Ecosystem Structure and Dynamics Notes

Ecosystem Structure and Dynamics

Levels of Ecological Study

• Ecologists study environmental interactions at various levels:

  • Organismal ecology

  • Population ecology

  • Community ecology

  • Ecosystem ecology

Ecosystem Components and Thermodynamics

• Ecosystems consist of biotic (living) and abiotic (non-living) components linked by energy and nutrient flows through the biosphere.

• First law of thermodynamics: Energy cannot be created or destroyed, only transferred or transformed.

• Law of conservation of mass: Matter cannot be created or destroyed.

• If nutrient outputs exceed inputs, that nutrient limits production.

• Decomposers (heterotrophs) obtain energy from detritus (nonliving organic matter).

• Prokaryotes and fungi are main decomposers.

Energy Flow in Ecosystems

• Energy flows from autotrophs through other organisms as biomass.

• Energy dissipates as heat and is no longer usable; nutrients cycle.

• Organisms at the same energy source level occupy the same trophic level.

Energy Flow Metrics

• Primary producer: organism that synthesizes food from inorganic sources.

• Gross Primary Productivity (GPP): Total chemical energy produced in an area.

• R: Energy used in cellular respiration.

• Net Primary Productivity (NPP): Energy invested in growth and reproduction (biomass).

• NPP = GPP - R

Energy and Biomass

• Plants use 0.8% of incoming sunlight (compared to 22% for solar panels).

• 45% of GPP becomes biomass; 55% is lost through respiration.

Factors Affecting Photosynthesis

• Photopigments absorb only a fraction of available light wavelengths.

• Photosynthetic rates in temperate biomes reduce drastically in winter.

• Enzyme efficiency is temperature dependent.

• Photosynthesis stalls in dry conditions.

NPP in Various Biomes

• Tropical wet/dry forests cover <5% of Earth but account for >30% of total NPP.

• Most productive aquatic biomes: algal beds, coral reefs, wetlands, estuaries.

• Open ocean NPP is low, but its vastness results in high total NPP.

• Humans appropriate 24% of potential NPP: 53% is harvested, 40% prevented by land use, 7% destroyed by fires.

Climate Change and NPP

• Climate change affects whether ecosystems store or lose carbon.

• Terrestrial NPP increased from 1982-1999 due to decreased cloud cover over tropical forests.

• If Net Ecosystem Production (NEP) > 0, the ecosystem stores carbon (carbon sink).

• If NEP < 0, the ecosystem is a carbon source.

Monitoring and Impacts of Climate Change on NPP

• Ecologists monitor climate change effects on NPP.

• While NPP increased in some regions, it has decreased globally.

• Increased CO_2 should increase photosynthesis, but drought has decreased NPP in vast areas.

Ocean NPP

• NPP is decreasing in oceans.

• Increased temperature causes surface water density to be lower than benthic water density, preventing nutrient transfer from the benthic zone to the surface.

• Changes in NPP, precipitation, and ocean acidification alter food web dynamics and ecosystem function.

Biomass Pyramids and Efficiency

• Biomass pyramid: tiers represent dry mass of organisms in one trophic level.

• Productivity: biomass produced per unit area per year.

• Efficiency: fraction of biomass transferred from one trophic level to next (typically about 10%).

Production Efficiency

• Production efficiency: fraction of energy stored in food not used for respiration.

• Production \ efficiency = \frac{Net \ secondary \ production}{Assimilation \ of \ primary \ production} \times 100\%

• Net secondary production: energy consumed and used for growth and reproduction.

• Assimilation: total energy consumed and used for growth, reproduction, and respiration.

Trophic Efficiency

• Trophic efficiency: percentage of production transferred from one trophic level to the next.

Variation in Ecological Efficiency

• "10 percent rule" masks variation.

• Large mammals: smaller surface-area-to-volume ratio → less heat loss → more efficient biomass producers.

• Ectotherms: spend less energy on temperature homeostasis → more efficient biomass producers.

• It is more efficient for humans to feed at lower trophic levels.

