bio326 : Ecosystem : Ecosystem Patterns : Lesson

Glossary terms that are important in this lesson: Eutrophication, nutrient use efficiency, ozone, ozone holes, steady-state, system, systems ecology.

I. Regulation of ecosystem function

1. Introduction
   • Ecosystem function is influenced by energy assimilation and nutrient assimilation which in turn depend on physical conditions
   • Primary production is correlated with temperature, precipitation, nutrient availability, and other physical factors but how they act alone and in concert is poorly understood
   • Models of real systems provide insight into ecosystem function
     • Examine output of model with respect to different inputs and conditions
     • Requires detailed knowledge of critical processes
     • Model's equations are validated by observation and experiment
     • Provide a concise and rigorous statement of understanding of a system
     • Guides experimentation and observation
     • Basis of systems of ecology: how interdependent parts contribute to the functioning of an entire system

2. External factors and internal controls
   • A system is a group of interacting components making a unified whole
   • External factors are energetic and material inputs from outside the system and physical conditions of the environment that influence the system's structure and function
   • Internal controls result from the way the components interact and, so, are part of the system's structure
   • Behavior of the system results from variation in external factors and the unique internal properties of the system

3. Energy and element fluxes in ecosystem function
   • Energy is a common currency of ecosystem structure and function but seems to cause little variation among ecosystems
   • Ecosystem structure and function respond to external factors, particularly temperature and precipitation
     • Hypothesis: Predominant control of energy flow is exerted by the element whose cycling is most strongly correlated with primary production
     • Evidence: measurements of litter production and several elements in the litter showed the strongest correlation between litter production and nitrogen
     • Conclusion: factors regulating nitrogen cycling predominantly control primary production in forests
   • Production per unit of nutrient cycled increases with lower rates of cycling, i.e., trees are more efficient with less nutrient
     • Hypothesis 1: trees assimilate more energy per unit of nutrient assimilated
     • Hypothesis 2: trees conserve nutrients before dropping their leaves

4. Development of systems models
   • A (nutrient) compartment contains an amount with inflows and outflows
   • Each flux (flow) may be constant or vary depending on other factors
   • The change in the amount of material or energy in a compartment depends on the relative sizes of the fluxes
   • Steady-state occurs when inflow equals outflow
   • The flux through the compartment depends only on the inflow
   • The global water cycle can be represented by a simple model
   • Lotka's model of ecosystem function
     • Model of nutrient cycling with three compartments
     • The structure and function of the system are defined completely by the transfer functions, which incorporate external factors and internal controls
     • The lowest transfer rate defines the character of the system by creating a bottleneck in the flow
     • Change in one segment of a nutrient cycle can alter the function of the entire system
   • An aquatic systems model
     • The sizes of some compartments (particulate and dissolved organic nitrogen, nitrate) changed dramatically with the seasons in Lake Ontario
     • A simple first-order model estimated values for transfer rates among forms of nitrogen and focused on the interactions among compartments responsible for observed changes
     • Simple models can direct inquiry into more general comparisons of function between ecosystems
   • A terrestrial systems model
     • Model of nitrogen cycling in an English scots pine plantation and in a tropical forest in Ghana in which the annual shedding of leaves and mineralization of detritus are represented by first order equations while nitrogen assimilation is a second order equation
     • Transfer rates for scots pine were equivalent but mineralization rate in Ghana was 100 times greater: external factors influence ecosystem function in Ghana through effects on microorganisms rather than on trees
     • Assimilation coefficient for phosphorus in Ghana is much higher than the rate of loss from vegetation: tropical trees are more efficient in their use of phosphorus than temperate trees are

II. Development and global ecology

1. Introduction
   • Biodiversity requires natural habitat
   • Conflict between human population growth and survival of other species
   • Rational development of natural resources
     • Human population will continue to grow and put increasing pressure on the planet
     • Healthy natural state depends on basic ecological principles
     • Some exploitation and management practices are more friendly than others
     • Greater cooperation among humans will be necessary
     • Both short-term and long-term costs of population growth and ecological mismanagement should be evaluated and assigned to the goods and services that produce them

