Ecosystems and Biogeochemical Cycles

Ecosystems

• An ecosystem includes all the organisms in a particular place, plus the abiotic environment.

Biogeochemical Cycles

• Biogeochemical cycles involve chemicals cycling within ecosystems, influenced by both biotic and abiotic processes.

Energy Flow

• Energy is converted to biological energy (usually carbon fixation by photosynthesis) and flows through an ecosystem.

Water Cycling

• Water largely determines the composition of communities.

• Water in the atmosphere exists as a gas.

• In terrestrial ecosystems, 90% of evaporation is through plants (transpiration).

• Water cools and falls to the surface as precipitation.

• It then flows to the ocean or is trapped as groundwater.

• Aquifers hold 95% of the fresh water used in the United States.

Carbon Cycle Changes

• Earth's reserves of coal and fossil fuels built up over geological time.

• Human burning of fossil fuels is creating imbalances in the carbon cycle.

• The concentration of CO_2 in the atmosphere is increasing rapidly.

Geologic Cycling of CO_2

• Early atmosphere had high CO_2 levels.

• CO2 + water in the air -> carbonic acid (H2CO_3).

• Carbonic acid + rocks removes CO_2 from the atmosphere and flows to the ocean floor to form rock.

• Volcanism releases CO_2 and other greenhouse gasses back into the atmosphere.

• CO2 combines with H2O to form carbonic acid: CO2 + H2O
  ightharpoonup H2CO3.

• Carbonic acid reacts with rocks.

• Calcium and bicarbonate form calcium carbonate which precipitates: Ca^{2+} + HCO3 ightharpoonup CaCO3.

Nutrients

• Living things need more than water and carbon.

• Limiting nutrients are in shortest supply relative to organism needs.

• Nitrogen and phosphorus are common limiting nutrients for terrestrial and aquatic ecosystems.

• Iron is the limiting nutrient for algal populations in about 1/3 of the world's oceans.

Nitrogen

• Nitrogen is a component of all proteins and nucleic acids.

• The atmosphere is 78% nitrogen, but most plants and animals cannot use N_2 (gas).

• They get nitrogen from ammonia or nitrates.

• Microbes are relied on for nitrification: Nitrogen (N2) -> Ammonia (NH3) -> Nitrates (NH_3).

• Recycling occurs by denitrification.

Nitrogen and Agriculture

• Nitrogen gets removed when crops are harvested.

• Legumes (peas, soybeans, peanuts) have symbiotic relationships with nitrogen-fixing bacteria in their roots.

• Crop rotation is used to naturally reintroduce Nitrogen.

• Nitrogenous fertilizers are produced using natural gas (a source of greenhouse gas).

• Humans have doubled the rate of transfer of N_2 in usable forms into soils and water.

Runoff

• Fertilizer overuse and runoff -> extra nutrition -> too much marine algae -> not enough Oxygen for other life.

• The Gulf of Mexico dead zone is caused by Mississippi runoff.

Phosphorus

• Phosphorus is required by all organisms.

• It occurs in nucleic acids, membranes, and ATP.

• Phosphorus has no significant gases that form in ecosystems; it exists as phosphate.

• Plants and algae use free inorganic phosphorus.

• Often affected by water flow.

• Animals eat plants to obtain their phosphorus.

Iron as a Limiting Nutrient

• When wind brings in iron-rich dust, algal populations proliferate, provided the iron is in a usable chemical form.

• Sand storms in the Sahara Desert can increase algal productivity in Pacific waters.

First Law of Thermodynamics

• Energy is neither created nor destroyed; it changes forms (light, chemical-bond energy, motion, heat).

Second Law of Thermodynamics

• Whenever organisms use chemical-bond or light energy, some is converted to heat (entropy).

• Earth functions as an open system for energy.

• The sun is our major source of energy.

Trophism

• Autotrophs (“self-feeders”) synthesize organic compounds from inorganic precursors.

  • Photoautotrophs use light as an energy source.

  • Chemoautotrophs use energy from inorganic oxidation reactions (prokaryotic).

• Heterotrophs cannot synthesize organic compounds from inorganic precursors.

