush moth (Coleophora alticolella) in England. The moth lays its eggs on the flowers of the rush Juncus squarrosus and the caterpillars feed on the developing seeds. Above 600 m, the moths and caterpillars are little affected by the low temperatures, but the rush, although it grows, fails to ripen its seeds. This, in turn, limits the distribution of the moth, because caterpillars that hatch in the colder elevations will starve as a result of insufficient food (Randall, 1982).
The effects of conditions on disease may also be important. Conditions may favor the spread of infection (winds carrying fungal spores), or favor the growth of the parasite, or weaken the defenses of the host. For example, during an epidemic of southern corn leaf blight (Helminthosporium 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 (Figure 2.15).
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 (see also Section 8.2.1). A reversal, by a change in temperature, of the outcome of competition between the species appears to play a key role in this. For example, in experimental streams supporting the two species maintained at 6°C over a 191-day period (a typical high altitude temperature), the survival of S. malma was far superior to that of S. leucomaenis; whereas at 12°C (typical low altitude), both species survived less well, but the outcome was so far reversed that by around 90 days all of the S. malma had died (Figure 2.16). Both species are quite capable, alone, of living at either temperature.
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 disease competition temperature and humidity
Figure 2.16 Changing temperature reverses the outcome of competition. At low temperature (6°C) on the left, the salmonid fish Salvelinus malma outsurvives cohabiting S. leucomaenis, whereas at 12°C, on the right, S. leucomaenis drives S. malma to extinction. Both species are quite capable, alone, of living at either temperature. (After Taniguchi & Nakano, 2000.)
Experiment period (days)
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.
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