Pedodiversity and Island Soil Geography: Testing the Driving Forces for Pedological Assemblages in Archipelagos of Different Origins.
William R. Effland, USDA/NRCS Soil Survey Division, 1400 Independence Ave., SW, Washington, DC 20250, A. Rodriguez-Rodriguez, Dpto. Edafologia y Geologia, Fac. Biologia, Univ. de La Laguna, 38204 La Laguna, Tenerife, I. Canarias, Spain, and Juan Jose Ibanez, CIDE, CSIC-UV, Valencia, Spain.
Although island biodiversity and biological assemblages has been a topic of widespread interest in ecology and conservation biology, soil scientists have conducted few studies of pedodiversity and pedological assemblages in archipelagos. In this contribution we analyze similarities and differences in the pedosphere for three archipelagos with different origins: (i) oceanic-shield islands (Hawaiian Islands), (ii) oceanic-intra-plate chains (Canary Islands), and (iii) non-marine land-bridge islands with volcanism associated with plate boundary conditions (Aegean Islands). In the Canary Islands, we also compiled selected biological data (some target biotaxa, number of ecosystems and bioclimatic belts), geological data (number and extension covered by parent materials with different lithologies), and relief for further analyses of pedodiversity, climatic diversity, geodiversity and biodiversity. Our pedogeographic analysis of the three archipelagos indicates archipelagos of varying origins contain different soil assemblages. We tested the hypothesis that soil taxa distributions on island chains can be modeled as a power function similar to the famous Species-Area Relationship (SAR) discussed by McArthur and Wilson (1962). The SAR functional relationship has been widely corroborated in numerous biodiversity studies. It also occurs in spatial pattern analysis for soil landscapes or pedological assemblages with similar results obtained for the British Isles. It is important to remember that this SAR relationship with the same exponential value is a core theory, in both biogeography and conservation biology. The Aegean Archipelago consists of several hundred islands and islets. Our results using soil taxa support application of the SAR that corresponds to a power law with an exponential value of 0.25. In the Canary Islands we obtained a SAR that conforms to a power law and has a similar exponent value. However, the statistical fit was less significant than shown for the former cases, probably because the data set is small (few islands). In the Canary archipelago, a strong relationship was obtained between pedodiversity, biodiversity, climate diversity, habitat heterogeneity and relief. For the Hawaii archipelago, we did not obtain a statistically significant relationship because we only studied six islands since soil inventories are not published for the smallest islands and islets. Overall, the dynamics of the underlying plate tectonic hot spots may be an important environmental variable affecting the various diversity types. In the Hawaiian Islands with the 1:250,000 State Soil Geographic (STATSGO) database, a soil chronosequence among the major islands was clearly observed. Island area and pedodiversity are negatively correlated, whereas island age and pedodiversity exhibit strong positive correlations. Older islands are the most pedodiverse, with strongly contrasting soils. They also have the older, most developed pedotaxa based on the USDA Soil Taxonomy Soil Order, Suborder and Great Group categories. Our analysis also corroborates at small spatial scales the divergent pedogenesis hypothesis proposed by Phillips for detailed scales. Conversely, a positive pedodiversity-area relationship was obtained for the lower hierarchical levels of Soil Families and Soil Series. For the intermediate level of Subgroups, relief appears as an important variable to help explain pedodiversity values. These observations suggest, at a minimum, multiple rationale underlying the categorical concepts of USDA Soil Taxonomy, which has been applied to the entire United States using STATSGO. Pedodiversity values using the taxonomic categories of Soil Orders, Suborders and Great Groups for the United States and the Hawaiian Islands do not positively correlate with area. In contrast, a strong positive correlation was observed for Soil Families and Soil Series with area in both cases, with the Subgroups intermediate between the highest and lowest hierarchical categories. We hypothesize that Soil Families and Soil Series categories may represent a classical-typological approach to spatial variability (for some properties of agronomic interest) when compared to the more abstract and broader pedotaxa (Soil Orders, Suborders, Great Groups and Subgroups). We further postulate that the conceptual rationale for Soil Families and Soil Series more closely resembles biogeographical races or ecotypes, in contrast to the other higher hierarchical levels of the USDA Soil Taxonomy. Based on the published literature, this study may be the first example in which pedologists outlined a theoretical proposal of “Island pedogeography” consistent with studies in previous decades conducted by ecologists and conservation biologists. This study illustrates several possibilities for further research in pedodiversity integrating methods from soil geography, landscape ecology, and spatial pattern analysis.