Ecology Lecture No. 24: Global Ecology
Thursday November 29 , 2012h
-Movements of biologically important elements are linked at a global scale that transcends ecological
boundaries. Atmospheric emissions of pollutants, dust, and greenhouse gases have caused widespread
environmental problems. A major focus of global ecology is the study of the environmental effects of
Global Biogeochemical Cycle:
-Elements move among geological, atmospheric, and biological pools at a global scale. The global cycling
of carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) are emphasized because of their biological
importance, and their roles in human alteration of the global environment. The pool, or reservoir, is the
amount of an element in a component of the biosphere. The flux is the rate of movement of an element
between pools. For example, terrestrial plants are a pool for carbon, while photosynthesis represents a
The Carbon Cycle:
-Major pools of C are in the atmosphere, oceans, land surface (includes soils and vegetation), and
sediments and rock. 99% of global C is in sediments and rock, the most stable pool; fluxes occur on
geological time scales. Ocean surface water takes up CO from the atmosphere by diffusion. C is
transferred to deeper water mostly as organic detritus and carbonate shells. Upwellings bring C-rich
water to the surface, releasing CO to2the atmosphere. In a terrestrial pool soils contain twice as much C
as plants. CO 2s exchanged with the atmosphere mostly by photosynthesis and respiration. Prior to the
Industrial Revolution, these two fluxes were roughly equal, with no net change in atmospheric CO2.
Anthropogenic Release Of Carbon:
-Anthropogenic release of C to the atmosphere from the terrestrial pool results from land use change,
mostly deforestation (20%); and from the burning of fossil fuels (80%). Before the mid-nineteenth
century, deforestation was the main anthropogenic flux. Removing the forest canopy warms the soil,
increasing rates of decomposition and respiration. Burning trees also releases CO , and s2all amounts of
CO and CH . 4n the twentieth century, major deforestation shifted from the mid-latitudes to the tropics.
About half of CO e2issions is taken up by the oceans and terrestrial biota. But this proportion will
decrease because terrestrial and ocean uptake will not keep pace with the rate of atmospheric increase.
Rising CO L2vels:
-Higher concentrations of CO may2stimulate photosynthesis. But experiments have shown that
increased photosynthetic rates may be short lived, and plants will acclimate to higher concentrations.
For forest trees, increased CO up2ake may be sustained longer. One method of testing effects of elevated CO u2es free-air CO enr2chment, or FACE. CO injecte2 into the air through vertical pipes that
surround a stand of trees. Rate of injection is controlled to achieve a particular concentration of CO . 2
Atmospheric CO aff2cts ocean pH by diffusing in and forming carbonic acid. Ocean acidity has increased
by 30% over the last century. Further increase is predicted by models. Many marine organisms form
shells of carbonate. Increasing acidity will dissolve existing shells and lower carbonate concentrations
will decrease the ability to synthesize new shells.
The Nitrogen Cycle:
-N is a constituent of enzymes and proteins, and often limits primary productivity. N and C cycles are
tightly coupled through the processes of photosynthesis and decomposition. The largest N pool is
atmospheric N , 2hich is not available to most organisms. Nitrogen-fixing bacteria are able to convert it
to a useable form. Fixed N compounds are called reactive (they can participate in chemical reactions).
Humans have altered the N cycle even more than the C cycle. The rate of N fixatio2 by humans now
exceeds natural terrestrial biological rates. Emissions of N from industrial and agricultural activities
cause widespread environmental changes, including acid precipitation.
The Phosphorous Cycle:
-P can be limiting to primary productivity in aquatic ecosystems and some terrestrial ecosystems. P
availability can control the rate of nitrogen-fixation, which has a high metabolic demand for P. The C, N,
and P cycles are linked through photosynthesis and NPP, decomposition, and N fixation2 P has
essentially no atmospheric pool, except as dust. The largest pools are in terrestrial soils and marine
sediments. P is internally cycled in ecosystems between uptake by plants and microorganisms and
release by decomposition. In terrestrial ecosystems, most P loss occurs by occlusion (transformation to
insoluble, biologically unavailable forms).
-P in aquatic systems is lost to the sediments. This is cycled again w