The Effect of Spatial and Environmental Drivers on Patterns in Species Richness and Composition
This thesis includes eight manuscripts with methodological, empirical and theoretical contributions that aim to enhance the understanding of species richness and composition patterns and their underlying drivers. Islands and isolated systems are in the focus of this work. Islands provide optimal conditions to study biogeographic patterns. Theoretical advances in ecology have been initiated by island biogeography. Theory on island biogeography has particularly been improved by a better representation of time related components including speciation and environmental change. Oceanic islands are not stable systems but follow a characteristic ontogeny. After the volcanic emergence over the sea surface, erosion processes, shaping the island first more heterogeneous and then flatter, transform islands. This thesis shows how particular characteristics of the classic theory of island biogeography can be included in the currently most advanced theoretical framework. While MacArthur & Wilson (1963) particulary focussed on processes (colonisation and extinction) for generating species richness patterns, current theory assumes a defined upper limit for species richness (“carrying capacity”). By reinforcing the importance of processes in the current theory, as suggested in this thesis, it is much simpler to formulate hypothesis that can be tested by empirical data. Carrying capacity is linked to “habitat heterogeneity”, both, in the meaning of topographic variability as well as the number of vegetation units that are present in a given area. This thesis demonstrates that a clear terminology is a prerequisite for a profound understanding of the effects of “heterogeneity” on species diversity patterns in general and the underlying biogeographic processes in particular. The heterogeneity of surfaces influences species diversity not only on scales larger than kilometres but also is important on very fine scales of meters and smaller. Novel methods to measure different aspects of surface variability are introduced and discussed and their effect on species richness and composition of plant species groups in different ecological systems is presented. Furthermore, this thesis highlights the isolating effect of elevation (elevation-driven ecological isolation hypothesis). Environmental filtering along an elevational gradient differentiates ecosystems. Isolation increases with elevation, as comparable ecosystems are much farther apart at high elevations than is the case for lowland ecosystems. In addition, ecosystems on neighbouring islands or on the continent that serve as source regions for colonising species are smaller in area in high elevations in comparison to low elevation ecosystems. Consequently, an above average speciation rate reflected in a high percentage of endemic species can be expected for higher elevations on islands and high mountains. The elevation driven ecological isolation hypothesis is tested for a number of islands and a new hypothesis indicating a complex interaction with isolation is developed. The difference in isolation between low and high elevation ecosystem diminishes as the overall isolation of the island increases. Thus the relation between the percentage of endemic species and elevation should reverse with an increase in isolation. On very isolated islands, low and high elevation ecosystems are alike isolated but low elevation ecosystems should have an above average speciation rate as they provide more area and higher temperatures relative to the ecosystems above (e.g. metabolic theory of ecology). The scale dependence of diversity patterns are attributed to ecological processes that operate differently over varying extents and grain sizes. This thesis demonstrates that scale dependencies in distance-decay analyses cannot be traced back to processes that are specific for the ecological scale, but can largely be attributed to sampling design and are highly sensitive to grain size and study extent. Distance-decay analyses are an adequate method to assess spatial turnover in species composition. However, this thesis shows that frequent practise of making comparisons among studies is not possible within the current methodological framework. Finally, this thesis provides an overview on patterns in species richness and composition and elaborates interconnections between associated theories and underling drivers. Promising novel research questions and directions are identified in the field of island biogeography and in an adequate formalisation of a “heterogeneity” concept.
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