Corridoi ripariali dei corsi d' acqua effimeri mediterranei: valutazione delle funzioni ecologiche nel paesaggio e potenzialità per la tutela ambientale nella pianificazione territoriale. Il caso della Lama San Giorgio nella Puglia centrale
Author(s)
Mairota, Paola
Date Issued
October 14, 2008
Type
Doctoral Thesis
Abstract
Ephemeral streams are typical of Mediterranean environments. Their characteristic flow regime as well as the strong human pressure on the land at the stream/upland interface condition the spatial configuration of the stream corridor. This in turn affects the ecological efficiency of such complex linear landscape element with regard to its major functions. These involve movement (either passive or active) of materials, energy and organisms along or across the corridor.
This work focuses on the assessment of the subsistence of the habitat function in the case of the stream corridor of an ephemeral stream, representative for karstic torrents in the Puglia region (southern Italy). The habitat function is relevant to key issues of ecological research, such as connectivity, plant dispersal and animal metapopulation dynamics. Insights on these issues are crucial as on them relies planning for management and conservation of natural resources and biodiversity maintenance. Moreover, one of the major threats to biodiversity in agricultural Mediterranean landscapes is habitat fragmentation, which causes populations decline and enhances local extinction risk.
To the ends of the work, a multiple scale analysis of landscape configuration for the stream corridor was performed. Such a need descends from the, by now unquestionable, notion of the hierarchical organisation of ecological systems and from the hierarchical nature or watershed systems. In addition, to the hierarchy of landscape pattern corresponds a hierarchy of decisions of animal organisms. Hence, the concept of scale is not alien from the notion of biodiversity. Finally, hierarchical planning is steadily gaining momentum as an approach to forest planning.
A range of spatial scales was identified that corresponds to the hierarchical organisation of the stream system, as well as to the levels of the hierarchy of the patchiness for the stream corridor understood as a complex and multifunctional landscape subsystem as well as a vulnerable resource for mankind.
For the detection of the stream corridor and its stratification into segments, at the scale of the hydrological system, a methodology was adopted that is based on the association of both extrinsic and intrinsic classificatory attributes (e.g. hierarchy within the hydrological system, lithology, climatic conditions as well as the presence/absence and the physiognomy of natural vegetation) to each component of the hydrological system. This procedure yielded the objective identification of both the stream corridor and the limits of the portion of the landscape more likely to affects its structural and functional features. In addition it brougth forward information on characteristics of the corridor, such as continuity/discontinuity, heterogeneity/homogeneity of sub-natural vegetation and their degree of isolation and distribution.
At the scale of stream corridor the optimal limit analysis of a line transect was performed that has allowed for the recognition of a micro-heterogeneous landscape pattern, as well as the spatial relations among land cover types. Also the spatial variations of the landscape matrix were analysed based on the moving split-window (MSW) technique applied to a plane transect. Different landscape types were thus identified and their characteristic structure described both in terms of composition and of spatial arrangement of sub-natural vegetation.
Segments’ spatial configuration was analised by means of selected landscape pattern indices (LPI). At the landscape scale segments were ordered according to a fragmentation gradient. At the class level the relations among the LPI depend on land cover type, thus providing information on the kind of restoration strategy needed to reduce habitat fragmentation.
For the assessment of the habitat function, birds were used as bioindicators, consistently with the most significant literature on this topic, given their sensitivity to environmental change in connection with their specialisation and vagility at the landscape scale. Bird species habitat needs, degree of specialisation, population and community traits were investigated at the different hierarchical levels and insights on the interaction between pattern (landscape structure) and process (bird distribution) were gained. The stream corridor performs as a refuge for many specialised species which prefer forest type vegetation as well as less intensively managed agricultural land cover types. Bird communities diversity is affected by the degree of fragmentation of suitable habitat types.
The environmental quality of the stream corridor was also assessed with respect of specialised bird species and a comparison was made with respect to carabid beetles, whose importance as bioindicators is also recognised, but for which analytical data are not available for the study area. At higher levels both taxonomic groups yield comparable results, yet the lower levels the need of operating with specific bioindicators emerges. Environmental quality was also compared to landscape configuration and it was confirmed that the relative dominance and pervasiveness of land cover types with a high anthropogenic determinism, landscape fragmentation and diversity negatively affect environmental quality at all levels of the hierarchy. Finally a zoning of the stream corridor was proposed with respect to both environmental quality and landscape configuration that represents the base to built a restoration strategy at this level.
Thus the importance of even ephemeral, depleted, stream corridors for metapopulation viability and their effectiveness in the performance of the habitat function is confirmed, which are crucial for biodiversity maintainance at the landscape scale. However the high fragmentation degree, the narrowness of the stream corridor and possibly the simple structure of such phytocoenoses, seem to explain the diversification within bird communities. This could be a signal that the turning point from source to sink for this subsystem may have been reached. Methodological issues are discussed, such as those relevant to the wide stream of landscape ecological research aimed at exploring both ecological and planning/management implications of the space-time hierarchical nature of complex ecological systems, to the legitimacy of the use of landscape indices to draw inferences on processes underlying the ecology of landscapes, and to the scale dependence of landscape heterogeneity.
Additional information
Dottorato di ricerca in Ecologia forestale
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