Hierarchical Organization of Ecological Mosaics

It is a priority to link and to integrate information from the structural analysis and the species-specific perception of mosaics.

A. Farina, Ecology, Cognition and Landscape, Landscape Series 11, 55

DOI 10.1007/978-90-481-3138-9_4, © Springer Science+Business Media B.V. 2010

Fig. 4.1 Mosaics appear at every level of resolution alternated with heterogeneity. In cases A and C the heterogeneity is the dominant pattern and it is not possible to observe mosaics. In cases B and D the mosaics are dominant. A and C are considered disordered images in which it is not possible to observe patterns Linkable to explicit ecological processes. In B and D the ordered vision dominates and if we consider such mosaics created by plant cover we can address the question: what were the factors responsible for such patterns?

Fig. 4.1 Mosaics appear at every level of resolution alternated with heterogeneity. In cases A and C the heterogeneity is the dominant pattern and it is not possible to observe mosaics. In cases B and D the mosaics are dominant. A and C are considered disordered images in which it is not possible to observe patterns Linkable to explicit ecological processes. In B and D the ordered vision dominates and if we consider such mosaics created by plant cover we can address the question: what were the factors responsible for such patterns?

When we analyze ecological mosaics, it is clear that, according to the scale utilized to observe such forms, different figures appear. Often this is possible, especially for plants, but this analysis can be carried out also using animals to distinguish sub-systems inside each mosaic.

It is reasonable to imagine that when we can observe two different types of patches inside a mosaic, it is possible to extract more detail on each patch type and distinguish another more fine-grained level inside each patch. This character of patches can be explained in terms of the hierarchical theory.

Well argued by O'Neill et al. (1986) hierarchical theory is useful to interpret the complexity of ecological systems, especially when we try to investigate the

Estension

Fig. 4.2 Hypothetical table

of contingency between

extension and resolution

n o

measuring the value of

Is

heterogeneity and contagion

"e s

considering paired changes in

e tt

resolution and extension. An

alternate highest score for

heterogeneity and patchiness

is expected

H=Heterogeneity C=Contagion

Estension

J1

J2

J3

J4

I1

H>C

I2

C>H

I3

H>C

I4

C>H

H=Heterogeneity C=Contagion mechanisms that create self-maintaining structures. At every level of hierarchy we observe sub-systems that are created and regulated by specific processes.

I hope the example I report is clear enough.

At Cerreto Pass, in the Northern Apennines there is a chalk evaporitic deposit from the Mesozoic era. The surface of this deposit has been modified by karstic processes creating dolines. Focusing on the forms of the relief, we observe different types of dolines, distinguishable by size, shape, and depth (Fig. 4.3).

Such dolines are the result of dissolution of chalk, and the mosaic created by such a process produces steep micro-relief. If we move in to a finer scale, investigating the dynamics inside each doline, the scenario appears more complicated, and to the geomorphological processes we have also to add biological processes that act on soil, modifying the dissolution rate of the chalk and the entire hydrological regime.

Moving further inside such a mosaic and focusing on the vegetation a completely different scenario appears. In fact it is possible to distinguish between tall shrubs and open prairies. This mosaic is the result of human intervention over a period of more than 5,000 years. Human activity has created clearings used for livestock grazing or for cultivation since protohistorical time.

In the past such clearings were probably abandoned during periods of famine, disease, or war, but again and again have been maintained by different degrees of disturbance. Such types of stewardship have so deeply influenced the landscape that today all processes linked to biological components are affected by this footprint (or memory).

So in conclusion, looking at vegetation on a broad scale, we can distinguish two types of patches: open areas and shrublands. Both types are the result of a long-term regime of disturbance through human use.

The mosaic created by geomorphological processes (doline) is independent of this last process created by humans, but moving up the scale something changes rapidly.

Moving inside the open prairies created by human intervention and changing the scale of resolution, we can observe another mosaic composed of different types of herbaceous plants. It is possible to distinguish between Ericaceae, Brachypodium pinnatum, and other grasses. Such a mosaic is largely affected by the grazing pressure of domestic (cows, horses, sheep, and goats) and wild grazers (Roe deer, hare, and insects).

A fourth level can be discovered inside each prairie patch type, and again we can distinguish different levels of standing biomass. The process involved is represented by soil fertility, in turn created by input of organic material (urines, feces) or by the activity of soil microbes and fungi.

The relationships between the different levels are not direct, and this is the main source of complexity. The mosaic is created by independent processes acting in the same place at different time lags.

According to the hierarchical theory, every system is the result of the contribution of concurring sub-systems, and this mechanism persists across different levels of spatial and temporal scale (Fig. 4.4).

Fig. 4.3 Example of a scaled image interpretation of a sub-montane pasture in which geomor-phology (a: karstic processes), land use (b: open areas, shrublands), and fine-grained use of the vegetation (c: plant composition, d: standing biomass) create a complex mosaic. See text for explanation c d

Fig. 4.3 Example of a scaled image interpretation of a sub-montane pasture in which geomor-phology (a: karstic processes), land use (b: open areas, shrublands), and fine-grained use of the vegetation (c: plant composition, d: standing biomass) create a complex mosaic. See text for explanation

The dynamics of a system depend on the hierarchical rank of the system: Highrank systems have slow dynamics. For instance, a biome changes over the long term while the plant community of our garden can change completely in one season. The different speed at which different hierarchical mosaics modify their structure is a fundamental force contributing to the overall complexity (Fig. 4.5).

Fig. 4.4 The transition from one level to a level of higher rank produces emerging characters from a filter zone

Fig. 4.5 Dynamics increase as one moves down the hierarchical structure of every system

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