  • Animals eat plants and other animals.

  • Bacteria and fungi decompose.

Trophic Cascade

*Sun -> Primary producers -> Herbivores -> Primary carnivores -> Secondary carnivores -> Detritivores.

Productivity

• Productivity: the rate at which the organisms in the trophic level collectively synthesizes organic matter to be used for the next trophic level.

• Primary productivity: productivity of the primary producers.

  • Sets energy budget for an ecosystem.

  • All organisms must rely on this source of energy.

Ecosystem Productivity

• Examples with NPP per Unit Area (g dry matter/m²/yr) and World NPP (10^{12} kg dry matter/m²/yr):

  • Extreme desert to algal beds and reefs.

GPP and NPP

• Gross primary productivity (GPP): raw rate at which primary producers synthesize new organic matter (around 1% of all solar energy).

• Net primary productivity (NPP): GPP less the respiration of the primary producers.

• Secondary productivity: productivity of a heterotroph trophic level.

10% Rule

• The amount of chemical-bond energy decreases as energy is passed from one trophic level to the next.

• Rule of thumb - about 10% of energy at one level made available to next level.

• 17% ingested energy is converted into insect biomass. Some is available to next consumer.

  • 17% growth, 33% cellular respiration, 50% feces.

Limits on Top Carnivores

• The number of trophic levels is limited by energy availability.

• Exponential decline of chemical-bond energy limits the lengths of trophic chains.

• Only about 1/1000 of the energy captured by photosynthesis passes all the way through to secondary carnivores.

10% Rule and Diet

• Humans are omnivores - get food from many trophic levels.

10% Rule and Diet

• Warm-blooded animals use up lots of respiration to regulate body temperature.

  • Microbes: ~40%

  • Insects: 10-40%

  • Fish & Reptiles: ~10%

  • Endotherms (birds and mammals): 1-3%

Ecological Pyramids

• Trophic relationships often depicted as pyramids.

• Energy flow/productivity must decrease per level and look like a pyramid.

Ecological Pyramids

• Pyramid of Energy Flow (Productivity).

• Energy hard to measure - easier to estimate biomass (total dry weight of organisms) or count number of organisms.

• Often are similar to energy pyramids.

Inverted Pyramids

• Sometimes biomass or numbers pyramids can be inverted, with lower levels being smaller.

  • Numbers – very large organisms have low numbers.

  • Trees - Primary producers in old growth forest have much smaller numbers than herbivore insects.

Inverted Pyramids

• Marine biomass pyramids often inverted.

• Resource constrained, so phytoplankton are often consumed immediately - quickly converted to primary consumers.

Trophic-Level Interactions

• Trophic cascade: effects exerted at one level affect two or more nearby levels.

• Top-down effects: when effects flow down.

• Bottom-up effects: when effect flows up.

Top-Down Effects: Simple

• Stream enclosures with large carnivorous fish have fewer primary carnivores, more herbivorous insects, and a lower level of algae

Top-Down Effects: Four-Level

• Primary producers - Algae.

• Primary consumer - herbivorous insects.

• Secondary consumers - carnivorous damselfly nymph.

• Tertiary consumers - fish.

Trophic Cascade: Two States

• Along the West Coast the sea otter/sea urchin/kelp system exists with low or high sea otter populations.

Tropical Regions

• Tropical regions have the highest diversity.

• Species diversity cline: biogeographic gradient in number of species correlated with latitude.

• Reported for plants and animals.

Island Biogeography

• Larger islands have more species.

• MacArthur-Wilson equilibrium model (Robert MacArthur and Edward O. Wilson).

• Islands have a tendency to accumulate more and more species through dispersion from mainland.

• Pool of potential colonizing species becomes depleted over time.

• More species on an island mean more to go extinct.

• At some point, extinctions and colonizations should be equal.

MacArthur and Wilson Equilibrium Model

• Further distance to mainland -> less dispersion -> less species diversity.

• Greater Island size -> larger populations -> harder to drive to extinction -> more species at equilibrium.

• True more diversity on larger islands.

• But also more speciation after colonization.

• Habitat diversity may also play a role.