Vital attributes

Noble and Slatyer (1981) were also interested in defining the qualities that determine the place of a species in a succession. They called these properties vital attributes. The two most important relate to: (i) the method of recovery after

Tilman's resource-ratio hypothesis emphasizes changing competitive abilities beyond just competitive ability: Noble and Slatyer's 'vital attributes'

Resource Ratio Hypothesis Succession

Figure 16.14 Tilman's (1988) resource-ratio hypothesis of succession. Five hypothetical plant species are assumed to be differentiated in their requirements for a limiting soil nutrient and light. During the succession, the habitat starts with a nutrient-poor soil but high light availability, changing gradually into a habitat with a rich soil but low availability of light at the soil surface. Relative competitive abilities change as conditions vary, and first one species and then another comes to dominate.

Time

Figure 16.14 Tilman's (1988) resource-ratio hypothesis of succession. Five hypothetical plant species are assumed to be differentiated in their requirements for a limiting soil nutrient and light. During the succession, the habitat starts with a nutrient-poor soil but high light availability, changing gradually into a habitat with a rich soil but low availability of light at the soil surface. Relative competitive abilities change as conditions vary, and first one species and then another comes to dominate.

disturbance (four classes are defined: vegetative spread, V; seedling pulse from a seed bank, S; seedling pulse from abundant dispersal from the surrounding area, D; no special mechanism with just moderate dispersal from only a small seed bank, N); and (ii) the ability of individuals to reproduce in the face of competition (defined in terms of tolerance T at one extreme and intolerance I at the other). Thus, for example, a species may be classed as SI if disturbance releases a seedling pulse from a seed bank, and if the plants are intolerant of competition (being unable to germinate or grow in competition with older or more advanced individuals of either their own or another species). Seedlings of such a species could establish themselves only immediately after a disturbance, when competitors are rare. Of course, a seedling pulse fits well with such a pioneer existence. An example is the annual Ambrosia artemisiifolia which often figures early in old-field successions. In contrast, the American beech (Fagus grandifolia) could be classed as VT (being able to regenerate vegetatively from root stumps, and tolerant of competition since it is able to establish itself and reproduce in competition with older or more advanced individuals of either its own or another species) or NT (if no stumps remain, it would invade slowly via seed dispersal). In either case, it would eventually displace other species and form part of the

'climax' vegetation. Noble and Slatyer argue that it should be possible to classify all the species in an area according to these two vital attributes (to which relative longevity might be added as a third). Given this information, quite precise predictions about successional sequences should be possible.

Lightning-induced fires produce regular and natural disturbances in many ecosystems in arid parts of the world and two fire-response syndromes, analogous to two of Noble and Slatyer's disturbance recovery classes, can be identified. Resprouters have massive, deeply penetrating root systems, and survive fires as individuals, whereas reseeders are killed by the fire but re-establish through heat-stimulated germination and growth of seedlings (Bell, 2001). The proportion of species that can be classified as resprouters is higher in forest and shrubland vegetation of southwest Western Australia (Mediterranean-type climate) than in more arid areas of the continent. Bell suggests that this is because the Western Australian communities have been subject to more frequent fires than other areas, conforming to the hypothesis that short intervals between fires (averaging 20 years or less in many areas of Western Australia) promote the success of resprouters. Longer intervals between fires, on the other hand, allow fuel loads to build up so that fires are more intense, killing resprouters and favoring the reseeding strategy.

The consideration of vital attributes from an evolutionary point of view suggests that certain attributes are likely to occur together more often than by chance. We can envisage two alternatives that might increase the fitness of an organism in a succession (Harper, 1977), either: (i) the species reacts to the competitive selection pressures and evolves characteristics that enable it to persist longer in the succession, i.e. it responds to K selection; or (ii) it may develop more efficient mechanisms of escape from the succession, and discover and colonize suitable early stages of succession elsewhere, i.e. it responds to r selection (see Section 4.12). Thus, from an evolutionary point of view, good colonizers can be expected to be poor competitors and vice versa. This is evident in Table 16.3, which lists some physiological characteristics that tend to go together in early and late successional plants.

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