Nutrient Limitation

• Nutrients limit primary production in most oceans and lakes.

• Nitrogen and phosphorus often limit marine production.

• Eutrophication: primary production increases as an ecosystem changes from nutrient-poor to nutrient-rich.

• Upwelling: deep, nutrient-rich waters circulate to the ocean surface, stimulating phytoplankton growth.

Decomposition

• Decomposers play a key role in chemical cycling.

• Decomposition rate is controlled by temperature and precipitation.

• Rapid decomposition areas can have low soil nutrient levels due to rapid cycling.

• Decomposition is slow in anaerobic bottom sediments and cold, wet ecosystems like peatlands.

Biogeochemical Cycles

• Biogeochemical cycle: path an element takes as it moves from abiotic systems through organisms and back.

• Energy is transferred when one organism eats another.

• Essential nutrients transferred: C, N, P, S, Ca, etc.

Cycling of Chemicals

• Chemicals cycle between organic matter and abiotic reservoirs.

• Ecosystems are supplied with energy from the Sun and Earth's interior.

• Life depends on recycling of chemicals.

• Biogeochemical cycles cycle chemicals between organisms and the Earth, locally or globally. (breathing out CO2 which contributes to biogeochemical cycle)

• Decomposers play a central role.

Water Cycle

• The global water cycle starts with water evaporating from the ocean and precipitating back.

• Water vapor moves over continents, augmented by evaporation and transpiration.

• Water moves from land to oceans via streams and groundwater.

• Oceans contain 97% of biosphere's water; 2% is in glaciers/ice caps, 1% in lakes, rivers and groundwater.

• The water cycle can amplify the accumulation of nutrients.

Groundwater

• Water table: upper limit of underground soil saturated with water.

• Aquifer: underground layer of water-bearing materials; groundwater extracted via wells.

• Asphalt and concrete reduce water reaching aquifers.

• Irrigated agriculture, household, and industrial use remove vast amounts of groundwater.

Nutrient Cycling Studies

• Hubbard Brook Experimental Forest: Study of nutrient cycling.

• Plant roots prevent soil erosion.

• Soil loss is hard to reverse; soil takes a long time to form.

• Clear-cutting experiment: watershed was clear-cut to determine effects on drainage and nutrient cycling; a dam site monitored water and mineral loss.

Ecological Restoration

• Biological communities can recover from disturbances over time.

• Restoration ecology: initiates or speeds up degraded ecosystem recovery.

• Physical structure restoration may be needed before biological restoration.

Nitrogen Cycle (slide 25)

• The nitrogen cycle depends on bacteria.

• Nitrogen is essential for proteins and nucleic acids.

• Nitrogen abiotic reservoirs: air and soil.

• Nitrogen fixation converts N2 to nitrogen used by plants (NH4, NO_3) via bacteria and cyanobacteria.

• Moves nitrogen from place to place.

Eutrophication & Nitrogen Pollution

• Eutrophication: overfertilization → algal blooms in aquatic ecosystems → oxygen-free “dead zones”.

• Nitrogen pollution from fossil fuels: acid rain, climate change, ozone depletion.

• Added nitrogen can increase productivity short term but decrease species diversity.

Phosphorus Cycle

• The global phosphorus cycle tracks phosphorus movement among terrestrial and aquatic ecosystems.

• Main phosphorus reservoir: Earth’s crust, mobilized by rock weathering.

• Human activity (mining, fertilizing) increases phosphorus in biogeochemical cycles, often causing eutrophication.

Carbon Cycle

• The carbon cycle depends on photosynthesis and respiration.

• Carbon is the main ingredient of all organic molecules.

• CO_2 returns to the atmosphere via respiration, balancing removal by photosynthesis.

• The carbon cycle is affected by burning wood and fossil fuels.

• unfixed carbon—→ to Biomass

Global Carbon Cycle Reservoirs

• The global carbon cycle involves carbon movement among terrestrial ecosystems, oceans, and the atmosphere.

• The ocean is the largest reservoir.

• The atmospheric reservoir is small but important due to the rapid carbon movement into and out of it.