Concept 42.1 Ecological Systems Vary in Space and over Time

• An ecological system consists of one or more organisms and the biotic and abiotic components of the environment with which they interact.
   
• Ecological systems can be studied at any scale in the biological hierarchy, from an individual organism and its immediate surroundings to populations, communities, landscapes, or the entire biosphere. Review Figure 42.1
   
• Because the components of ecological systems and their interactions are distributed unevenly in space and change over time, each system, at each time, is potentially unique.
   

Concept 42.2 Climate and Topography Shape Earth's Physical Environments

• Weather is the state of atmospheric conditions in a particular place at a particular time, and climate is their average state and pattern of variation over longer periods.
   
• Latitudinal differences in solar energy input are caused by differences in the angle of the sun's incoming rays. Seasonality results from the tilt of Earth's axis of rotation relative to its orbit around the sun. Review Figures 42.3 and 42.4
   
• Latitudinal differences in solar energy input drive north–south patterns of atmospheric circulation. These patterns, in turn, influence latitudinal patterns of precipitation. Review Figure 42.5
   
• Prevailing surface winds result from the interaction of the north–south atmospheric circulation cells with Earth's eastward rotation. Review Figure 42.6
   
• Surface currents in the oceans are driven by prevailing winds and deflected by continents. Review Figure 42.7
   
• Climate diagrams summarize climate at a particular location in an ecologically relevant way. Review Figure 42.8
   
• Variation in the elevation of Earth's surface, known as topography, affects the physical environment. Review Figure 42.9 and ANIMATED TUTORIAL 42.1
   

Concept 42.3 Physical Geography Provides the Template for Biogeography

• A biome is a distinct physical environment that is inhabited by ecologically similar organisms with similar adaptations. The species that occupy the same biome in geographically separate regions may not be closely related phylogenetically but often show convergent adaptations. See ANIMATED TUTORIAL 42.2
   
• The distribution of terrestrial biomes is broadly determined by annual patterns of temperature and precipitation. Review Figures 42.10 and 42.11
   
• Other features of the physical environment—particularly soil characteristics—interact with climate to influence the character of vegetation.
   
• Climate is less important in distinguishing aquatic biomes than are water depth and flow, temperature, pressure, salinity, and characteristics of the substrate. Review Figure 42.13 and Table 42.1
   

Concept 42.4 Geological History Has Shaped the Distributions of Organisms

• Alfred Russel Wallace first noticed a boundary, now known as Wallace's line, between two distinct assemblages of species that was not explained by climate or soil. Review Figure 42.14
   
• The world can be divided into biogeographic regions, each of which contains distinct assemblages of species. The boundaries of biogeographic regions generally correspond to present or past barriers to dispersal and can be explained by continental drift. Review Figures 42.15, 42.16, and WEB ACTIVITY 42.1
   
• Biogeographers use phylogenetic information, in conjunction with the fossil record and geological history, to determine how the modern distributions of organisms came about.
   

Concept 42.5 Human Activities Affect Ecological Systems on a Global Scale

• Humans have converted almost half of Earth's land area into human-dominated ecosystems, which are much more homogenous and less complex than natural ecosystems, and human activities are reducing the complexity of the natural ecological systems that remain.
   
• Humans move species around the globe without regard to natural barriers to dispersal. These movements are homogenizing the biota of the planet.
   

Concept 42.6 Ecological Investigation Depends on Natural History Knowledge and Modeling

• Accurate and reliable knowledge of natural history—that is, information about the components of ecological systems, their interactions, and the environmental context of those interactions—is crucial to the study of ecology.
   
• Developing computer models of the major components of an ecological system and their interactions is often needed to generate testable predictions about these complex systems.