Heterogeneity and Mosaic

Heterogeneity is a common attribute of every ecological system and a lot of scientific attention has been paid to this theme (Kolasa and Pickett 1991, Hutchings et al. 2000). Species are living in environments that are highly heterogeneous in space and time. The heterogeneity is created by the variation in abiotic factors

(physical and chemical properties of the soil, microtopography, microclimate) as well as by organisms (plants via leaf deposit, exudate, root growth; animals by grazing, trampling, burrowing). Some organisms, like termites, are real ecosystem engineers promoting heterogeneity and providing in such a way habitats for many other species that have a less direct relationship with the environment. Finally, another cause of heterogeneity is represented by stochastic events like earthquakes, hurricanes, fires, volcanic eruptions, flooding, etc.

Heterogeneity changes in time and space and according to age every organism may experience a different type of heterogeneity. For instance, a poplar seed experiences a different type of heterogeneity compared with an adult plant. A dramatic difference appears when we compare the heterogeneity perceived by a less mobile species like a caterpillar to that of a very mobile butterfly.

Heterogeneity is not a novel concept in ecology; nevertheless, the interest of ecologists is growing, thanks to the use of new technologies to explore this relevant pattern of ecological complexity.

The nature of heterogeneity is not as clearly evident as we believe. A heterogeneous medium means that the composing elements are different or have different spatial characters. Heterogeneity has been distinguished by Kolasa and Rollo (1991) into two types: measured heterogeneity and functional heterogeneity. The measured heterogeneity is "a product of the observer's arbitrary perspective" (Kolasa and Rollo 1991). The functional heterogeneity is the heterogeneity perceived by the ecological entity (organism or process).

According to a general perspective we can also describe two further types of heterogeneity: spatial and temporal. Spatial heterogeneity means a different distribution in space of ecological entities. Temporal heterogeneity means the change in characteristics for the same point across a period of time. Recently, Wiens (2000) recognized four forms of heterogeneity: spatial variance, patterned variance, compositional variance, and locational variance. As we will see later, the locational variance represents de facto the mosaic (Fig. 3.3).

Spatial variance, therefore, intercepts the irregular position of objects in the matrix, and patterned variance considers the importance of the nearest elements from a focal object. The compositional variance is expressed either by quality or quantity of the objects. Finally, the locational variance is the explicit spatial arrangement of patches in a matrix.

Heterogeneity and patchiness in some cases have been considered as synonymous. Heterogeneity normally is used to indicate the nonhomogeneous nature of

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Fig. 3.3 The four main types of heterogeneity: (a) Spatial variance, (b) Patterned variance, (c) Compositional variance, (d) Locational variance (from Wiens 2000)

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Fig. 3.3 The four main types of heterogeneity: (a) Spatial variance, (b) Patterned variance, (c) Compositional variance, (d) Locational variance (from Wiens 2000)

the environment in which a species is living. In other words heterogeneity is the grain of the habitat, the minimum dimension of objects perceived as distinct by an observer.

In general, a mosaic is a heterogeneous system. So in conclusion heterogeneity is a more general definition of variant attributes of an ecological system, and a mosaic is an entity pertaining to our cognitive perception.

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