Measuring geomorphological diversity on coastal environments: A new approach to geodiversity

Abstract

Geodiversity can be defined as the spatial variability of geo-elements. Its emergence as a research field is relatively recent, with a wide range of approaches and methods of evaluation and quantification already in use. The work described in this paper was carried out from a novel perspective. Firstly, the emphasis is specifically centred on geomorphological diversity. Secondly, mathematical ecology calculations are applied which tackle the problem on the basis of a larger number of diversity parameters. In addition, whereas most authors have focussed on mountainous and/or continental areas, for the first time a study of this type is applied to coastal landscapes. The analysis covers the full 459 km of volcanic coastline of the islands of Gran Canaria and La Palma (Canary Islands). These two oceanic islands were chosen for purposes of diversity comparison given their differing stages of geomorphic development. In accordance with an ad hoc geomorphological taxonomy, as developed in this study, 14 geomorphic groups and 42 subgroups were identified and quantified according to their presence [1] or absence [0] along the coastline. A cartographic-analytical method (coastline data-storing or CDS), based on the computation of binary information in the cartographic vector of the coastline, enabled calculation of the occurrence probability of these groups and subgroups. Both at local level (analyses of coastal morphoassemblages) and general level, three components of geomorphological diversity (richness, evenness and dissimilarity) were estimated, through indices of richness (R), Shannon's entropy (H′) and Rao's quadratic entropy (Q). The data reveal that geomorphological coastal diversity is between 11.1% and 48.2% higher in Gran Canaria than in La Palma. The method was subjected to a fractal stress test through iterative mapping procedures to assess the effect of scale on the behaviour of the relative frequencies and indices. The variability that was obtained allowed the extraction of corrected values in the indices for La Palma (H′c = 1.88 ± 0.008 and Qc = 0.183 ± 0.004) and Gran Canaria (H′c = 2.11 ± 0.019 and Qc = 0.232 ± 0.003). A moving-window test was also applied as a rarefaction method to confirm the contrast in diversity in coasts of different size. The results enabled to advance hypotheses about the relation between coastal geomorphic diversity and geological age of islands.