Ecology. Michael Begon

Ecology - Michael  Begon


Скачать книгу
(tips) of existing shoots and in the leaf axils. Within the buds, the meristematic cells are the most sensitive part of the whole shoot – the ‘Achilles’ heel’ of plants. Raunkiaer argued that the ways in which these buds are protected in different plants are powerful indicators of the hazards in their environments and may be used to define the different plant forms (Figure 1.24). Thus, trees expose their buds high in the air, fully exposed to the wind, cold and drought; Raunkiaer called them phanerophytes (Greek phanero, ‘visible’; phyte, ‘plant’). By contrast, many perennial herbs form cushions or tussocks in which buds are borne above ground but are protected from drought and cold in the dense mass of old leaves and shoots (chamaephytes: ‘on the ground plants’). Buds are even better protected when they are formed at or in the soil surface (hemicryptophytes: ‘half hidden plants’) or on buried dormant storage organs (bulbs, corms and rhizomes – cryptophytes: ‘hidden plants’; or geophytes: ‘earth plants’). These allow the plants to make rapid growth and to flower before they die back to a dormant state. A final major category consists of annual plants that depend wholly on dormant seeds to carry their populations through seasons of drought and cold (therophytes: ‘summer plants’). Therophytes are the plants of deserts (they make up nearly 50% of the flora of Death Valley, USA), sand dunes and repeatedly disturbed habitats. They also include the annual weeds of arable lands, gardens and urban wastelands.

      But there is, of course, no vegetation that consists entirely of one growth form. All vegetation contains a mixture, a spectrum, of Raunkiaer’s life forms. The composition of the spectrum in any particular habitat is as good a shorthand description of its vegetation as ecologists have yet managed to devise. Raunkiaer compared these with a ‘global spectrum’ obtained by sampling from a compendium of all species known and described in his time (the Index Kewensis), biased by the fact that the tropics were, and still are, relatively unexplored. Thus, for example, we recognise a chaparral type of vegetation when we see it in Chile, Australia, California or Crete because the life form spectrums are similar. Their detailed taxonomies would only emphasise how different they are.

      APPLICATION 1.6 Stream invertebrate species traits and agricultural pollution

      

Schematic illustration of the species traits in streams. Relationships between the representation of species traits of stream invertebrates and the intensity of agriculture in the catchment area of the stream.

      Source: From Doledec et al. (2006).

      environments are heterogeneous

      A completely homogeneous environment might well become dominated by one or a very few species that are well adapted to the conditions and resources there. But there are no homogeneous environments in nature. Even a continuously stirred culture of microorganisms is heterogeneous 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 members of species 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 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 2 and 3), and in part because heterogeneity can alter communities by interrupting what would otherwise be a steady march to an equilibrium state of a few species (Скачать книгу