Concept 45.1   Climate and Nutrients Affect Ecosystem Function

• The term ecosystem is commonly applied to ecological communities plus the abiotic environments with which they exchange energy and materials.
• A community’s net primary productivity (NPP) reflects the rate at which it exchanges energy and materials with its abiotic surroundings, and is thus a measure of ecosystem function.
• NPP varies considerably among ecosystem types. Review Figure 45.1
• The global pattern of variation in terrestrial NPP shows that it is influenced by temperature and precipitation. Soil nutrients also play a role in terrestrial NPP. Review Figure 45.2 and Figure 45.3
• Aquatic NPP varies most strongly with availability of light and nutrients. Review Figure 45.4

Concept 45.2   Biological, Geological, and Chemical Processes Move Materials through Ecosystems

• The chemical form in which an element exists and where it occurs on Earth determine whether the element is accessible to life. Alternative forms and locations of matter can be thought of as compartments of the global ecosystem.
• The chemical transformations and physical transport of elements or molecules that move them among compartments are called biogeochemical cycles. Review Figure 45.5
• The pool, or amount, of an element or molecule in a compartment depends on its flows, or fluxes, into and out of that compartment.

Concept 45.3   Certain Biogeochemical Cycles Are Especially Critical for Ecosystems

• Solar-powered evaporation drives the water cycle, moving water into the atmosphere in gaseous form. Condensation and precipitation return water to Earth’s surface. Gravity-driven flows move water from land to lakes and oceans. Review Figure 45.6 and ANIMATED TUTORIAL 45.1
• The availability of nitrogen to living organisms depends on nitrogen fixation and other biochemical processes carried out by microbes. These processes convert gaseous N₂ into forms that other organisms can use. Other microbial processes return N₂ to the atmosphere. Review Figure 45.7 and ANIMATED TUTORIAL 45.2
• Human activities add various chemical forms of nitrogen to the atmosphere, where they contribute to smog and acid rain, to terrestrial systems, where they act as fertilizers, and to aquatic systems, where they cause eutrophication. Review Figure 45.9
• Photosynthesis and respiration move carbon between inorganic and organic compartments. As a result, energy flow and the flux of carbon through biological communities are intimately linked.
• Some carbon absorbed from the atmosphere by ocean waters is taken up through photosynthesis, some precipitates as carbonate compounds, and some sinks to the benthic zone as dead organic material. Carbonate compounds and organic material that accumulate in sediments and soils are transformed into carbonate rocks and fossil fuels.
• Humans are increasing the carbon pool in the atmosphere by burning fossil fuels, manufacturing cement, raising livestock, and raising wetland crops. Review Figure 45.10 and ANIMATED TUTORIAL 45.3
• The biogeochemical cycles of different materials are linked when the same physical, chemical, or biological processes drive their fluxes. Review ACTIVITY 45.1

Concept 45.4   Biogeochemical Cycles Affect Global Climate

• Earth absorbs incoming solar radiation and reemits it at infrared wavelengths. Some of this infrared radiation is absorbed by atmospheric gases and is reradiated back to Earth’s surface. The resulting retention of heat energy within the Earth system is called the greenhouse effect. Review Figure 45.11 and ANIMATED TUTORIAL 45.4
• Gases such as water vapor, carbon dioxide, methane, and nitrous oxide that absorb infrared radiation are called greenhouse gases. The pools of these molecules in the atmosphere influence Earth’s radiation budget.
• Atmospheric concentrations of CO₂ and other greenhouse gases have been increasing rapidly since about 1880, and average annual global temperatures have followed suit. Review Figure 45.12 and Figure 45.13
• Global warming is changing Earth’s climates. High latitudes are warming more than low latitudes, precipitation patterns are changing, and storm intensities are increasing. Review Figure 45.14
• Computer models of the Earth system show that human activities have contributed significantly to the recent warming of the climate.

Concept 45.5   Rapid Climate Change Affects Species and Communities

• Climate change is altering the timing of some seasonal environmental cues but not others. Although species sometimes can evolve new responses to such cues, the rate of evolution will not keep up with an environment that changes too rapidly.
• Because different species respond to different seasonal cues, altered cues result in timing mismatches among species in a community and thus disrupt their interactions.
• Climate change is altering the distributions and abundances of species, resulting in the assembly of novel communities.
• Climate change is increasing the frequency of extreme weather events, which can cause sudden shifts in species distributions and in community composition.

Concept 45.6   Ecological Challenges Can Be Addressed through Science and International Cooperation

• Humans are causing major changes in the biosphere and in other aspects of the Earth system. However, we are also uniquely equipped to address these changes, not only because science enables us to understand the natural world and to devise solutions to problems, but also because of our capacity for cooperative action. Review ACTIVITY 45.2

See ACTIVITY 45.3 for a concept review of this chapter.