Rancho Grande Trees with 0 > 10 cm
Fig. 3.15 Species/area curves for four tropical forests in Venezuela. A Cloud forest La Carbonera (minimal quadrat 2,500 m2); B moist trade-wind forest Rio Caura (minimal quadrat 1,100 m2); C and D cloud forest of Rancho Grande, C all species, D only trees with a stem diameter > 10 cm, for which the minimal quadrat has not been attained at 7,500 m2. (After data of Vareschi 1980, from Luttge 1995)
• at the level of communities or coenoses, a-diversity,
• at the level of ecosystems with different vegetation types, for example at ridges, on hillsides, in valleys etc., (-diversity,
• at the level of landscapes comprising several ecosystems, 7-diversity.
Another approach is the evaluation of diversity in the spatial structure of the physical environment in relation to environmental factors. Bell et al. (1993) have suggested the use of log-log regressions of environmental factors, such as edaphic variables, water chemistry and climate, vs distance over large spatial scales (106 m). Plotting the log variance of environmental factors, e.g. the soil nitrogen, soil phosphorus etc., on log distance allows comparisons of both the heterogeneity of different environments with respect to that given factor and the different factors within a particular environment. With consideration of environmental factors, of course, eco-physiological aspects are introduced into the discussion of diversity. Varying ecophysiological behaviour of given genotypes is expressed as phenotypic plasticity. In the log-log analysis of regression of variance of environmental factors on distance by Bell et al. (1993) there was no indication that the variance of the physical environment tended to approach some maximal value as the distance increased. In contrast, it increased continuously with distance. Thus, the slopes of the log-log regressions provide a means to compare heterogeneity of environments and factors in relation to each other, i.e. slopes of the log-log regressions indicate the correlations of nearby sites between each other. Where nearby sites are highly correlated, selection will tend to favour specialization because dispersing offspring find conditions for growth similar to their parents. This leads to diversity of genotypes. On the other hand, where nearby sites show little correlation between each other, offspring tend to find conditions different to their parents. This favours plasticity.
Fig. 3.16 Floristic diversity related to species-area diagrams. (After Whitmore 1990, from Lüttge 1995). aß... Species-poor community (low a-diversity) with low ß-diversity, viz. few species, small minimal quadrat; aß... species-poor community (low a-diversity) with high ß-diversity, viz. few species, large minimal quadrat; aß... species-rich community (high a-diversity) with low ß-diversity, viz. many species, small minimal quadrat; aß... species-rich community (high a -
diversity) with high ß-diversity, viz. many species, large minimal quadrat; y____bi- (or multiphasic)
curves indicating y -diversity
3.3.2 Diversity and Plasticity and the Biological Stress Concept
Environmental factors direct us to the consideration of the role of stress, because any environmental factor can become a stress factor or stressor if its dosage is too high or too low. This is explained by the biological stress concept as described in Box 3.1. We may ask the questions as to how stress may be involved in regulating plasticity and diversity and whether in fact, plasticity and diversity are related.
In an experiment applying different degrees of stress to experimental microcosms, Grime et al. (1987) have demonstrated that high diversity is given only within a rather narrow range of stressed conditions. For conditions in the British Isles high species diversity occurred at stress intensities which allowed no less than 350 g m-2 dry biomass, but no more than 750 g m-2. At lower stress there is dominance of one or only a few robust and competitive species. At higher stress only a few highly adapted "specialists" survive as competition peaks in circumstances of abundance. In relation to plant nutrition, despite the abundance of resources available to vegetation established on fertile soil, plant growth results in the simultaneous development of depletion zones which creates circumstances of unequal access and traps subordinates in the depletion zones while supporting the monopoly by growth forms with the genetic potential for coarse-grained foraging (Campbell et al. 1991).
In a Malaysian rainforest it was shown that the species diversity of trees and vines strongly responded to combined phosphorus and potassium concentrations in the soil. Diversity was highest on soils with a medium P + K index, it declined towards both very poor soils and soils with more ample nutrient supply (Fig. 3.17). The relationship between diversity and environmental richness can also be demonstrated for tropical birds and mammals (Reichholf 1994).
A good physical analogue for developing the terminology is a spring. Stress is put on the spring by strain. Reversible stress is brought about by strain in the elastic range of the spring material: elastic strain. Irreversible stress is due to strain beyond the elastic range of the spring material: plastic strain.
The biological stress concept was developed by Selye (1973), Levitt (1980) and Larcher (1987). Any external factor (biotic or abiotic) and internal factor can induce stress, i.e. become a stressor, if its dosage is too high or too low. The terminology of the biological stress concept is explained in the diagram giving four different possible cases for the development of a biological system with time (abscissa).
Positive Stress Effects
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