IV.6 Spatial Patterns of Species Diversity in Terrestrial Environments
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IV.6 Spatial Patterns of Species Diversity in Terrestrial Environments Brian A. Maurer OUTLINE 1. The physical environment 2. Dynamics of geographic populations 3. Patterns in genetic variation and adaptation 4. Synthesis: How spatial patterns in species diversity are generated and maintained 5. Empirical sampling protocols 6. Species abundance and relative abundance distributions 7. Species–area relationships 8. Abundance–distribution relationships 9. Nested subsets community pattern 10. Species diversity along environmental gradients 11. Understanding species diversity Spatial patterns of species diversity have intrigued ecologists since European natural historians discovered that the flora and fauna of the world varied dramatically across the face of the Earth. It is only within the last few decades that a clear understanding of the processes underlying these patterns has arisen. Variation in the number of species found across space depends on several interacting sets of processes. The first set of processes affect the physical and chemical properties of the hydrosphere, atmosphere, and lithosphere. The second set of processes comprises the demographic responses of individual organisms interacting with their physical and biological environment summed up within geographic populations of different species. The final set of processes are the long-term adaptive responses of populations as natural selection shapes gene pools of different species over evolutionary time. The complex interactions of these sets of factors occur across a wide range of spatial and temporal scales, making it difficult to isolate simple explanations for data collected at single spatial and/ or temporal scales. In what follows, I provide an outline for how the three sets of processes work together to set the broad patterns of species diversity seen across geographic space. Here I focus on terrestrial patterns, although a similar argument applies to marine patterns of species diversity. After outlining the processes underlying species diversity variation, I show how different methods of sampling species diversity across geographic space produce the variety of patterns documented by ecologists and biogeographers. GLOSSARY adaptive syndrome. The suite of morphological, physiological , and behavioral characters that determine an organism’s ability to survive and reproduce a diversity. Thespecies diversityofa locallysampledsite b diversity. The turnover in species diversity among different sites within a landscape, generally referring to sites that share the same metacommunity c diversity. Turnover in species diversity among different metacommunities geographic population. All viable populations of a species found within the species’ geographic range geographic range. The spatial region that includes all viable populations of a species metacommunity. For any given local community, the assemblage of all geographic populations that contribute immigrants to the community metapopulation. A group of local populations linked together by dispersal species diversity. The number and relative abundances of species within a specified geographic region, often divided into a, b, and g diversity viable population. Any population that can persist through time by a combination of local recruitment and immigration 1. THE PHYSICAL ENVIRONMENT Spatial patterns of species diversity result from the interplay of biology with large-scale patterns in the physical properties of the Earth. For a complete de- scription of diversity patterns, then, it is necessary to describe the dynamics of the physical system that comprises the thin layer of materials covering the Earth. Several important sources of energy drive the geophysical environment. Of these, the most important from the perspective of living systems is the sun. Energy from the sun not reflected back into space is absorbed by the surface of the Earth and the atmosphere. The absorbed energy heats air masses in the atmosphere, causing large vertical circulation patterns that distribute water in the atmosphere unevenly across the surface of the Earth. These movements result in broad patterns of climate with latitude, with wet tropical climates near the equator, bands of arid climates at approximately 308 latitude, and wetter temperate climates at 608 latitude (see chapter IV.7 for a more detailed account ). Modifications of the general patterns of heat distribution across the face of the Earth and the consequent movements of air masses across its surface incorporate a number of different processes. The rotational energy of the Earth modifies these general patterns, particularly near coastal regions. The tilt of the Earth results in seasonal patterns in climate as the Earth travels around the sun. Topographic features of the Earth’s crust deflect movements of air masses upward, changing nearby patterns of precipitation. Gravitational energy from the moon causes large movements of water masses in the oceans, resulting in tidal patterns along continental margins. These factors combined result in...