Dynamics of Ecosystems

Chapter 55

Chemical Components of Ecosystems

•Living organisms composed primarily of “rare” elements and molecules

–CHNOPS

–Water

•Cycling of components allows them to be reused nearly indefinitely

–Otherwise would have “run out” long ago

•Often limit growth when in short supply

The Water Cycle

•Oceans - H₂O evaporates into atmosphere.

•Land – transpiration of water from plants.

–98% of water circulating between the atmosphere and the oceans.

–2% in living continental ice, lakes, rivers, groundwater, living organisms

•<0.0001% as river/lake water

Water cycle

•Evaporation – water from oceans & surface water converted to water vapor

–Transpiration – plants converting ground water/soil moisture into water vapor

•Condensation – water vapor condenses into clouds

•Precipitation – rain, snow, sleet or hail falling to surface of Earth

–Most falls back into oceans

–On land, either stored in snow/ice, lakes, groundwater or flows toward oceans in streams

Stream Water

•Mostly surface runoff following rain events

–~42% of North American precipitation becomes river water

–Melting snow/glaciers in some areas

–Most surface water takes <20 days to reach ocean

•Groundwater contributes to stream flow

–Stream beds usually at or slightly below water table

–Increased proportion of stream flow farther from rain events

The Water Cycle

•Groundwater

–Large proportion of fresh water many parts of US.

•Upper portion (water table) is partially accessible to plants

•Recharges rivers during drought

•Deep layers “mined” by humans.

–Slowly recharged from rainfall

–Being withdrawn faster than recharged

Altered Water Cycle

•Forest ecosystems transpire majority of moisture back into the atmosphere.

–Keeps water vapor levels high, increases rainfall

–Deforestation causes rapid water drainage

•Increased flooding

•Erosion of topsoil

•Reduced atmospheric water vapor = Desertification

The Carbon Cycle

•CO₂ = 0.03% of the atmosphere.

–Heterotrophs obtain carbon indirectly from photosynthetic organisms.

•organic compounds formed from carbon dioxide fixation

•broken down by metabolism

•released back into atmosphere as CO₂

•re-deposited in soil/sediment during decomposition

Atmospheric Carbon

•~ 700 billion metric tons CO₂ located in atmosphere

•~ 1 trillion metric tons dissolved in oceans.

–Fossil fuels contain 5 trillion metric tons.

•Burning as fuel increasing atmospheric CO₂

The Nitrogen Cycle

•Few organisms (bacteria) fix N₂ into useful forms.

–Requires much energy

•Nitrogen Fixation: N₂+ 3H₂ à 2NH₃

–Ammonification/Nitrification

–Component of Amino acids and Nucleic acids

•re-deposited in soil/sediment during decomposition.

The Phosphorus Cycle

•PO₄ released from weathering rocks into soils & water

•Absorbed by plants (eaten by animals)

–Component of Nucleic acids and phospholipids

•re-deposited in soil/sediment during decomposition.

–Fertilizers (usually w/ P & N) increase crop production (excess drains into streams)

Trophic Levels

•Autotrophs - Capture light energy and manufacture own chemical energy.

–Producers

•Heterotrophs - Must obtain organic molecules by consuming autotrophs or other heterotrophs.

–Consumers

Trophic Levels

–Primary Consumers - Herbivores

–Secondary Consumers - Organisms that feed primary consumers.

–Tertiary Consumers – Feed on secondary consumers

–Detritivores & Decomposers – Feed on dead material

•recycle complex organic material into simpler compounds.

Trophic Levels

•Trophic Level = feeding level of an organism.

–Organisms from each trophic level = food chain.

•Interconnected food chains = food web.

•~ 10% of energy transfers from one trophic level to the next.

Primary Productivity

•Primary Productivity = amount of organic matter produced from solar energy

•Secondary Productivity = production by heterotrophs

The Energy in Food Chains

•Second Law of Thermodynamics

•food chains 3-4 steps (trophic levels)

–Usually more in aquatic systems than terrestrial

Ecological Pyramids

•Fewer individuals at each successive trophic level.

–Total biomass decreases with increasing trophic levels

Ecological Pyramids

•Top Carnivores

–relatively large = biomass available at the top of the pyramid concentrated in few individuals.

Interactions Among Trophic Levels

•Trophic Cascading = change in one trophic level affects other trophic levels.

Effects of Species Richness

•Species richness = # of species in given area

•Species-rich communities resist disturbance

–More stable

Variables Associated w/ Species Richness

•Ecosystem Productivity = intermediate levels of productivity with most species.

•High productivity often due to domination and exploitation by a few species

Habitat Diversity and Complexity

•Spatial Heterogeneity = greater variety of microhabitats and microclimates.

Warmer temperatures

•Climate = warm temperatures associated w/ greater species richness

•Long term stability important

Biogeographic Patterns of Species Diversity

•General increase in species richness from the arctic to the tropics.

–Not hard & fast rule

•Diversity in the Tropics

–Evolutionary Age (geologic stability)**

•Fewer “extinctions”

–Predictability (climatic stability)

–Spatial Heterogeneity

–Predation

–Higher Productivity