Environments are heterogeneous

There are no homogeneous environments in nature. Even a continuously stirred culture of microorganisms is heterogeneous

Raunkiaer's classification w

Phanerophytes

Annuals (therophytes)

Cryptophytes

Hemicryptophytes or m

Chamaephytes

Tropical

Desert

Mediterranean

Temperate

Arctic er 20

Figure 1.19 The drawings above depict the variety of plant forms distinguished by Raunkiaer on the basis of where they bear their buds (shown in color). Below are life form spectrums for five different biomes. The colored bars show the percentage of the total flora that is composed of species with each of the five different life forms. The gray bars are the proportions of the various life forms in the world flora for comparison. (From Crawley, 1986.)

because it has a boundary - the walls of the culture vessel -and cultured microorganisms often subdivide into two forms: one that sticks to the walls and the other that remains free in the medium.

The extent to which an environment is heterogeneous depends on the scale of the organism that senses it. To a mustard seed, a grain of soil is a mountain; and to a caterpillar, a single leaf may represent a lifetime's diet. A seed lying in the shadow of a leaf may be inhibited in its germination while a seed lying outside that shadow germinates freely. What appears to the human observer as a homogeneous environment may, to an organism within it, be a mosaic of the intolerable and the adequate.

There may also be gradients in space (e.g. altitude) or gradients in time, and the latter, in their turn, may be rhythmic (like

Figure 1.20 The percentages of forest mammals in various locomotory and feeding habitat categories in communities in: (a) Malaya, all forested areas (161 species), (b) Panama dry forest (70 species), (c) Australia, Cape York forest (50 species), and (d) Zaire, Irangi forest (96 species). C, carnivores; HF, herbivores and fructivores; I, insectivores; M, mixed feeders; (-) aerial; (-----) arboreal;

(---) scansorial; (--) small ground mammals. (After Andrews et al., 1979.)

Figure 1.20 The percentages of forest mammals in various locomotory and feeding habitat categories in communities in: (a) Malaya, all forested areas (161 species), (b) Panama dry forest (70 species), (c) Australia, Cape York forest (50 species), and (d) Zaire, Irangi forest (96 species). C, carnivores; HF, herbivores and fructivores; I, insectivores; M, mixed feeders; (-) aerial; (-----) arboreal;

(---) scansorial; (--) small ground mammals. (After Andrews et al., 1979.)

daily and seasonal cycles), directional (like the accumulation of a pollutant in a lake) or erratic (like fires, hailstorms and typhoons).

Heterogeneity crops up again and again in later chapters - in part because of the challenges it poses to organisms in moving from patch to patch (Chapter 6), in part because of the variety of opportunities it provides for different species (Chapters 8 and 19), and in part because heterogeneity can alter communities by interrupting what would otherwise be a steady march to an equilibrium state (Chapters 10 and 19).

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