Concept 46.1 Climate and Nutrients Affect Ecosystem Function

• An ecological community in its abiotic context is an ecosystem.
   
• A community's rate of biomass production per unit of area—its net primary productivity (NPP)—is related to its exchange of energy and materials with its abiotic surroundings, and thus is a measure of ecosystem function.
   
• NPP varies considerably among ecosystem types. Review Figure 46.1
   
• Variation of terrestrial NPP shows that it is influenced by both temperature and precipitation. Soil nutrients also play a role in terrestrial NPP. Review Figures 46.2 and 46.3
   
• Aquatic NPP varies most strongly with availability of light and nutrients. Review Figure 46.4
   

Concept 46.2 Biological, Geological, and Chemical Processes Move Materials through Ecosystems

• The molecular form and physical state in which an element exists and the physical compartment of the biosphere in which it is located determine whether the element is accessible to life.
   
• The transformations of materials and their movements among compartments are called biogeochemical cycles. Review Figure 46.5
   
• 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 46.3 Certain Biogeochemical Cycles Are Especially Critical for Ecosystems

• Evaporation drives the water cycle, moving water into the atmosphere in gaseous form; precipitation returns liquid water to Earth's surface. These transitions between gaseous and liquid states involve exchanges of heat energy between water molecules and their surroundings. Gravity-driven flows move water from land to the oceans, transporting many other materials as well. Review Figure 46.6 and ANIMATED TUTORIAL 46.1
   
• The availability of nitrogen to living organisms depends on nitrogen fixation and other biochemical processes carried out by microbes. These processes change gaseous N₂ into forms that primary producers can absorb and that are recycled locally, and return N₂ to the atmosphere. Review Figure 46.7 and ANIMATED TUTORIAL 46.2
   
• Human activities add various chemical forms of nitrogen to the atmosphere and to aquatic systems, contributing to smog and causing eutrophication and oxygen-poor dead zones devoid of life. Review Figure 46.9
   
• Photosynthesis and respiration move carbon between the inorganic and organic compartments of the biosphere. As a result, energy flow and the flux of carbon through biological communities are intimately linked.
   
• Some carbon absorbed from the atmosphere by surface waters is taken up through photosynthesis, some precipitates as calcium carbonate, and some falls to the benthic zone as organic detritus. Calcium carbonate and detritus that accumulate in sediments and soils are transformed into carbonate rocks and fossil fuels.
   
• Use of fossil fuels by humans increases the carbon content of the atmosphere. Review Figure 46.10 and ANIMATED TUTORIAL 46.3
   
• The biogeochemical cycles of different materials can interact in ways that are hard to predict.
   

Concept 46.4 Biogeochemical Cycles Affect Global Climate

• Earth absorbs incoming solar energy and re-emits it in infrared wavelengths. Some of this infrared radiation is absorbed by atmospheric gases and is re-radiated back to Earth's surface. The resulting retention of heat within the Earth system is called the greenhouse effect. Review Figure 46.11 and ANIMATED TUTORIAL 46.4
   
• Gases such as water vapor, carbon dioxide, methane, and nitrous oxide that absorb strongly in infrared wavelengths are called greenhouse gases. Fluxes of these molecules into and out of the atmosphere influence Earth's radiation balance.
   
• Atmospheric concentrations of CO₂ and other greenhouse gases have been increasing since about 1880, and average annual global temperatures have followed suit. Review Figures 46.12 and 46.13
   
• 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 46.14
   
• Computer models of the Earth system show that human activities have contributed significantly to the recent warming of the climate.
   

Concept 46.5 Rapid Climate Change Affects Species and Communities

• Climate change is altering the timing of some seasonal environmental cues but not others. Although species 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 distribution and abundance 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 46.6 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 WEB ACTIVITY 46.1
   

See WEB ACTIVITY 46.2 for a concept review of this chapter.