2. Ecological processes
   • Energy flow and nutrient cycling are important in both natural and managed ecosystems
   • Negative-feedback mechanisms restore systems after perturbation
     • Decomposition of detritus stimulates decomposers
     • Increase in herbivores leads to declining birth rates and rising mortality
   • Sustainable biosphere requires the conservation of the ecological processes responsible for its productivity

3. Threats to ecological processes
   • Direct: maximizing the harvest from fisheries potentially exterminates fish populations
   • Indirect: clear-cutting increases erosion which degrades streams and coastal marine areas
   • Overexploitation
     • Classic consumer-resource interactions maintain a steady-state balance between exploitation and replenishment
     • Human technology interrupts negative-feedback, permitting exploitation to continue to exhaust a resource beyond recovery
       • Tropical productivity requires vegetative cover to maintain fertility
     • Human activities need to be tied to the ability of local resources to support them
   • Introduction of alien species
     • Disrupt local biota and ecosystem function in unpredictable ways
     • Islands are particularly vulnerable because of their comparatively low diversity and simple community structure
   • Habitat conversion
     • Upsets natural processes of regeneration and control, leads to ecological disaster
       • Cutting tropical forests increases erosion and eliminates fertility
       • Altering soil structure by plowing American prairies led to a dust bowl
       • Clearing mangroves removed natural protection of coastline from hurricanes
       • Damming rivers changes sediment transport and blocks fish migrations
   • Irrigation
     • Tremendous short-term benefits: increases productivity
     • Extensive long-term costs: altered environment, lowered water table, pollution, salinization, spread of water-borne disease
   • Fertilization and eutrophication
     • Agricultural fertilizers increase productivity
     • Enter ground water, rivers, lakes, increasing productivity and upsetting natural cycles of nutrient regeneration and energy flow
     • Effects can often be reversed by blocking inputs
   • Accumulation of toxins in the environment: poison the habitat
     • Acids
       • Direct effects on organisms by interfering with physiological functions
       • Indirect effects by influence on nutrient availability and regeneration
       • Acid mine drainage results when sulfur bacteria oxidize sulfur and thiol forms of sulfur to sulfates
       • Acid rain: sulfur and nitrogen in fuels are converted to acids in the atmosphere and are deposited at low pH
     • Heavy metals from numerous human activities are toxic to plants and animals
     • Organic compounds kill pests but accumulate in non-target organisms and persist in the environment
       • Some toxic organics can be removed by bioremediation
       • Biological control often provides effective regulation of pests
       • Oil spills disrupt biological membranes and effects can last for decades
     • Radiation
       • Background levels occur everywhere but are higher at certain locations
       • Human activities concentrate radioactive materials with long lifetimes
       • UV is increasing as the protective ozone layer diminishes

4. Global threats
   • Impact extends far beyond the source area
   • Ozone layer and UV radiation
     • Ozone is a highly reactive molecule of oxygen which is toxic to life even in small concentrations
     • Ozone is a photochemical product of air pollution containing NO₂
     • Ozone in the upper atmosphere absorbs UV radiation that damages organisms on the earth's surface
       • Equilibrium concentration of ozone is reduced by chloroflurocarbons (CFC's) from human activities and contribute to ozone holes
       • Societal agreements to eliminate CFC's may restore the natural equilibrium of ozone in the upper atmosphere in 100-200 years
   • Carbon dioxide and the greenhouse effect
     • Greenhouse gases (CO₂, CH₄, CFCl₃) capture infrared radiation heading from earth to space and reradiate part back to earth
       • Allows shortwave radiation from the sun to reach the earth
       • Prevents some longwave radiation from leaving and helps maintain a "liveable" surface temperature
     • Warming effect is increasing more rapidly than organisms may be able to accomodate
     • Possible consequences
       • Rise in sea level
       • Changes in agriculture
       • Increasing desertification

5. Human ecology
   • Human population is out of balance with its environment
     • Resources are being consumed faster than they are being regenerated
     • Waste production exceeds the ability of the biosphere to maintain environmental quality
   • Human population has temporarily escaped ecological feedback mechanisms
   • The ability of the human population to restore balance depends on education and understanding of basic ecological principles
   • If humans do not restore the balance voluntarily, nature will restore it

When you have completed this lesson, go on to Review Questions